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THE INVENTIONS

RESEARCHES AND WRITINGS

OF

NIKOLA TESLA



TO HIS COUNTRYMEN

  IN EASTERN EUROPE THIS RECORD OF
  THE WORK ALREADY ACCOMPLISHED BY

  NIKOLA TESLA

  IS RESPECTFULLY DEDICATED



[Illustration: Nikola Tesla]



  THE INVENTIONS
  RESEARCHES AND WRITINGS

  OF

  NIKOLA TESLA


  WITH SPECIAL REFERENCE TO HIS WORK IN POLYPHASE
  CURRENTS AND HIGH POTENTIAL LIGHTING


  BY

  THOMAS COMMERFORD MARTIN

  Editor THE ELECTRICAL ENGINEER; Past-President American Institute
  Electrical Engineers


  1894
  THE ELECTRICAL ENGINEER
  NEW YORK

  D. VAN NOSTRAND COMPANY,
  NEW YORK.



  Entered according to Act of Congress in the year 1893 by
  T. C. MARTIN
  in the office of the Librarian of Congress at Washington


  Press of McIlroy & Emmet, 36 Cortlandt St., N. Y.




PREFACE.


The electrical problems of the present day lie largely in the economical
transmission of power and in the radical improvement of the means and
methods of illumination. To many workers and thinkers in the domain of
electrical invention, the apparatus and devices that are familiar,
appear cumbrous and wasteful, and subject to severe limitations. They
believe that the principles of current generation must be changed, the
area of current supply be enlarged, and the appliances used by the
consumer be at once cheapened and simplified. The brilliant successes of
the past justify them in every expectancy of still more generous
fruition.

The present volume is a simple record of the pioneer work done in such
departments up to date, by Mr. Nikola Tesla, in whom the world has
already recognized one of the foremost of modern electrical
investigators and inventors. No attempt whatever has been made here to
emphasize the importance of his researches and discoveries. Great ideas
and real inventions win their own way, determining their own place by
intrinsic merit. But with the conviction that Mr. Tesla is blazing a
path that electrical development must follow for many years to come, the
compiler has endeavored to bring together all that bears the impress of
Mr. Tesla's genius, and is worthy of preservation. Aside from its value
as showing the scope of his inventions, this volume may be of service as
indicating the range of his thought. There is intellectual profit in
studying the push and play of a vigorous and original mind.

Although the lively interest of the public in Mr. Tesla's work is
perhaps of recent growth, this volume covers the results of full ten
years. It includes his lectures, miscellaneous articles and
discussions, and makes note of all his inventions thus far known,
particularly those bearing on polyphase motors and the effects obtained
with currents of high potential and high frequency. It will be seen that
Mr. Tesla has ever pressed forward, barely pausing for an instant to
work out in detail the utilizations that have at once been obvious to
him of the new principles he has elucidated. Wherever possible his own
language has been employed.

It may be added that this volume is issued with Mr. Tesla's sanction and
approval, and that permission has been obtained for the re-publication
in it of such papers as have been read before various technical
societies of this country and Europe. Mr. Tesla has kindly favored the
author by looking over the proof sheets of the sections embodying his
latest researches. The work has also enjoyed the careful revision of the
author's friend and editorial associate, Mr. Joseph Wetzler, through
whose hands all the proofs have passed.

DECEMBER, 1893.

                                                              T. C. M.




CONTENTS.


PART I.

POLYPHASE CURRENTS.

  CHAPTER I.
  BIOGRAPHICAL AND INTRODUCTORY.                                       3

  CHAPTER II.
  A NEW SYSTEM OF ALTERNATING CURRENT MOTORS AND TRANSFORMERS.         7

  CHAPTER III.
  THE TESLA ROTATING MAGNETIC FIELD.--MOTORS WITH CLOSED
  CONDUCTORS.--SYNCHRONIZING MOTORS.--ROTATING FIELD TRANSFORMERS.     9

  CHAPTER IV.
  MODIFICATIONS AND EXPANSIONS OF THE TESLA POLYPHASE SYSTEMS.        26

  CHAPTER V.
  UTILIZING FAMILIAR TYPES OF GENERATORS OF THE CONTINUOUS CURRENT
  TYPE.                                                               31

  CHAPTER VI.
  METHOD OF OBTAINING DESIRED SPEED OF MOTOR OR GENERATOR.            36

  CHAPTER VII.
  REGULATOR FOR ROTARY CURRENT MOTORS.                                45

  CHAPTER VIII.
  SINGLE CIRCUIT, SELF-STARTING SYNCHRONIZING MOTORS.                 50

  CHAPTER IX.
  CHANGE FROM DOUBLE CURRENT TO SINGLE CURRENT MOTORS.                56

  CHAPTER X.
  MOTOR WITH "CURRENT LAG" ARTIFICIALLY SECURED.                      58

  CHAPTER XI.
  ANOTHER METHOD OF TRANSFORMATION FROM A TORQUE TO A SYNCHRONIZING
  MOTOR.                                                              62

  CHAPTER XII.
  "MAGNETIC LAG" MOTOR.                                               67

  CHAPTER XIII.
  METHOD OF OBTAINING DIFFERENCE OF PHASE BY MAGNETIC SHIELDING.      71

  CHAPTER XIV.
  TYPE OF TESLA SINGLE-PHASE MOTOR.                                   76

  CHAPTER XV.
  MOTORS WITH CIRCUITS OF DIFFERENT RESISTANCE.                       79

  CHAPTER XVI.
  MOTOR WITH EQUAL MAGNETIC ENERGIES IN FIELD AND ARMATURE.           81

  CHAPTER XVII.
  MOTORS WITH COINCIDING MAXIMA OF MAGNETIC EFFECT IN ARMATURE AND
  FIELD.                                                              83

  CHAPTER XVIII.
  MOTOR BASED ON THE DIFFERENCE OF PHASE IN THE MAGNETIZATION OF
  THE INNER AND OUTER PARTS OF AN IRON CORE.                          88

  CHAPTER XIX.
  ANOTHER TYPE OF TESLA INDUCTION MOTOR.                              92

  CHAPTER XX.
  COMBINATIONS OF SYNCHRONIZING MOTOR AND TORQUE MOTOR.               95

  CHAPTER XXI.
  MOTOR WITH A CONDENSER IN THE ARMATURE CIRCUIT.                    101

  CHAPTER XXII.
  MOTOR WITH CONDENSER IN ONE OF THE FIELD CIRCUITS.                 106

  CHAPTER XXIII.
  TESLA POLYPHASE TRANSFORMER.                                       109

  CHAPTER XXIV.
  A CONSTANT CURRENT TRANSFORMER WITH MAGNETIC SHIELD BETWEEN
  COILS OF PRIMARY AND SECONDARY.                                    113


PART II.

THE TESLA EFFECTS WITH HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS.

  CHAPTER XXV.
  INTRODUCTORY.--THE SCOPE OF THE TESLA LECTURES.                    119

  CHAPTER XXVI.
  THE NEW YORK LECTURE. EXPERIMENTS WITH ALTERNATE CURRENTS OF VERY
  HIGH FREQUENCY, AND THEIR APPLICATION TO METHODS OF ARTIFICIAL
  ILLUMINATION, MAY 20, 1891.                                        145

  CHAPTER XXVII.
  THE LONDON LECTURE. EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH
  POTENTIAL AND HIGH FREQUENCY, FEBRUARY 3, 1892.                    198

  CHAPTER XXVIII.
  THE PHILADELPHIA AND ST. LOUIS LECTURE. ON LIGHT AND OTHER HIGH
  FREQUENCY PHENOMENA, FEBRUARY AND MARCH, 1893.                     294

  CHAPTER XXIX.
  TESLA ALTERNATING CURRENT GENERATORS FOR HIGH FREQUENCY.           374

  CHAPTER XXX.
  ALTERNATE CURRENT ELECTROSTATIC INDUCTION APPARATUS.               392

  CHAPTER XXXI.
  "MASSAGE" WITH CURRENTS OF HIGH FREQUENCY.                         394

  CHAPTER XXXII.
  ELECTRIC DISCHARGE IN VACUUM TUBES.                                396


PART III.

MISCELLANEOUS INVENTIONS AND WRITINGS.

  CHAPTER XXXIII.
  METHOD OF OBTAINING DIRECT FROM ALTERNATING CURRENTS.              409

  CHAPTER XXXIV.
  CONDENSERS WITH PLATES IN OIL.                                     418

  CHAPTER XXXV.
  ELECTROLYTIC REGISTERING METER.                                    420

  CHAPTER XXXVI.
  THERMO-MAGNETIC MOTORS AND PYRO-MAGNETIC GENERATORS.               424

  CHAPTER XXXVII.
  ANTI-SPARKING DYNAMO BRUSH AND COMMUTATOR.                         432

  CHAPTER XXXVIII.
  AUXILIARY BRUSH REGULATION OF DIRECT CURRENT DYNAMOS.              438

  CHAPTER XXXIX.
  IMPROVEMENT IN DYNAMO AND MOTOR CONSTRUCTION.                      448

  CHAPTER XL.
  TESLA DIRECT CURRENT ARC LIGHTING SYSTEM.                          451

  CHAPTER XLI.
  IMPROVEMENT IN UNIPOLAR GENERATORS.                                465


PART IV.

APPENDIX: EARLY PHASE MOTORS AND THE TESLA OSCILLATORS.

  CHAPTER XLII.
  MR. TESLA'S PERSONAL EXHIBIT AT THE WORLD'S FAIR.                  477

  CHAPTER XLIII.
  THE TESLA MECHANICAL AND ELECTRICAL OSCILLATORS.                   486




PART I.

POLYPHASE CURRENTS.




CHAPTER I.

BIOGRAPHICAL AND INTRODUCTORY.


As an introduction to the record contained in this volume of Mr. Tesla's
investigations and discoveries, a few words of a biographical nature
will, it is deemed, not be out of place, nor other than welcome.

Nikola Tesla was born in 1857 at Smiljan, Lika, a borderland region of
Austro-Hungary, of the Serbian race, which has maintained against Turkey
and all comers so unceasing a struggle for freedom. His family is an old
and representative one among these Switzers of Eastern Europe, and his
father was an eloquent clergyman in the Greek Church. An uncle is to-day
Metropolitan in Bosnia. His mother was a woman of inherited ingenuity,
and delighted not only in skilful work of the ordinary household
character, but in the construction of such mechanical appliances as
looms and churns and other machinery required in a rural community.
Nikola was educated at Gospich in the public school for four years, and
then spent three years in the Real Schule. He was then sent to Carstatt,
Croatia, where he continued his studies for three years in the Higher
Real Schule. There for the first time he saw a steam locomotive. He
graduated in 1873, and, surviving an attack of cholera, devoted himself
to experimentation, especially in electricity and magnetism. His father
would have had him maintain the family tradition by entering the Church,
but native genius was too strong, and he was allowed to enter the
Polytechnic School at Gratz, to finish his studies, and with the object
of becoming a professor of mathematics and physics. One of the machines
there experimented with was a Gramme dynamo, used as a motor. Despite
his instructor's perfect demonstration of the fact that it was
impossible to operate a dynamo without commutator or brushes, Mr. Tesla
could not be convinced that such accessories were necessary or
desirable. He had already seen with quick intuition that a way could be
found to dispense with them; and from that time he may be said to have
begun work on the ideas that fructified ultimately in his rotating field
motors.

In the second year of his Gratz course, Mr. Tesla gave up the notion of
becoming a teacher, and took up the engineering curriculum. His studies
ended, he returned home in time to see his father die, and then went to
Prague and Buda-Pesth to study languages, with the object of qualifying
himself broadly for the practice of the engineering profession. For a
short time he served as an assistant in the Government Telegraph
Engineering Department, and then became associated with M. Puskas, a
personal and family friend, and other exploiters of the telephone in
Hungary. He made a number of telephonic inventions, but found his
opportunities of benefiting by them limited in various ways. To gain a
wider field of action, he pushed on to Paris and there secured
employment as an electrical engineer with one of the large companies in
the new industry of electric lighting.

It was during this period, and as early as 1882, that he began serious
and continued efforts to embody the rotating field principle in
operative apparatus. He was enthusiastic about it; believed it to mark a
new departure in the electrical arts, and could think of nothing else.
In fact, but for the solicitations of a few friends in commercial
circles who urged him to form a company to exploit the invention, Mr.
Tesla, then a youth of little worldly experience, would have sought an
immediate opportunity to publish his ideas, believing them to be worthy
of note as a novel and radical advance in electrical theory as well as
destined to have a profound influence on all dynamo electric machinery.

At last he determined that it would be best to try his fortunes in
America. In France he had met many Americans, and in contact with them
learned the desirability of turning every new idea in electricity to
practical use. He learned also of the ready encouragement given in the
United States to any inventor who could attain some new and valuable
result. The resolution was formed with characteristic quickness, and
abandoning all his prospects in Europe, he at once set his face
westward.

Arrived in the United States, Mr. Tesla took off his coat the day he
arrived, in the Edison Works. That place had been a goal of his
ambition, and one can readily imagine the benefit and stimulus derived
from association with Mr. Edison, for whom Mr. Tesla has always had the
strongest admiration. It was impossible, however, that, with his own
ideas to carry out, and his own inventions to develop, Mr. Tesla could
long remain in even the most delightful employ; and, his work now
attracting attention, he left the Edison ranks to join a company
intended to make and sell an arc lighting system based on some of his
inventions in that branch of the art. With unceasing diligence he
brought the system to perfection, and saw it placed on the market. But
the thing which most occupied his time and thoughts, however, all
through this period, was his old discovery of the rotating field
principle for alternating current work, and the application of it in
motors that have now become known the world over.

Strong as his convictions on the subject then were, it is a fact that
he stood very much alone, for the alternating current had no well
recognized place. Few electrical engineers had ever used it, and the
majority were entirely unfamiliar with its value, or even its essential
features. Even Mr. Tesla himself did not, until after protracted effort
and experimentation, learn how to construct alternating current
apparatus of fair efficiency. But that he had accomplished his purpose
was shown by the tests of Prof. Anthony, made in the of winter 1887-8,
when Tesla motors in the hands of that distinguished expert gave an
efficiency equal to that of direct current motors. Nothing now stood in
the way of the commercial development and introduction of such motors,
except that they had to be constructed with a view to operating on the
circuits then existing, which in this country were all of high
frequency.

The first full publication of his work in this direction--outside his
patents--was a paper read before the American Institute of Electrical
Engineers in New York, in May, 1888 (read at the suggestion of Prof.
Anthony and the present writer), when he exhibited motors that had been
in operation long previous, and with which his belief that brushes and
commutators could be dispensed with, was triumphantly proved to be
correct. The section of this volume devoted to Mr. Tesla's inventions in
the utilization of polyphase currents will show how thoroughly from the
outset he had mastered the fundamental idea and applied it in the
greatest variety of ways.

Having noted for years the many advantages obtainable with alternating
currents, Mr. Tesla was naturally led on to experiment with them at
higher potentials and higher frequencies than were common or approved
of. Ever pressing forward to determine in even the slightest degree the
outlines of the unknown, he was rewarded very quickly in this field
with results of the most surprising nature. A slight acquaintance with
some of these experiments led the compiler of this volume to urge Mr.
Tesla to repeat them before the American Institute of Electrical
Engineers. This was done in May, 1891, in a lecture that marked, beyond
question, a distinct departure in electrical theory and practice, and
all the results of which have not yet made themselves fully apparent.
The New York lecture, and its successors, two in number, are also
included in this volume, with a few supplementary notes.

Mr. Tesla's work ranges far beyond the vast departments of polyphase
currents and high potential lighting. The "Miscellaneous" section of
this volume includes a great many other inventions in arc lighting,
transformers, pyro-magnetic generators, thermo-magnetic motors,
third-brush regulation, improvements in dynamos, new forms of
incandescent lamps, electrical meters, condensers, unipolar dynamos, the
conversion of alternating into direct currents, etc. It is needless to
say that at this moment Mr. Tesla is engaged on a number of interesting
ideas and inventions, to be made public in due course. The present
volume deals simply with his work accomplished to date.




CHAPTER II.

A NEW SYSTEM OF ALTERNATING CURRENT MOTORS AND TRANSFORMERS.


The present section of this volume deals with polyphase currents, and
the inventions by Mr. Tesla, made known thus far, in which he has
embodied one feature or another of the broad principle of rotating field
poles or _resultant attraction_ exerted on the armature. It is needless
to remind electricians of the great interest aroused by the first
enunciation of the rotating field principle, or to dwell upon the
importance of the advance from a single alternating current, to methods
and apparatus which deal with more than one. Simply prefacing the
consideration here attempted of the subject, with the remark that in
nowise is the object of this volume of a polemic or controversial
nature, it may be pointed out that Mr. Tesla's work has not at all been
fully understood or realized up to date. To many readers, it is
believed, the analysis of what he has done in this department will be a
revelation, while it will at the same time illustrate the beautiful
flexibility and range of the principles involved. It will be seen that,
as just suggested, Mr. Tesla did not stop short at a mere rotating
field, but dealt broadly with the shifting of the resultant attraction
of the magnets. It will be seen that he went on to evolve the
"multiphase" system with many ramifications and turns; that he showed
the broad idea of motors employing currents of differing phase in the
armature with direct currents in the field; that he first described and
worked out the idea of an armature with a body of iron and coils closed
upon themselves; that he worked out both synchronizing and torque
motors; that he explained and illustrated how machines of ordinary
construction might be adapted to his system; that he employed condensers
in field and armature circuits, and went to the bottom of the
fundamental principles, testing, approving or rejecting, it would
appear, every detail that inventive ingenuity could hit upon.

Now that opinion is turning so emphatically in favor of lower
frequencies, it deserves special note that Mr. Tesla early recognized
the importance of the low frequency feature in motor work. In fact his
first motors exhibited publicly--and which, as Prof. Anthony showed in
his tests in the winter of 1887-8, were the equal of direct current
motors in efficiency, output and starting torque--were of the low
frequency type. The necessity arising, however, to utilize these motors
in connection with the existing high frequency circuits, our survey
reveals in an interesting manner Mr. Tesla's fertility of resource in
this direction. But that, after exhausting all the possibilities of this
field, Mr. Tesla returns to low frequencies, and insists on the
superiority of his polyphase system in alternating current distribution,
need not at all surprise us, in view of the strength of his convictions,
so often expressed, on this subject. This is, indeed, significant, and
may be regarded as indicative of the probable development next to be
witnessed.

Incidental reference has been made to the efficiency of rotating field
motors, a matter of much importance, though it is not the intention to
dwell upon it here. Prof. Anthony in his remarks before the American
Institute of Electrical Engineers, in May, 1888, on the two small Tesla
motors then shown, which he had tested, stated that one gave an
efficiency of about 50 per cent. and the other a little over sixty per
cent. In 1889, some tests were reported from Pittsburgh, made by Mr.
Tesla and Mr. Albert Schmid, on motors up to 10 H. P. and weighing about
850 pounds. These machines showed an efficiency of nearly 90 per cent.
With some larger motors it was then found practicable to obtain an
efficiency, with the three wire system, up to as high as 94 and 95 per
cent. These interesting figures, which, of course, might be supplemented
by others more elaborate and of later date, are cited to show that the
efficiency of the system has not had to wait until the present late day
for any demonstration of its commercial usefulness. An invention is none
the less beautiful because it may lack utility, but it must be a
pleasure to any inventor to know that the ideas he is advancing are
fraught with substantial benefits to the public.




CHAPTER III.

THE TESLA ROTATING MAGNETIC FIELD.--MOTORS WITH CLOSED
CONDUCTORS.--SYNCHRONIZING MOTORS.--ROTATING FIELD TRANSFORMERS.


The best description that can be given of what he attempted, and
succeeded in doing, with the rotating magnetic field, is to be found in
Mr. Tesla's brief paper explanatory of his rotary current, polyphase
system, read before the American Institute of Electrical Engineers, in
New York, in May, 1888, under the title "A New System of Alternate
Current Motors and Transformers." As a matter of fact, which a perusal
of the paper will establish, Mr. Tesla made no attempt in that paper to
describe all his work. It dealt in reality with the few topics
enumerated in the caption of this chapter. Mr. Tesla's reticence was no
doubt due largely to the fact that his action was governed by the wishes
of others with whom he was associated, but it may be worth mention that
the compiler of this volume--who had seen the motors running, and who
was then chairman of the Institute Committee on Papers and Meetings--had
great difficulty in inducing Mr. Tesla to give the Institute any paper
at all. Mr. Tesla was overworked and ill, and manifested the greatest
reluctance to an exhibition of his motors, but his objections were at
last overcome. The paper was written the night previous to the meeting,
in pencil, very hastily, and under the pressure just mentioned.

In this paper casual reference was made to two special forms of motors
not within the group to be considered. These two forms were: 1. A motor
with one of its circuits in series with a transformer, and the other in
the secondary of the transformer. 2. A motor having its armature circuit
connected to the generator, and the field coils closed upon themselves.
The paper in its essence is as follows, dealing with a few leading
features of the Tesla system, namely, the rotating magnetic field,
motors with closed conductors, synchronizing motors, and rotating field
transformers:--

The subject which I now have the pleasure of bringing to your notice is
a novel system of electric distribution and transmission of power by
means of alternate currents, affording peculiar advantages, particularly
in the way of motors, which I am confident will at once establish the
superior adaptability of these currents to the transmission of power and
will show that many results heretofore unattainable can be reached by
their use; results which are very much desired in the practical
operation of such systems, and which cannot be accomplished by means of
continuous currents.

Before going into a detailed description of this system, I think it
necessary to make a few remarks with reference to certain conditions
existing in continuous current generators and motors, which, although
generally known, are frequently disregarded.

In our dynamo machines, it is well known, we generate alternate currents
which we direct by means of a commutator, a complicated device and, it
may be justly said, the source of most of the troubles experienced in
the operation of the machines. Now, the currents so directed cannot be
utilized in the motor, but they must--again by means of a similar
unreliable device--be reconverted into their original state of alternate
currents. The function of the commutator is entirely external, and in no
way does it affect the internal working of the machines. In reality,
therefore, all machines are alternate current machines, the currents
appearing as continuous only in the external circuit during their
transit from generator to motor. In view simply of this fact, alternate
currents would commend themselves as a more direct application of
electrical energy, and the employment of continuous currents would only
be justified if we had dynamos which would primarily generate, and
motors which would be directly actuated by, such currents.

But the operation of the commutator on a motor is twofold; first, it
reverses the currents through the motor, and secondly, it effects
automatically, a progressive shifting of the poles of one of its
magnetic constituents. Assuming, therefore, that both of the useless
operations in the systems, that is to say, the directing of the
alternate currents on the generator and reversing the direct currents on
the motor, be eliminated, it would still be necessary, in order to cause
a rotation of the motor, to produce a progressive shifting of the poles
of one of its elements, and the question presented itself--How to
perform this operation by the direct action of alternate currents? I
will now proceed to show how this result was accomplished.

[Illustration: FIG. 1.]

[Illustration: FIG. 1a.]

[Illustration: FIG. 2.]

[Illustration: FIG. 2a.]

In the first experiment a drum-armature was provided with two coils at
right angles to each other, and the ends of these coils were connected
to two pairs of insulated contact-rings as usual. A ring was then made
of thin insulated plates of sheet-iron and wound with four coils, each
two opposite coils being connected together so as to produce free poles
on diametrically opposite sides of the ring. The remaining free ends of
the coils were then connected to the contact-rings of the generator
armature so as to form two independent circuits, as indicated in Fig. 9.
It may now be seen what results were secured in this combination, and
with this view I would refer to the diagrams, Figs. 1 to 8_a_. The field
of the generator being independently excited, the rotation of the
armature sets up currents in the coils C C_{1}, varying in strength and
direction in the well-known manner. In the position shown in Fig. 1, the
current in coil C is nil, while coil C_{1} is traversed by its maximum
current, and the connections may be such that the ring is magnetized by
the coils c_{1} c_{1}, as indicated by the letters N S in Fig. 1_a_,
the magnetizing effect of the coils c c being nil, since these coils
are included in the circuit of coil C.

[Illustration: FIG. 3.]

[Illustration: FIG. 3a.]

In Fig. 2, the armature coils are shown in a more advanced position,
one-eighth of one revolution being completed. Fig. 2_a_ illustrates the
corresponding magnetic condition of the ring. At this moment the coil
C_{1} generates a current of the same direction as previously, but
weaker, producing the poles n_{1} s_{1} upon the ring; the coil C also
generates a current of the same direction, and the connections may be
such that the coils c c produce the poles n s, as shown in Fig. 2_a_.
The resulting polarity is indicated by the letters N S, and it will be
observed that the poles of the ring have been shifted one-eighth of the
periphery of the same.

[Illustration: FIG. 4.]

[Illustration: FIG. 4a.]

In Fig. 3 the armature has completed one quarter of one revolution. In
this phase the current in coil C is a maximum, and of such direction as
to produce the poles N S in Fig. 3_a_, whereas the current in coil C_{1}
is nil, this coil being at its neutral position. The poles N S in Fig.
3_a_ are thus shifted one quarter of the circumference of the ring.

Fig. 4 shows the coils C C in a still more advanced position, the
armature having completed three-eighths of one revolution. At that
moment the coil C still generates a current of the same direction as
before, but of less strength, producing the comparatively weaker poles
n s in Fig. 4_a_. The current in the coil C_{1} is of the same strength,
but opposite direction. Its effect is, therefore, to produce upon the
ring the poles n_{1} s_{1}, as indicated, and a polarity, N S, results,
the poles now being shifted three-eighths of the periphery of the ring.

[Illustration: FIG. 5.]

[Illustration: FIG. 5a.]

In Fig. 5 one half of one revolution of the armature is completed, and
the resulting magnetic condition of the ring is indicated in Fig. 5_a_.
Now the current in coil C is nil, while the coil C_{1} yields its
maximum current, which is of the same direction as previously; the
magnetizing effect is, therefore, due to the coils, c_{1} c_{1} alone,
and, referring to Fig. 5_a_, it will be observed that the poles N S are
shifted one half of the circumference of the ring. During the next half
revolution the operations are repeated, as represented in the Figs. 6 to
8_a_.

[Illustration: FIG. 6.]

[Illustration: FIG. 6a.]

A reference to the diagrams will make it clear that during one
revolution of the armature the poles of the ring are shifted once around
its periphery, and, each revolution producing like effects, a rapid
whirling of the poles in harmony with the rotation of the armature is
the result. If the connections of either one of the circuits in the ring
are reversed, the shifting of the poles is made to progress in the
opposite direction, but the operation is identically the same. Instead
of using four wires, with like result, three wires may be used, one
forming a common return for both circuits.

[Illustration: FIG. 7.]

[Illustration: FIG. 7_a_.]

This rotation or whirling of the poles manifests itself in a series of
curious phenomena. If a delicately pivoted disc of steel or other
magnetic metal is approached to the ring it is set in rapid rotation,
the direction of rotation varying with the position of the disc. For
instance, noting the direction outside of the ring it will be found that
inside the ring it turns in an opposite direction, while it is
unaffected if placed in a position symmetrical to the ring. This is
easily explained. Each time that a pole approaches, it induces an
opposite pole in the nearest point on the disc, and an attraction is
produced upon that point; owing to this, as the pole is shifted further
away from the disc a tangential pull is exerted upon the same, and the
action being constantly repeated, a more or less rapid rotation of the
disc is the result. As the pull is exerted mainly upon that part which
is nearest to the ring, the rotation outside and inside, or right and
left, respectively, is in opposite directions, Fig. 9. When placed
symmetrically to the ring, the pull on the opposite sides of the disc
being equal, no rotation results. The action is based on the magnetic
inertia of iron; for this reason a disc of hard steel is much more
affected than a disc of soft iron, the latter being capable of very
rapid variations of magnetism. Such a disc has proved to be a very
useful instrument in all these investigations, as it has enabled me to
detect any irregularity in the action. A curious effect is also produced
upon iron filings. By placing some upon a paper and holding them
externally quite close to the ring, they are set in a vibrating motion,
remaining in the same place, although the paper may be moved back and
forth; but in lifting the paper to a certain height which seems to be
dependent on the intensity of the poles and the speed of rotation, they
are thrown away in a direction always opposite to the supposed movement
of the poles. If a paper with filings is put flat upon the ring and the
current turned on suddenly, the existence of a magnetic whirl may easily
be observed.

To demonstrate the complete analogy between the ring and a revolving
magnet, a strongly energized electro-magnet was rotated by mechanical
power, and phenomena identical in every particular to those mentioned
above were observed.

Obviously, the rotation of the poles produces corresponding inductive
effects and may be utilized to generate currents in a closed conductor
placed within the influence of the poles. For this purpose it is
convenient to wind a ring with two sets of superimposed coils forming
respectively the primary and secondary circuits, as shown in Fig. 10. In
order to secure the most economical results the magnetic circuit should
be completely closed, and with this object in view the construction may
be modified at will.

[Illustration: FIG. 8.]

[Illustration: FIG. 8_a_.]

The inductive effect exerted upon the secondary coils will be mainly due
to the shifting or movement of the magnetic action; but there may also
be currents set up in the circuits in consequence of the variations in
the intensity of the poles. However, by properly designing the generator
and determining the magnetizing effect of the primary coils, the latter
element may be made to disappear. The intensity of the poles being
maintained constant, the action of the apparatus will be perfect, and
the same result will be secured as though the shifting were effected by
means of a commutator with an infinite number of bars. In such case the
theoretical relation between the energizing effect of each set of
primary coils and their resultant magnetizing effect may be expressed by
the equation of a circle having its centre coinciding with that of an
orthogonal system of axes, and in which the radius represents the
resultant and the co-ordinates both of its components. These are then
respectively the sine and cosine of the angle _a_ between the radius and
one of the axes (_OX_). Referring to Fig. 11, we have r^2 = x^2 + y^2;
where x = r cos _a_, and y = r sin _a_.

Assuming the magnetizing effect of each set of coils in the transformer
to be proportional to the current--which may be admitted for weak
degrees of magnetization--then x = Kc and y = Kc^1, where K is a
constant and c and c^1 the current in both sets of coils respectively.
Supposing, further, the field of the generator to be uniform, we have
for constant speed

  c^1 = K^1 sin _a_ and
  c = K^1 sin (90° + _a_) = K^1 cos _a_,

where K^1 is a constant. See Fig. 12.

Therefore,

  x = Kc = K K^1 cos _a_;
  y = Kc^1 = K K^1 sin _a_; and
  K K^1 = r.

[Illustration: FIG. 9.]

That is, for a uniform field the disposition of the two coils at right
angles will secure the theoretical result, and the intensity of the
shifting poles will be constant. But from r^2 = x^2 + y^2 it follows
that for y = 0, r = x; it follows that the joint magnetizing effect
of both sets of coils should be equal to the effect of one set when at
its maximum action. In transformers and in a certain class of motors the
fluctuation of the poles is not of great importance, but in another
class of these motors it is desirable to obtain the theoretical result.

In applying this principle to the construction of motors, two typical
forms of motor have been developed. First, a form having a comparatively
small rotary effort at the start but maintaining a perfectly uniform
speed at all loads, which motor has been termed synchronous. Second, a
form possessing a great rotary effort at the start, the speed being
dependent on the load.

These motors may be operated in three different ways: 1. By the
alternate currents of the source only. 2. By a combined action of these
and of induced currents. 3. By the joint action of alternate and
continuous currents.

[Illustration: FIG. 10.]

The simplest form of a synchronous motor is obtained by winding a
laminated ring provided with pole projections with four coils, and
connecting the same in the manner before indicated. An iron disc having
a segment cut away on each side may be used as an armature. Such a motor
is shown in Fig. 9. The disc being arranged to rotate freely within the
ring in close proximity to the projections, it is evident that as the
poles are shifted it will, owing to its tendency to place itself in such
a position as to embrace the greatest number of the lines of force,
closely follow the movement of the poles, and its motion will be
synchronous with that of the armature of the generator; that is, in the
peculiar disposition shown in Fig. 9, in which the armature produces by
one revolution two current impulses in each of the circuits. It is
evident that if, by one revolution of the armature, a greater number of
impulses is produced, the speed of the motor will be correspondingly
increased. Considering that the attraction exerted upon the disc is
greatest when the same is in close proximity to the poles, it follows
that such a motor will maintain exactly the same speed at all loads
within the limits of its capacity.

To facilitate the starting, the disc may be provided with a coil closed
upon itself. The advantage secured by such a coil is evident. On the
start the currents set up in the coil strongly energize the disc and
increase the attraction exerted upon the same by the ring, and currents
being generated in the coil as long as the speed of the armature is
inferior to that of the poles, considerable work may be performed by
such a motor even if the speed be below normal. The intensity of the
poles being constant, no currents will be generated in the coil when the
motor is turning at its normal speed.

Instead of closing the coil upon itself, its ends may be connected to
two insulated sliding rings, and a continuous current supplied to these
from a suitable generator. The proper way to start such a motor is to
close the coil upon itself until the normal speed is reached, or nearly
so, and then turn on the continuous current. If the disc be very
strongly energized by a continuous current the motor may not be able to
start, but if it be weakly energized, or generally so that the
magnetizing effect of the ring is preponderating, it will start and
reach the normal speed. Such a motor will maintain absolutely the same
speed at all loads. It has also been found that if the motive power of
the generator is not excessive, by checking the motor the speed of the
generator is diminished in synchronism with that of the motor. It is
characteristic of this form of motor that it cannot be reversed by
reversing the continuous current through the coil.

[Illustration: FIG. 11.]

[Illustration: FIG. 12.]

The synchronism of these motors may be demonstrated experimentally in a
variety of ways. For this purpose it is best to employ a motor
consisting of a stationary field magnet and an armature arranged to
rotate within the same, as indicated in Fig. 13. In this case the
shifting of the poles of the armature produces a rotation of the latter
in the opposite direction. It results therefrom that when the normal
speed is reached, the poles of the armature assume fixed positions
relatively to the field magnet, and the same is magnetized by
induction, exhibiting a distinct pole on each of the pole-pieces. If a
piece of soft iron is approached to the field magnet, it will at the
start be attracted with a rapid vibrating motion produced by the
reversals of polarity of the magnet, but as the speed of the armature
increases, the vibrations become less and less frequent and finally
entirely cease. Then the iron is weakly but permanently attracted,
showing that synchronism is reached and the field magnet energized by
induction.

The disc may also be used for the experiment. If held quite close to the
armature it will turn as long as the speed of rotation of the poles
exceeds that of the armature; but when the normal speed is reached, or
very nearly so, it ceases to rotate and is permanently attracted.

[Illustration: FIG. 13.]

A crude but illustrative experiment is made with an incandescent lamp.
Placing the lamp in circuit with the continuous current generator and in
series with the magnet coil, rapid fluctuations are observed in the
light in consequence of the induced currents set up in the coil at the
start; the speed increasing, the fluctuations occur at longer intervals,
until they entirely disappear, showing that the motor has attained its
normal speed. A telephone receiver affords a most sensitive instrument;
when connected to any circuit in the motor the synchronism may be easily
detected on the disappearance of the induced currents.

In motors of the synchronous type it is desirable to maintain the
quantity of the shifting magnetism constant, especially if the magnets
are not properly subdivided.

To obtain a rotary effort in these motors was the subject of long
thought. In order to secure this result it was necessary to make such a
disposition that while the poles of one element of the motor are shifted
by the alternate currents of the source, the poles produced upon the
other elements should always be maintained in the proper relation to the
former, irrespective of the speed of the motor. Such a condition exists
in a continuous current motor; but in a synchronous motor, such as
described, this condition is fulfilled only when the speed is normal.

[Illustration: FIG. 14.]

The object has been attained by placing within the ring a properly
subdivided cylindrical iron core wound with several independent coils
closed upon themselves. Two coils at right angles as in Fig. 14, are
sufficient, but a greater number may be advantageously employed. It
results from this disposition that when the poles of the ring are
shifted, currents are generated in the closed armature coils. These
currents are the most intense at or near the points of the greatest
density of the lines of force, and their effect is to produce poles upon
the armature at right angles to those of the ring, at least
theoretically so; and since this action is entirely independent of the
speed--that is, as far as the location of the poles is concerned--a
continuous pull is exerted upon the periphery of the armature. In many
respects these motors are similar to the continuous current motors. If
load is put on, the speed, and also the resistance of the motor, is
diminished and more current is made to pass through the energizing
coils, thus increasing the effort. Upon the load being taken off, the
counter-electromotive force increases and less current passes through
the primary or energizing coils. Without any load the speed is very
nearly equal to that of the shifting poles of the field magnet.

[Illustration: FIG. 15.]

[Illustration: FIG. 16.]

[Illustration: FIG. 17.]

It will be found that the rotary effort in these motors fully equals
that of the continuous current motors. The effort seems to be greatest
when both armature and field magnet are without any projections; but as
in such dispositions the field cannot be concentrated, probably the best
results will be obtained by leaving pole projections on one of the
elements only. Generally, it may be stated the projections diminish the
torque and produce a tendency to synchronism.

A characteristic feature of motors of this kind is their property of
being very rapidly reversed. This follows from the peculiar action of
the motor. Suppose the armature to be rotating and the direction of
rotation of the poles to be reversed. The apparatus then represents a
dynamo machine, the power to drive this machine being the momentum
stored up in the armature and its speed being the sum of the speeds of
the armature and the poles.

[Illustration: FIG. 18.]

[Illustration: FIG. 19.]

[Illustration: FIG. 20.]

[Illustration: FIG. 21.]

If we now consider that the power to drive such a dynamo would be very
nearly proportional to the third power of the speed, for that reason
alone the armature should be quickly reversed. But simultaneously with
the reversal another element is brought into action, namely, as the
movement of the poles with respect to the armature is reversed, the
motor acts like a transformer in which the resistance of the secondary
circuit would be abnormally diminished by producing in this circuit an
additional electromotive force. Owing to these causes the reversal is
instantaneous.

If it is desirable to secure a constant speed, and at the same time a
certain effort at the start, this result may be easily attained in a
variety of ways. For instance, two armatures, one for torque and the
other for synchronism, may be fastened on the same shaft and any desired
preponderance may be given to either one, or an armature may be wound
for rotary effort, but a more or less pronounced tendency to synchronism
may be given to it by properly constructing the iron core; and in many
other ways.

As a means of obtaining the required phase of the currents in both the
circuits, the disposition of the two coils at right angles is the
simplest, securing the most uniform action; but the phase may be
obtained in many other ways, varying with the machine employed. Any of
the dynamos at present in use may be easily adapted for this purpose by
making connections to proper points of the generating coils. In closed
circuit armatures, such as used in the continuous current systems, it is
best to make four derivations from equi-distant points or bars of the
commutator, and to connect the same to four insulated sliding rings on
the shaft. In this case each of the motor circuits is connected to two
diametrically opposite bars of the commutator. In such a disposition the
motor may also be operated at half the potential and on the three-wire
plan, by connecting the motor circuits in the proper order to three of
the contact rings.

In multipolar dynamo machines, such as used in the converter systems,
the phase is conveniently obtained by winding upon the armature two
series of coils in such a manner that while the coils of one set or
series are at their maximum production of current, the coils of the
other will be at their neutral position, or nearly so, whereby both sets
of coils may be subjected simultaneously or successively to the inducing
action of the field magnets.

Generally the circuits in the motor will be similarly disposed, and
various arrangements may be made to fulfill the requirements; but the
simplest and most practicable is to arrange primary circuits on
stationary parts of the motor, thereby obviating, at least in certain
forms, the employment of sliding contacts. In such a case the magnet
coils are connected alternately in both the circuits; that is, 1, 3,
5 ... in one, and 2, 4, 6 ... in the other, and the coils of each set
of series may be connected all in the same manner, or alternately in
opposition; in the latter case a motor with half the number of poles
will result, and its action will be correspondingly modified. The Figs.
15, 16, and 17, show three different phases, the magnet coils in each
circuit being connected alternately in opposition. In this case there
will be always four poles, as in Figs. 15 and 17; four pole projections
will be neutral; and in Fig. 16 two adjacent pole projections will have
the same polarity. If the coils are connected in the same manner there
will be eight alternating poles, as indicated by the letters n' s'
in Fig. 15.

The employment of multipolar motors secures in this system an advantage
much desired and unattainable in the continuous current system, and that
is, that a motor may be made to run exactly at a predetermined speed
irrespective of imperfections in construction, of the load, and, within
certain limits, of electromotive force and current strength.

In a general distribution system of this kind the following plan should
be adopted. At the central station of supply a generator should be
provided having a considerable number of poles. The motors operated from
this generator should be of the synchronous type, but possessing
sufficient rotary effort to insure their starting. With the observance
of proper rules of construction it may be admitted that the speed of
each motor will be in some inverse proportion to its size, and the
number of poles should be chosen accordingly. Still, exceptional demands
may modify this rule. In view of this, it will be advantageous to
provide each motor with a greater number of pole projections or coils,
the number being preferably a multiple of two and three. By this means,
by simply changing the connections of the coils, the motor may be
adapted to any probable demands.

If the number of the poles in the motor is even, the action will be
harmonious and the proper result will be obtained; if this is not the
case, the best plan to be followed is to make a motor with a double
number of poles and connect the same in the manner before indicated, so
that half the number of poles result. Suppose, for instance, that the
generator has twelve poles, and it would be desired to obtain a speed
equal to 12/7 of the speed of the generator. This would require a motor
with seven pole projections or magnets, and such a motor could not be
properly connected in the circuits unless fourteen armature coils would
be provided, which would necessitate the employment of sliding
contacts. To avoid this, the motor should be provided with fourteen
magnets and seven connected in each circuit, the magnets in each circuit
alternating among themselves. The armature should have fourteen closed
coils. The action of the motor will not be quite as perfect as in the
case of an even number of poles, but the drawback will not be of a
serious nature.

However, the disadvantages resulting from this unsymmetrical form will
be reduced in the same proportion as the number of the poles is
augmented.

If the generator has, say, n, and the motor n_{1} poles, the speed of
the motor will be equal to that of the generator multiplied by n/n_{1}.

The speed of the motor will generally be dependent on the number of the
poles, but there may be exceptions to this rule. The speed may be
modified by the phase of the currents in the circuit or by the character
of the current impulses or by intervals between each or between groups
of impulses. Some of the possible cases are indicated in the diagrams,
Figs. 18, 19, 20 and 21, which are self-explanatory. Fig. 18 represents
the condition generally existing, and which secures the best result. In
such a case, if the typical form of motor illustrated in Fig. 9 is
employed, one complete wave in each circuit will produce one revolution
of the motor. In Fig. 19 the same result will be effected by one wave in
each circuit, the impulses being successive; in Fig. 20 by four, and in
Fig. 21 by eight waves.

By such means any desired speed may be attained, that is, at least
within the limits of practical demands. This system possesses this
advantage, besides others, resulting from simplicity. At full loads the
motors show an efficiency fully equal to that of the continuous current
motors. The transformers present an additional advantage in their
capability of operating motors. They are capable of similar
modifications in construction, and will facilitate the introduction of
motors and their adaptation to practical demands. Their efficiency
should be higher than that of the present transformers, and I base my
assertion on the following:

In a transformer, as constructed at present, we produce the currents in
the secondary circuit by varying the strength of the primary or exciting
currents. If we admit proportionality with respect to the iron core the
inductive effect exerted upon the secondary coil will be proportional
to the numerical sum of the variations in the strength of the exciting
current per unit of time; whence it follows that for a given variation
any prolongation of the primary current will result in a proportional
loss. In order to obtain rapid variations in the strength of the
current, essential to efficient induction, a great number of undulations
are employed; from this practice various disadvantages result. These
are: Increased cost and diminished efficiency of the generator; more
waste of energy in heating the cores, and also diminished output of the
transformer, since the core is not properly utilized, the reversals
being too rapid. The inductive effect is also very small in certain
phases, as will be apparent from a graphic representation, and there may
be periods of inaction, if there are intervals between the succeeding
current impulses or waves. In producing a shifting of the poles in a
transformer, and thereby inducing currents, the induction is of the
ideal character, being always maintained at its maximum action. It is
also reasonable to assume that by a shifting of the poles less energy
will be wasted than by reversals.




CHAPTER IV.

MODIFICATIONS AND EXPANSIONS OF THE TESLA POLYPHASE SYSTEMS.


In his earlier papers and patents relative to polyphase currents, Mr.
Tesla devoted himself chiefly to an enunciation of the broad lines and
ideas lying at the basis of this new work; but he supplemented this
immediately by a series of other striking inventions which may be
regarded as modifications and expansions of certain features of the
Tesla systems. These we shall now proceed to deal with.

In the preceding chapters we have thus shown and described the Tesla
electrical systems for the transmission of power and the conversion and
distribution of electrical energy, in which the motors and the
transformers contain two or more coils or sets of coils, which were
connected up in independent circuits with corresponding coils of an
alternating current generator, the operation of the system being brought
about by the co-operation of the alternating currents in the independent
circuits in progressively moving or shifting the poles or points of
maximum magnetic effect of the motors or converters. In these systems
two independent conductors are employed for each of the independent
circuits connecting the generator with the devices for converting the
transmitted currents into mechanical energy or into electric currents of
another character. This, however, is not always necessary. The two or
more circuits may have a single return path or wire in common, with a
loss, if any, which is so extremely slight that it may be disregarded
entirely. For the sake of illustration, if the generator have two
independent coils and the motor two coils or two sets of coils in
corresponding relations to its operative elements one terminal of each
generator coil is connected to the corresponding terminals of the motor
coils through two independent conductors, while the opposite terminals
of the respective coils are both connected to one return wire. The
following description deals with the modification. Fig. 22 is a
diagrammatic illustration of a generator and single motor constructed
and electrically connected in accordance with the invention. Fig. 23 is
a diagram of the system as it is used in operating motors or converters,
or both, in parallel, while Fig. 24 illustrates diagrammatically the
manner of operating two or more motors or converters, or both, in
series. Referring to Fig. 22, A A designate the poles of the field
magnets of an alternating-current generator, the armature of which,
being in this case cylindrical in form and mounted on a shaft, C, is
wound longitudinally with coils B B'. The shaft C carries three
insulated contact-rings, _a b c_, to two of which, as _b c_, one
terminal of each coil, as _e d_, is connected. The remaining terminals,
_f g_, are both connected to the third ring, _a_.

[Illustration: FIG. 22.]

[Illustration: FIG. 24.]

A motor in this case is shown as composed of a ring, H, wound with four
coils, I I J J, electrically connected, so as to co-operate in pairs,
with a tendency to fix the poles of the ring at four points ninety
degrees apart. Within the magnetic ring H is a disc or cylindrical core
wound with two coils, G G', which may be connected to form two closed
circuits. The terminals _j k_ of the two sets or pairs of coils are
connected, respectively, to the binding-posts E' F', and the other
terminals, _h i_, are connected to a single binding-post, D'. To operate
the motor, three line-wires are used to connect the terminals of the
generator with those of the motor.

[Illustration: FIG. 23.]

So far as the apparent action or mode of operation of this arrangement
is concerned, the single wire D, which is, so to speak, a common
return-wire for both circuits, may be regarded as two independent wires.
In the illustration, with the order of connection shown, coil B' of the
generator is producing its maximum current and coil B its minimum; hence
the current which passes through wire e, ring b, brush b', line-wire E,
terminal E', wire j, coils I I, wire or terminal D', line-wire D, brush
_a'_, ring _a_, and wire _f_, fixes the polar line of the motor midway
between the two coils I I; but as the coil B' moves from the position
indicated it generates less current, while coil B, moving into the
field, generates more. The current from coil B passes through the
devices and wires designated by the letters _d_, _c_, C' F, F' _k_, J J,
_i_, D', D, _a'_, _a_, and _g_, and the position of the poles of the
motor will be due to the resultant effect of the currents in the two
sets of coils--that is, it will be advanced in proportion to the advance
or forward movement of the armature coils. The movement of the
generator-armature through one-quarter of a revolution will obviously
bring coil B' into its neutral position and coil B into its position of
maximum effect, and this shifts the poles ninety degrees, as they are
fixed solely by coils B. This action is repeated for each quarter of a
complete revolution.

When more than one motor or other device is employed, they may be run
either in parallel or series. In Fig. 23 the former arrangement is
shown. The electrical device is shown as a converter, L, of which the
two sets of primary coils _p r_ are connected, respectively, to the
mains F E, which are electrically connected with the two coils of the
generator. The cross-circuit wires _l m_, making these connections, are
then connected to the common return-wire D. The secondary coils _p' p''_
are in circuits _n o_, including, for example, incandescent lamps. Only
one converter is shown entire in this figure, the others being
illustrated diagrammatically.

When motors or converters are to be run in series, the two wires E F are
led from the generator to the coils of the first motor or converter,
then continued on to the next, and so on through the whole series, and
are then joined to the single wire D, which completes both circuits
through the generator. This is shown in Fig. 24, in which J I represent
the two coils or sets of coils of the motors.

There are, of course, other conditions under which the same idea may be
carried out. For example, in case the motor and generator each has three
independent circuits, one terminal of each circuit is connected to a
line-wire, and the other three terminals to a common return-conductor.
This arrangement will secure similar results to those attained with a
generator and motor having but two independent circuits, as above
described.

When applied to such machines and motors as have three or more induced
circuits with a common electrical joint, the three or more terminals of
the generator would be simply connected to those of the motor. Mr.
Tesla states, however, that the results obtained in this manner show a
lower efficiency than do the forms dwelt upon more fully above.




CHAPTER V.

UTILIZING FAMILIAR TYPES OF GENERATOR OF THE CONTINUOUS CURRENT TYPE.


The preceding descriptions have assumed the use of alternating current
generators in which, in order to produce the progressive movement of the
magnetic poles, or of the resultant attraction of independent field
magnets, the current generating coils are independent or separate. The
ordinary forms of continuous current dynamos may, however, be employed
for the same work, in accordance with a method of adaptation devised by
Mr. Tesla. As will be seen, the modification involves but slight changes
in their construction, and presents other elements of economy.

On the shaft of a given generator, either in place of or in addition to
the regular commutator, are secured as many pairs of insulated
collecting-rings as there are circuits to be operated. Now, it will be
understood that in the operation of any dynamo electric generator the
currents in the coils in their movement through the field of force
undergo different phases--that is to say, at different positions of the
coils the currents have certain directions and certain strengths--and
that in the Tesla motors or transformers it is necessary that the
currents in the energizing coils should undergo a certain order of
variations in strength and direction. Hence, the further step--viz., the
connection between the induced or generating coils of the machine and
the contact-rings from which the currents are to be taken off--will be
determined solely by what order of variations of strength and direction
in the currents is desired for producing a given result in the
electrical translating device. This may be accomplished in various ways;
but in the drawings we give typical instances only of the best and most
practicable ways of applying the invention to three of the leading types
of machines in widespread use, in order to illustrate the principle.

Fig. 25 is a diagram illustrative of the mode of applying the invention
to the well-known type of "closed" or continuous circuit machines. Fig.
26 is a similar diagram embodying an armature with separate coils
connected diametrically, or what is generally called an "open-circuit"
machine. Fig. 27 is a diagram showing the application of the invention
to a machine the armature-coils of which have a common joint.

[Illustration: FIG. 25.]

Referring to Fig. 25, let A represent a Tesla motor or transformer
which, for convenience, we will designate as a "converter." It consists
of an annular core, B, wound with four independent coils, C and D, those
diametrically opposite being connected together so as to co-operate in
pairs in establishing free poles in the ring, the tendency of each pair
being to fix the poles at ninety degrees from the other. There may be an
armature, E, within the ring, which is wound with coils closed upon
themselves. The object is to pass through coils C D currents of such
relative strength and direction as to produce a progressive shifting or
movement of the points of maximum magnetic effect around the ring, and
to thereby maintain a rotary movement of the armature. There are
therefore secured to the shaft F of the generator, four insulated
contact-rings, _a b c d_, upon which bear the collecting-brushes
_a' b' c' d'_, connected by wires G G H H, respectively, with the
terminals of coils C and D.

Assume, for sake of illustration, that the coils D D are to receive the
maximum and coils C C at the same instant the minimum current, so that
the polar line may be midway between the coils D D. The rings _a b_
would therefore be connected to the continuous armature-coil at its
neutral points with respect to the field, or the point corresponding
with that of the ordinary commutator brushes, and between which exists
the greatest difference of potential; while rings _c d_ would be
connected to two points in the coil, between which exists no difference
of potential. The best results will be obtained by making these
connections at points equidistant from one another, as shown. These
connections are easiest made by using wires L between the rings and the
loops or wires J, connecting the coil I to the segments of the
commutator K. When the converters are made in this manner, it is evident
that the phases of the currents in the sections of the generator coil
will be reproduced in the converter coils. For example, after turning
through an arc of ninety degrees the conductors L L, which before
conveyed the maximum current, will receive the minimum current by reason
of the change in the position of their coils, and it is evident that for
the same reason the current in these coils has gradually fallen from the
maximum to the minimum in passing through the arc of ninety degrees. In
this special plan of connections, the rotation of the magnetic poles of
the converter will be synchronous with that of the armature coils of the
generator, and the result will be the same, whether the energizing
circuits are derivations from a continuous armature coil or from
independent coils, as in Mr. Tesla's other devices.

In Fig. 25, the brushes M M are shown in dotted lines in their proper
normal position. In practice these brushes may be removed from the
commutator and the field of the generator excited by an external source
of current; or the brushes may be allowed to remain on the commutator
and to take off a converted current to excite the field, or to be used
for other purposes.

In a certain well-known class of machines known as the "open circuit,"
the armature contains a number of coils the terminals of which connect
to commutator segments, the coils being connected across the armature in
pairs. This type of machine is represented in Fig. 26. In this machine
each pair of coils goes through the same phases as the coils in some of
the generators already shown, and it is obviously only necessary to
utilize them in pairs or sets to operate a Tesla converter by extending
the segments of the commutators belonging to each pair of coils and
causing a collecting brush to bear on the continuous portion of each
segment. In this way two or more circuits may be taken off from the
generator, each including one or more pairs or sets of coils as may be
desired.

[Illustration: FIG. 26.]

[Illustration: FIG. 27.]

In Fig. 26 I I represent the armature coils, T T the poles of the field
magnet, and F the shaft carrying the commutators, which are extended to
form continuous portions _a b c d_. The brushes bearing on the
continuous portions for taking off the alternating currents are
represented by _a' b' c' d'_. The collecting brushes, or those which may
be used to take off the direct current, are designated by M M. Two pairs
of the armature coils and their commutators are shown in the figure as
being utilized; but all may be utilized in a similar manner.

There is another well-known type of machine in which three or more
coils, A' B' C', on the armature have a common joint, the free ends
being connected to the segments of a commutator. This form of generator
is illustrated in Fig. 27. In this case each terminal of the generator
is connected directly or in derivation to a continuous ring, _a b c_,
and collecting brushes, _a' b' c'_, bearing thereon, take off the
alternating currents that operate the motor. It is preferable in this
case to employ a motor or transformer with three energizing coils, A''
B'' C'', placed symmetrically with those of the generator, and the
circuits from the latter are connected to the terminals of such coils
either directly--as when they are stationary--or by means of brushes
_e'_ and contact rings _e_. In this, as in the other cases, the ordinary
commutator may be used on the generator, and the current taken from it
utilized for exciting the generator field-magnets or for other
purposes.




CHAPTER VI.

METHOD OF OBTAINING DESIRED SPEED OF MOTOR OR GENERATOR.


With the object of obtaining the desired speed in motors operated by
means of alternating currents of differing phase, Mr. Tesla has devised
various plans intended to meet the practical requirements of the case,
in adapting his system to types of multipolar alternating current
machines yielding a large number of current reversals for each
revolution.

For example, Mr. Tesla has pointed out that to adapt a given type of
alternating current generator, you may couple rigidly two complete
machines, securing them together in such a way that the requisite
difference in phase will be produced; or you may fasten two armatures to
the same shaft within the influence of the same field and with the
requisite angular displacement to yield the proper difference in phase
between the two currents; or two armatures may be attached to the same
shaft with their coils symmetrically disposed, but subject to the
influence of two sets of field magnets duly displaced; or the two sets
of coils may be wound on the same armature alternately or in such manner
that they will develop currents the phases of which differ in time
sufficiently to produce the rotation of the motor.

Another method included in the scope of the same idea, whereby a single
generator may run a number of motors either at its own rate of speed or
all at different speeds, is to construct the motors with fewer poles
than the generator, in which case their speed will be greater than that
of the generator, the rate of speed being higher as the number of their
poles is relatively less. This may be understood from an example, taking
a generator that has two independent generating coils which revolve
between two pole pieces oppositely magnetized; and a motor with
energizing coils that produce at any given time two magnetic poles in
one element that tend to set up a rotation of the motor. A generator
thus constructed yields four reversals, or impulses, in each
revolution, two in each of its independent circuits; and the effect upon
the motor is to shift the magnetic poles through three hundred and sixty
degrees. It is obvious that if the four reversals in the same order
could be produced by each half-revolution of the generator the motor
would make two revolutions to the generator's one. This would be readily
accomplished by adding two intermediate poles to the generator or
altering it in any of the other equivalent ways above indicated. The
same rule applies to generators and motors with multiple poles. For
instance, if a generator be constructed with two circuits, each of which
produces twelve reversals of current to a revolution, and these currents
be directed through the independent energizing-coils of a motor, the
coils of which are so applied as to produce twelve magnetic poles at all
times, the rotation of the two will be synchronous; but if the
motor-coils produce but six poles, the movable element will be rotated
twice while the generator rotates once; or if the motor have four poles,
its rotation will be three times as fast as that of the generator.

[Illustration: FIG. 28.]

[Illustration: FIG. 29.]

These features, so far as necessary to an understanding of the
principle, are here illustrated. Fig. 28 is a diagrammatic illustration
of a generator constructed in accordance with the invention. Fig. 29 is
a similar view of a correspondingly constructed motor. Fig. 30 is a
diagram of a generator of modified construction. Fig. 31 is a diagram of
a motor of corresponding character. Fig. 32 is a diagram of a system
containing a generator and several motors adapted to run at various
speeds.

In Fig. 28, let C represent a cylindrical armature core wound
longitudinally with insulated coils A A, which are connected up in
series, the terminals of the series being connected to collecting-rings
_a a_ on the shaft G. By means of this shaft the armature is mounted to
rotate between the poles of an annular field-magnet D, formed with polar
projections wound with coils E, that magnetize the said projections. The
coils E are included in the circuit of a generator F, by means of which
the field-magnet is energized. If thus constructed, the machine is a
well-known form of alternating-current generator. To adapt it to his
system, however, Mr. Tesla winds on armature C a second set of coils B B
intermediate to the first, or, in other words, in such positions that
while the coils of one set are in the relative positions to the poles of
the field-magnet to produce the maximum current, those of the other set
will be in the position in which they produce the minimum current. The
coils B are connected, also, in series and to two connecting-rings,
secured generally to the shaft at the opposite end of the armature.

[Illustration: FIG. 30.]

[Illustration: FIG. 31.]

The motor shown in Fig. 29 has an annular field-magnet H, with four
pole-pieces wound with coils I. The armature is constructed similarly to
the generator, but with two sets of two coils in closed circuits to
correspond with the reduced number of magnetic poles in the field. From
the foregoing it is evident that one revolution of the armature of the
generator producing eight current impulses in each circuit will produce
two revolutions of the motor-armature.

The application of the principle of this invention is not, however,
confined to any particular form of machine. In Figs. 30 and 31 a
generator and motor of another well-known type are shown. In Fig. 30, J
J are magnets disposed in a circle and wound with coils K, which are in
circuit with a generator which supplies the current that maintains the
field of force. In the usual construction of these machines the
armature-conductor L is carried by a suitable frame, so as to be rotated
in face of the magnets J J, or between these magnets and another similar
set in front of them. The magnets are energized so as to be of
alternately opposite polarity throughout the series, so that as the
conductor C is rotated the current impulses combine or are added to one
another, those produced by the conductor in any given position being all
in the same direction. To adapt such a machine to his system, Mr. Tesla
adds a second set of induced conductors M, in all respects similar to
the first, but so placed in reference to it that the currents produced
in each will differ by a quarter-phase. With such relations it is
evident that as the current decreases in conductor L it increases in
conductor M, and conversely, and that any of the forms of Tesla motor
invented for use in this system may be operated by such a generator.

Fig. 31 is intended to show a motor corresponding to the machine in Fig.
30. The construction of the motor is identical with that of the
generator, and if coupled thereto it will run synchronously therewith.
J' J' are the field-magnets, and K' the coils thereon. L' is one of the
armature-conductors and M' the other.

Fig. 32 shows in diagram other forms of machine. The generator N in this
case is shown as consisting of a stationary ring O, wound with
twenty-four coils P P', alternate coils being connected in series in two
circuits. Within this ring is a disc or drum Q, with projections Q'
wound with energizing-coils included in circuit with a generator R. By
driving this disc or cylinder alternating currents are produced in the
coils P and P', which are carried off to run the several motors.

The motors are composed of a ring or annular field-magnet S, wound with
two sets of energizing-coils T T', and armatures U, having projections
U' wound with coils V, all connected in series in a closed circuit or
each closed independently on itself.

Suppose the twelve generator-coils P are wound alternately in opposite
directions, so that any two adjacent coils of the same set tend to
produce a free pole in the ring O between them and the twelve coils P'
to be similarly wound. A single revolution of the disc or cylinder Q,
the twelve polar projections of which are of opposite polarity, will
therefore produce twelve current impulses in each of the circuits W W'.
Hence the motor X, which has sixteen coils or eight free poles, will
make one and a half turns to the generator's one. The motor Y, with
twelve coils or six poles, will rotate with twice the speed of the
generator, and the motor Z, with eight coils or four poles, will revolve
three times as fast as the generator. These multipolar motors have a
peculiarity which may be often utilized to great advantage. For example,
in the motor X, Fig. 32, the eight poles may be either alternately
opposite or there may be at any given time alternately two like and two
opposite poles. This is readily attained by making the proper electrical
connections. The effect of such a change, however, would be the same as
reducing the number of poles one-half, and thereby doubling the speed
of any given motor.

[Illustration: FIG. 32.]

It is obvious that the Tesla electrical transformers which have
independent primary currents may be used with the generators described.
It may also be stated with respect to the devices we now describe that
the most perfect and harmonious action of the generators and motors is
obtained when the numbers of the poles of each are even and not odd. If
this is not the case, there will be a certain unevenness of action which
is the less appreciable as the number of poles is greater; although this
may be in a measure corrected by special provisions which it is not here
necessary to explain. It also follows, as a matter of course, that if
the number of the poles of the motor be greater than that of the
generator the motor will revolve at a slower speed than the generator.

In this chapter, we may include a method devised by Mr. Tesla for
avoiding the very high speeds which would be necessary with large
generators. In lieu of revolving the generator armature at a high rate
of speed, he secures the desired result by a rotation of the magnetic
poles of one element of the generator, while driving the other at a
different speed. The effect is the same as that yielded by a very high
rate of rotation.

In this instance, the generator which supplies the current for operating
the motors or transformers consists of a subdivided ring or annular core
wound with four diametrically-opposite coils, E E', Fig. 33. Within the
ring is mounted a cylindrical armature-core wound longitudinally with
two independent coils, F F', the ends of which lead, respectively, to
two pairs of insulated contact or collecting rings, D D' G G', on the
armature shaft. Collecting brushes _d d' g g'_ bear upon these rings,
respectively, and convey the currents through the two independent
line-circuits M M'. In the main line there may be included one or more
motors or transformers, or both. If motors be used, they are of the
usual form of Tesla construction with independent coils or sets of coils
J J', included, respectively, in the circuits M M'. These
energizing-coils are wound on a ring or annular field or on pole pieces
thereon, and produce by the action of the alternating currents passing
through them a progressive shifting of the magnetism from pole to pole.
The cylindrical armature H of the motor is wound with two coils at right
angles, which form independent closed circuits.

If transformers be employed, one set of the primary coils, as N N, wound
on a ring or annular core is connected to one circuit, as M', and the
other primary coils, N N', to the circuit M. The secondary coils K K'
may then be utilized for running groups of incandescent lamps P P'.

[Illustration: FIG. 33.]

With this generator an exciter is employed. This consists of two poles,
A A, of steel permanently magnetized, or of iron excited by a battery or
other generator of continuous currents, and a cylindrical armature core
mounted on a shaft, B, and wound with two longitudinal coils, C C'. One
end of each of these coils is connected to the collecting-rings _b c_,
respectively, while the other ends are both connected to a ring, _a_.
Collecting-brushes _b' c'_ bear on the rings _b c_, respectively, and
conductors L L convey the currents therefrom through the coils E and E
of the generator. L' is a common return-wire to brush _a'_. Two
independent circuits are thus formed, one including coils C of the
exciter and E E of the generator, the other coils C' of the exciter and
E' E' of the generator. It results from this that the operation of the
exciter produces a progressive movement of the magnetic poles of the
annular field-core of the generator, the shifting or rotary movement of
the poles being synchronous with the rotation of the exciter armature.
Considering the operative conditions of a system thus established, it
will be found that when the exciter is driven so as to energize the
field of the generator, the armature of the latter, if left free to
turn, would rotate at a speed practically the same as that of the
exciter. If under such conditions the coils F F' of the generator
armature be closed upon themselves or short-circuited, no currents, at
least theoretically, will be generated in these armature coils. In
practice the presence of slight currents is observed, the existence of
which is attributable to more or less pronounced fluctuations in the
intensity of the magnetic poles of the generator ring. So, if the
armature-coils F F' be closed through the motor, the latter will not be
turned as long as the movement of the generator armature is synchronous
with that of the exciter or of the magnetic poles of its field. If, on
the contrary, the speed of the generator armature be in any way checked,
so that the shifting or rotation of the poles of the field becomes
relatively more rapid, currents will be induced in the armature coils.
This obviously follows from the passing of the lines of force across the
armature conductors. The greater the speed of rotation of the magnetic
poles relatively to that of the armature the more rapidly the currents
developed in the coils of the latter will follow one another, and the
more rapidly the motor will revolve in response thereto, and this
continues until the armature generator is stopped entirely, as by a
brake, when the motor, if properly constructed, runs at the speed with
which the magnetic poles of the generator rotate.

The effective strength of the currents developed in the armature coils
of the generator is dependent upon the strength of the currents
energizing the generator and upon the number of rotations per unit of
time of the magnetic poles of the generator; hence the speed of the
motor armature will depend in all cases upon the relative speeds of the
armature of the generator and of its magnetic poles. For example, if the
poles are turned two thousand times per unit of time and the armature is
turned eight hundred, the motor will turn twelve hundred times, or
nearly so. Very slight differences of speed may be indicated by a
delicately balanced motor.

Let it now be assumed that power is applied to the generator armature to
turn it in a direction opposite to that in which its magnetic poles
rotate. In such case the result would be similar to that produced by a
generator the armature and field magnets of which are rotated in
opposite directions, and by reason of these conditions the motor
armature will turn at a rate of speed equal to the sum of the speeds of
the armature and magnetic poles of the generator, so that a
comparatively low speed of the generator armature will produce a high
speed in the motor.

It will be observed in connection with this system that on diminishing
the resistance of the external circuit of the generator armature by
checking the speed of the motor or by adding translating devices in
multiple arc in the secondary circuit or circuits of the transformer the
strength of the current in the armature circuit is greatly increased.
This is due to two causes: first, to the great differences in the speeds
of the motor and generator, and, secondly, to the fact that the
apparatus follows the analogy of a transformer, for, in proportion as
the resistance of the armature or secondary circuits is reduced, the
strength of the currents in the field or primary circuits of the
generator is increased and the currents in the armature are augmented
correspondingly. For similar reasons the currents in the armature-coils
of the generator increase very rapidly when the speed of the armature is
reduced when running in the same direction as the magnetic poles or
conversely.

It will be understood from the above description that the
generator-armature may be run in the direction of the shifting of the
magnetic poles, but more rapidly, and that in such case the speed of the
motor will be equal to the difference between the two rates.




CHAPTER VII.

REGULATOR FOR ROTARY CURRENT MOTORS.


An interesting device for regulating and reversing has been devised by
Mr. Tesla for the purpose of varying the speed of polyphase motors. It
consists of a form of converter or transformer with one element capable
of movement with respect to the other, whereby the inductive relations
may be altered, either manually or automatically, for the purpose of
varying the strength of the induced current. Mr. Tesla prefers to
construct this device in such manner that the induced or secondary
element may be movable with respect to the other; and the invention, so
far as relates merely to the construction of the device itself,
consists, essentially, in the combination, with two opposite magnetic
poles, of an armature wound with an insulated coil and mounted on a
shaft, whereby it may be turned to the desired extent within the field
produced by the poles. The normal position of the core of the secondary
element is that in which it most completely closes the magnetic circuit
between the poles of the primary element, and in this position its coil
is in its most effective position for the inductive action upon it of
the primary coils; but by turning the movable core to either side, the
induced currents delivered by its coil become weaker until, by a
movement of the said core and coil through 90°, there will be no current
delivered.

Fig. 34 is a view in side elevation of the regulator. Fig. 35 is a
broken section on line _x x_ of Fig. 34. Fig. 36 is a diagram
illustrating the most convenient manner of applying the regulator to
ordinary forms of motors, and Fig. 37 is a similar diagram illustrating
the application of the device to the Tesla alternating-current motors.
The regulator may be constructed in many ways to secure the desired
result; but that which is, perhaps, its best form is shown in Figs. 34
and 35.

A represents a frame of iron. B B are the cores of the inducing or
primary coils C C. D is a shaft mounted on the side bars, D', and on
which is secured a sectional iron core, E, wound with an induced or
secondary coil, F, the convolutions of which are parallel with the axis
of the shaft. The ends of the core are rounded off so as to fit closely
in the space between the two poles and permit the core E to be turned to
and held at any desired point. A handle, G, secured to the projecting
end of the shaft D, is provided for this purpose.

[Illustration: FIG. 34.]

[Illustration: FIG. 35.]

In Fig. 36 let H represent an ordinary alternating current generator,
the field-magnets of which are excited by a suitable source of current,
I. Let J designate an ordinary form of electromagnetic motor provided
with an armature, K, commutator L, and field-magnets M. It is well known
that such a motor, if its field-magnet cores be divided up into
insulated sections, may be practically operated by an alternating
current; but in using this regulator with such a motor, Mr. Tesla
includes one element of the motor only--say the armature-coils--in the
main circuit of the generator, making the connections through the
brushes and the commutator in the usual way. He also includes one of the
elements of the regulator--say the stationary coils--in the same
circuit, and in the circuit with the secondary or movable coil of the
regulator he connects up the field-coils of the motor. He also prefers
to use flexible conductors to make the connections from the secondary
coil of the regulator, as he thereby avoids the use of sliding contacts
or rings without interfering with the requisite movement of the core E.

If the regulator be in its normal position, or that in which its
magnetic circuit is most nearly closed, it delivers its maximum induced
current, the phases of which so correspond with those of the primary
current that the motor will run as though both field and armature were
excited by the main current.

[Illustration: FIG. 36.]

To vary the speed of the motor to any rate between the minimum and
maximum rates, the core E and coils F are turned in either direction to
an extent which produces the desired result, for in its normal position
the convolutions of coil F embrace the maximum number of lines of force,
all of which act with the same effect upon the coil; hence it will
deliver its maximum current; but by turning the coil F out of its
position of maximum effect the number of lines of force embraced by it
is diminished. The inductive effect is therefore impaired, and the
current delivered by coil F will continue to diminish in proportion to
the angle at which the coil F is turned until, after passing through an
angle of ninety degrees, the convolutions of the coil will be at right
angles to those of coils C C, and the inductive effect reduced to a
minimum.

Incidentally to certain constructions, other causes may influence the
variation in the strength of the induced currents. For example, in the
present case it will be observed that by the first movement of coil F a
certain portion of its convolutions are carried beyond the line of the
direct influence of the lines of force, and that the magnetic path or
circuit for the lines is impaired; hence the inductive effect would be
reduced. Next, that after moving through a certain angle, which is
obviously determined by the relative dimensions of the bobbin or coil F,
diagonally opposite portions of the coil will be simultaneously included
in the field, but in such positions that the lines which produce a
current-impulse in one portion of the coil in a certain direction will
produce in the diagonally opposite portion a corresponding impulse in
the opposite direction; hence portions of the current will neutralize
one another.

As before stated, the mechanical construction of the device may be
greatly varied; but the essential conditions of the principle will be
fulfilled in any apparatus in which the movement of the elements with
respect to one another effects the same results by varying the inductive
relations of the two elements in a manner similar to that described.

[Illustration: FIG. 37.]

It may also be stated that the core E is not indispensable to the
operation of the regulator; but its presence is obviously beneficial.
This regulator, however, has another valuable property in its capability
of reversing the motor, for if the coil F be turned through a
half-revolution, the position of its convolutions relatively to the two
coils C C and to the lines of force is reversed, and consequently the
phases of the current will be reversed. This will produce a rotation of
the motor in an opposite direction. This form of regulator is also
applied with great advantage to Mr. Tesla's system of utilizing
alternating currents, in which the magnetic poles of the field of a
motor are progressively shifted by means of the combined effects upon
the field of magnetizing coils included in independent circuits, through
which pass alternating currents in proper order and relations to each
other.

In Fig. 37, let P represent a Tesla generator having two independent
coils, P' and P'', on the armature, and T a diagram of a motor having
two independent energizing coils or sets of coils, R R'. One of the
circuits from the generator, as S' S', includes one set, R' R', of the
energizing coils of the motor, while the other circuit, as S S, includes
the primary coils of the regulator. The secondary coil of the regulator
includes the other coils, R R, of the motor.

While the secondary coil of the regulator is in its normal position, it
produces its maximum current, and the maximum rotary effect is imparted
to the motor; but this effect will be diminished in proportion to the
angle at which the coil F of the regulator is turned. The motor will
also be reversed by reversing the position of the coil with reference to
the coils C C, and thereby reversing the phases of the current produced
by the generator. This changes the direction of the movement of the
shifting poles which the armature follows.

One of the main advantages of this plan of regulation is its economy of
power. When the induced coil is generating its maximum current, the
maximum amount of energy in the primary coils is absorbed; but as the
induced coil is turned from its normal position the self-induction of
the primary-coils reduces the expenditure of energy and saves power.

It is obvious that in practice either coils C C or coil F may be used as
primary or secondary, and it is well understood that their relative
proportions may be varied to produce any desired difference or
similarity in the inducing and induced currents.




CHAPTER VIII.

SINGLE CIRCUIT, SELF-STARTING SYNCHRONIZING MOTORS.


In the first chapters of this section we have, bearing in mind the broad
underlying principle, considered a distinct class of motors, namely,
such as require for their operation a special generator capable of
yielding currents of differing phase. As a matter of course, Mr. Tesla
recognizing the desirability of utilizing his motors in connection with
ordinary systems of distribution, addressed himself to the task of
inventing various methods and ways of achieving this object. In the
succeeding chapters, therefore, we witness the evolution of a number of
ideas bearing upon this important branch of work. It must be obvious to
a careful reader, from a number of hints encountered here and there,
that even the inventions described in these chapters to follow do not
represent the full scope of the work done in these lines. They might,
indeed, be regarded as exemplifications.

We will present these various inventions in the order which to us
appears the most helpful to an understanding of the subject by the
majority of readers. It will be naturally perceived that in offering a
series of ideas of this nature, wherein some of the steps or links are
missing, the descriptions are not altogether sequential; but any one who
follows carefully the main drift of the thoughts now brought together
will find that a satisfactory comprehension of the principles can be
gained.

As is well known, certain forms of alternating-current machines have the
property, when connected in circuit with an alternating current
generator, of running as a motor in synchronism therewith; but, while
the alternating current will run the motor after it has attained a rate
of speed synchronous with that of the generator, it will not start it.
Hence, in all instances heretofore where these "synchronizing motors,"
as they are termed, have been run, some means have been adopted to bring
the motors up to synchronism with the generator, or approximately so,
before the alternating current of the generator is applied to drive
them. In some instances mechanical appliances have been utilized for
this purpose. In others special and complicated forms of motor have been
constructed. Mr. Tesla has discovered a much more simple method or plan
of operating synchronizing motors, which requires practically no other
apparatus than the motor itself. In other words, by a certain change in
the circuit connections of the motor he converts it at will from a
double circuit motor, or such as have been already described, and which
will start under the action of an alternating current, into a
synchronizing motor, or one which will be run by the generator only when
it has reached a certain speed of rotation synchronous with that of the
generator. In this manner he is enabled to extend very greatly the
applications of his system and to secure all the advantages of both
forms of alternating current motor.

The expression "synchronous with that of the generator," is used here in
its ordinary acceptation--that is to say, a motor is said to synchronize
with the generator when it preserves a certain relative speed determined
by its number of poles and the number of alternations produced per
revolution of the generator. Its actual speed, therefore, may be faster
or slower than that of the generator; but it is said to be synchronous
so long as it preserves the same relative speed.

In carrying out this invention Mr. Tesla constructs a motor which has a
strong tendency to synchronism with the generator. The construction
preferred is that in which the armature is provided with polar
projections. The field-magnets are wound with two sets of coils, the
terminals of which are connected to a switch mechanism, by means of
which the line-current may be carried directly through these coils or
indirectly through paths by which its phases are modified. To start such
a motor, the switch is turned on to a set of contacts which includes in
one motor circuit a dead resistance, in the other an inductive
resistance, and, the two circuits being in derivation, it is obvious
that the difference in phase of the current in such circuits will set up
a rotation of the motor. When the speed of the motor has thus been
brought to the desired rate the switch is shifted to throw the main
current directly through the motor-circuits, and although the currents
in both circuits will now be of the same phase the motor will continue
to revolve, becoming a true synchronous motor. To secure greater
efficiency, the armature or its polar projections are wound with coils
closed on themselves.

In the accompanying diagrams, Fig. 38 illustrates the details of the
plan above set forth, and Figs. 39 and 40 modifications of the same.

[Illustration: FIGS. 38, 39 and 40.]

Referring to Fig. 38, let A designate the field-magnets of a motor, the
polar projections of which are wound with coils B C included in
independent circuits, and D the armature with polar projections wound
with coils E closed upon themselves, the motor in these respects being
similar in construction to those described already, but having on
account of the polar projections on the armature core, or other similar
and well-known features, the properties of a synchronizing-motor. L L'
represents the conductors of a line from an alternating current
generator G.

Near the motor is placed a switch the action of which is that of the one
shown in the diagrams, which is constructed as follows: F F' are two
conducting plates or arms, pivoted at their ends and connected by an
insulating cross-bar, H, so as to be shifted in parallelism. In the path
of the bars F F' is the contact 2, which forms one terminal of the
circuit through coils C, and the contact 4, which is one terminal of the
circuit through coils B. The opposite end of the wire of coils C is
connected to the wire L or bar F', and the corresponding end of coils B
is connected to wire L' and bar F; hence if the bars be shifted so as to
bear on contacts 2 and 4 both sets of coils B C will be included in the
circuit L L' in multiple arc or derivation. In the path of the levers F
F' are two other contact terminals, 1 and 3. The contact 1 is connected
to contact 2 through an artificial resistance, I, and contact 3 with
contact 4 through a self-induction coil, J, so that when the switch
levers are shifted upon the points 1 and 3 the circuits of coils B and C
will be connected in multiple arc or derivation to the circuit L L', and
will include the resistance and self-induction coil respectively. A
third position of the switch is that in which the levers F and F' are
shifted out of contact with both sets of points. In this case the motor
is entirely out of circuit.

The purpose and manner of operating the motor by these devices are as
follows: The normal position of the switch, the motor being out of
circuit, is off the contact points. Assuming the generator to be
running, and that it is desired to start the motor, the switch is
shifted until its levers rest upon points 1 and 3. The two
motor-circuits are thus connected with the generator circuit; but by
reason of the presence of the resistance I in one and the self-induction
coil J in the other the coincidence of the phases of the current is
disturbed sufficiently to produce a progression of the poles, which
starts the motor in rotation. When the speed of the motor has run up to
synchronism with the generator, or approximately so, the switch is
shifted over upon the points 2 and 4, thus cutting out the coils I and
J, so that the currents in both circuits have the same phase; but the
motor now runs as a synchronous motor.

It will be understood that when brought up to speed the motor will run
with only one of the circuits B or C connected with the main or
generator circuit, or the two circuits may be connected in series. This
latter plan is preferable when a current having a high number of
alternations per unit of time is employed to drive the motor. In such
case the starting of the motor is more difficult, and the dead and
inductive resistances must take up a considerable proportion of the
electromotive force of the circuits. Generally the conditions are so
adjusted that the electromotive force used in each of the motor circuits
is that which is required to operate the motor when its circuits are in
series. The plan followed in this case is illustrated in Fig. 39. In
this instance the motor has twelve poles and the armature has polar
projections D wound with closed coils E. The switch used is of
substantially the same construction as that shown in the previous
figure. There are, however, five contacts, designated as 5, 6, 7, 8, and
9. The motor-circuits B C, which include alternate field-coils, are
connected to the terminals in the following order: One end of circuit C
is connected to contact 9 and to contact 5 through a dead resistance, I.
One terminal of circuit B is connected to contact 7 and to contact 6
through a self-induction coil, J. The opposite terminals of both
circuits are connected to contact 8.

One of the levers, as F, of the switch is made with an extension, _f_,
or otherwise, so as to cover both contacts 5 and 6 when shifted into the
position to start the motor. It will be observed that when in this
position and with lever F' on contact 8 the current divides between the
two circuits B C, which from their difference in electrical character
produce a progression of the poles that starts the motor in rotation.
When the motor has attained the proper speed, the switch is shifted so
that the levers cover the contacts 7 and 9, thereby connecting circuits
B and C in series. It is found that by this disposition the motor is
maintained in rotation in synchronism with the generator. This principle
of operation, which consists in converting by a change of connections or
otherwise a double-circuit motor, or one operating by a progressive
shifting of the poles, into an ordinary synchronizing motor may be
carried out in many other ways. For instance, instead of using the
switch shown in the previous figures, we may use a temporary ground
circuit between the generator and motor, in order to start the motor, in
substantially the manner indicated in Fig. 40. Let G in this figure
represent an ordinary alternating-current generator with, say, two
poles, M M', and an armature wound with two coils, N N', at right angles
and connected in series. The motor has, for example, four poles wound
with coils B C, which are connected in series, and an armature with
polar projections D wound with closed coils E E. From the common joint
or union between the two circuits of both the generator and the motor an
earth connection is established, while the terminals or ends of these
circuits are connected to the line. Assuming that the motor is a
synchronizing motor or one that has the capability of running in
synchronism with the generator, but not of starting, it may be started
by the above-described apparatus by closing the ground connection from
both generator and motor. The system thus becomes one with a two-circuit
generator and motor, the ground forming a common return for the currents
in the two circuits L and L'. When by this arrangement of circuits the
motor is brought to speed, the ground connection is broken between the
motor or generator, or both, ground-switches P P' being employed for
this purpose. The motor then runs as a synchronizing motor.

In describing the main features which constitute this invention
illustrations have necessarily been omitted of the appliances used in
conjunction with the electrical devices of similar systems--such, for
instance, as driving-belts, fixed and loose pulleys for the motor, and
the like; but these are matters well understood.

Mr. Tesla believes he is the first to operate electro-magnetic motors by
alternating currents in any of the ways herein described--that is to
say, by producing a progressive movement or rotation of their poles or
points of greatest magnetic attraction by the alternating currents until
they have reached a given speed, and then by the same currents producing
a simple alternation of their poles, or, in other words, by a change in
the order or character of the circuit connections to convert a motor
operating on one principle to one operating on another.




CHAPTER IX.

CHANGE FROM DOUBLE CURRENT TO SINGLE CURRENT MOTOR.


A description is given elsewhere of a method of operating alternating
current motors by first rotating their magnetic poles until they have
attained synchronous speed, and then alternating the poles. The motor is
thus transformed, by a simple change of circuit connections from one
operated by the action of two or more independent energizing currents to
one operated either by a single current or by several currents acting as
one. Another way of doing this will now be described.

At the start the magnetic poles of one element or field of the motor are
progressively shifted by alternating currents differing in phase and
passed through independent energizing circuits, and short circuit the
coils of the other element. When the motor thus started reaches or
passes the limit of speed synchronous with the generator, Mr. Tesla
connects up the coils previously short-circuited with a source of direct
current and by a change of the circuit connections produces a simple
alternation of the poles. The motor then continues to run in synchronism
with the generator. The motor here shown in Fig. 41 is one of the
ordinary forms, with field-cores either laminated or solid and with a
cylindrical laminated armature wound, for example, with the coils A B at
right angles. The shaft of the armature carries three collecting or
contact rings C D E. (Shown, for better illustration, as of different
diameters.)

One end of coil A connects to one ring, as C, and one end of coil B
connects with ring D. The remaining ends are connected to ring E.
Collecting springs or brushes F G H bear upon the rings and lead to the
contacts of a switch, to be presently described. The field-coils have
their terminals in binding-posts K K, and may be either closed upon
themselves or connected with a source of direct current L, by means of a
switch M. The main or controlling switch has five contacts _a b c d e_
and two levers _f g_, pivoted and connected by an insulating cross-bar
_h_, so as to move in parallelism. These levers are connected to the
line wires from a source of alternating currents N. Contact _a_ is
connected to brush G and coil B through a dead resistance R and wire P.
Contact _b_ is connected with brush F and coil A through a
self-induction coil S and wire O. Contacts _c_ and _e_ are connected to
brushes G F, respectively, through the wires P O, and contact _d_ is
directly connected with brush H. The lever _f_ has a widened end, which
may span the contacts _a b_. When in such position and with lever _g_ on
contact _d_, the alternating currents divide between the two
motor-coils, and by reason of their different self-induction a
difference of current-phase is obtained that starts the motor in
rotation. In starting, the field-coils are short circuited.

[Illustration: FIG. 41.]

When the motor has attained the desired speed, the switch is shifted to
the position shown in dotted lines--that is to say, with the levers _f
g_ resting on points _c e_. This connects up the two armature coils in
series, and the motor will then run as a synchronous motor. The
field-coils are thrown into circuit with the direct current source when
the main switch is shifted.




CHAPTER X.

MOTOR WITH "CURRENT LAG" ARTIFICIALLY SECURED.


One of the general ways followed by Mr. Tesla in developing his rotary
phase motors is to produce practically independent currents differing
primarily in phase and to pass these through the motor-circuits. Another
way is to produce a single alternating current, to divide it between the
motor-circuits, and to effect artificially a lag in one of these
circuits or branches, as by giving to the circuits different
self-inductive capacity, and in other ways. In the former case, in which
the necessary difference of phase is primarily effected in the
generation of currents, in some instances, the currents are passed
through the energizing coils of both elements of the motor--the field
and armature; but a further result or modification may be obtained by
doing this under the conditions hereinafter specified in the case of
motors in which the lag, as above stated, is artificially secured.

Figs. 42 to 47, inclusive, are diagrams of different ways in which the
invention is carried out; and Fig. 48, a side view of a form of motor
used by Mr. Tesla for this purpose.

[Illustration: FIGS. 42, 43 and 44.]

A B in Fig. 42 indicate the two energizing circuits of a motor, and C D
two circuits on the armature. Circuit or coil A is connected in series
with circuit or coil C, and the two circuits B D are similarly
connected. Between coils A and C is a contact-ring _e_, forming one
terminal of the latter, and a brush _a_, forming one terminal of the
former. A ring _d_ and brush _c_ similarly connect coils B and D. The
opposite terminals of the field-coils connect to one binding post _h_ of
the motor, and those of the armature coils are similarly connected to
the opposite binding post _i_ through a contact-ring _f_ and brush _g_.
Thus each motor-circuit while in derivation to the other includes one
armature and one field coil. These circuits are of different
self-induction, and may be made so in various ways. For the sake
of clearness, an artificial resistance R is shown in one of these
circuits, and in the other a self-induction coil S. When an alternating
current is passed through this motor it divides between its two
energizing-circuits. The higher self-induction of one circuit produces a
greater retardation or lag in the current therein than in the other. The
difference of phase between the two currents effects the rotation or
shifting of the points of maximum magnetic effect that secures the
rotation of the armature. In certain respects this plan of including
both armature and field coils in circuit is a marked improvement. Such a
motor has a good torque at starting; yet it has also considerable
tendency to synchronism, owing to the fact that when properly
constructed the maximum magnetic effects in both armature and field
coincide--a condition which in the usual construction of these motors
with closed armature coils is not readily attained. The motor thus
constructed exhibits too, a better regulation of current from no load to
load, and there is less difference between the apparent and real energy
expended in running it. The true synchronous speed of this form of motor
is that of the generator when both are alike--that is to say, if the
number of the coils on the armature and on the field is _x_, the motor
will run normally at the same speed as a generator driving it if the
number of field magnets or poles of the same be also _x_.

[Illustration: FIGS. 45, 46 and 47.]

Fig. 43 shows a somewhat modified arrangement of circuits. There is in
this case but one armature coil E, the winding of which maintains
effects corresponding to the resultant poles produced by the two
field-circuits.

Fig. 44 represents a disposition in which both armature and field are
wound with two sets of coils, all in multiple arc to the line or main
circuit. The armature coils are wound to correspond with the field-coils
with respect to their self-induction. A modification of this plan is
shown in Fig. 45--that is to say, the two field coils and two armature
coils are in derivation to themselves and in series with one another.
The armature coils in this case, as in the previous figure, are wound
for different self-induction to correspond with the field coils.

Another modification is shown in Fig. 46. In this case only one
armature-coil, as D, is included in the line-circuit, while the other,
as C, is short-circuited.

In such a disposition as that shown in Fig. 43, or where only one
armature-coil is employed, the torque on the start is somewhat reduced,
while the tendency to synchronism is somewhat increased. In such a
disposition as shown in Fig. 46, the opposite conditions would exist. In
both instances, however, there is the advantage of dispensing with one
contact-ring.

[Illustration: FIG. 48.]

In Fig. 46 the two field-coils and the armature-coil D are in multiple
arc. In Fig. 47 this disposition is modified, coil D being shown in
series with the two field-coils.

Fig. 48 is an outline of the general form of motor in which this
invention is embodied. The circuit connections between the armature and
field coils are made, as indicated in the previous figures, through
brushes and rings, which are not shown.




CHAPTER XI.

ANOTHER METHOD OF TRANSFORMATION FROM A TORQUE TO A SYNCHRONIZING MOTOR.


In a preceding chapter we have described a method by which Mr. Tesla
accomplishes the change in his type of rotating field motor from a
torque to a synchronizing motor. As will be observed, the desired end is
there reached by a change in the circuit connections at the proper
moment. We will now proceed to describe another way of bringing about
the same result. The principle involved in this method is as follows:--

If an alternating current be passed through the field coils only of a
motor having two energizing circuits of different self-induction and the
armature coils be short-circuited, the motor will have a strong torque,
but little or no tendency to synchronism with the generator; but if the
same current which energizes the field be passed also through the
armature coils the tendency to remain in synchronism is very
considerably increased. This is due to the fact that the maximum
magnetic effects produced in the field and armature more nearly
coincide. On this principle Mr. Tesla constructs a motor having
independent field circuits of different self-induction, which are joined
in derivation to a source of alternating currents. The armature is wound
with one or more coils, which are connected with the field coils through
contact rings and brushes, and around the armature coils a shunt is
arranged with means for opening or closing the same. In starting this
motor the shunt is closed around the armature coils, which will
therefore be in closed circuit. When the current is directed through the
motor, it divides between the two circuits, (it is not necessary to
consider any case where there are more than two circuits used), which,
by reason of their different self-induction, secure a difference of
phase between the two currents in the two branches, that produces a
shifting or rotation of the poles. By the alternations of current, other
currents are induced in the closed--or short-circuited--armature coils
and the motor has a strong torque. When the desired speed is reached,
the shunt around the armature-coils is opened and the current directed
through both armature and field coils. Under these conditions the motor
has a strong tendency to synchronism.

[Illustration: FIGS. 49, 50 and 51.]

In Fig. 49, A and B designate the field coils of the motor. As the
circuits including these coils are of different self-induction, this is
represented by a resistance coil R in circuit with A, and a
self-induction coil S in circuit with B. The same result may of course
be secured by the winding of the coils. C is the armature circuit, the
terminals of which are rings _a b_. Brushes _c d_ bear on these rings
and connect with the line and field circuits. D is the shunt or short
circuit around the armature. E is the switch in the shunt.

It will be observed that in such a disposition as is illustrated in
Fig. 49, the field circuits A and B being of different self-induction,
there will always be a greater lag of the current in one than the other,
and that, generally, the armature phases will not correspond with
either, but with the resultant of both. It is therefore important to
observe the proper rule in winding the armature. For instance, if the
motor have eight poles--four in each circuit--there will be four
resultant poles, and hence the armature winding should be such as to
produce four poles, in order to constitute a true synchronizing motor.

[Illustration: FIG. 52.]

The diagram, Fig. 50, differs from the previous one only in respect to
the order of connections. In the present case the armature-coil, instead
of being in series with the field-coils, is in multiple arc therewith.
The armature-winding may be similar to that of the field--that is to
say, the armature may have two or more coils wound or adapted for
different self-induction and adapted, preferably, to produce the same
difference of phase as the field-coils. On starting the motor the shunt
is closed around both coils. This is shown in Fig. 51, in which the
armature coils are F G. To indicate their different electrical
character, there are shown in circuit with them, respectively, the
resistance R' and the self-induction coil S'. The two armature coils are
in series with the field-coils and the same disposition of the shunt or
short-circuit D is used. It is of advantage in the operation of motors
of this kind to construct or wind the armature in such manner that when
short-circuited on the start it will have a tendency to reach a higher
speed than that which synchronizes with the generator. For example, a
given motor having, say, eight poles should run, with the armature coil
short-circuited, at two thousand revolutions per minute to bring it up
to synchronism. It will generally happen, however, that this speed is
not reached, owing to the fact that the armature and field currents do
not properly correspond, so that when the current is passed through the
armature (the motor not being quite up to synchronism) there is a
liability that it will not "hold on," as it is termed. It is preferable,
therefore, to so wind or construct the motor that on the start, when the
armature coils are short-circuited, the motor will tend to reach a speed
higher than the synchronous--as for instance, double the latter. In such
case the difficulty above alluded to is not felt, for the motor will
always hold up to synchronism if the synchronous speed--in the case
supposed of two thousand revolutions--is reached or passed. This may be
accomplished in various ways; but for all practical purposes the
following will suffice: On the armature are wound two sets of coils. At
the start only one of these is short-circuited, thereby producing a
number of poles on the armature, which will tend to run the speed up
above the synchronous limit. When such limit is reached or passed, the
current is directed through the other coil, which, by increasing the
number of armature poles, tends to maintain synchronism.

[Illustration: FIG. 53.]

In Fig. 52, such a disposition is shown. The motor having, say, eight
poles contains two field-circuits A and B, of different self-induction.
The armature has two coils F and G. The former is closed upon itself,
the latter connected with the field and line through contact-rings _a
b_, brushes _c d_, and a switch E. On the start the coil F alone is
active and the motor tends to run at a speed above the synchronous; but
when the coil G is connected to the circuit the number of armature poles
is increased, while the motor is made a true synchronous motor. This
disposition has the advantage that the closed armature-circuit imparts
to the motor torque when the speed falls off, but at the same time the
conditions are such that the motor comes out of synchronism more
readily. To increase the tendency to synchronism, two circuits may be
used on the armature, one of which is short-circuited on the start and
both connected with the external circuit after the synchronous speed is
reached or passed. This disposition is shown in Fig. 53. There are three
contact-rings _a b e_ and three brushes _c d f_, which connect the
armature circuits with the external circuit. On starting, the switch H
is turned to complete the connection between one binding-post P and the
field-coils. This short-circuits one of the armature-coils, as G. The
other coil F is out of circuit and open. When the motor is up to speed,
the switch H is turned back, so that the connection from binding-post P
to the field coils is through the coil G, and switch K is closed,
thereby including coil F in multiple arc with the field coils. Both
armature coils are thus active.

From the above-described instances it is evident that many other
dispositions for carrying out the invention are possible.




CHAPTER XII.

"MAGNETIC LAG" MOTOR.


The following description deals with another form of motor, namely,
depending on "magnetic lag" or hysteresis, its peculiarity being that in
it the attractive effects or phases while lagging behind the phases of
current which produce them, are manifested simultaneously and not
successively. The phenomenon utilized thus at an early stage by Mr.
Tesla, was not generally believed in by scientific men, and Prof. Ayrton
was probably first to advocate it or to elucidate the reason of its
supposed existence.

Fig. 54 is a side view of the motor, in elevation. Fig. 55 is a
part-sectional view at right angles to Fig. 54. Fig. 56 is an end view
in elevation and part section of a modification, and Fig. 57 is a
similar view of another modification.

In Figs. 54 and 55, A designates a base or stand, and B B the
supporting-frame of the motor. Bolted to the supporting-frame are two
magnetic cores or pole-pieces C C', of iron or soft steel. These may be
subdivided or laminated, in which case hard iron or steel plates or bars
should be used, or they should be wound with closed coils. D is a
circular disc armature, built up of sections or plates of iron and
mounted in the frame between the pole-pieces C C', curved to conform to
the circular shape thereof. This disc may be wound with a number of
closed coils E. F F are the main energizing coils, supported by the
supporting-frame, so as to include within their magnetizing influence
both the pole-pieces C C' and the armature D. The pole-pieces C C'
project out beyond the coils F F on opposite sides, as indicated in the
drawings. If an alternating current be passed through the coils F F,
rotation of the armature will be produced, and this rotation is
explained by the following apparent action, or mode of operation: An
impulse of current in the coils F F establishes two polarities in the
motor. The protruding end of pole-piece C, for instance, will be of one
sign, and the corresponding end of pole-piece C' will be of the opposite
sign. The armature also exhibits two poles at right angles to the coils
F F, like poles to those in the pole-pieces being on the same side of
the coils. While the current is flowing there is no appreciable tendency
to rotation developed; but after each current impulse ceases or begins
to fall, the magnetism in the armature and in the ends of the
pole-pieces C C' lags or continues to manifest itself, which produces a
rotation of the armature by the repellent force between the more closely
approximating points of maximum magnetic effect. This effect is
continued by the reversal of current, the polarities of field and
armature being simply reversed. One or both of the elements--the
armature or field--may be wound with closed induced coils to intensify
this effect. Although in the illustrations but one of the fields is
shown, each element of the motor really constitutes a field, wound with
the closed coils, the currents being induced mainly in those
convolutions or coils which are parallel to the coils F F.

[Illustration: FIG. 54.]

[Illustration: FIG. 55.]

A modified form of this motor is shown in Fig. 56. In this form G is one
of two standards that support the bearings for the armature-shaft. H H
are uprights or sides of a frame, preferably magnetic, the ends C C' of
which are bent in the manner indicated, to conform to the shape of the
armature D and form field-magnet poles. The construction of the armature
may be the same as in the previous figure, or it may be simply a
magnetic disc or cylinder, as shown, and a coil or coils F F are
secured in position to surround both the armature and the poles C C'.
The armature is detachable from its shaft, the latter being passed
through the armature after it has been inserted in position. The
operation of this form of motor is the same in principle as that
previously described and needs no further explanation.

[Illustration: FIG. 56.]

[Illustration: FIG. 57.]

One of the most important features in alternating current motors is,
however, that they should be adapted to and capable of running
efficiently on the alternating circuits in present use, in which almost
without exception the generators yield a very high number of
alternations. Such a motor, of the type under consideration, Mr. Tesla
has designed by a development of the principle of the motor shown in
Fig. 56, making a multipolar motor, which is illustrated in Fig. 57. In
the construction of this motor he employs an annular magnetic frame J,
with inwardly-extending ribs or projections K, the ends of which all
bend or turn in one direction and are generally shaped to conform to the
curved surface of the armature. Coils F F are wound from one part K to
the one next adjacent, the ends or loops of each coil or group of wires
being carried over toward the shaft, so as to form U-shaped groups
of convolutions at each end of the armature. The pole-pieces C C', being
substantially concentric with the armature, form ledges, along which the
coils are laid and should project to some extent beyond the the coils,
as shown. The cylindrical or drum armature D is of the same construction
as in the other motors described, and is mounted to rotate within the
annular frame J and between the U-shaped ends or bends of the
coils F. The coils F are connected in multiple or in series with a
source of alternating currents, and are so wound that with a current or
current impulse of given direction they will make the alternate
pole-pieces C of one polarity and the other pole-pieces C' of the
opposite polarity. The principle of the operation of this motor is the
same as the other above described, for, considering any two pole-pieces
C C', a current impulse passing in the coil which bridges them or is
wound over both tends to establish polarities in their ends of opposite
sign and to set up in the armature core between them a polarity of the
same sign as that of the nearest pole-piece C. Upon the fall or
cessation of the current impulse that established these polarities the
magnetism which lags behind the current phase, and which continues to
manifest itself in the polar projections C C' and the armature, produces
by repulsion a rotation of the armature. The effect is continued by each
reversal of the current. What occurs in the case of one pair of
pole-pieces occurs simultaneously in all, so that the tendency to
rotation of the armature is measured by the sum of all the forces
exerted by the pole-pieces, as above described. In this motor also the
magnetic lag or effect is intensified by winding one or both cores with
closed induced coils. The armature core is shown as thus wound. When
closed coils are used, the cores should be laminated.

It is evident that a pulsatory as well as an alternating current might
be used to drive or operate the motors above described.

It will be understood that the degree of subdivision, the mass of the
iron in the cores, their size and the number of alternations in the
current employed to run the motor, must be taken into consideration in
order to properly construct this motor. In other words, in all such
motors the proper relations between the number of alternations and the
mass, size, or quality of the iron must be preserved in order to secure
the best results.




CHAPTER XIII.

METHOD OF OBTAINING DIFFERENCE OF PHASE BY MAGNETIC SHIELDING.


In that class of motors in which two or more sets of energizing magnets
are employed, and in which by artificial means a certain interval of
time is made to elapse between the respective maximum or minimum periods
or phases of their magnetic attraction or effect, the interval or
difference in phase between the two sets of magnets is limited in
extent. It is desirable, however, for the economical working of such
motors that the strength or attraction of one set of magnets should be
maximum, at the time when that of the other set is minimum, and
conversely; but these conditions have not heretofore been realized
except in cases where the two currents have been obtained from
independent sources in the same or different machines. Mr. Tesla has
therefore devised a motor embodying conditions that approach more nearly
the theoretical requirements of perfect working, or in other words, he
produces artificially a difference of magnetic phase by means of a
current from a single primary source sufficient in extent to meet the
requirements of practical and economical working. He employs a motor
with two sets of energizing or field magnets, each wound with coils
connected with a source of alternating or rapidly-varying currents, but
forming two separate paths or circuits. The magnets of one set are
protected to a certain extent from the energizing action of the current
by means of a magnetic shield or screen interposed between the magnet
and its energizing coil. This shield is properly adapted to the
conditions of particular cases, so as to shield or protect the main core
from magnetization until it has become itself saturated and no longer
capable of containing all the lines of force produced by the current. It
will be seen that by this means the energizing action begins in the
protected set of magnets a certain arbitrarily-determined period of time
later than in the other, and that by this means alone or in conjunction
with other means or devices heretofore employed a practical difference
of magnetic phase may readily be secured.

Fig. 58 is a view of a motor, partly in section, with a diagram
illustrating the invention. Fig. 59 is a similar view of a modification
of the same.

[Illustration: FIG. 58.]

[Illustration: FIG. 59.]

In Fig. 58, which exhibits the simplest form of the invention, A A is
the field-magnet of a motor, having, say, eight poles or
inwardly-projecting cores B and C. The cores B form one set of magnets
and are energized by coils D. The cores C, forming the other set are
energized by coils E, and the coils are connected, preferably, in series
with one another, in two derived or branched circuits, F G,
respectively, from a suitable source of current. Each coil E is
surrounded by a magnetic shield H, which is preferably composed of an
annealed, insulated, or oxidized iron wire wrapped or wound on the coils
in the manner indicated so as to form a closed magnetic circuit around
the coils and between the same and the magnetic cores C. Between the
pole pieces or cores B C is mounted the armature K, which, as is usual
in this type of machines, is wound with coils L closed upon themselves.
The operation resulting from this disposition is as follows: If a
current impulse be directed through the two circuits of the motor, it
will quickly energize the cores B, but not so the cores C, for the
reason that in passing through the coils E there is encountered the
influence of the closed magnetic circuits formed by the shields H. The
first effect is to retard effectively the current impulse in circuit G,
while at the same time the proportion of current which does pass does
not magnetize the cores C, which are shielded or screened by the
shields H. As the increasing electromotive force then urges more current
through the coils E, the iron wire H becomes magnetically saturated and
incapable of carrying all the lines of force, and hence ceases to
protect the cores C, which becomes magnetized, developing their maximum
effect after an interval of time subsequent to the similar manifestation
of strength in the other set of magnets, the extent of which is
arbitrarily determined by the thickness of the shield H, and other
well-understood conditions.

From the above it will be seen that the apparatus or device acts in two
ways. First, by retarding the current, and, second, by retarding the
magnetization of one set of the cores, from which its effectiveness will
readily appear.

Many modifications of the principle of this invention are possible. One
useful and efficient application of the invention is shown in Fig. 59.
In this figure a motor is shown similar in all respects to that above
described, except that the iron wire H, which is wrapped around the
coils E, is in this case connected in series with the coils D. The
iron-wire coils H, are connected and wound, so as to have little or no
self-induction, and being added to the resistance of the circuit F, the
action of the current in that circuit will be accelerated, while in the
other circuit G it will be retarded. The shield H may be made in many
forms, as will be understood, and used in different ways, as appears
from the foregoing description.

As a modification of his type of motor with "shielded" fields, Mr. Tesla
has constructed a motor with a field-magnet having two sets of poles or
inwardly-projecting cores and placed side by side, so as practically to
form two fields of force and alternately disposed--that is to say, with
the poles of one set or field opposite the spaces between the other. He
then connects the free ends of one set of poles by means of laminated
iron bands or bridge-pieces of considerably smaller cross-section than
the cores themselves, whereby the cores will all form parts of complete
magnetic circuits. When the coils on each set of magnets are connected
in multiple circuits or branches from a source of alternating currents,
electromotive forces are set up in or impressed upon each circuit
simultaneously; but the coils on the magnetically bridged or shunted
cores will have, by reason of the closed magnetic circuits, a high
self-induction, which retards the current, permitting at the beginning
of each impulse but little current to pass. On the other hand, no such
opposition being encountered in the other set of coils, the current
passes freely through them, magnetizing the poles on which they are
wound. As soon, however, as the laminated bridges become saturated and
incapable of carrying all the lines of force which the rising
electromotive force, and consequently increased current, produce, free
poles are developed at the ends of the cores, which, acting in
conjunction with the others, produce rotation of the armature.

The construction in detail by which this invention is illustrated is
shown in the accompanying drawings.

[Illustration: FIG. 60.]

[Illustration: FIG. 61.]

Fig. 60 is a view in side elevation of a motor embodying the principle.
Fig. 61 is a vertical cross-section of the motor. A is the frame of the
motor, which should be built up of sheets of iron punched out to the
desired shape and bolted together with insulation between the sheets.
When complete, the frame makes a field-magnet with inwardly projecting
pole-pieces B and C. To adapt them to the requirements of this
particular case these pole-pieces are out of line with one another,
those marked B surrounding one end of the armature and the others, as C,
the opposite end, and they are disposed alternately--that is to say, the
pole-pieces of one set occur in line with the spaces between those of
the other sets.

The armature D is of cylindrical form, and is also laminated in the
usual way and is wound longitudinally with coils closed upon themselves.
The pole-pieces C are connected or shunted by bridge-pieces E. These may
be made independently and attached to the pole-pieces, or they may be
parts of the forms or blanks stamped or punched out of sheet-iron. Their
size or mass is determined by various conditions, such as the strength
of the current to be employed, the mass or size of the cores to which
they are applied, and other familiar conditions.

Coils F surround the pole-pieces B, and other coils G are wound on the
pole-pieces C. These coils are connected in series in two circuits,
which are branches of a circuit from a generator of alternating
currents, and they may be so wound, or the respective circuits in which
they are included may be so arranged, that the circuit of coils G will
have, independently of the particular construction described, a higher
self-induction than the other circuit or branch.

The function of the shunts or bridges E is that they shall form with the
cores C a closed magnetic circuit for a current up to a predetermined
strength, so that when saturated by such current and unable to carry
more lines of force than such a current produces they will to no further
appreciable extent interfere with the development, by a stronger
current, of free magnetic poles at the ends of the cores C.

In such a motor the current is so retarded in the coils G, and the
manifestation of the free magnetism in the poles C is so delayed beyond
the period of maximum magnetic effect in poles B, that a strong torque
is produced and the motor operates with approximately the power
developed in a motor of this kind energized by independently generated
currents differing by a full quarter phase.




CHAPTER XIV.

TYPE OF TESLA SINGLE-PHASE MOTOR.


Up to this point, two principal types of Tesla motors have been
described: First, those containing two or more energizing circuits
through which are caused to pass alternating currents differing from one
another in phase to an extent sufficient to produce a continuous
progression or shifting of the poles or points of greatest magnetic
effect, in obedience to which the movable element of the motor is
maintained in rotation; second, those containing poles, or parts of
different magnetic susceptibility, which under the energizing influence
of the same current or two currents coinciding in phase will exhibit
differences in their magnetic periods or phases. In the first class of
motors the torque is due to the magnetism established in different
portions of the motor by currents from the same or from independent
sources, and exhibiting time differences in phase. In the second class
the torque results from the energizing effects of a current upon
different parts of the motor which differ in magnetic susceptibility--in
other words, parts which respond in the same relative degree to the
action of a current, not simultaneously, but after different intervals
of time.

In another Tesla motor, however, the torque, instead of being solely the
result of a time difference in the magnetic periods or phases of the
poles or attractive parts to whatever cause due, is produced by an
angular displacement of the parts which, though movable with respect to
one another, are magnetized simultaneously, or approximately so, by the
same currents. This principle of operation has been embodied practically
in a motor in which the necessary angular displacement between the
points of greatest magnetic attraction in the two elements of the
motor--the armature and field--is obtained by the direction of the
lamination of the magnetic cores of the elements.

Fig. 62 is a side view of such a motor with a portion of its armature
core exposed. Fig. 63 is an end or edge view of the same. Fig. 64 is a
central cross-section of the same, the armature being shown mainly in
elevation.

[Illustration: FIG. 62.]

[Illustration: FIG. 63.]

[Illustration: FIG. 64.]

Let A A designate two plates built up of thin sections or laminæ of soft
iron insulated more or less from one another and held together by bolts
_a_ and secured to a base B. The inner faces of these plates contain
recesses or grooves in which a coil or coils D are secured obliquely to
the direction of the laminations. Within the coils D is a disc E,
preferably composed of a spirally-wound iron wire or ribbon or a series
of concentric rings and mounted on a shaft F, having bearings in the
plates A A. Such a device when acted upon by an alternating current is
capable of rotation and constitutes a motor, the operation of which may
be explained in the following manner: A current or current-impulse
traversing the coils D tends to magnetize the cores A A and E, all of
which are within the influence of the field of the coils. The poles thus
established would naturally lie in the same line at right angles to the
coils D, but in the plates A they are deflected by reason of the
direction of the laminations, and appear at or near the extremities of
these plates. In the disc, however, where these conditions are not
present, the poles or points of greatest attraction are on a line at
right angles to the plane of the coils; hence there will be a torque
established by this angular displacement of the poles or magnetic lines,
which starts the disc in rotation, the magnetic lines of the armature
and field tending toward a position of parallelism. This rotation is
continued and maintained by the reversals of the current in coils D D,
which change alternately the polarity of the field-cores A A. This
rotary tendency or effect will be greatly increased by winding the disc
with conductors G, closed upon themselves and having a radial direction,
whereby the magnetic intensity of the poles of the disc will be greatly
increased by the energizing effect of the currents induced in the coils
G by the alternating currents in coils D.

The cores of the disc and field may or may not be of different magnetic
susceptibility--that is to say, they may both be of the same kind of
iron, so as to be magnetized at approximately the same instant by the
coils D; or one may be of soft iron and the other of hard, in order that
a certain time may elapse between the periods of their magnetization. In
either case rotation will be produced; but unless the disc is provided
with the closed energizing coils it is desirable that the
above-described difference of magnetic susceptibility be utilized to
assist in its rotation.

The cores of the field and armature may be made in various ways, as will
be well understood, it being only requisite that the laminations in each
be in such direction as to secure the necessary angular displacement of
the points of greatest attraction. Moreover, since the disc may be
considered as made up of an infinite number of radial arms, it is
obvious that what is true of a disc holds for many other forms of
armature.




CHAPTER XV.

MOTORS WITH CIRCUITS OF DIFFERENT RESISTANCE.


As has been pointed out elsewhere, the lag or retardation of the phases
of an alternating current is directly proportional to the self-induction
and inversely proportional to the resistance of the circuit through
which the current flows. Hence, in order to secure the proper
differences of phase between the two motor-circuits, it is desirable to
make the self-induction in one much higher and the resistance much lower
than the self-induction and resistance, respectively, in the other. At
the same time the magnetic quantities of the two poles or sets of poles
which the two circuits produce should be approximately equal. These
requirements have led Mr. Tesla to the invention of a motor having the
following general characteristics: The coils which are included in that
energizing circuit which is to have the higher self-induction are made
of coarse wire, or a conductor of relatively low resistance, and with
the greatest possible length or number of turns. In the other set of
coils a comparatively few turns of finer wire are used, or a wire of
higher resistance. Furthermore, in order to approximate the magnetic
quantities of the poles excited by these coils, Mr. Tesla employs in the
self-induction circuit cores much longer than those in the other or
resistance circuit.

Fig. 65 is a part sectional view of the motor at right angles to the
shaft. Fig. 66 is a diagram of the field circuits.

In Fig. 66, let A represent the coils in one motor circuit, and B those
in the other. The circuit A is to have the higher self-induction. There
are, therefore, used a long length or a large number of turns of coarse
wire in forming the coils of this circuit. For the circuit B, a smaller
conductor is employed, or a conductor of a higher resistance than
copper, such as German silver or iron, and the coils are wound with
fewer turns. In applying these coils to a motor, Mr. Tesla builds up a
field-magnet of plates C, of iron and steel, secured together in the
usual manner by bolts D. Each plate is formed with four (more or less)
long cores E, around which is a space to receive the coil and an equal
number of short projections F to receive the coils of the
resistance-circuit. The plates are generally annular in shape, having an
open space in the centre for receiving the armature G, which Mr. Tesla
prefers to wind with closed coils. An alternating current divided
between the two circuits is retarded as to its phases in the circuit A
to a much greater extent than in the circuit B. By reason of the
relative sizes and disposition of the cores and coils the magnetic
effect of the poles E and F upon the armature closely approximate.

[Illustration: FIG. 65.]

[Illustration: FIG. 66.]

An important result secured by the construction shown here is that these
coils which are designed to have the higher self-induction are almost
completely surrounded by iron, and that the retardation is thus very
materially increased.




CHAPTER XVI.

MOTOR WITH EQUAL MAGNETIC ENERGIES IN FIELD AND ARMATURE.


Let it be assumed that the energy as represented in the magnetism in the
field of a given rotating field motor is ninety and that of the armature
ten. The sum of these quantities, which represents the total energy
expended in driving the motor, is one hundred; but, assuming that the
motor be so constructed that the energy in the field is represented by
fifty, and that in the armature by fifty, the sum is still one hundred;
but while in the first instance the product is nine hundred, in the
second it is two thousand five hundred, and as the energy developed is
in proportion to these products it is clear that those motors are the
most efficient--other things being equal--in which the magnetic energies
developed in the armature and field are equal. These results Mr. Tesla
obtains by using the same amount of copper or ampere turns in both
elements when the cores of both are equal, or approximately so, and the
same current energizes both; or in cases where the currents in one
element are induced to those of the other he uses in the induced coils
an excess of copper over that in the primary element or conductor.

[Illustration: FIG. 67.]

The conventional figure of a motor here introduced, Fig. 67, will give
an idea of the solution furnished by Mr. Tesla for the specific problem.
Referring to the drawing, A is the field-magnet, B the armature, C the
field coils, and D the armature-coils of the motor.

Generally speaking, if the mass of the cores of armature and field be
equal, the amount of copper or ampere turns of the energizing coils on
both should also be equal; but these conditions will be modified in
different forms of machine. It will be understood that these results are
most advantageous when existing under the conditions presented where the
motor is running with its normal load, a point to be well borne in
mind.




CHAPTER XVII.

MOTORS WITH COINCIDING MAXIMA OF MAGNETIC EFFECT IN ARMATURE AND FIELD.


In this form of motor, Mr. Tesla's object is to design and build
machines wherein the maxima of the magnetic effects of the armature and
field will more nearly coincide than in some of the types previously
under consideration. These types are: First, motors having two or more
energizing circuits of the same electrical character, and in the
operation of which the currents used differ primarily in phase; second,
motors with a plurality of energizing circuits of different electrical
character, in or by means of which the difference of phase is produced
artificially, and, third, motors with a plurality of energizing
circuits, the currents in one being induced from currents in another.
Considering the structural and operative conditions of any one of
them--as, for example, that first named--the armature which is mounted
to rotate in obedience to the co-operative influence or action of the
energizing circuits has coils wound upon it which are closed upon
themselves and in which currents are induced by the energizing-currents
with the object and result of energizing the armature-core; but under
any such conditions as must exist in these motors, it is obvious that a
certain time must elapse between the manifestations of an energizing
current impulse in the field coils, and the corresponding magnetic state
or phase in the armature established by the current induced thereby;
consequently a given magnetic influence or effect in the field which is
the direct result of a primary current impulse will have become more or
less weakened or lost before the corresponding effect in the armature
indirectly produced has reached its maximum. This is a condition
unfavorable to efficient working in certain cases--as, for instance,
when the progress of the resultant poles or points of maximum attraction
is very great, or when a very high number of alternations is
employed--for it is apparent that a stronger tendency to rotation will
be maintained if the maximum magnetic attractions or conditions in both
armature and field coincide, the energy developed by a motor being
measured by the product of the magnetic quantities of the armature and
field.

To secure this coincidence of maximum magnetic effects, Mr. Tesla has
devised various means, as explained below. Fig. 68 is a diagrammatic
illustration of a Tesla motor system in which the alternating currents
proceed from independent sources and differ primarily in phase.

[Illustration: FIG. 68.]

[Illustration: FIG. 69.]

A designates the field-magnet or magnetic frame of the motor; B B,
oppositely located pole-pieces adapted to receive the coils of one
energizing circuit; and C C, similar pole-pieces for the coils of the
other energizing circuit. These circuits are designated, respectively,
by D E, the conductor D'' forming a common return to the generator G.
Between these poles is mounted an armature--for example, a ring or
annular armature, wound with a series of coils F, forming a closed
circuit or circuits. The action or operation of a motor thus constructed
is now well understood. It will be observed, however, that the magnetism
of poles B, for example, established by a current impulse in the coils
thereon, precedes the magnetic effect set up in the armature by the
induced current in coils F. Consequently the mutual attraction between
the armature and field-poles is considerably reduced. The same
conditions will be found to exist if, instead of assuming the poles B or
C as acting independently, we regard the ideal resultant of both acting
together, which is the real condition. To remedy this, the motor field
is constructed with secondary poles B' C', which are situated between
the others. These pole-pieces are wound with coils D' E', the former in
derivation to the coils D, the latter to coils E. The main or primary
coils D and E are wound for a different self-induction from that of the
coils D' and E', the relations being so fixed that if the currents in D
and E differ, for example, by a quarter-phase, the currents in each
secondary coil, as D' E', will differ from those in its appropriate
primary D or E by, say, forty-five degrees, or one-eighth of a period.

Now, assuming that an impulse or alternation in circuit or branch E is
just beginning, while in the branch D it is just falling from maximum,
the conditions are those of a quarter-phase difference. The ideal
resultant of the attractive forces of the two sets of poles B C
therefore may be considered as progressing from poles B to poles C,
while the impulse in E is rising to maximum, and that in D is falling to
zero or minimum. The polarity set up in the armature, however, lags
behind the manifestations of field magnetism, and hence the maximum
points of attraction in armature and field, instead of coinciding, are
angularly displaced. This effect is counteracted by the supplemental
poles B' C'. The magnetic phases of these poles succeed those of poles B
C by the same, or nearly the same, period of time as elapses between the
effect of the poles B C and the corresponding induced effect in the
armature; hence the magnetic conditions of poles B' C' and of the
armature more nearly coincide and a better result is obtained. As poles
B' C' act in conjunction with the poles in the armature established by
poles B C, so in turn poles C B act similarly with the poles set up by
B' C', respectively. Under such conditions the retardation of the
magnetic effect of the armature and that of the secondary poles will
bring the maximum of the two more nearly into coincidence and a
correspondingly stronger torque or magnetic attraction secured.

In such a disposition as is shown in Fig. 68 it will be observed that
as the adjacent pole-pieces of either circuit are of like polarity they
will have a certain weakening effect upon one another. Mr. Tesla
therefore prefers to remove the secondary poles from the direct
influence of the others. This may be done by constructing a motor with
two independent sets of fields, and with either one or two armatures
electrically connected, or by using two armatures and one field. These
modifications are illustrated further on.

[Illustration: FIG. 70.]

[Illustration: FIG. 71.]

Fig. 69 is a diagrammatic illustration of a motor and system in which
the difference of phase is artificially produced. There are two coils D
D in one branch and two coils E E in another branch of the main circuit
from the generator G. These two circuits or branches are of different
self-induction, one, as D, being higher than the other. This is
graphically indicated by making coils D much larger than coils E. By
reason of the difference in the electrical character of the two
circuits, the phases of current in one are retarded to a greater extent
than the other. Let this difference be thirty degrees. A motor thus
constructed will rotate under the action of an alternating current; but
as happens in the case previously described the corresponding magnetic
effects of the armature and field do not coincide owing to the time that
elapses between a given magnetic effect in the armature and the
condition of the field that produces it. The secondary or supplemental
poles B' C' are therefore availed of. There being thirty degrees
difference of phase between the currents in coils D E, the magnetic
effect of poles B' C' should correspond to that produced by a current
differing from the current in coils D or E by fifteen degrees. This we
can attain by winding each supplemental pole B' C' with two coils H H'.
The coils H are included in a derived circuit having the same
self-induction as circuit D, and coils H' in a circuit having the same
self-induction as circuit E, so that if these circuits differ by thirty
degrees the magnetism of poles B' C' will correspond to that produced by
a current differing from that in either D or E by fifteen degrees. This
is true in all other cases. For example, if in Fig. 68 the coils D' E'
be replaced by the coils H H' included in the derived circuits, the
magnetism of the poles B' C' will correspond in effect or phase, if it
may be so termed, to that produced by a current differing from that in
either circuit D or E by forty-five degrees, or one-eighth of a period.

This invention as applied to a derived circuit motor is illustrated in
Figs. 70 and 71. The former is an end view of the motor with the
armature in section and a diagram of connections, and Fig. 71 a vertical
section through the field. These figures are also drawn to show one of
the dispositions of two fields that may be adopted in carrying out the
principle. The poles B B C C are in one field, the remaining poles in
the other. The former are wound with primary coils I J and secondary
coils I' J', the latter with coils K L. The primary coils I J are in
derived circuits, between which, by reason of their different
self-induction, there is a difference of phase, say, of thirty degrees.
The coils I' K are in circuit with one another, as also are coils J' L,
and there should be a difference of phase between the currents in coils
K and L and their corresponding primaries of, say, fifteen degrees. If
the poles B C are at right angles, the armature-coils should be
connected directly across, or a single armature core wound from end to
end may be used; but if the poles B C be in line there should be an
angular displacement of the armature coils, as will be well understood.

The operation will be understood from the foregoing. The maximum
magnetic condition of a pair of poles, as B' B', coincides closely with
the maximum effect in the armature, which lags behind the corresponding
condition in poles B B.




CHAPTER XVIII.

MOTOR BASED ON THE DIFFERENCE OF PHASE IN THE MAGNETIZATION OF THE INNER
AND OUTER PARTS OF AN IRON CORE.


It is well known that if a magnetic core, even if laminated or
subdivided, be wound with an insulated coil and a current of electricity
be directed through the coil, the magnetization of the entire core does
not immediately ensue, the magnetizing effect not being exhibited in all
parts simultaneously. This may be attributed to the fact that the action
of the current is to energize first those laminæ or parts of the core
nearest the surface and adjacent to the exciting-coil, and from thence
the action progresses toward the interior. A certain interval of time
therefore elapses between the manifestation of magnetism in the external
and the internal sections or layers of the core. If the core be thin or
of small mass, this effect may be inappreciable; but in the case of a
thick core, or even of a comparatively thin one, if the number of
alternations or rate of change of the current strength be very great,
the time interval occurring between the manifestations of magnetism in
the interior of the core and in those parts adjacent to the coil is more
marked. In the construction of such apparatus as motors which are
designed to be run by alternating or equivalent currents--such as
pulsating or undulating currents generally--Mr. Tesla found it desirable
and even necessary to give due consideration to this phenomenon and to
make special provisions in order to obviate its consequences. With the
specific object of taking advantage of this action or effect, and to
render it more pronounced, he constructs a field magnet in which the
parts of the core or cores that exhibit at different intervals of time
the magnetic effect imparted to them by alternating or equivalent
currents in an energizing coil or coils, are so placed with relation to
a rotating armature as to exert thereon their attractive effect
successively in the order of their magnetization. By this means he
secures a result similar to that which he had previously attained in
other forms or types of motor in which by means of one or more
alternating currents he has produced the rotation or progression of the
magnetic poles.

This new mode of operation will now be described. Fig. 72 is a side
elevation of such motor. Fig. 73 is a side elevation of a more
practicable and efficient embodiment of the invention. Fig. 74 is a
central vertical section of the same in the plane of the axis of
rotation.

[Illustration: FIGS. 72 and 73.]

Referring to Fig. 72, let X represent a large iron core, which may be
composed of a number of sheets or laminæ of soft iron or steel.
Surrounding this core is a coil Y, which is connected with a source E of
rapidly varying currents. Let us consider now the magnetic conditions
existing in this core at any point, as _b_, at or near the centre, and
any other point, as _a_, nearer the surface. When a current impulse is
started in the magnetizing coil Y, the section or part at _a_, being
close to the coil, is immediately energized, while the section or part
at _b_, which, to use a convenient expression, is "protected" by the
intervening sections or layers between _a_ and _b_, does not at once
exhibit its magnetism. However, as the magnetization of _a_ increases,
_b_ becomes also affected, reaching finally its maximum strength some
time later than _a_. Upon the weakening of the current the magnetization
of _a_ first diminishes, while _b_ still exhibits its maximum strength;
but the continued weakening of _a_ is attended by a subsequent weakening
of _b_. Assuming the current to be an alternating one, _a_ will now be
reversed, while _b_ still continues of the first imparted polarity. This
action continues the magnetic condition of _b_, following that of _a_ in
the manner above described. If an armature--for instance, a simple disc
F, mounted to rotate freely on an axis--be brought into proximity to the
core, a movement of rotation will be imparted to the disc, the direction
depending upon its position relatively to the core, the tendency being
to turn the portion of the disc nearest to the core from _a_ to _b_, as
indicated in Fig. 72.

[Illustration: FIG. 74.]

This action or principle of operation has been embodied in a practicable
form of motor, which is illustrated in Fig. 73. Let A in that figure
represent a circular frame of iron, from diametrically opposite points
of the interior of which the cores project. Each core is composed of
three main parts B, B and C, and they are similarly formed with a
straight portion or body _e_, around which the energizing coil is wound,
a curved arm or extension _c_, and an inwardly projecting pole or end
_d_. Each core is made up of two parts B B, with their polar extensions
reaching in one direction, and a part C between the other two, and with
its polar extension reaching in the opposite direction. In order to
lessen in the cores the circulation of currents induced therein, the
several sections are insulated from one another in the manner usually
followed in such cases. These cores are wound with coils D, which are
connected in the same circuit, either in parallel or series, and
supplied with an alternating or a pulsating current, preferably the
former, by a generator E, represented diagrammatically. Between the
cores or their polar extensions is mounted a cylindrical or similar
armature F, wound with magnetizing coils G, closed upon themselves.

The operation of this motor is as follows: When a current impulse or
alternation is directed through the coils D, the sections B B of the
cores, being on the surface and in close proximity to the coils, are
immediately energized. The sections C, on the other hand, are protected
from the magnetizing influence of the coil by the interposed layers of
iron B B. As the magnetism of B B increases, however, the sections C are
also energized; but they do not attain their maximum strength until a
certain time subsequent to the exhibition by the sections B B of their
maximum. Upon the weakening of the current the magnetic strength of B B
first diminishes, while the sections C have still their maximum
strength; but as B B continue to weaken the interior sections are
similarly weakened. B B may then begin to exhibit an opposite polarity,
which is followed later by a similar change on C, and this action
continues. B B and C may therefore be considered as separate
field-magnets, being extended so as to act on the armature in the most
efficient positions, and the effect is similar to that in the other
forms of Tesla motor--viz., a rotation or progression of the maximum
points of the field of force. Any armature--such, for instance, as a
disc--mounted in this field would rotate from the pole first to exhibit
its magnetism to that which exhibits it later.

It is evident that the principle here described may be carried out in
conjunction with other means for securing a more favorable or efficient
action of the motor. For example, the polar extensions of the sections C
may be wound or surrounded by closed coils. The effect of these coils
will be to still more effectively retard the magnetization of the polar
extensions of C.




CHAPTER XIX.

ANOTHER TYPE OF TESLA INDUCTION MOTOR.


It will have been gathered by all who are interested in the advance of
the electrical arts, and who follow carefully, step by step, the work of
pioneers, that Mr. Tesla has been foremost to utilize inductive effects
in permanently closed circuits, in the operation of alternating motors.
In this chapter one simple type of such a motor is described and
illustrated, which will serve as an exemplification of the principle.

Let it be assumed that an ordinary alternating current generator is
connected up in a circuit of practically no self-induction, such, for
example, as a circuit containing incandescent lamps only. On the
operation of the machine, alternating currents will be developed in the
circuit, and the phases of these currents will theoretically coincide
with the phases of the impressed electromotive force. Such currents may
be regarded and designated as the "unretarded currents."

It will be understood, of course, that in practice there is always more
or less self-induction in the circuit, which modifies to a corresponding
extent these conditions; but for convenience this may be disregarded in
the consideration of the principle of operation, since the same laws
apply. Assume next that a path of currents be formed across any two
points of the above circuit, consisting, for example, of the primary of
an induction device. The phases of the currents passing through the
primary, owing to the self-induction of the same, will not coincide with
the phases of the impressed electromotive force, but will lag behind,
such lag being directly proportional to the self-induction and inversely
proportional to the resistance of the said coil. The insertion of this
coil will also cause a lagging or retardation of the currents traversing
and delivered by the generator behind the impressed electromotive force,
such lag being the mean or resultant of the lag of the current through
the primary alone and of the "unretarded current" in the entire working
circuit. Next consider the conditions imposed by the association in
inductive relation with the primary coil, of a secondary coil. The
current generated in the secondary coil will react upon the primary
current, modifying the retardation of the same, according to the amount
of self-induction and resistance in the secondary circuit. If the
secondary circuit has but little self-induction--as, for instance, when
it contains incandescent lamps only--it will increase the actual
difference of phase between its own and the primary current, first, by
diminishing the lag between the primary current and the impressed
electromotive force, and, second, by its own lag or retardation behind
the impressed electromotive force. On the other hand, if the secondary
circuit have a high self-induction, its lag behind the current in the
primary is directly increased, while it will be still further increased
if the primary have a very low self-induction. The better results are
obtained when the primary has a low self-induction.

[Illustration: FIG. 75.]

[Illustration: FIG. 76.]

Fig. 75 is a diagram of a Tesla motor embodying this principle. Fig. 76
is a similar diagram of a modification of the same. In Fig. 75 let A
designate the field-magnet of a motor which, as in all these motors, is
built up of sections or plates. B C are polar projections upon which the
coils are wound. Upon one pair of these poles, as C, are wound primary
coils D, which are directly connected to the circuit of an alternating
current generator G. On the same poles are also wound secondary coils F,
either side by side or over or under the primary coils, and these are
connected with other coils E, which surround the poles B B. The
currents in both primary and secondary coils in such a motor will be
retarded or will lag behind the impressed electromotive force; but to
secure a proper difference in phase between the primary and secondary
currents themselves, Mr. Tesla increases the resistance of the circuit
of the secondary and reduces as much as practicable its self-induction.
This is done by using for the secondary circuit, particularly in the
coils E, wire of comparatively small diameter and having but few turns
around the cores; or by using some conductor of higher specific
resistance, such as German silver; or by introducing at some point in
the secondary circuit an artificial resistance R. Thus the
self-induction of the secondary is kept down and its resistance
increased, with the result of decreasing the lag between the impressed
electro-motive force and the current in the primary coils and increasing
the difference of phase between the primary and secondary currents.

In the disposition shown in Fig. 76, the lag in the secondary is
increased by increasing the self-induction of that circuit, while the
increasing tendency of the primary to lag is counteracted by inserting
therein a dead resistance. The primary coils D in this case have a low
self-induction and high resistance, while the coils E F, included in the
secondary circuit, have a high self-induction and low resistance. This
may be done by the proper winding of the coils; or in the circuit
including the secondary coils E F, we may introduce a self-induction
coil S, while in the primary circuit from the generator G and including
coils D, there may be inserted a dead resistance R. By this means the
difference of phase between the primary and secondary is increased. It
is evident that both means of increasing the difference of
phase--namely, by the special winding as well as by the supplemental or
external inductive and dead resistance--may be employed conjointly.

In the operation of this motor the current impulses in the primary coils
induce currents in the secondary coils, and by the conjoint action of
the two the points of greatest magnetic attraction are shifted or
rotated.

In practice it is found desirable to wind the armature with closed coils
in which currents are induced by the action thereon of the primaries.




CHAPTER XX.

COMBINATIONS OF SYNCHRONIZING MOTOR AND TORQUE MOTOR.


In the preceding descriptions relative to synchronizing motors and
methods of operating them, reference has been made to the plan adopted
by Mr. Tesla, which consists broadly in winding or arranging the motor
in such manner that by means of suitable switches it could be started as
a multiple-circuit motor, or one operating by a progression of its
magnetic poles, and then, when up to speed, or nearly so, converted into
an ordinary synchronizing motor, or one in which the magnetic poles were
simply alternated. In some cases, as when a large motor is used and when
the number of alternations is very high, there is more or less
difficulty in bringing the motor to speed as a double or
multiple-circuit motor, for the plan of construction which renders the
motor best adapted to run as a synchronizing motor impairs its
efficiency as a torque or double-circuit motor under the assumed
conditions on the start. This will be readily understood, for in a large
synchronizing motor the length of the magnetic circuit of the polar
projections, and their mass, are so great that apparently considerable
time is required for magnetization and demagnetization. Hence with a
current of a very high number of alternations the motor may not respond
properly. To avoid this objection and to start up a synchronizing motor
in which these conditions obtain, Mr. Tesla has combined two motors, one
a synchronizing motor, the other a multiple-circuit or torque motor, and
by the latter he brings the first-named up to speed, and then either
throws the whole current into the synchronizing motor or operates
jointly both of the motors.

This invention involves several novel and useful features. It will be
observed, in the first place, that both motors are run, without
commutators of any kind, and, secondly, that the speed of the torque
motor may be higher than that of the synchronizing motor, as will be the
case when it contains a fewer number of poles or sets of poles, so that
the motor will be more readily and easily brought up to speed. Thirdly,
the synchronizing motor may be constructed so as to have a much more
pronounced tendency to synchronism without lessening the facility with
which it is started.

Fig. 77 is a part sectional view of the two motors; Fig. 78 an end view
of the synchronizing motor; Fig. 79 an end view and part section of the
torque or double-circuit motor; Fig. 80 a diagram of the circuit
connections employed; and Figs. 81, 82, 83, 84 and 85 are diagrams of
modified dispositions of the two motors.

[Illustration: FIG. 77.]

Inasmuch as neither motor is doing any work while the current is acting
upon the other, the two armatures are rigidly connected, both being
mounted upon the same shaft A, the field-magnets B of the synchronizing
and C of the torque motor being secured to the same base D. The
preferably larger synchronizing motor has polar projections on its
armature, which rotate in very close proximity to the poles of the
field, and in other respects it conforms to the conditions that are
necessary to secure synchronous action. The pole-pieces of the armature
are, however, wound with closed coils E, as this obviates the employment
of sliding contacts. The smaller or torque motor, on the other hand,
has, preferably, a cylindrical armature F, without polar projections and
wound with closed coils G. The field-coils of the torque motor are
connected up in two series H and I, and the alternating current from the
generator is directed through or divided between these two circuits in
any manner to produce a progression of the poles or points of maximum
magnetic effect. This result is secured by connecting the two
motor-circuits in derivation with the circuit from the generator,
inserting in one motor circuit a dead resistance and in the other a
self-induction coil, by which means a difference in phase between the
two divisions of the current is secured. If both motors have the same
number of field poles, the torque motor for a given number of
alternations will tend to run at double the speed of the other, for,
assuming the connections to be such as to give the best results, its
poles are divided into two series and the number of poles is virtually
reduced one-half, which being acted upon by the same number of
alternations tend to rotate the armature at twice the speed. By this
means the main armature is more easily brought to or above the required
speed. When the speed necessary for synchronism is imparted to the main
motor, the current is shifted from the torque motor into the other.

[Illustration: FIG. 78.]

[Illustration: FIG. 79.]

A convenient arrangement for carrying out this invention is shown in
Fig. 80, in which J J are the field coils of the synchronizing, and H I
the field coils of the torque motor. L L' are the conductors of the main
line. One end of, say, coils H is connected to wire L through a
self-induction coil M. One end of the other set of coils I is connected
to the same wire through a dead resistance N. The opposite ends of these
two circuits are connected to the contact _m_ of a switch, the handle or
lever of which is in connection with the line-wire L'. One end of the
field circuit of the synchronizing motor is connected to the wire L. The
other terminates in the switch-contact _n_. From the diagram it will be
readily seen that if the lever P be turned upon contact _m_, the torque
motor will start by reason of the difference of phase between the
currents in its two energizing circuits. Then when the desired speed is
attained, if the lever P be shifted upon contact _n_ the entire current
will pass through the field coils of the synchronizing motor and the
other will be doing no work.

The torque motor may be constructed and operated in various ways, many
of which have already been touched upon. It is not necessary that one
motor be cut out of circuit while the other is in, for both may be acted
upon by current at the same time, and Mr. Tesla has devised various
dispositions or arrangements of the two motors for accomplishing this.
Some of these arrangements are illustrated in Figs. 81 to 85.

[Illustration: FIG. 80.]

Referring to Fig. 81, let T designate the torque or multiple circuit
motor and S the synchronizing motor, L L' being the line-wires from a
source of alternating current. The two circuits of the torque motor of
different degrees of self-induction, and designated by N M, are
connected in derivation to the wire L. They are then joined and
connected to the energizing circuit of the synchronizing motor, the
opposite terminal of which is connected to wire L'. The two motors are
thus in series. To start them Mr. Tesla short-circuits the synchronizing
motor by a switch P', throwing the whole current through the torque
motor. Then when the desired speed is reached the switch P' is opened,
so that the current passes through both motors. In such an arrangement
as this it is obviously desirable for economical and other reasons that
a proper relation between the speeds of the two motors should be
observed.

In Fig. 82 another disposition is illustrated. S is the synchronizing
motor and T the torque motor, the circuits of both being in parallel. W
is a circuit also in derivation to the motor circuits and containing a
switch P''. S' is a switch in the synchronizing motor circuit. On the
start the switch S' is opened, cutting out the motor S. Then P'' is
opened, throwing the entire current through the motor T, giving it a
very strong torque. When the desired speed is reached, switch S' is
closed and the current divides between both motors. By means of switch
P'' both motors may be cut out.

[Illustration: FIGS. 81, 82, 83, 84 and 85.]

In Fig. 83 the arrangement is substantially the same, except that a
switch T' is placed in the circuit which includes the two circuits of
the torque motor. Fig. 84 shows the two motors in series, with a shunt
around both containing a switch S T. There is also a shunt around the
synchronizing motor S, with a switch P'. In Fig. 85 the same disposition
is shown; but each motor is provided with a shunt, in which are switches
P' and T'', as shown.




CHAPTER XXI.

MOTOR WITH A CONDENSER IN THE ARMATURE CIRCUIT.


We now come to a new class of motors in which resort is had to
condensers for the purpose of developing the required difference of
phase and neutralizing the effects of self-induction. Mr. Tesla early
began to apply the condenser to alternating apparatus, in just how many
ways can only be learned from a perusal of other portions of this
volume, especially those dealing with his high frequency work.

Certain laws govern the action or effects produced by a condenser when
connected to an electric circuit through which an alternating or in
general an undulating current is made to pass. Some of the most
important of such effects are as follows: First, if the terminals or
plates of a condenser be connected with two points of a circuit, the
potentials of which are made to rise and fall in rapid succession, the
condenser allows the passage, or more strictly speaking, the
transference of a current, although its plates or armatures may be so
carefully insulated as to prevent almost completely the passage of a
current of unvarying strength or direction and of moderate electromotive
force. Second, if a circuit, the terminals of which are connected with
the plates of the condenser, possess a certain self-induction, the
condenser will overcome or counteract to a greater or less degree,
dependent upon well-understood conditions, the effects of such
self-induction. Third, if two points of a closed or complete circuit
through which a rapidly rising and falling current flows be shunted or
bridged by a condenser, a variation in the strength of the currents in
the branches and also a difference of phase of the currents therein is
produced. These effects Mr. Tesla has utilized and applied in a variety
of ways in the construction and operation of his motors, such as by
producing a difference in phase in the two energizing circuits of an
alternating current motor by connecting the two circuits in derivation
and connecting up a condenser in series in one of the circuits. A
further development, however, possesses certain novel features of
practical value and involves a knowledge of facts less generally
understood. It comprises the use of a condenser or condensers in
connection with the induced or armature circuit of a motor and certain
details of the construction of such motors. In an alternating current
motor of the type particularly referred to above, or in any other which
has an armature coil or circuit closed upon itself, the latter
represents not only an inductive resistance, but one which is
periodically varying in value, both of which facts complicate and
render difficult the attainment of the conditions best suited to the
most efficient working conditions; in other words, they require, first,
that for a given inductive effect upon the armature there should be the
greatest possible current through the armature or induced coils, and,
second, that there should always exist between the currents in the
energizing and the induced circuits a given relation of phase. Hence
whatever tends to decrease the self-induction and increase the current
in the induced circuits will, other things being equal, increase the
output and efficiency of the motor, and the same will be true of causes
that operate to maintain the mutual attractive effect between the field
magnets and armature at its maximum. Mr. Tesla secures these results by
connecting with the induced circuit or circuits a condenser, in the
manner described below, and he also, with this purpose in view,
constructs the motor in a special manner.

[Illustration: FIG. 86.]

[Illustration: FIG. 88.]

[Illustration: FIG. 89.]

[Illustration: FIG. 87.]

[Illustration: FIG. 90.]

Referring to the drawings, Fig. 86, is a view, mainly diagrammatic, of
an alternating current motor, in which the present principle is applied.
Fig. 87 is a central section, in line with the shaft, of a special form
of armature core. Fig. 88 is a similar section of a modification of the
same. Fig. 89 is one of the sections of the core detached. Fig. 90 is a
diagram showing a modified disposition of the armature or induced
circuits.

The general plan of the invention is illustrated in Fig. 86. A A in this
figure represent the the frame and field magnets of an alternating
current motor, the poles or projections of which are wound with coils B
and C, forming independent energizing circuits connected either to the
same or to independent sources of alternating currents, so that the
currents flowing through the circuits, respectively, will have a
difference of phase. Within the influence of this field is an armature
core D, wound with coils E. In motors of this description heretofore
these coils have been closed upon themselves, or connected in a closed
series; but in the present case each coil or the connected series of
coils terminates in the opposite plates of a condenser F. For this
purpose the ends of the series of coils are brought out through the
shaft to collecting rings G, which are connected to the condenser by
contact brushes H and suitable conductors, the condenser being
independent of the machine. The armature coils are wound or connected in
such manner that adjacent coils produce opposite poles.

The action of this motor and the effect of the plan followed in its
construction are as follows: The motor being started in operation and
the coils of the field magnets being traversed by alternating currents,
currents are induced in the armature coils by one set of field coils, as
B, and the poles thus established are acted upon by the other set, as C.
The armature coils, however, have necessarily a high self-induction,
which opposes the flow of the currents thus set up. The condenser F not
only permits the passage or transference of these currents, but also
counteracts the effects of self-induction, and by a proper adjustment of
the capacity of the condenser, the self-induction of the coils, and the
periods of the currents, the condenser may be made to overcome entirely
the effect of self-induction.

It is preferable on account of the undesirability of using sliding
contacts of any kind, to associate the condenser with the armature
directly, or make it a part of the armature. In some cases Mr. Tesla
builds up the armature of annular plates K K, held by bolts L between
heads M, which are secured to the driving shaft, and in the hollow space
thus formed he places a condenser F, generally by winding the two
insulated plates spirally around the shaft. In other cases he utilizes
the plates of the core itself as the plates of the condenser. For
example, in Figs. 88 and 89, N is the driving shaft, M M are the heads
of the armature-core, and K K' the iron plates of which the core is
built up. These plates are insulated from the shaft and from one
another, and are held together by rods or bolts L. The bolts pass
through a large hole in one plate and a small hole in the one next
adjacent, and so on, connecting electrically all of plates K, as one
armature of a condenser, and all of plates K' as the other.

To either of the condensers above described the armature coils may be
connected, as explained by reference to Fig. 86.

In motors in which the armature coils are closed upon themselves--as,
for example, in any form of alternating current motor in which one
armature coil or set of coils is in the position of maximum induction
with respect to the field coils or poles, while the other is in the
position of minimum induction--the coils are best connected in one
series, and two points of the circuit thus formed are bridged by a
condenser. This is illustrated in Fig. 90, in which E represents one set
of armature coils and E' the other. Their points of union are joined
through a condenser F. It will be observed that in this disposition the
self-induction of the two branches E and E' varies with their position
relatively to the field magnet, and that each branch is alternately the
predominating source of the induced current. Hence the effect of the
condenser F is twofold. First, it increases the current in each of the
branches alternately, and, secondly, it alters the phase of the currents
in the branches, this being the well-known effect which results from
such a disposition of a condenser with a circuit, as above described.
This effect is favorable to the proper working of the motor, because it
increases the flow of current in the armature circuits due to a given
inductive effect, and also because it brings more nearly into
coincidence the maximum magnetic effects of the coacting field and
armature poles.

It will be understood, of course, that the causes that contribute to the
efficiency of condensers when applied to such uses as the above must be
given due consideration in determining the practicability and efficiency
of the motors. Chief among these is, as is well known, the periodicity
of the current, and hence the improvements described are more
particularly adapted to systems in which a very high rate of alternation
or change is maintained.

Although this invention has been illustrated in connection with a
special form of motor, it will be understood that it is equally
applicable to any other alternating current motor in which there is a
closed armature coil wherein the currents are induced by the action of
the field, and the feature of utilizing the plates or sections of a
magnetic core for forming the condenser is applicable, generally, to
other kinds of alternating current apparatus.




CHAPTER XXII.

MOTOR WITH CONDENSER IN ONE OF THE FIELD CIRCUITS.


If the field or energizing circuits of a rotary phase motor be both
derived from the same source of alternating currents and a condenser of
proper capacity be included in one of the same, approximately, the
desired difference of phase may be obtained between the currents flowing
directly from the source and those flowing through the condenser; but
the great size and expense of condensers for this purpose that would
meet the requirements of the ordinary systems of comparatively low
potential are particularly prohibitory to their employment.

Another, now well-known, method or plan of securing a difference of
phase between the energizing currents of motors of this kind is to
induce by the currents in one circuit those in the other circuit or
circuits; but as no means had been proposed that would secure in this
way between the phases of the primary or inducing and the secondary or
induced currents that difference--theoretically ninety degrees--that is
best adapted for practical and economical working, Mr. Tesla devised a
means which renders practicable both the above described plans or
methods, and by which he is enabled to obtain an economical and
efficient alternating current motor. His invention consists in placing a
condenser in the secondary or induced circuit of the motor above
described and raising the potential of the secondary currents to such a
degree that the capacity of the condenser, which is in part dependent on
the potential, need be quite small. The value of this condenser is
determined in a well-understood manner with reference to the
self-induction and other conditions of the circuit, so as to cause the
currents which pass through it to differ from the primary currents by a
quarter phase.

Fig. 91 illustrates the invention as embodied in a motor in which the
inductive relation of the primary and secondary circuits is secured by
winding them inside the motor partly upon the same cores; but the
invention applies, generally, to other forms of motor in which one of
the energizing currents is induced in any way from the other.

Let A B represent the poles of an alternating current motor, of which C
is the armature wound with coils D, closed upon themselves, as is now
the general practice in motors of this kind. The poles A, which
alternate with poles B, are wound with coils of ordinary or coarse wire
E in such direction as to make them of alternate north and south
polarity, as indicated in the diagram by the characters N S. Over these
coils, or in other inductive relation to the same, are wound long
fine-wire coils F F, and in the same direction throughout as the coils
E. These coils are secondaries, in which currents of very high potential
are induced. All the coils E in one series are connected, and all the
secondaries F in another.

[Illustration: FIG. 91.]

On the intermediate poles B are wound fine-wire energizing coils G,
which are connected in series with one another, and also with the series
of secondary coils F, the direction of winding being such that a
current-impulse induced from the primary coils E imparts the same
magnetism to the poles B as that produced in poles A by the primary
impulse. This condition is indicated by the characters N' S'.

In the circuit formed by the two sets of coils F and G is introduced a
condenser H; otherwise this circuit is closed upon itself, while the
free ends of the circuit of coils E are connected to a source of
alternating currents. As the condenser capacity which is needed in any
particular motor of this kind is dependent upon the rate of alternation
or the potential, or both, its size or cost, as before explained, may be
brought within economical limits for use with the ordinary circuits if
the potential of the secondary circuit in the motor be sufficiently
high. By giving to the condenser proper values, any desired difference
of phase between the primary and secondary energizing circuits may be
obtained.




CHAPTER XXIII.

TESLA POLYPHASE TRANSFORMER.


Applying the polyphase principle to the construction of transformers as
well to the motors already noticed, Mr. Tesla has invented some very
interesting forms, which he considers free from the defects of earlier
and, at present, more familiar forms. In these transformers he provides
a series of inducing coils and corresponding induced coils, which are
generally wound upon a core closed upon itself, usually a ring of
laminated iron.

The two sets of coils are wound side by side or superposed or otherwise
placed in well-known ways to bring them into the most effective
relations to one another and to the core. The inducing or primary coils
wound on the core are divided into pairs or sets by the proper
electrical connections, so that while the coils of one pair or set
co-operate in fixing the magnetic poles of the core at two given
diametrically opposite points, the coils of the other pair or
set--assuming, for sake of illustration, that there are but two--tend to
fix the poles ninety degrees from such points. With this induction
device is used an alternating current generator with coils or sets of
coils to correspond with those of the converter, and the corresponding
coils of the generator and converter are then connected up in
independent circuits. It results from this that the different electrical
phases in the generator are attended by corresponding magnetic changes
in the converter; or, in other words, that as the generator coils
revolve, the points of greatest magnetic intensity in the converter will
be progressively shifted or whirled around.

Fig. 92 is a diagrammatic illustration of the converter and the
electrical connections of the same. Fig. 93 is a horizontal central
cross-section of Fig. 92. Fig. 94 is a diagram of the circuits of the
entire system, the generator being shown in section.

Mr. Tesla uses a core, A, which is closed upon itself--that is to say,
of an annular cylindrical or equivalent form--and as the efficiency of
the apparatus is largely increased by the subdivision of this core, he
makes it of thin strips, plates, or wires of soft iron electrically
insulated as far as practicable. Upon this core are wound, say, four
coils, B B B' B', used as primary coils, and for which long lengths of
comparatively fine wire are employed. Over these coils are then wound
shorter coils of coarser wire, C C C' C', to constitute the induced or
secondary coils. The construction of this or any equivalent form of
converter may be carried further, as above pointed out, by inclosing
these coils with iron--as, for example, by winding over the coils layers
of insulated iron wire.

[Illustration: FIGS. 92 and 93.]

[Illustration: FIG. 94.]

The device is provided with suitable binding posts, to which the ends of
the coils are led. The diametrically opposite coils B B and B' B' are
connected, respectively, in series, and the four terminals are connected
to the binding posts. The induced coils are connected together in any
desired manner. For example, as shown in Fig. 94, C C may be connected
in multiple arc when a quantity current is desired--as for running a
group of incandescent lamps--while C' C' may be independently connected
in series in a circuit including arc lamps or the like. The generator in
this system will be adapted to the converter in the manner illustrated.
For example, in the present case there are employed a pair of ordinary
permanent or electro-magnets, E E, between which is mounted a
cylindrical armature on a shaft, F, and wound with two coils, G G'. The
terminals of these coils are connected, respectively, to four insulated
contact or collecting rings, H H H' H', and the four line circuit wires
L connect the brushes K, bearing on these rings, to the converter in the
order shown. Noting the results of this combination, it will be observed
that at a given point of time the coil G is in its neutral position and
is generating little or no current, while the other coil, G', is in a
position where it exerts its maximum effect. Assuming coil G to be
connected in circuit with coils B B of the converter, and coil G' with
coils B' B', it is evident that the poles of the ring A will be
determined by coils B' B' alone; but as the armature of the generator
revolves, coil G develops more current and coil G' less, until G reaches
its maximum and G' its neutral position. The obvious result will be to
shift the poles of the ring A through one-quarter of its periphery. The
movement of the coils through the next quarter of a turn--during which
coil G' enters a field of opposite polarity and generates a current of
opposite direction and increasing strength, while coil G, in passing
from its maximum to its neutral position generates a current of
decreasing strength and same direction as before--causes a further
shifting of the poles through the second quarter of the ring. The second
half-revolution will obviously be a repetition of the same action. By
the shifting of the poles of the ring A, a powerful dynamic inductive
effect on the coils C C' is produced. Besides the currents generated in
the secondary coils by dynamo-magnetic induction, other currents will be
set up in the same coils in consequence of many variations in the
intensity of the poles in the ring A. This should be avoided by
maintaining the intensity of the poles constant, to accomplish which
care should be taken in designing and proportioning the generator and in
distributing the coils in the ring A, and balancing their effect. When
this is done, the currents are produced by dynamo-magnetic induction
only, the same result being obtained as though the poles were shifted by
a commutator with an infinite number of segments.

The modifications which are applicable to other forms of converter are
in many respects applicable to this, such as those pertaining more
particularly to the form of the core, the relative lengths and
resistances of the primary and secondary coils, and the arrangements for
running or operating the same.




CHAPTER XXIV.

A CONSTANT CURRENT TRANSFORMER WITH MAGNETIC SHIELD BETWEEN COILS OF
PRIMARY AND SECONDARY.


Mr. Tesla has applied his principle of magnetic shielding of parts to
the construction also of transformers, the shield being interposed
between the primary and secondary coils. In transformers of the ordinary
type it will be found that the wave of electromotive force of the
secondary very nearly coincides with that of the primary, being,
however, in opposite sign. At the same time the currents, both primary
and secondary, lag behind their respective electromotive forces; but as
this lag is practically or nearly the same in the case of each it
follows that the maximum and minimum of the primary and secondary
currents will nearly coincide, but differ in sign or direction, provided
the secondary be not loaded or if it contain devices having the property
of self-induction. On the other hand, the lag of the primary behind the
impressed electromotive force may be diminished by loading the secondary
with a non-inductive or dead resistance--such as incandescent
lamps--whereby the time interval between the maximum or minimum periods
of the primary and secondary currents is increased. This time interval,
however, is limited, and the results obtained by phase difference in the
operation of such devices as the Tesla alternating current motors can
only be approximately realized by such means of producing or securing
this difference, as above indicated, for it is desirable in such cases
that there should exist between the primary and secondary currents, or
those which, however produced, pass through the two circuits of the
motor, a difference of phase of ninety degrees; or, in other words, the
current in one circuit should be a maximum when that in the other
circuit is a minimum. To attain to this condition more perfectly, an
increased retardation of the secondary current is secured in the
following manner: Instead of bringing the primary and secondary coils or
circuits of a transformer into the closest possible relations, as has
hitherto been done, Mr. Tesla protects in a measure the secondary from
the inductive action or effect of the primary by surrounding either the
primary or the secondary with a comparatively thin magnetic shield or
screen. Under these modified conditions, as long as the primary current
has a small value, the shield protects the secondary; but as soon as the
primary current has reached a certain strength, which is arbitrarily
determined, the protecting magnetic shield becomes saturated and the
inductive action upon the secondary begins. It results, therefore, that
the secondary current begins to flow at a certain fraction of a period
later than it would without the interposed shield, and since this
retardation may be obtained without necessarily retarding the primary
current also, an additional lag is secured, and the time interval
between the maximum or minimum periods of the primary and secondary
currents is increased. Such a transformer may, by properly proportioning
its several elements and determining the proper relations between the
primary and secondary windings, the thickness of the magnetic shield,
and other conditions, be constructed to yield a constant current at all
loads.

[Illustration: FIG. 95.]

Fig. 95 is a cross-section of a transformer embodying this improvement.
Fig. 96 is a similar view of a modified form of transformer, showing
diagrammatically the manner of using the same.

A A is the main core of the transformer, composed of a ring of soft
annealed and insulated or oxidized iron wire. Upon this core is wound
the secondary circuit or coil B B. This latter is then covered with a
layer or layers of annealed and insulated iron wires C C, wound in a
direction at right angles to the secondary coil. Over the whole is then
wound the primary coil or wire D D. From the nature of this construction
it will be obvious that as long as the shield formed by the wires C is
below magnetic saturation the secondary coil or circuit is effectually
protected or shielded from the inductive influence of the primary,
although on open circuit it may exhibit some electromotive force. When
the strength of the primary reaches a certain value, the shield C,
becoming saturated, ceases to protect the secondary from inductive
action, and current is in consequence developed therein. For similar
reasons, when the primary current weakens, the weakening of the
secondary is retarded to the same or approximately the same extent.

[Illustration: FIG. 96.]

The specific construction of the transformer is largely immaterial. In
Fig. 96, for example, the core A is built up of thin insulated iron
plates or discs. The primary circuit D is wound next the core A. Over
this is applied the shield C, which in this case is made up of thin
strips or plates of iron properly insulated and surrounding the primary,
forming a closed magnetic circuit. The secondary B is wound over the
shield C. In Fig. 96, also, E is a source of alternating or rapidly
changing currents. The primary of the transformer is connected with the
circuit of the generator. F is a two-circuit alternating current motor,
one of the circuits being connected with the main circuit from the
source E, and the other being supplied with currents from the secondary
of the transformer.




PART II.

THE TESLA EFFECTS WITH HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS.




CHAPTER XXV.

INTRODUCTION.--THE SCOPE OF THE TESLA LECTURES.


Before proceeding to study the three Tesla lectures here presented, the
reader may find it of some assistance to have his attention directed to
the main points of interest and significance therein. The first of these
lectures was delivered in New York, at Columbia College, before the
American Institute of Electrical Engineers, May 20, 1891. The urgent
desire expressed immediately from all parts of Europe for an opportunity
to witness the brilliant and unusual experiments with which the lecture
was accompanied, induced Mr. Tesla to go to England early in 1892, when
he appeared before the Institution of Electrical Engineers, and a day
later, by special request, before the Royal Institution. His reception
was of the most enthusiastic and flattering nature on both occasions. He
then went, by invitation, to France, and repeated his novel
demonstrations before the Société Internationale des Electriciens, and
the Société Française de Physique. Mr. Tesla returned to America in the
fall of 1892, and in February, 1893, delivered his third lecture before
the Franklin Institute of Philadelphia, in fulfilment of a long standing
promise to Prof. Houston. The following week, at the request of
President James I. Ayer, of the National Electric Light Association, the
same lecture was re-delivered in St. Louis. It had been intended to
limit the invitations to members, but the appeals from residents in the
city were so numerous and pressing that it became necessary to secure a
very large hall. Hence it came about that the lecture was listened to by
an audience of over 5,000 people, and was in some parts of a more
popular nature than either of its predecessors. Despite this concession
to the need of the hour and occasion, Mr. Tesla did not hesitate to show
many new and brilliant experiments, and to advance the frontier of
discovery far beyond any point he had theretofore marked publicly.

We may now proceed to a running review of the lectures themselves. The
ground covered by them is so vast that only the leading ideas and
experiments can here be touched upon; besides, it is preferable that the
lectures should be carefully gone over for their own sake, it being more
than likely that each student will discover a new beauty or stimulus in
them. Taking up the course of reasoning followed by Mr. Tesla in his
first lecture, it will be noted that he started out with the recognition
of the fact, which he has now experimentally demonstrated, that for the
production of light waves, primarily, electrostatic effects must be
brought into play, and continued study has led him to the opinion that
all electrical and magnetic effects may be referred to electrostatic
molecular forces. This opinion finds a singular confirmation in one of
the most striking experiments which he describes, namely, the production
of a veritable flame by the agitation of electrostatically charged
molecules. It is of the highest interest to observe that this result
points out a way of obtaining a flame which consumes no material and in
which no chemical action whatever takes place. It also throws a light on
the nature of the ordinary flame, which Mr. Tesla believes to be due to
electrostatic molecular actions, which, if true, would lead directly to
the idea that even chemical affinities might be electrostatic in their
nature and that, as has already been suggested, molecular forces in
general may be referable to one and the same cause. This singular
phenomenon accounts in a plausible manner for the unexplained fact that
buildings are frequently set on fire during thunder storms without
having been at all struck by lightning. It may also explain the total
disappearance of ships at sea.

One of the striking proofs of the correctness of the ideas advanced by
Mr. Tesla is the fact that, notwithstanding the employment of the most
powerful electromagnetic inductive effects, but feeble luminosity is
obtainable, and this only in close proximity to the source of
disturbance; whereas, when the electrostatic effects are intensified,
the same initial energy suffices to excite luminosity at considerable
distances from the source. That there are only electrostatic effects
active seems to be clearly proved by Mr. Tesla's experiments with an
induction coil operated with alternating currents of very high
frequency. He shows how tubes may be made to glow brilliantly at
considerable distances from any object when placed in a powerful,
rapidly alternating, electrostatic field, and he describes many
interesting phenomena observed in such a field. His experiments open up
the possibility of lighting an apartment by simply creating in it such
an electrostatic field, and this, in a certain way, would appear to be
the ideal method of lighting a room, as it would allow the illuminating
device to be freely moved about. The power with which these exhausted
tubes, devoid of any electrodes, light up is certainly remarkable.

That the principle propounded by Mr. Tesla is a broad one is evident
from the many ways in which it may be practically applied. We need only
refer to the variety of the devices shown or described, all of which are
novel in character and will, without doubt, lead to further important
results at the hands of Mr. Tesla and other investigators. The
experiment, for instance, of lighting up a single filament or block of
refractory material with a single wire, is in itself sufficient to give
Mr. Tesla's work the stamp of originality, and the numerous other
experiments and effects which may be varied at will, are equally new and
interesting. Thus, the incandescent filament spinning in an unexhausted
globe, the well-known Crookes experiment on open circuit, and the many
others suggested, will not fail to interest the reader. Mr. Tesla has
made an exhaustive study of the various forms of the discharge presented
by an induction coil when operated with these rapidly alternating
currents, starting from the thread-like discharge and passing through
various stages to the true electric flame.

A point of great importance in the introduction of high tension
alternating current which Mr. Tesla brings out is the necessity of
carefully avoiding all gaseous matter in the high tension apparatus. He
shows that, at least with very rapidly alternating currents of high
potential, the discharge may work through almost any practicable
thickness of the best insulators, if air is present. In such cases the
air included within the apparatus is violently agitated and by molecular
bombardment the parts may be so greatly heated as to cause a rupture of
the insulation. The practical outcome of this is, that, whereas with
steady currents, any kind of insulation may be used, with rapidly
alternating currents oils will probably be the best to employ, a fact
which has been observed, but not until now satisfactorily explained. The
recognition of the above fact is of special importance in the
construction of the costly commercial induction coils which are often
rendered useless in an unaccountable manner. The truth of these views of
Mr. Tesla is made evident by the interesting experiments illustrative
of the behavior of the air between charged surfaces, the luminous
streams formed by the charged molecules appearing even when great
thicknesses of the best insulators are interposed between the charged
surfaces. These luminous streams afford in themselves a very interesting
study for the experimenter. With these rapidly alternating currents they
become far more powerful and produce beautiful light effects when they
issue from a wire, pinwheel or other object attached to a terminal of
the coil; and it is interesting to note that they issue from a ball
almost as freely as from a point, when the frequency is very high.

From these experiments we also obtain a better idea of the importance of
taking into account the capacity and self-induction in the apparatus
employed and the possibilities offered by the use of condensers in
conjunction with alternate currents, the employment of currents of high
frequency, among other things, making it possible to reduce the
condenser to practicable dimensions. Another point of interest and
practical bearing is the fact, proved by Mr. Tesla, that for alternate
currents, especially those of high frequency, insulators are required
possessing a small specific inductive capacity, which at the same time
have a high insulating power.

Mr. Tesla also makes interesting and valuable suggestion in regard to
the economical utilization of iron in machines and transformers. He
shows how, by maintaining by continuous magnetization a flow of lines
through the iron, the latter may be kept near its maximum permeability
and a higher output and economy may be secured in such apparatus. This
principle may prove of considerable commercial importance in the
development of alternating systems. Mr. Tesla's suggestion that the same
result can be secured by heating the iron by hysteresis and eddy
currents, and increasing the permeability in this manner, while it may
appear less practical, nevertheless opens another direction for
investigation and improvement.

The demonstration of the fact that with alternating currents of high
frequency, sufficient energy may be transmitted under practicable
conditions through the glass of an incandescent lamp by electrostatic or
electromagnetic induction may lead to a departure in the construction of
such devices. Another important experimental result achieved is the
operation of lamps, and even motors, with the discharges of condensers,
this method affording a means of converting direct or alternating
currents. In this connection Mr. Tesla advocates the perfecting of
apparatus capable of generating electricity of high tension from heat
energy, believing this to be a better way of obtaining electrical energy
for practical purposes, particularly for the production of light.

While many were probably prepared to encounter curious phenomena of
impedance in the use of a condenser discharged disruptively, the
experiments shown were extremely interesting on account of their
paradoxical character. The burning of an incandescent lamp at any candle
power when connected across a heavy metal bar, the existence of nodes on
the bar and the possibility of exploring the bar by means of an ordinary
Cardew voltmeter, are all peculiar developments, but perhaps the most
interesting observation is the phenomenon of impedance observed in the
lamp with a straight filament, which remains dark while the bulb glows.

Mr. Tesla's manner of operating an induction coil by means of the
disruptive discharge, and thus obtaining enormous differences of
potential from comparatively small and inexpensive coils, will be
appreciated by experimenters and will find valuable application in
laboratories. Indeed, his many suggestions and hints in regard to the
construction and use of apparatus in these investigations will be highly
valued and will aid materially in future research.

The London lecture was delivered twice. In its first form, before the
Institution of Electrical Engineers, it was in some respects an
amplification of several points not specially enlarged upon in the New
York lecture, but brought forward many additional discoveries and new
investigations. Its repetition, in another form, at the Royal
Institution, was due to Prof. Dewar, who with Lord Rayleigh, manifested
a most lively interest in Mr. Tesla's work, and whose kindness
illustrated once more the strong English love of scientific truth and
appreciation of its votaries. As an indefatigable experimenter, Mr.
Tesla was certainly nowhere more at home than in the haunts of Faraday,
and as the guest of Faraday's successor. This Royal Institution lecture
summed up the leading points of Mr. Tesla's work, in the high potential,
high frequency field, and we may here avail ourselves of so valuable a
summarization, in a simple form, of a subject by no means easy of
comprehension until it has been thoroughly studied.

In these London lectures, among the many notable points made was first,
the difficulty of constructing the alternators to obtain the very high
frequencies needed. To obtain the high frequencies it was necessary to
provide several hundred polar projections, which were necessarily small
and offered many drawbacks, and this the more as exceedingly high
peripheral speeds had to be resorted to. In some of the first machines
both armature and field had polar projections. These machines produced a
curious noise, especially when the armature was started from the state
of rest, the field being charged. The most efficient machine was found
to be one with a drum armature, the iron body of which consisted of very
thin wire annealed with special care. It was, of course, desirable to
avoid the employment of iron in the armature, and several machines of
this kind, with moving or stationary conductors were constructed, but
the results obtained were not quite satisfactory, on account of the
great mechanical and other difficulties encountered.

The study of the properties of the high frequency currents obtained from
these machines is very interesting, as nearly every experiment discloses
something new. Two coils traversed by such a current attract or repel
each other with a force which, owing to the imperfection of our sense of
touch, seems continuous. An interesting observation, already noted under
another form, is that a piece of iron, surrounded by a coil through
which the current is passing appears to be continuously magnetized. This
apparent continuity might be ascribed to the deficiency of the sense of
touch, but there is evidence that in currents of such high frequencies
one of the impulses preponderates over the other.

As might be expected, conductors traversed by such currents are rapidly
heated, owing to the increase of the resistance, and the heating effects
are relatively much greater in the iron. The hysteresis losses in iron
are so great that an iron core, even if finely subdivided, is heated in
an incredibly short time. To give an idea of this, an ordinary iron wire
1/16 inch in diameter inserted within a coil having 250 turns, with a
current estimated to be five amperes passing through the coil, becomes
within two seconds' time so hot as to scorch wood. Beyond a certain
frequency, an iron core, no matter how finely subdivided, exercises a
dampening effect, and it was easy to find a point at which the
impedance of a coil was not affected by the presence of a core
consisting of a bundle of very thin well annealed and varnished iron
wires.

Experiments with a telephone, a conductor in a strong magnetic field, or
with a condenser or arc, seem to afford certain proof that sounds far
above the usually accepted limit of hearing would be perceived if
produced with sufficient power. The arc produced by these currents
possesses several interesting features. Usually it emits a note the
pitch of which corresponds to twice the frequency of the current, but if
the frequency be sufficiently high it becomes noiseless, the limit of
audition being determined principally by the linear dimensions of the
arc. A curious feature of the arc is its persistency, which is due
partly to the inability of the gaseous column to cool and increase
considerably in resistance, as is the case with low frequencies, and
partly to the tendency of such a high frequency machine to maintain a
constant current.

In connection with these machines the condenser affords a particularly
interesting study. Striking effects are produced by proper adjustments
of capacity and self-induction. It is easy to raise the electromotive
force of the machine to many times the original value by simply
adjusting the capacity of a condenser connected in the induced circuit.
If the condenser be at some distance from the machine, the difference of
potential on the terminals of the latter may be only a small fraction of
that on the condenser.

But the most interesting experiences are gained when the tension of the
currents from the machine is raised by means of an induction coil. In
consequence of the enormous rate of change obtainable in the primary
current, much higher potential differences are obtained than with coils
operated in the usual ways, and, owing to the high frequency, the
secondary discharge possesses many striking peculiarities. Both the
electrodes behave generally alike, though it appears from some
observations that one current impulse preponderates over the other, as
before mentioned.

The physiological effects of the high tension discharge are found to be
so small that the shock of the coil can be supported without any
inconvenience, except perhaps a small burn produced by the discharge
upon approaching the hand to one of the terminals. The decidedly smaller
physiological effects of these currents are thought to be due either to
a different distribution through the body or to the tissues acting as
condensers. But in the case of an induction coil with a great many turns
the harmlessness is principally due to the fact that but little energy
is available in the external circuit when the same is closed through the
experimenter's body, on account of the great impedance of the coil.

In varying the frequency and strength of the currents through the
primary of the coil, the character of the secondary discharge is greatly
varied, and no less than five distinct forms are observed:--A weak,
sensitive thread discharge, a powerful flaming discharge, and three
forms of brush or streaming discharges. Each of these possesses certain
noteworthy features, but the most interesting to study are the latter.

Under certain conditions the streams, which are presumably due to the
violent agitation of the air molecules, issue freely from all points of
the coil, even through a thick insulation. If there is the smallest air
space between the primary and secondary, they will form there and surely
injure the coil by slowly warming the insulation. As they form even with
ordinary frequencies when the potential is excessive, the air-space must
be most carefully avoided. These high frequency streamers differ in
aspect and properties from those produced by a static machine. The wind
produced by them is small and should altogether cease if still
considerably higher frequencies could be obtained. A peculiarity is that
they issue as freely from surfaces as from points. Owing to this, a
metallic vane, mounted in one of the terminals of the coil so as to
rotate freely, and having one of its sides covered with insulation, is
spun rapidly around. Such a vane would not rotate with a steady
potential, but with a high frequency coil it will spin, even if it be
entirely covered with insulation, provided the insulation on one side be
either thicker or of a higher specific inductive capacity. A Crookes
electric radiometer is also spun around when connected to one of the
terminals of the coil, but only at very high exhaustion or at ordinary
pressures.

There is still another and more striking peculiarity of such a high
frequency streamer, namely, it is hot. The heat is easily perceptible
with frequencies of about 10,000, even if the potential is not
excessively high. The heating effect is, of course, due to the molecular
impacts and collisions. Could the frequency and potential be pushed far
enough, then a brush could be produced resembling in every particular a
flame and giving light and heat, yet without a chemical process taking
place.

The hot brush, when properly produced, resembles a jet of burning gas
escaping under great pressure, and it emits an extraordinary strong
smell of ozone. The great ozonizing action is ascribed to the fact that
the agitation of the molecules of the air is more violent in such a
brush than in the ordinary streamer of a static machine. But the most
powerful brush discharges were produced by employing currents of much
higher frequencies than it was possible to obtain by means of the
alternators. These currents were obtained by disruptively discharging a
condenser and setting up oscillations. In this manner currents of a
frequency of several hundred thousand were obtained.

Currents of this kind, Mr. Tesla pointed out, produce striking effects.
At these frequencies, the impedance of a copper bar is so great that a
potential difference of several hundred volts can be maintained between
two points of a short and thick bar, and it is possible to keep an
ordinary incandescent lamp burning at full candle power by attaching the
terminals of the lamp to two points of the bar no more than a few inches
apart. When the frequency is extremely high, nodes are found to exist on
such a bar, and it is easy to locate them by means of a lamp.

By converting the high tension discharges of a low frequency coil in
this manner, it was found practicable to keep a few lamps burning on the
ordinary circuit in the laboratory, and by bringing the undulation to a
low pitch, it was possible to operate small motors.

This plan likewise allows of converting high tension discharges of one
direction into low tension unidirectional currents, by adjusting the
circuit so that there are no oscillations. In passing the oscillating
discharges through the primary of a specially constructed coil, it is
easy to obtain enormous potential differences with only few turns of the
secondary.

Great difficulties were at first experienced in producing a successful
coil on this plan. It was found necessary to keep all air, or gaseous
matter in general, away from the charged surfaces, and oil immersion was
resorted to. The wires used were heavily covered with gutta-percha and
wound in oil, or the air was pumped out by means of a Sprengel pump. The
general arrangement was the following:--An ordinary induction coil,
operated from a low frequency alternator, was used to charge Leyden
jars. The jars were made to discharge over a single or multiple gap
through the primary of the second coil. To insure the action of the gap,
the arc was blown out by a magnet or air blast. To adjust the potential
in the secondary a small oil condenser was used, or polished brass
spheres of different sizes were screwed on the terminals and their
distance adjusted.

When the conditions were carefully determined to suit each experiment,
magnificent effects were obtained. Two wires, stretched through the
room, each being connected to one of the terminals of the coil, emitted
streams so powerful that the light from them allowed distinguishing the
objects in the room; the wires became luminous even though covered with
thick and most excellent insulation. When two straight wires, or two
concentric circles of wire, are connected to the terminals, and set at
the proper distance, a uniform luminous sheet is produced between them.
It was possible in this way to cover an area of more than one meter
square completely with the streams. By attaching to one terminal a large
circle of wire and to the other terminal a small sphere, the streams are
focused upon the sphere, produce a strongly lighted spot upon the same,
and present the appearance of a luminous cone. A very thin wire glued
upon a plate of hard rubber of great thickness, on the opposite side of
which is fastened a tinfoil coating, is rendered intensely luminous when
the coating is connected to the other terminal of the coil. Such an
experiment can be performed also with low frequency currents, but much
less satisfactorily.

When the terminals of such a coil, even of a very small one, are
separated by a rubber or glass plate, the discharge spreads over the
plate in the form of streams, threads or brilliant sparks, and affords a
magnificent display, which cannot be equaled by the largest coil
operated in the usual ways. By a simple adjustment it is possible to
produce with the coil a succession of brilliant sparks, exactly as with
a Holtz machine.

Under certain conditions, when the frequency of the oscillation is very
great, white, phantom-like streams are seen to break forth from the
terminals of the coil. The chief interesting feature about them is, that
they stream freely against the outstretched hand or other conducting
object without producing any sensation, and the hand may be approached
very near to the terminal without a spark being induced to jump. This is
due presumably to the fact that a considerable portion of the energy is
carried away or dissipated in the streamers, and the difference of
potential between the terminal and the hand is diminished.

It is found in such experiments that the frequency of the vibration and
the quickness of succession of the sparks between the knobs affect to a
marked degree the appearance of the streams. When the frequency is very
low, the air gives way in more or less the same manner as by a steady
difference of potential, and the streams consist of distinct threads,
generally mingled with thin sparks, which probably correspond to the
successive discharges occurring between the knobs. But when the
frequency is very high, and the arc of the discharge produces a sound
which is loud and smooth (which indicates both that oscillation takes
place and that the sparks succeed each other with great rapidity), then
the luminous streams formed are perfectly uniform. They are generally of
a purplish hue, but when the molecular vibration is increased by raising
the potential, they assume a white color.

The luminous intensity of the streams increases rapidly when the
potential is increased; and with frequencies of only a few hundred
thousand, could the coil be made to withstand a sufficiently high
potential difference, there is no doubt that the space around a wire
could be made to emit a strong light, merely by the agitation of the
molecules of the air at ordinary pressure.

Such discharges of very high frequency which render luminous the air at
ordinary pressure we have very likely occasion to witness in the aurora
borealis. From many of these experiments it seems reasonable to infer
that sudden cosmic disturbances, such as eruptions on the sun, set the
electrostatic charge of the earth in an extremely rapid vibration, and
produce the glow by the violent agitation of the air in the upper and
even in the lower strata. It is thought that if the frequency were low,
or even more so if the charge were not at all vibrating, the lower dense
strata would break down as in a lightning discharge. Indications of such
breaking down have been repeatedly observed, but they can be attributed
to the fundamental disturbances, which are few in number, for the
superimposed vibration would be so rapid as not to allow a disruptive
break.

The study of these discharge phenomena has led Mr. Tesla to the
recognition of some important facts. It was found, as already stated,
that gaseous matter must be most carefully excluded from any dielectric
which is subjected to great, rapidly changing electrostatic stresses.
Since it is difficult to exclude the gas perfectly when solid insulators
are used, it is necessary to resort to liquid dielectrics. When a solid
dielectric is used, it matters little how thick and how good it is; if
air be present, streamers form, which gradually heat the dielectric and
impair its insulating power, and the discharge finally breaks through.
Under ordinary conditions the best insulators are those which possess
the highest specific inductive capacity, but such insulators are not the
best to employ when working with these high frequency currents, for in
most cases the higher specific inductive capacity is rather a
disadvantage. The prime quality of the insulating medium for these
currents is continuity. For this reason principally it is necessary to
employ liquid insulators, such as oils. If two metal plates, connected
to the terminals of the coil, are immersed in oil and set a distance
apart, the coil may be kept working for any length of time without a
break occurring, or without the oil being warmed, but if air bubbles are
introduced, they become luminous; the air molecules, by their impact
against the oil, heat it, and after some time cause the insulation to
give way. If, instead of the oil, a solid plate of the best dielectric,
even several times thicker than the oil intervening between the metal
plates, is inserted between the latter, the air having free access to
the charged surfaces, the dielectric invariably is warmed and breaks
down.

The employment of oil is advisable or necessary even with low
frequencies, if the potentials are such that streamers form, but only in
such cases, as is evident from the theory of the action. If the
potentials are so low that streamers do not form, then it is even
disadvantageous to employ oil, for it may, principally by confining the
heat, be the cause of the breaking down of the insulation.

The exclusion of gaseous matter is not only desirable on account of the
safety of the apparatus, but also on account of economy, especially in a
condenser, in which considerable waste of power may occur merely owing
to the presence of air, if the electric density on the charged surfaces
is great.

In the course of these investigations a phenomenon of special scientific
interest was observed. It may be ranked among the brush phenomena, in
fact it is a kind of brush which forms at, or near, a single terminal in
high vacuum. In a bulb with a conducting electrode, even if the latter
be of aluminum, the brush has only a very short existence, but it can be
preserved for a considerable length of time in a bulb devoid of any
conducting electrode. To observe the phenomenon it is found best to
employ a large spherical bulb having in its centre a small bulb
supported on a tube sealed to the neck of the former. The large bulb
being exhausted to a high degree, and the inside of the small bulb being
connected to one of the terminals of the coil, under certain conditions
there appears a misty haze around the small bulb, which, after passing
through some stages, assumes the form of a brush, generally at right
angles to the tube supporting the small bulb. When the brush assumes
this form it may be brought to a state of extreme sensitiveness to
electrostatic and magnetic influence. The bulb hanging straight down,
and all objects being remote from it, the approach of the observer
within a few paces will cause the brush to fly to the opposite side, and
if he walks around the bulb it will always keep on the opposite side. It
may begin to spin around the terminal long before it reaches that
sensitive stage. When it begins to turn around, principally, but also
before, it is affected by a magnet, and at a certain stage it is
susceptible to magnetic influence to an astonishing degree. A small
permanent magnet, with its poles at a distance of no more than two
centimetres will affect it visibly at a distance of two metres, slowing
down or accelerating the rotation according to how it is held relatively
to the brush.

When the bulb hangs with the globe down, the rotation is always
clockwise. In the southern hemisphere it would occur in the opposite
direction, and on the (magnetic) equator the brush should not turn at
all. The rotation may be reversed by a magnet kept at some distance. The
brush rotates best, seemingly, when it is at right angles to the lines
of force of the earth. It very likely rotates, when at its maximum
speed, in synchronism with the alternations, say, 10,000 times a second.
The rotation can be slowed down or accelerated by the approach or
recession of the observer, or any conducting body, but it cannot be
reversed by putting the bulb in any position. Very curious experiments
may be performed with the brush when in its most sensitive state. For
instance, the brush resting in one position, the experimenter may, by
selecting a proper position, approach the hand at a certain considerable
distance to the bulb, and he may cause the brush to pass off by merely
stiffening the muscles of the arm, the mere change of configuration of
the arm and the consequent imperceptible displacement being sufficient
to disturb the delicate balance. When it begins to rotate slowly, and
the hands are held at a proper distance, it is impossible to make even
the slightest motion without producing a visible effect upon the brush.
A metal plate connected to the other terminal of the coil affects it at
a great distance, slowing down the rotation often to one turn a second.

Mr. Tesla hopes that this phenomenon will prove a valuable aid in the
investigation of the nature of the forces acting in an electrostatic or
magnetic field. If there is any motion which is measurable going on in
the space, such a brush would be apt to reveal it. It is, so to speak, a
beam of light, frictionless, devoid of inertia. On account of its
marvellous sensitiveness to electrostatic or magnetic disturbances it
may be the means of sending signals through submarine cables with any
speed, and even of transmitting intelligence to a distance without
wires.

In operating an induction coil with these rapidly alternating currents,
it is astonishing to note, for the first time, the great importance of
the relation of capacity, self-induction, and frequency as bearing upon
the general result. The combined effect of these elements produces many
curious effects. For instance, two metal plates are connected to the
terminals and set at a small distance, so that an arc is formed between
them. This arc _prevents_ a strong current from flowing through the
coil. If the arc be interrupted by the interposition of a glass plate,
the capacity of the condenser obtained counteracts the self-induction,
and a stronger current is made to pass. The effects of capacity are the
most striking, for in these experiments, since the self-induction and
frequency both are high, the critical capacity is very small, and need
be but slightly varied to produce a very considerable change. The
experimenter brings his body in contact with the terminals of the
secondary of the coil, or attaches to one or both terminals insulated
bodies of very small bulk, such as exhausted bulbs, and he produces a
considerable rise or fall of potential on the secondary, and greatly
affects the flow of the current through the primary coil.

In many of the phenomena observed, the presence of the air, or,
generally speaking, of a medium of a gaseous nature (using this term not
to imply specific properties, but in contradistinction to homogeneity or
perfect continuity) plays an important part, as it allows energy to be
dissipated by molecular impact or bombardment. The action is thus
explained:--When an insulated body connected to a terminal of the coil
is suddenly charged to high potential, it acts inductively upon the
surrounding air, or whatever gaseous medium there might be. The
molecules or atoms which are near it are, of course, more attracted, and
move through a greater distance than the further ones. When the nearest
molecules strike the body they are repelled, and collisions occur at all
distances within the inductive distance. It is now clear that, if the
potential be steady, but little loss of energy can be caused in this
way, for the molecules which are nearest to the body having had an
additional charge imparted to them by contact, are not attracted until
they have parted, if not with all, at least with most of the additional
charge, which can be accomplished only after a great many collisions.
This is inferred from the fact that with a steady potential there is but
little loss in dry air. When the potential, instead of being steady, is
alternating, the conditions are entirely different. In this case a
rhythmical bombardment occurs, no matter whether the molecules after
coming in contact with the body lose the imparted charge or not, and,
what is more, if the charge is not lost, the impacts are all the more
violent. Still, if the frequency of the impulses be very small, the loss
caused by the impacts and collisions would not be serious unless the
potential was excessive. But when extremely high frequencies and more or
less high potentials are used, the loss may be very great. The total
energy lost per unit of time is proportionate to the product of the
number of impacts per second, or the frequency and the energy lost in
each impact. But the energy of an impact must be proportionate to the
square of the electric density of the body, on the assumption that the
charge imparted to the molecule is proportionate to that density. It is
concluded from this that the total energy lost must be proportionate to
the product of the frequency and the square of the electric density; but
this law needs experimental confirmation. Assuming the preceding
considerations to be true, then, by rapidly alternating the potential of
a body immersed in an insulating gaseous medium, any amount of energy
may be dissipated into space. Most of that energy, then, is not
dissipated in the form of long ether waves, propagated to considerable
distance, as is thought most generally, but is consumed in impact and
collisional losses--that is, heat vibrations--on the surface and in the
vicinity of the body. To reduce the dissipation it is necessary to work
with a small electric density--the smaller, the higher the frequency.

The behavior of a gaseous medium to such rapid alternations of potential
makes it appear plausible that electrostatic disturbances of the earth,
produced by cosmic events, may have great influence upon the
meteorological conditions. When such disturbances occur both the
frequency of the vibrations of the charge and the potential are in all
probability excessive, and the energy converted into heat may be
considerable. Since the density must be unevenly distributed, either in
consequence of the irregularity of the earth's surface, or on account of
the condition of the atmosphere in various places, the effect produced
would accordingly vary from place to place. Considerable variations in
the temperature and pressure of the atmosphere may in this manner be
caused at any point of the surface of the earth. The variations may be
gradual or very sudden, according to the nature of the original
disturbance, and may produce rain and storms, or locally modify the
weather in any way.

From many experiences gathered in the course of these investigations it
appears certain that in lightning discharges the air is an element of
importance. For instance, during a storm a stream may form on a nail or
pointed projection of a building. If lightning strikes somewhere in the
neighborhood, the harmless static discharge may, in consequence of the
oscillations set up, assume the character of a high-frequency streamer,
and the nail or projection may be brought to a high temperature by the
violent impact of the air molecules. Thus, it is thought, a building may
be set on fire without the lightning striking it. In like manner small
metallic objects may be fused and volatilized--as frequently occurs in
lightning discharges--merely because they are surrounded by air. Were
they immersed in a practically continuous medium, such as oil, they
would probably be safe, as the energy would have to spend itself
elsewhere.

An instructive experience having a bearing on this subject is the
following:--A glass tube of an inch or so in diameter and several inches
long is taken, and a platinum wire sealed into it, the wire running
through the center of the tube from end to end. The tube is exhausted to
a moderate degree. If a steady current is passed through the wire it is
heated uniformly in all parts and the gas in the tube is of no
consequence. But if high frequency discharges are directed through the
wire, it is heated more on the ends than in the middle portion, and if
the frequency, or rate of charge, is high enough, the wire might as well
be cut in the middle as not, for most of the heating on the ends is due
to the rarefied gas. Here the gas might only act as a conductor of no
impedance, diverting the current from the wire as the impedance of the
latter is enormously increased, and merely heating the ends of the wire
by reason of their resistance to the passage of the discharge. But it is
not at all necessary that the gas in the tube should be conducting; it
might be at an extremely low pressure, still the ends of the wire would
be heated; however, as is ascertained by experience, only the two ends
would in such case not be electrically connected through the gaseous
medium. Now, what with these frequencies and potentials occurs in an
exhausted tube, occurs in the lightning discharge at ordinary pressure.

From the facility with which any amount of energy may be carried off
through a gas, Mr. Tesla infers that the best way to render harmless a
lightning discharge is to afford it in some way a passage through a
volume of gas.

The recognition of some of the above facts has a bearing upon
far-reaching scientific investigations in which extremely high
frequencies and potentials are used. In such cases the air is an
important factor to be considered. So, for instance, if two wires are
attached to the terminals of the coil, and the streamers issue from
them, there is dissipation of energy in the form of heat and light, and
the wires behave like a condenser of larger capacity. If the wires be
immersed in oil, the dissipation of energy is prevented, or at least
reduced, and the apparent capacity is diminished. The action of the air
would seem to make it very difficult to tell, from the measured or
computed capacity of a condenser in which the air is acted upon, its
actual capacity or vibration period, especially if the condenser is of
very small surface and is charged to a very high potential. As many
important results are dependant upon the correctness of the estimation
of the vibration period, this subject demands the most careful scrutiny
of investigators.

In Leyden jars the loss due to the presence of air is comparatively
small, principally on account of the great surface of the coatings and
the small external action, but if there are streamers on the top, the
loss may be considerable, and the period of vibration is affected. In a
resonator, the density is small, but the frequency is extreme, and may
introduce a considerable error. It appears certain, at any rate, that
the periods of vibration of a charged body in a gaseous and in a
continuous medium, such as oil, are different, on account of the action
of the former, as explained.

Another fact recognized, which is of some consequence, is, that in
similar investigations the general considerations of static screening
are not applicable when a gaseous medium is present. This is evident
from the following experiment:--A short and wide glass tube is taken and
covered with a substantial coating of bronze powder, barely allowing the
light to shine a little through. The tube is highly exhausted and
suspended on a metallic clasp from the end of a wire. When the wire is
connected with one of the terminals of the coil, the gas inside of the
tube is lighted in spite of the metal coating. Here the metal evidently
does not screen the gas inside as it ought to, even if it be very thin
and poorly conducting. Yet, in a condition of rest the metal coating,
however thin, screens the inside perfectly.

One of the most interesting results arrived at in pursuing these
experiments, is the demonstration of the fact that a gaseous medium,
upon which vibration is impressed by rapid changes of electrostatic
potential, is rigid. In illustration of this result an experiment made
by Mr. Tesla may by cited:--A glass tube about one inch in diameter and
three feet long, with outside condenser coatings on the ends, was
exhausted to a certain point, when, the tube being suspended freely from
a wire connecting the upper coating to one of the terminals of the coil,
the discharge appeared in the form of a luminous thread passing through
the axis of the tube. Usually the thread was sharply defined in the
upper part of the tube and lost itself in the lower part. When a magnet
or the finger was quickly passed near the upper part of the luminous
thread, it was brought out of position by magnetic or electrostatic
influence, and a transversal vibration like that of a suspended cord,
with one or more distinct nodes, was set up, which lasted for a few
minutes and gradually died out. By suspending from the lower condenser
coating metal plates of different sizes, the speed of the vibration was
varied. This vibration would seem to show beyond doubt that the thread
possessed rigidity, at least to transversal displacements.

Many experiments were tried to demonstrate this property in air at
ordinary pressure. Though no positive evidence has been obtained, it is
thought, nevertheless, that a high frequency brush or streamer, if the
frequency could be pushed far enough, would be decidedly rigid. A small
sphere might then be moved within it quite freely, but if thrown against
it the sphere would rebound. An ordinary flame cannot possess rigidity
to a marked degree because the vibration is directionless; but an
electric arc, it is believed, must possess that property more or less. A
luminous band excited in a bulb by repeated discharges of a Leyden jar
must also possess rigidity, and if deformed and suddenly released should
vibrate.

From like considerations other conclusions of interest are reached. The
most probable medium filling the space is one consisting of independent
carriers immersed in an insulating fluid. If through this medium
enormous electrostatic stresses are assumed to act, which vary rapidly
in intensity, it would allow the motion of a body through it, yet it
would be rigid and elastic, although the fluid itself might be devoid of
these properties. Furthermore, on the assumption that the independent
carriers are of any configuration such that the fluid resistance to
motion in one direction is greater than in another, a stress of that
nature would cause the carriers to arrange themselves in groups, since
they would turn to each other their sides of the greatest electric
density, in which position the fluid resistance to approach would be
smaller than to receding. If in a medium of the above characteristics a
brush would be formed by a steady potential, an exchange of the carriers
would go on continually, and there would be less carriers per unit of
volume in the brush than in the space at some distance from the
electrode, this corresponding to rarefaction. If the potential were
rapidly changing, the result would be very different; the higher the
frequency of the pulses, the slower would be the exchange of the
carriers; finally, the motion of translation through measurable space
would cease, and, with a sufficiently high frequency and intensity of
the stress, the carriers would be drawn towards the electrode, and
compression would result.

An interesting feature of these high frequency currents is that they
allow of operating all kinds of devices by connecting the device with
only one leading wire to the electric source. In fact, under certain
conditions it may be more economical to supply the electrical energy
with one lead than with two.

An experiment of special interest shown by Mr. Tesla, is the running, by
the use of only one insulated line, of a motor operating on the
principle of the rotating magnetic field enunciated by Mr. Tesla. A
simple form of such a motor is obtained by winding upon a laminated iron
core a primary and close to it a secondary coil, closing the ends of the
latter and placing a freely movable metal disc within the influence of
the moving field. The secondary coil may, however, be omitted. When one
of the ends of the primary coil of the motor is connected to one of the
terminals of the high frequency coil and the other end to an insulated
metal plate, which, it should be stated, is not absolutely necessary for
the success of the experiment, the disc is set in rotation.

Experiments of this kind seem to bring it within possibility to operate
a motor at any point of the earth's surface from a central source,
without any connection to the same except through the earth. If, by
means of powerful machinery, rapid variations of the earth's potential
were produced, a grounded wire reaching up to some height would be
traversed by a current which could be increased by connecting the free
end of the wire to a body of some size. The current might be converted
to low tension and used to operate a motor or other device. The
experiment, which would be one of great scientific interest, would
probably best succeed on a ship at sea. In this manner, even if it were
not possible to operate machinery, intelligence might be transmitted
quite certainly.

In the course of this experimental study special attention was devoted
to the heating effects produced by these currents, which are not only
striking, but open up the possibility of producing a more efficient
illuminant. It is sufficient to attach to the coil terminal a thin wire
or filament, to have the temperature of the latter perceptibly raised.
If the wire or filament be enclosed in a bulb, the heating effect is
increased by preventing the circulation of the air. If the air in the
bulb be strongly compressed, the displacements are smaller, the impacts
less violent, and the heating effect is diminished. On the contrary, if
the air in the bulb be exhausted, an inclosed lamp filament is brought
to incandescence, and any amount of light may thus be produced.

The heating of the inclosed lamp filament depends on so many things of a
different nature, that it is difficult to give a generally applicable
rule under which the maximum heating occurs. As regards the size of the
bulb, it is ascertained that at ordinary or only slightly differing
atmospheric pressures, when air is a good insulator, the filament is
heated more in a small bulb, because of the better confinement of heat
in this case. At lower pressures, when air becomes conducting, the
heating effect is greater in a large bulb, but at excessively high
degrees of exhaustion there seems to be, beyond a certain and rather
small size of the vessel, no perceptible difference in the heating.

The shape of the vessel is also of some importance, and it has been
found of advantage for reasons of economy to employ a spherical bulb
with the electrode mounted in its centre, where the rebounding molecules
collide.

It is desirable on account of economy that all the energy supplied to
the bulb from the source should reach without loss the body to be
heated. The loss in conveying the energy from the source to the body may
be reduced by employing thin wires heavily coated with insulation, and
by the use of electrostatic screens. It is to be remarked, that the
screen cannot be connected to the ground as under ordinary conditions.

In the bulb itself a large portion of the energy supplied may be lost by
molecular bombardment against the wire connecting the body to be heated
with the source. Considerable improvement was effected by covering the
glass stem containing the wire with a closely fitting conducting tube.
This tube is made to project a little above the glass, and prevents the
cracking of the latter near the heated body. The effectiveness of the
conducting tube is limited to very high degrees of exhaustion. It
diminishes the energy lost in bombardment for two reasons; first, the
charge given up by the atoms spreads over a greater area, and hence the
electric density at any point is small, and the atoms are repelled with
less energy than if they would strike against a good insulator;
secondly, as the tube is electrified by the atoms which first come in
contact with it, the progress of the following atoms against the tube is
more or less checked by the repulsion which the electrified tube must
exert upon the similarly electrified atoms. This, it is thought,
explains why the discharge through a bulb is established with much
greater facility when an insulator, than when a conductor, is present.

During the investigations a great many bulbs of different construction,
with electrodes of different material, were experimented upon, and a
number of observations of interest were made. Mr. Tesla has found that
the deterioration of the electrode is the less, the higher the
frequency. This was to be expected, as then the heating is effected by
many small impacts, instead by fewer and more violent ones, which
quickly shatter the structure. The deterioration is also smaller when
the vibration is harmonic. Thus an electrode, maintained at a certain
degree of heat, lasts much longer with currents obtained from an
alternator, than with those obtained by means of a disruptive discharge.
One of the most durable electrodes was obtained from strongly compressed
carborundum, which is a kind of carbon recently produced by Mr. E. G.
Acheson, of Monongahela City, Pa. From experience, it is inferred, that
to be most durable, the electrode should be in the form of a sphere with
a highly polished surface.

In some bulbs refractory bodies were mounted in a carbon cup and put
under the molecular impact. It was observed in such experiments that the
carbon cup was heated at first, until a higher temperature was reached;
then most of the bombardment was directed against the refractory body,
and the carbon was relieved. In general, when different bodies were
mounted in the bulb, the hardest fusible would be relieved, and would
remain at a considerably lower temperature. This was necessitated by the
fact that most of the energy supplied would find its way through the
body which was more easily fused or "evaporated."

Curiously enough it appeared in some of the experiments made, that a
body was fused in a bulb under the molecular impact by evolution of less
light than when fused by the application of heat in ordinary ways. This
may be ascribed to a loosening of the structure of the body under the
violent impacts and changing stresses.

Some experiments seem to indicate that under certain conditions a body,
conducting or nonconducting, may, when bombarded, emit light, which to
all appearances is due to phosphorescence, but may in reality be caused
by the incandescence of an infinitesimal layer, the mean temperature of
the body being comparatively small. Such might be the case if each
single rhythmical impact were capable of instantaneously exciting the
retina, and the rhythm were just high enough to cause a continuous
impression in the eye. According to this view, a coil operated by
disruptive discharge would be eminently adapted to produce such a
result, and it is found by experience that its power of exciting
phosphorescence is extraordinarily great. It is capable of exciting
phosphorescence at comparatively low degrees of exhaustion, and also
projects shadows at pressures far greater than those at which the mean
free path is comparable to the dimensions of the vessel. The latter
observation is of some importance, inasmuch as it may modify the
generally accepted views in regard to the "radiant state" phenomena.

A thought which early and naturally suggested itself to Mr. Tesla, was
to utilize the great inductive effects of high frequency currents to
produce light in a sealed glass vessel without the use of leading in
wires. Accordingly, many bulbs were constructed in which the energy
necessary to maintain a button or filament at high incandescence, was
supplied through the glass by either electrostatic or electrodynamic
induction. It was easy to regulate the intensity of the light emitted by
means of an externally applied condenser coating connected to an
insulated plate, or simply by means of a plate attached to the bulb
which at the same time performed the function of a shade.

A subject of experiment, which has been exhaustively treated in England
by Prof. J. J. Thomson, has been followed up independently by Mr. Tesla
from the beginning of this study, namely, to excite by electrodynamic
induction a luminous band in a closed tube or bulb. In observing the
behavior of gases, and the luminous phenomena obtained, the importance
of the electrostatic effects was noted and it appeared desirable to
produce enormous potential differences, alternating with extreme
rapidity. Experiments in this direction led to some of the most
interesting results arrived at in the course of these investigations. It
was found that by rapid alternations of a high electrostatic potential,
exhausted tubes could be lighted at considerable distances from a
conductor connected to a properly constructed coil, and that it was
practicable to establish with the coil an alternating electrostatic
field, acting through the whole room and lighting a tube wherever it was
placed within the four walls. Phosphorescent bulbs may be excited in
such a field, and it is easy to regulate the effect by connecting to the
bulb a small insulated metal plate. It was likewise possible to maintain
a filament or button mounted in a tube at bright incandescence, and, in
one experiment, a mica vane was spun by the incandescence of a platinum
wire.

Coming now to the lecture delivered in Philadelphia and St. Louis, it
may be remarked that to the superficial reader, Mr. Tesla's
introduction, dealing with the importance of the eye, might appear as a
digression, but the thoughtful reader will find therein much food for
meditation and speculation. Throughout his discourse one can trace Mr.
Tesla's effort to present in a popular way thoughts and views on the
electrical phenomena which have in recent years captivated the
scientific world, but of which the general public has even yet merely
received an inkling. Mr. Tesla also dwells rather extensively on his
well-known method of high-frequency conversion; and the large amount of
detail information will be gratefully received by students and
experimenters in this virgin field. The employment of apt analogies in
explaining the fundamental principles involved makes it easy for all to
gain a clear idea of their nature. Again, the ease with which, thanks to
Mr. Tesla's efforts, these high-frequency currents may now be obtained
from circuits carrying almost any kind of current, cannot fail to result
in an extensive broadening of this field of research, which offers so
many possibilities. Mr. Tesla, true philosopher as he is, does not
hesitate to point out defects in some of his methods, and indicates the
lines which to him seem the most promising. Particular stress is laid by
him upon the employment of a medium in which the discharge electrodes
should be immersed in order that this method of conversion may be
brought to the highest perfection. He has evidently taken pains to give
as much useful information as possible to those who wish to follow in
his path, as he shows in detail the circuit arrangements to be adopted
in all ordinary cases met with in practice, and although some of these
methods were described by him two years before, the additional
information is still timely and welcome.

In his experiments he dwells first on some phenomena produced by
electrostatic force, which he considers in the light of modern theories
to be the most important force in nature for us to investigate. At the
very outset he shows a strikingly novel experiment illustrating the
effect of a rapidly varying electrostatic force in a gaseous medium, by
touching with one hand one of the terminals of a 200,000 volt
transformer and bringing the other hand to the opposite terminal. The
powerful streamers which issued from his hand and astonished his
audiences formed a capital illustration of some of the views advanced,
and afforded Mr. Tesla an opportunity of pointing out the true reasons
why, with these currents, such an amount of energy can be passed
through the body with impunity. He then showed by experiment the
difference between a steady and a rapidly varying force upon the
dielectric. This difference is most strikingly illustrated in the
experiment in which a bulb attached to the end of a wire in connection
with one of the terminals of the transformer is ruptured, although all
extraneous bodies are remote from the bulb. He next illustrates how
mechanical motions are produced by a varying electrostatic force acting
through a gaseous medium. The importance of the action of the air is
particularly illustrated by an interesting experiment.

Taking up another class of phenomena, namely, those of dynamic
electricity, Mr. Tesla produced in a number of experiments a variety of
effects by the employment of only a single wire with the evident intent
of impressing upon his audience the idea that electric vibration or
current can be transmitted with ease, without any return circuit; also
how currents so transmitted can be converted and used for many practical
purposes. A number of experiments are then shown, illustrating the
effects of frequency, self-induction and capacity; then a number of ways
of operating motive and other devices by the use of a single lead. A
number of novel impedance phenomena are also shown which cannot fail to
arouse interest.

Mr. Tesla next dwelt upon a subject which he thinks of great importance,
that is, electrical resonance, which he explained in a popular way. He
expressed his firm conviction that by observing proper conditions,
intelligence, and possibly even power, can be transmitted through the
medium or through the earth; and he considers this problem worthy of
serious and immediate consideration.

Coming now to the light phenomena in particular, he illustrated the four
distinct kinds of these phenomena in an original way, which to many must
have been a revelation. Mr. Tesla attributes these light effects to
molecular or atomic impacts produced by a varying electrostatic stress
in a gaseous medium. He illustrated in a series of novel experiments the
effect of the gas surrounding the conductor and shows beyond a doubt
that with high frequency and high potential currents, the surrounding
gas is of paramount importance in the heating of the conductor. He
attributes the heating partially to a conduction current and partially
to bombardment, and demonstrates that in many cases the heating may be
practically due to the bombardment alone. He pointed out also that the
skin effect is largely modified by the presence of the gas or of an
atomic medium in general. He showed also some interesting experiments in
which the effect of convection is illustrated. Probably one of the most
curious experiments in this connection is that in which a thin platinum
wire stretched along the axis of an exhausted tube is brought to
incandescence at certain points corresponding to the position of the
striæ, while at others it remains dark. This experiment throws an
interesting light upon the nature of the striæ and may lead to important
revelations.

Mr. Tesla also demonstrated the dissipation of energy through an atomic
medium and dwelt upon the behavior of vacuous space in conveying heat,
and in this connection showed the curious behavior of an electrode
stream, from which he concludes that the molecules of a gas probably
cannot be acted upon directly at measurable distances.

Mr. Tesla summarized the chief results arrived at in pursuing his
investigations in a manner which will serve as a valuable guide to all
who may engage in this work. Perhaps most interest will centre on his
general statements regarding the phenomena of phosphorescence, the most
important fact revealed in this direction being that when exciting a
phosphorescent bulb a certain definite potential gives the most
economical result.

The lectures will now be presented in the order of their date of
delivery.




CHAPTER XXVI.

EXPERIMENTS WITH ALTERNATE CURRENTS OF VERY HIGH FREQUENCY AND THEIR
APPLICATION TO METHODS OF ARTIFICIAL ILLUMINATION.[1]

  [1] A lecture delivered before the American Institute of
      Electrical Engineers, at Columbia College, N. Y.,
      May 20, 1891.


There is no subject more captivating, more worthy of study, than nature.
To understand this great mechanism, to discover the forces which are
active, and the laws which govern them, is the highest aim of the
intellect of man.

Nature has stored up in the universe infinite energy. The eternal
recipient and transmitter of this infinite energy is the ether. The
recognition of the existence of ether, and of the functions it performs,
is one of the most important results of modern scientific research. The
mere abandoning of the idea of action at a distance, the assumption of a
medium pervading all space and connecting all gross matter, has freed
the minds of thinkers of an ever present doubt, and, by opening a new
horizon--new and unforeseen possibilities--has given fresh interest to
phenomena with which we are familiar of old. It has been a great step
towards the understanding of the forces of nature and their multifold
manifestations to our senses. It has been for the enlightened student of
physics what the understanding of the mechanism of the firearm or of the
steam engine is for the barbarian. Phenomena upon which we used to look
as wonders baffling explanation, we now see in a different light. The
spark of an induction coil, the glow of an incandescent lamp, the
manifestations of the mechanical forces of currents and magnets are no
longer beyond our grasp; instead of the incomprehensible, as before,
their observation suggests now in our minds a simple mechanism, and
although as to its precise nature all is still conjecture, yet we know
that the truth cannot be much longer hidden, and instinctively we feel
that the understanding is dawning upon us. We still admire these
beautiful phenomena, these strange forces, but we are helpless no
longer; we can in a certain measure explain them, account for them, and
we are hopeful of finally succeeding in unraveling the mystery which
surrounds them.

In how far we can understand the world around us is the ultimate thought
of every student of nature. The coarseness of our senses prevents us
from recognizing the ulterior construction of matter, and astronomy,
this grandest and most positive of natural sciences, can only teach us
something that happens, as it were, in our immediate neighborhood: of
the remoter portions of the boundless universe, with its numberless
stars and suns, we know nothing. But far beyond the limit of perception
of our senses the spirit still can guide us, and so we may hope that
even these unknown worlds--infinitely small and great--may in a measure
become known to us. Still, even if this knowledge should reach us, the
searching mind will find a barrier, perhaps forever unsurpassable, to
the _true_ recognition of that which _seems_ to be, the mere
_appearance_ of which is the only and slender basis of all our
philosophy.

Of all the forms of nature's immeasurable, all-pervading energy, which
ever and ever changing and moving, like a soul animates the inert
universe, electricity and magnetism are perhaps the most fascinating.
The effects of gravitation, of heat and light we observe daily, and soon
we get accustomed to them, and soon they lose for us the character of
the marvelous and wonderful; but electricity and magnetism, with their
singular relationship, with their seemingly dual character, unique among
the forces in nature, with their phenomena of attractions, repulsions
and rotations, strange manifestations of mysterious agents, stimulate
and excite the mind to thought and research. What is electricity, and
what is magnetism? These questions have been asked again and again. The
most able intellects have ceaselessly wrestled with the problem; still
the question has not as yet been fully answered. But while we cannot
even to-day state what these singular forces are, we have made good
headway towards the solution of the problem. We are now confident that
electric and magnetic phenomena are attributable to ether, and we are
perhaps justified in saying that the effects of static electricity are
effects of ether under strain, and those of dynamic electricity and
electro-magnetism effects of ether in motion. But this still leaves the
question, as to what electricity and magnetism are, unanswered.

First, we naturally inquire, What is electricity, and is there such a
thing as electricity? In interpreting electric phenomena, we may speak
of electricity or of an electric condition, state or effect. If we speak
of electric effects we must distinguish two such effects, opposite in
character and neutralizing each other, as observation shows that two
such opposite effects exist. This is unavoidable, for in a medium of the
properties of ether, we cannot possibly exert a strain, or produce a
displacement or motion of any kind, without causing in the surrounding
medium an equivalent and opposite effect. But if we speak of
electricity, meaning a _thing_, we must, I think, abandon the idea of
two electricities, as the existence of two such things is highly
improbable. For how can we imagine that there should be two things,
equivalent in amount, alike in their properties, but of opposite
character, both clinging to matter, both attracting and completely
neutralizing each other? Such an assumption, though suggested by many
phenomena, though most convenient for explaining them, has little to
commend it. If there _is_ such a thing as electricity, there can be only
_one_ such thing, and excess and want of that one thing, possibly; but
more probably its condition determines the positive and negative
character. The old theory of Franklin, though falling short in some
respects, is, from a certain point of view, after all, the most
plausible one. Still, in spite of this, the theory of the two
electricities is generally accepted, as it apparently explains electric
phenomena in a more satisfactory manner. But a theory which better
explains the facts is not necessarily true. Ingenious minds will invent
theories to suit observation, and almost every independent thinker has
his own views on the subject.

It is not with the object of advancing an opinion, but with the desire
of acquainting you better with some of the results, which I will
describe, to show you the reasoning I have followed, the departures I
have made--that I venture to express, in a few words, the views and
convictions which have led me to these results.

I adhere to the idea that there is a thing which we have been in the
habit of calling electricity. The question is, What is that thing? or,
What, of all things, the existence of which we know, have we the best
reason to call electricity? We know that it acts like an incompressible
fluid; that there must be a constant quantity of it in nature; that it
can be neither produced nor destroyed; and, what is more important, the
electro-magnetic theory of light and all facts observed teach us that
electric and ether phenomena are identical. The idea at once suggests
itself, therefore, that electricity might be called ether. In fact, this
view has in a certain sense been advanced by Dr. Lodge. His interesting
work has been read by everyone and many have been convinced by his
arguments. His great ability and the interesting nature of the subject,
keep the reader spellbound; but when the impressions fade, one realizes
that he has to deal only with ingenious explanations. I must confess,
that I cannot believe in two electricities, much less in a
doubly-constituted ether. The puzzling behavior of the ether as a solid
to waves of light and heat, and as a fluid to the motion of bodies
through it, is certainly explained in the most natural and satisfactory
manner by assuming it to be in motion, as Sir William Thomson has
suggested; but regardless of this, there is nothing which would enable
us to conclude with certainty that, while a fluid is not capable of
transmitting transverse vibrations of a few hundred or thousand per
second, it might not be capable of transmitting such vibrations when
they range into hundreds of million millions per second. Nor can anyone
prove that there are transverse ether waves emitted from an alternate
current machine, giving a small number of alternations per second; to
such slow disturbances, the ether, if at rest, may behave as a true
fluid.

Returning to the subject, and bearing in mind that the existence of two
electricities is, to say the least, highly improbable, we must remember,
that we have no evidence of electricity, nor can we hope to get it,
unless gross matter is present. Electricity, therefore, cannot be called
ether in the broad sense of the term; but nothing would seem to stand in
the way of calling electricity ether associated with matter, or bound
ether; or, in other words, that the so-called static charge of the
molecule is ether associated in some way with the molecule. Looking at
it in that light, we would be justified in saying, that electricity is
concerned in all molecular actions.

Now, precisely what the ether surrounding the molecules is, wherein it
differs from ether in general, can only be conjectured. It cannot differ
in density, ether being incompressible: it must, therefore, be under
some strain or in motion, and the latter is the most probable. To
understand its functions, it would be necessary to have an exact idea of
the physical construction of matter, of which, of course, we can only
form a mental picture.

But of all the views on nature, the one which assumes one matter and one
force, and a perfect uniformity throughout, is the most scientific and
most likely to be true. An infinitesimal world, with the molecules and
their atoms spinning and moving in orbits, in much the same manner as
celestial bodies, carrying with them and probably spinning with them
ether, or in other words, carrying with them static charges, seems to my
mind the most probable view, and one which, in a plausible manner,
accounts for most of the phenomena observed. The spinning of the
molecules and their ether sets up the ether tensions or electrostatic
strains; the equalization of ether tensions sets up ether motions or
electric currents, and the orbital movements produce the effects of
electro and permanent magnetism.

About fifteen years ago, Prof. Rowland demonstrated a most interesting
and important fact, namely, that a static charge carried around produces
the effects of an electric current. Leaving out of consideration the
precise nature of the mechanism, which produces the attraction and
repulsion of currents, and conceiving the electrostatically charged
molecules in motion, this experimental fact gives us a fair idea of
magnetism. We can conceive lines or tubes of force which physically
exist, being formed of rows of directed moving molecules; we can see
that these lines must be closed, that they must tend to shorten and
expand, etc. It likewise explains in a reasonable way, the most puzzling
phenomenon of all, permanent magnetism, and, in general, has all the
beauties of the Ampere theory without possessing the vital defect of the
same, namely, the assumption of molecular currents. Without enlarging
further upon the subject, I would say, that I look upon all
electrostatic, current and magnetic phenomena as being due to
electrostatic molecular forces.

The preceding remarks I have deemed necessary to a full understanding of
the subject as it presents itself to my mind.

Of all these phenomena the most important to study are the current
phenomena, on account of the already extensive and ever-growing use of
currents for industrial purposes. It is now a century since the first
practical source of current was produced, and, ever since, the phenomena
which accompany the flow of currents have been diligently studied, and
through the untiring efforts of scientific men the simple laws which
govern them have been discovered. But these laws are found to hold good
only when the currents are of a steady character. When the currents are
rapidly varying in strength, quite different phenomena, often
unexpected, present themselves, and quite different laws hold good,
which even now have not been determined as fully as is desirable, though
through the work, principally, of English scientists, enough knowledge
has been gained on the subject to enable us to treat simple cases which
now present themselves in daily practice.

The phenomena which are peculiar to the changing character of the
currents are greatly exalted when the rate of change is increased, hence
the study of these currents is considerably facilitated by the
employment of properly constructed apparatus. It was with this and other
objects in view that I constructed alternate current machines capable of
giving more than two million reversals of current per minute, and to
this circumstance it is principally due, that I am able to bring to your
attention some of the results thus far reached, which I hope will prove
to be a step in advance on account of their direct bearing upon one of
the most important problems, namely, the production of a practical and
efficient source of light.

The study of such rapidly alternating currents is very interesting.
Nearly every experiment discloses something new. Many results may, of
course, be predicted, but many more are unforeseen. The experimenter
makes many interesting observations. For instance, we take a piece of
iron and hold it against a magnet. Starting from low alternations and
running up higher and higher we feel the impulses succeed each other
faster and faster, get weaker and weaker, and finally disappear. We then
observe a continuous pull; the pull, of course, is not continuous; it
only appears so to us; our sense of touch is imperfect.

We may next establish an arc between the electrodes and observe, as the
alternations rise, that the note which accompanies alternating arcs gets
shriller and shriller, gradually weakens, and finally ceases. The air
vibrations, of course, continue, but they are too weak to be perceived;
our sense of hearing fails us.

We observe the small physiological effects, the rapid heating of the
iron cores and conductors, curious inductive effects, interesting
condenser phenomena, and still more interesting light phenomena with a
high tension induction coil. All these experiments and observations
would be of the greatest interest to the student, but their description
would lead me too far from the principal subject. Partly for this
reason, and partly on account of their vastly greater importance, I will
confine myself to the description of the light effects produced by these
currents.

In the experiments to this end a high tension induction coil or
equivalent apparatus for converting currents of comparatively low into
currents of high tension is used.

If you will be sufficiently interested in the results I shall describe
as to enter into an experimental study of this subject; if you will be
convinced of the truth of the arguments I shall advance--your aim will
be to produce high frequencies and high potentials; in other words,
powerful electrostatic effects. You will then encounter many
difficulties, which, if completely overcome, would allow us to produce
truly wonderful results.

First will be met the difficulty of obtaining the required frequencies
by means of mechanical apparatus, and, if they be obtained otherwise,
obstacles of a different nature will present themselves. Next it will be
found difficult to provide the requisite insulation without considerably
increasing the size of the apparatus, for the potentials required are
high, and, owing to the rapidity of the alternations, the insulation
presents peculiar difficulties. So, for instance, when a gas is present,
the discharge may work, by the molecular bombardment of the gas and
consequent heating, through as much as an inch of the best solid
insulating material, such as glass, hard rubber, porcelain, sealing wax,
etc.; in fact, through any known insulating substance. The chief
requisite in the insulation of the apparatus is, therefore, the
exclusion of any gaseous matter.

In general my experience tends to show that bodies which possess the
highest specific inductive capacity, such as glass, afford a rather
inferior insulation to others, which, while they are good insulators,
have a much smaller specific inductive capacity, such as oils, for
instance, the dielectric losses being no doubt greater in the former.
The difficulty of insulating, of course, only exists when the potentials
are excessively high, for with potentials such as a few thousand volts
there is no particular difficulty encountered in conveying currents from
a machine giving, say, 20,000 alternations per second, to quite a
distance. This number of alternations, however, is by far too small for
many purposes, though quite sufficient for some practical applications.
This difficulty of insulating is fortunately not a vital drawback; it
affects mostly the size of the apparatus, for, when excessively high
potentials would be used, the light-giving devices would be located not
far from the apparatus, and often they would be quite close to it. As
the air-bombardment of the insulated wire is dependent on condenser
action, the loss may be reduced to a trifle by using excessively thin
wires heavily insulated.

Another difficulty will be encountered in the capacity and
self-induction necessarily possessed by the coil. If the coil be large,
that is, if it contain a great length of wire, it will be generally
unsuited for excessively high frequencies; if it be small, it may be
well adapted for such frequencies, but the potential might then not be
as high as desired. A good insulator, and preferably one possessing a
small specific inductive capacity, would afford a two-fold advantage.
First, it would enable us to construct a very small coil capable of
withstanding enormous differences of potential; and secondly, such a
small coil, by reason of its smaller capacity and self-induction, would
be capable of a quicker and more vigorous vibration. The problem then of
constructing a coil or induction apparatus of any kind possessing the
requisite qualities I regard as one of no small importance, and it has
occupied me for a considerable time.

The investigator who desires to repeat the experiments which I will
describe, with an alternate current machine, capable of supplying
currents of the desired frequency, and an induction coil, will do well
to take the primary coil out and mount the secondary in such a manner as
to be able to look through the tube upon which the secondary is wound.
He will then be able to observe the streams which pass from the primary
to the insulating tube, and from their intensity he will know how far he
can strain the coil. Without this precaution he is sure to injure the
insulation. This arrangement permits, however, an easy exchange of the
primaries, which is desirable in these experiments.

The selection of the type of machine best suited for the purpose must be
left to the judgment of the experimenter. There are here illustrated
three distinct types of machines, which, besides others, I have used in
my experiments.

Fig. 97 represents the machine used in my experiments before this
Institute. The field magnet consists of a ring of wrought iron with 384
pole projections. The armature comprises a steel disc to which is
fastened a thin, carefully welded rim of wrought iron. Upon the rim are
wound several layers of fine, well annealed iron wire, which, when
wound, is passed through shellac. The armature wires are wound around
brass pins, wrapped with silk thread. The diameter of the armature wire
in this type of machine should not be more than 1/6 of the thickness of
the pole projections, else the local action will be considerable.

[Illustration: FIG. 97.]

Fig. 98 represents a larger machine of a different type. The field
magnet of this machine consists of two like parts which either enclose
an exciting coil, or else are independently wound. Each part has 480
pole projections, the projections of one facing those of the other. The
armature consists of a wheel of hard bronze, carrying the conductors
which revolve between the projections of the field magnet. To wind the
armature conductors, I have found it most convenient to proceed in the
following manner. I construct a ring of hard bronze of the required
size. This ring and the rim of the wheel are provided with the proper
number of pins, and both fastened upon a plate. The armature conductors
being wound, the pins are cut off and the ends of the conductors
fastened by two rings which screw to the bronze ring and the rim of the
wheel, respectively. The whole may then be taken off and forms a solid
structure. The conductors in such a type of machine should consist of
sheet copper, the thickness of which, of course, depends on the
thickness of the pole projections; or else twisted thin wires should be
employed.

Fig. 99 is a smaller machine, in many respects similar to the former,
only here the armature conductors and the exciting coil are kept
stationary, while only a block of wrought iron is revolved.

[Illustration: FIG. 98.]

It would be uselessly lengthening this description were I to dwell more
on the details of construction of these machines. Besides, they have
been described somewhat more elaborately in _The Electrical Engineer_,
of March 18, 1891. I deem it well, however, to call the attention of the
investigator to two things, the importance of which, though self
evident, he is nevertheless apt to underestimate; namely, to the local
action in the conductors which must be carefully avoided, and to the
clearance, which must be small. I may add, that since it is desirable to
use very high peripheral speeds, the armature should be of very large
diameter in order to avoid impracticable belt speeds. Of the several
types of these machines which have been constructed by me, I have found
that the type illustrated in Fig. 97 caused me the least trouble in
construction, as well as in maintenance, and on the whole, it has been a
good experimental machine.

In operating an induction coil with very rapidly alternating currents,
among the first luminous phenomena noticed are naturally those presented
by the high-tension discharge. As the number of alternations per second
is increased, or as--the number being high--the current through the
primary is varied, the discharge gradually changes in appearance. It
would be difficult to describe the minor changes which occur, and the
conditions which bring them about, but one may note five distinct forms
of the discharge.

[Illustration: FIG. 99.]

First, one may observe a weak, sensitive discharge in the form of a
thin, feeble-colored thread. (Fig. 100a.) It always occurs when, the
number of alternations per second being high, the current through the
primary is very small. In spite of the excessively small current, the
rate of change is great, and the difference of potential at the
terminals of the secondary is therefore considerable, so that the arc is
established at great distances; but the quantity of "electricity" set in
motion is insignificant, barely sufficient to maintain a thin,
threadlike arc. It is excessively sensitive and may be made so to such a
degree that the mere act of breathing near the coil will affect it, and
unless it is perfectly well protected from currents of air, it wriggles
around constantly. Nevertheless, it is in this form excessively
persistent, and when the terminals are approached to, say, one-third of
the striking distance, it can be blown out only with difficulty. This
exceptional persistency, when short, is largely due to the arc being
excessively thin; presenting, therefore, a very small surface to the
blast. Its great sensitiveness, when very long, is probably due to the
motion of the particles of dust suspended in the air.

[Illustration: FIG. 100a.]

[Illustration: FIG. 100b.]

When the current through the primary is increased, the discharge gets
broader and stronger, and the effect of the capacity of the coil becomes
visible until, finally, under proper conditions, a white flaming arc,
Fig. 100 B, often as thick as one's finger, and striking across the
whole coil, is produced. It develops remarkable heat, and may be further
characterized by the absence of the high note which accompanies the
less powerful discharges. To take a shock from the coil under these
conditions would not be advisable, although under different conditions,
the potential being much higher, a shock from the coil may be taken with
impunity. To produce this kind of discharge the number of alternations
per second must not be too great for the coil used; and, generally
speaking, certain relations between capacity, self-induction and
frequency must be observed.

The importance of these elements in an alternate current circuit is now
well-known, and under ordinary conditions, the general rules are
applicable. But in an induction coil exceptional conditions prevail.
First, the self-induction is of little importance before the arc is
established, when it asserts itself, but perhaps never as prominently as
in ordinary alternate current circuits, because the capacity is
distributed all along the coil, and by reason of the fact that the coil
usually discharges through very great resistances; hence the currents
are exceptionally small. Secondly, the capacity goes on increasing
continually as the potential rises, in consequence of absorption which
takes place to a considerable extent. Owing to this there exists no
critical relationship between these quantities, and ordinary rules would
not seem to be applicable. As the potential is increased either in
consequence of the increased frequency or of the increased current
through the primary, the amount of the energy stored becomes greater and
greater, and the capacity gains more and more in importance. Up to a
certain point the capacity is beneficial, but after that it begins to be
an enormous drawback. It follows from this that each coil gives the best
result with a given frequency and primary current. A very large coil,
when operated with currents of very high frequency, may not give as much
as 1/8 inch spark. By adding capacity to the terminals, the condition
may be improved, but what the coil really wants is a lower frequency.

When the flaming discharge occurs, the conditions are evidently such
that the greatest current is made to flow through the circuit. These
conditions may be attained by varying the frequency within wide limits,
but the highest frequency at which the flaming arc can still be
produced, determines, for a given primary current, the maximum striking
distance of the coil. In the flaming discharge the _eclat_ effect of the
capacity is not perceptible; the rate at which the energy is being
stored then just equals the rate at which it can be disposed of through
the circuit. This kind of discharge is the severest test for a coil; the
break, when it occurs, is of the nature of that in an overcharged Leyden
jar. To give a rough approximation I would state that, with an ordinary
coil of, say 10,000 ohms resistance, the most powerful arc would be
produced with about 12,000 alternations per second.

When the frequency is increased beyond that rate, the potential, of
course, rises, but the striking distance may, nevertheless, diminish,
paradoxical as it may seem. As the potential rises the coil attains more
and more the properties of a static machine until, finally, one may
observe the beautiful phenomenon of the streaming discharge, Fig. 101,
which may be produced across the whole length of the coil. At that stage
streams begin to issue freely from all points and projections. These
streams will also be seen to pass in abundance in the space between the
primary and the insulating tube. When the potential is excessively high
they will always appear, even if the frequency be low, and even if the
primary be surrounded by as much as an inch of wax, hard rubber, glass,
or any other insulating substance. This limits greatly the output of the
coil, but I will later show how I have been able to overcome to a
considerable extent this disadvantage in the ordinary coil.

Besides the potential, the intensity of the streams depends on the
frequency; but if the coil be very large they show themselves, no matter
how low the frequencies used. For instance, in a very large coil of a
resistance of 67,000 ohms, constructed by me some time ago, they appear
with as low as 100 alternations per second and less, the insulation of
the secondary being 3/4 inch of ebonite. When very intense they produce
a noise similar to that produced by the charging of a Holtz machine, but
much more powerful, and they emit a strong smell of ozone. The lower the
frequency, the more apt they are to suddenly injure the coil. With
excessively high frequencies they may pass freely without producing any
other effect than to heat the insulation slowly and uniformly.

[Illustration: FIG. 101.]

[Illustration: FIG. 102.]

The existence of these streams shows the importance of constructing an
expensive coil so as to permit of one's seeing through the tube
surrounding the primary, and the latter should be easily exchangeable;
or else the space between the primary and secondary should be completely
filled up with insulating material so as to exclude all air. The
non-observance of this simple rule in the construction of commercial
coils is responsible for the destruction of many an expensive coil.

At the stage when the streaming discharge occurs, or with somewhat
higher frequencies, one may, by approaching the terminals quite nearly,
and regulating properly the effect of capacity, produce a veritable
spray of small silver-white sparks, or a bunch of excessively thin
silvery threads (Fig. 102) amidst a powerful brush--each spark or thread
possibly corresponding to one alternation. This, when produced under
proper conditions, is probably the most beautiful discharge, and when an
air blast is directed against it, it presents a singular appearance. The
spray of sparks, when received through the body, causes some
inconvenience, whereas, when the discharge simply streams, nothing at
all is likely to be felt if large conducting objects are held in the
hands to protect them from receiving small burns.

If the frequency is still more increased, then the coil refuses to give
any spark unless at comparatively small distances, and the fifth typical
form of discharge may be observed (Fig. 103). The tendency to stream out
and dissipate is then so great that when the brush is produced at one
terminal no sparking occurs, even if, as I have repeatedly tried, the
hand, or any conducting object, is held within the stream; and, what is
more singular, the luminous stream is not at all easily deflected by the
approach of a conducting body.

[Illustration: FIG. 103.]

[Illustration: FIG. 104.]

At this stage the streams seemingly pass with the greatest freedom
through considerable thicknesses of insulators, and it is particularly
interesting to study their behavior. For this purpose it is convenient
to connect to the terminals of the coil two metallic spheres which may
be placed at any desired distance, Fig. 104. Spheres are preferable to
plates, as the discharge can be better observed. By inserting dielectric
bodies between the spheres, beautiful discharge phenomena may be
observed. If the spheres be quite close and a spark be playing between
them, by interposing a thin plate of ebonite between the spheres the
spark instantly ceases and the discharge spreads into an intensely
luminous circle several inches in diameter, provided the spheres are
sufficiently large. The passage of the streams heats, and, after a
while, softens, the rubber so much that two plates may be made to stick
together in this manner. If the spheres are so far apart that no spark
occurs, even if they are far beyond the striking distance, by inserting
a thick plate of glass the discharge is instantly induced to pass from
the spheres to the glass in the form of luminous streams. It appears
almost as though these streams pass _through_ the dielectric. In reality
this is not the case, as the streams are due to the molecules of the air
which are violently agitated in the space between the oppositely charged
surfaces of the spheres. When no dielectric other than air is present,
the bombardment goes on, but is too weak to be visible; by inserting a
dielectric the inductive effect is much increased, and besides, the
projected air molecules find an obstacle and the bombardment becomes so
intense that the streams become luminous. If by any mechanical means we
could effect such a violent agitation of the molecules we could produce
the same phenomenon. A jet of air escaping through a small hole under
enormous pressure and striking against an insulating substance, such as
glass, may be luminous in the dark, and it might be possible to produce
a phosphorescence of the glass or other insulators in this manner.

The greater the specific inductive capacity of the interposed
dielectric, the more powerful the effect produced. Owing to this, the
streams show themselves with excessively high potentials even if the
glass be as much as one and one-half to two inches thick. But besides
the heating due to bombardment, some heating goes on undoubtedly in the
dielectric, being apparently greater in glass than in ebonite. I
attribute this to the greater specific inductive capacity of the glass,
in consequence of which, with the same potential difference, a greater
amount of energy is taken up in it than in rubber. It is like connecting
to a battery a copper and a brass wire of the same dimensions. The
copper wire, though a more perfect conductor, would heat more by reason
of its taking more current. Thus what is otherwise considered a virtue
of the glass is here a defect. Glass usually gives way much quicker than
ebonite; when it is heated to a certain degree, the discharge suddenly
breaks through at one point, assuming then the ordinary form of an arc.

The heating effect produced by molecular bombardment of the dielectric
would, of course, diminish as the pressure of the air is increased, and
at enormous pressure it would be negligible, unless the frequency would
increase correspondingly.

It will be often observed in these experiments that when the spheres are
beyond the striking distance, the approach of a glass plate, for
instance, may induce the spark to jump between the spheres. This occurs
when the capacity of the spheres is somewhat below the critical value
which gives the greatest difference of potential at the terminals of the
coil. By approaching a dielectric, the specific inductive capacity of
the space between the spheres is increased, producing the same effect as
if the capacity of the spheres were increased. The potential at the
terminals may then rise so high that the air space is cracked. The
experiment is best performed with dense glass or mica.

Another interesting observation is that a plate of insulating material,
when the discharge is passing through it, is strongly attracted by
either of the spheres, that is by the nearer one, this being obviously
due to the smaller mechanical effect of the bombardment on that side,
and perhaps also to the greater electrification.

From the behavior of the dielectrics in these experiments, we may
conclude that the best insulator for these rapidly alternating currents
would be the one possessing the smallest specific inductive capacity and
at the same time one capable of withstanding the greatest differences of
potential; and thus two diametrically opposite ways of securing the
required insulation are indicated, namely, to use either a perfect
vacuum or a gas under great pressure; but the former would be
preferable. Unfortunately neither of these two ways is easily carried
out in practice.

It is especially interesting to note the behavior of an excessively high
vacuum in these experiments. If a test tube, provided with external
electrodes and exhausted to the highest possible degree, be connected to
the terminals of the coil, Fig. 105, the electrodes of the tube are
instantly brought to a high temperature and the glass at each end of the
tube is rendered intensely phosphorescent, but the middle appears
comparatively dark, and for a while remains cool.

When the frequency is so high that the discharge shown in Fig. 103 is
observed, considerable dissipation no doubt occurs in the coil.
Nevertheless the coil may be worked for a long time, as the heating is
gradual.

In spite of the fact that the difference of potential may be enormous,
little is felt when the discharge is passed through the body, provided
the hands are armed. This is to some extent due to the higher frequency,
but principally to the fact that less energy is available externally,
when the difference of potential reaches an enormous value, owing to the
circumstance that, with the rise of potential, the energy absorbed in
the coil increases as the square of the potential. Up to a certain point
the energy available externally increases with the rise of potential,
then it begins to fall off rapidly. Thus, with the ordinary high tension
induction coil, the curious paradox exists, that, while with a given
current through the primary the shock might be fatal, with many times
that current it might be perfectly harmless, even if the frequency be
the same. With high frequencies and excessively high potentials when the
terminals are not connected to bodies of some size, practically all the
energy supplied to the primary is taken up by the coil. There is no
breaking through, no local injury, but all the material, insulating and
conducting, is uniformly heated.

[Illustration: FIG. 105.]

[Illustration: FIG. 106.]

To avoid misunderstanding in regard to the physiological effect of
alternating currents of very high frequency, I think it necessary to
state that, while it is an undeniable fact that they are incomparably
less dangerous than currents of low frequencies, it should not be
thought that they are altogether harmless. What has just been said
refers only to currents from an ordinary high tension induction coil,
which currents are necessarily very small; if received directly from a
machine or from a secondary of low resistance, they produce more or less
powerful effects, and may cause serious injury, especially when used in
conjunction with condensers.

The streaming discharge of a high tension induction coil differs in many
respects from that of a powerful static machine. In color it has neither
the violet of the positive, nor the brightness of the negative, static
discharge, but lies somewhere between, being, of course, alternatively
positive and negative. But since the streaming is more powerful when the
point or terminal is electrified positively, than when electrified
negatively, it follows that the point of the brush is more like the
positive, and the root more like the negative, static discharge. In the
dark, when the brush is very powerful, the root may appear almost white.
The wind produced by the escaping streams, though it may be very
strong--often indeed to such a degree that it may be felt quite a
distance from the coil--is, nevertheless, considering the quantity of
the discharge, smaller than that produced by the positive brush of a
static machine, and it affects the flame much less powerfully. From the
nature of the phenomenon we can conclude that the higher the frequency,
the smaller must, of course, be the wind produced by the streams, and
with sufficiently high frequencies no wind at all would be produced at
the ordinary atmospheric pressures. With frequencies obtainable by means
of a machine, the mechanical effect is sufficiently great to revolve,
with considerable speed, large pin-wheels, which in the dark present a
beautiful appearance owing to the abundance of the streams (Fig. 106).

[Illustration: FIG. 107.]

[Illustration: FIG. 108.]

In general, most of the experiments usually performed with a static
machine can be performed with an induction coil when operated with very
rapidly alternating currents. The effects produced, however, are much
more striking, being of incomparably greater power. When a small length
of ordinary cotton covered wire, Fig. 107, is attached to one terminal
of the coil, the streams issuing from all points of the wire may be so
intense as to produce a considerable light effect. When the potentials
and frequencies are very high, a wire insulated with gutta percha or
rubber and attached to one of the terminals, appears to be covered with
a luminous film. A very thin bare wire when attached to a terminal emits
powerful streams and vibrates continually to and fro or spins in a
circle, producing a singular effect (Fig. 108). Some of these
experiments have been described by me in _The Electrical World_, of
February 21, 1891.

Another peculiarity of the rapidly alternating discharge of the
induction coil is its radically different behavior with respect to
points and rounded surfaces.

If a thick wire, provided with a ball at one end and with a point at the
other, be attached to the positive terminal of a static machine,
practically all the charge will be lost through the point, on account of
the enormously greater tension, dependent on the radius of curvature.
But if such a wire is attached to one of the terminals of the induction
coil, it will be observed that with very high frequencies streams issue
from the ball almost as copiously as from the point (Fig. 109).

It is hardly conceivable that we could produce such a condition to an
equal degree in a static machine, for the simple reason, that the
tension increases as the square of the density, which in turn is
proportional to the radius of curvature; hence, with a steady potential
an enormous charge would be required to make streams issue from a
polished ball while it is connected with a point. But with an induction
coil the discharge of which alternates with great rapidity it is
different. Here we have to deal with two distinct tendencies. First,
there is the tendency to escape which exists in a condition of rest, and
which depends on the radius of curvature; second, there is the tendency
to dissipate into the surrounding air by condenser action, which depends
on the surface. When one of these tendencies is a maximum, the other is
at a minimum. At the point the luminous stream is principally due to the
air molecules coming bodily in contact with the point; they are
attracted and repelled, charged and discharged, and, their atomic
charges being thus disturbed, vibrate and emit light waves. At the ball,
on the contrary, there is no doubt that the effect is to a great extent
produced inductively, the air molecules not _necessarily_ coming in
contact with the ball, though they undoubtedly do so. To convince
ourselves of this we only need to exalt the condenser action, for
instance, by enveloping the ball, at some distance, by a better
conductor than the surrounding medium, the conductor being, of course,
insulated; or else by surrounding it with a better dielectric and
approaching an insulated conductor; in both cases the streams will break
forth more copiously. Also, the larger the ball with a given frequency,
or the higher the frequency, the more will the ball have the advantage
over the point. But, since a certain intensity of action is required to
render the streams visible, it is obvious that in the experiment
described the ball should not be taken too large.

In consequence of this two-fold tendency, it is possible to produce by
means of points, effects identical to those produced by capacity. Thus,
for instance, by attaching to one terminal of the coil a small length of
soiled wire, presenting many points and offering great facility to
escape, the potential of the coil may be raised to the same value as by
attaching to the terminal a polished ball of a surface many times
greater than that of the wire.

[Illustration: FIG. 109.]

[Illustration: FIG. 110.]

An interesting experiment, showing the effect of the points, may be
performed in the following manner: Attach to one of the terminals of the
coil a cotton covered wire about two feet in length, and adjust the
conditions so that streams issue from the wire. In this experiment the
primary coil should be preferably placed so that it extends only about
half way into the secondary coil. Now touch the free terminal of the
secondary with a conducting object held in the hand, or else connect it
to an insulated body of some size. In this manner the potential on the
wire may be enormously raised. The effect of this will be either to
increase, or to diminish, the streams. If they increase, the wire is too
short; if they diminish, it is too long. By adjusting the length of the
wire, a point is found where the touching of the other terminal does not
at all affect the streams. In this case the rise of potential is exactly
counteracted by the drop through the coil. It will be observed that
small lengths of wire produce considerable difference in the magnitude
and luminosity of the streams. The primary coil is placed sidewise for
two reasons: First, to increase the potential at the wire; and, second,
to increase the drop through the coil. The sensitiveness is thus
augmented.

There is still another and far more striking peculiarity of the brush
discharge produced by very rapidly alternating currents. To observe this
it is best to replace the usual terminals of the coil by two metal
columns insulated with a good thickness of ebonite. It is also well to
close all fissures and cracks with wax so that the brushes cannot form
anywhere except at the tops of the columns. If the conditions are
carefully adjusted--which, of course, must be left to the skill of the
experimenter--so that the potential rises to an enormous value, one may
produce two powerful brushes several inches long, nearly white at their
roots, which in the dark bear a striking resemblance to two flames of a
gas escaping under pressure (Fig. 110). But they do not only _resemble_,
they _are_ veritable flames, for they are hot. Certainly they are not as
hot as a gas burner, _but they would be so if the frequency and the
potential would be sufficiently high_. Produced with, say, twenty
thousand alternations per second, the heat is easily perceptible even if
the potential is not excessively high. The heat developed is, of course,
due to the impact of the air molecules against the terminals and against
each other. As, at the ordinary pressures, the mean free path is
excessively small, it is possible that in spite of the enormous initial
speed imparted to each molecule upon coming in contact with the
terminal, its progress--by collision with other molecules--is retarded
to such an extent, that it does not get away far from the terminal, but
may strike the same many times in succession. The higher the frequency,
the less the molecule is able to get away, and this the more so, as for
a given effect the potential required is smaller; and a frequency is
conceivable--perhaps even obtainable--at which practically the same
molecules would strike the terminal. Under such conditions the exchange
of the molecules would be very slow, and the heat produced at, and very
near, the terminal would be excessive. But if the frequency would go on
increasing constantly, the heat produced would begin to diminish for
obvious reasons. In the positive brush of a static machine the exchange
of the molecules is very rapid, the stream is constantly of one
direction, and there are fewer collisions; hence the heating effect must
be very small. Anything that impairs the facility of exchange tends to
increase the local heat produced. Thus, if a bulb be held over the
terminal of the coil so as to enclose the brush, the air contained in
the bulb is very quickly brought to a high temperature. If a glass tube
be held over the brush so as to allow the draught to carry the brush
upwards, scorching hot air escapes at the top of the tube. Anything held
within the brush is, of course, rapidly heated, and the possibility of
using such heating effects for some purpose or other suggests itself.

When contemplating this singular phenomenon of the hot brush, we cannot
help being convinced that a similar process must take place in the
ordinary flame, and it seems strange that after all these centuries past
of familiarity with the flame, now, in this era of electric lighting and
heating, we are finally led to recognize, that since time immemorial we
have, after all, always had "electric light and heat" at our disposal.
It is also of no little interest to contemplate, that we have a possible
way of producing--by other than chemical means--a veritable flame, which
would give light and heat without any material being consumed, without
any chemical process taking place, and to accomplish this, we only need
to perfect methods of producing enormous frequencies and potentials. I
have no doubt that if the potential could be made to alternate with
sufficient rapidity and power, the brush formed at the end of a wire
would lose its electrical characteristics and would become flamelike.
The flame must be due to electrostatic molecular action.

This phenomenon now explains in a manner which can hardly be doubted the
frequent accidents occurring in storms. It is well known that objects
are often set on fire without the lightning striking them. We shall
presently see how this can happen. On a nail in a roof, for instance, or
on a projection of any kind, more or less conducting, or rendered so by
dampness, a powerful brush may appear. If the lightning strikes
somewhere in the neighborhood the enormous potential may be made to
alternate or fluctuate perhaps many million times a second. The air
molecules are violently attracted and repelled, and by their impact
produce such a powerful heating effect that a fire is started. It is
conceivable that a ship at sea may, in this manner, catch fire at many
points at once. When we consider, that even with the comparatively low
frequencies obtained from a dynamo machine, and with potentials of no
more than one or two hundred thousand volts, the heating effects are
considerable, we may imagine how much more powerful they must be with
frequencies and potentials many times greater; and the above explanation
seems, to say the least, very probable. Similar explanations may have
been suggested, but I am not aware that, up to the present, the heating
effects of a brush produced by a rapidly alternating potential have been
experimentally demonstrated, at least not to such a remarkable degree.

[Illustration: FIG. 111.]

By preventing completely the exchange of the air molecules, the local
heating effect may be so exalted as to bring a body to incandescence.
Thus, for instance, if a small button, or preferably a very thin wire or
filament be enclosed in an unexhausted globe and connected with the
terminal of the coil, it may be rendered incandescent. The phenomenon is
made much more interesting by the rapid spinning round in a circle of
the top of the filament, thus presenting the appearance of a luminous
funnel, Fig. 111, which widens when the potential is increased. When the
potential is small the end of the filament may perform irregular
motions, suddenly changing from one to the other, or it may describe an
ellipse; but when the potential is very high it always spins in a
circle; and so does generally a thin straight wire attached freely to
the terminal of the coil. These motions are, of course, due to the
impact of the molecules, and the irregularity in the distribution of the
potential, owing to the roughness and dissymmetry of the wire or
filament. With a perfectly symmetrical and polished wire such motions
would probably not occur. That the motion is not likely to be due to
others causes is evident from the fact that it is not of a definite
direction, and that in a very highly exhausted globe it ceases
altogether. The possibility of bringing a body to incandescence in an
exhausted globe, or even when not at all enclosed, would seem to afford
a possible way of obtaining light effects, which, in perfecting methods
of producing rapidly alternating potentials, might be rendered available
for useful purposes.

[Illustration: FIG. 112a.]

In employing a commercial coil, the production of very powerful brush
effects is attended with considerable difficulties, for when these high
frequencies and enormous potentials are used, the best insulation is apt
to give way. Usually the coil is insulated well enough to stand the
strain from convolution to convolution, since two double silk covered
paraffined wires will withstand a pressure of several thousand volts;
the difficulty lies principally in preventing the breaking through from
the secondary to the primary, which is greatly facilitated by the
streams issuing from the latter. In the coil, of course, the strain is
greatest from section to section, but usually in a larger coil there are
so many sections that the danger of a sudden giving way is not very
great. No difficulty will generally be encountered in that direction,
and besides, the liability of injuring the coil internally is very much
reduced by the fact that the effect most likely to be produced is simply
a gradual heating, which, when far enough advanced, could not fail to
be observed. The principal necessity is then to prevent the streams
between the primary and the tube, not only on account of the heating and
possible injury, but also because the streams may diminish very
considerably the potential difference available at the terminals. A few
hints as to how this may be accomplished will probably be found useful
in most of these experiments with the ordinary induction coil.

[Illustration: FIG. 112b.]

One of the ways is to wind a short primary, Fig. 112a, so that the
difference of potential is not at that length great enough to cause the
breaking forth of the streams through the insulating tube. The length of
the primary should be determined by experiment. Both the ends of the
coil should be brought out on one end through a plug of insulating
material fitting in the tube as illustrated. In such a disposition one
terminal of the secondary is attached to a body, the surface of which is
determined with the greatest care so as to produce the greatest rise in
the potential. At the other terminal a powerful brush appears, which may
be experimented upon.

The above plan necessitates the employment of a primary of comparatively
small size, and it is apt to heat when powerful effects are desirable
for a certain length of time. In such a case it is better to employ a
larger coil, Fig. 112b, and introduce it from one side of the tube,
until the streams begin to appear. In this case the nearest terminal of
the secondary may be connected to the primary or to the ground, which is
practically the same thing, if the primary is connected directly to the
machine. In the case of ground connections it is well to determine
experimentally the frequency which is best suited under the conditions
of the test. Another way of obviating the streams, more or less, is to
make the primary in sections and supply it from separate, well insulated
sources.

In many of these experiments, when powerful effects are wanted for a
short time, it is advantageous to use iron cores with the primaries. In
such case a very large primary coil may be wound and placed side by side
with the secondary, and, the nearest terminal of the latter being
connected to the primary, a laminated iron core is introduced through
the primary into the secondary as far as the streams will permit. Under
these conditions an excessively powerful brush, several inches long,
which may be appropriately called "St. Elmo's hot fire," may be caused
to appear at the other terminal of the secondary, producing striking
effects. It is a most powerful ozonizer, so powerful indeed, that only a
few minutes are sufficient to fill the whole room with the smell of
ozone, and it undoubtedly possesses the quality of exciting chemical
affinities.

For the production of ozone, alternating currents of very high frequency
are eminently suited, not only on account of the advantages they offer
in the way of conversion but also because of the fact, that the
ozonizing action of a discharge is dependent on the frequency as well as
on the potential, this being undoubtedly confirmed by observation.

In these experiments if an iron core is used it should be carefully
watched, as it is apt to get excessively hot in an incredibly short
time. To give an idea of the rapidity of the heating, I will state, that
by passing a powerful current through a coil with many turns, the
inserting within the same of a thin iron wire for no more than one
second's time is sufficient to heat the wire to something like 100° C.

But this rapid heating need not discourage us in the use of iron cores
in connection with rapidly alternating currents. I have for a long time
been convinced that in the industrial distribution by means of
transformers, some such plan as the following might be practicable. We
may use a comparatively small iron core, subdivided, or perhaps not even
subdivided. We may surround this core with a considerable thickness of
material which is fire-proof and conducts the heat poorly, and on top of
that we may place the primary and secondary windings. By using either
higher frequencies or greater magnetizing forces, we may by hysteresis
and eddy currents heat the iron core so far as to bring it nearly to its
maximum permeability, which, as Hopkinson has shown, may be as much as
sixteen times greater than that at ordinary temperatures. If the iron
core were perfectly enclosed, it would not be deteriorated by the heat,
and, if the enclosure of fire-proof material would be sufficiently
thick, only a limited amount of energy could be radiated in spite of the
high temperature. Transformers have been constructed by me on that plan,
but for lack of time, no thorough tests have as yet been made.

Another way of adapting the iron core to rapid alternations, or,
generally speaking, reducing the frictional losses, is to produce by
continuous magnetization a flow of something like seven thousand or
eight thousand lines per square centimetre through the core, and then
work with weak magnetizing forces and preferably high frequencies around
the point of greatest permeability. A higher efficiency of conversion
and greater output are obtainable in this manner. I have also employed
this principle in connection with machines in which there is no reversal
of polarity. In these types of machines, as long as there are only few
pole projections, there is no great gain, as the maxima and minima of
magnetization are far from the point of maximum permeability; but when
the number of the pole projections is very great, the required rate of
change may be obtained, without the magnetization varying so far as to
depart greatly from the point of maximum permeability, and the gain is
considerable.

The above described arrangements refer only to the use of commercial
coils as ordinarily constructed. If it is desired to construct a coil
for the express purpose of performing with it such experiments as I have
described, or, generally, rendering it capable of withstanding the
greatest possible difference of potential, then a construction as
indicated in Fig. 113 will be found of advantage. The coil in this case
is formed of two independent parts which are wound oppositely, the
connection between both being made near the primary. The potential in
the middle being zero, there is not much tendency to jump to the primary
and not much insulation is required. In some cases the middle point may,
however, be connected to the primary or to the ground. In such a coil
the places of greatest difference of potential are far apart and the
coil is capable of withstanding an enormous strain. The two parts may be
movable so as to allow a slight adjustment of the capacity effect.

As to the manner of insulating the coil, it will be found convenient to
proceed in the following way: First, the wire should be boiled in
paraffine until all the air is out; then the coil is wound by running
the wire through melted paraffine, merely for the purpose of fixing the
wire. The coil is then taken off from the spool, immersed in a
cylindrical vessel filled with pure melted wax and boiled for a long
time until the bubbles cease to appear. The whole is then left to cool
down thoroughly, and then the mass is taken out of the vessel and turned
up in a lathe. A coil made in this manner and with care is capable of
withstanding enormous potential differences.

[Illustration: FIG. 113.]

It may be found convenient to immerse the coil in paraffine oil or some
other kind of oil; it is a most effective way of insulating, principally
on account of the perfect exclusion of air, but it may be found that,
after all, a vessel filled with oil is not a very convenient thing to
handle in a laboratory.

If an ordinary coil can be dismounted, the primary may be taken out of
the tube and the latter plugged up at one end, filled with oil, and the
primary reinserted. This affords an excellent insulation and prevents
the formation of the streams.

Of all the experiments which may be performed with rapidly alternating
currents the most interesting are those which concern the production of
a practical illuminant. It cannot be denied that the present methods,
though they were brilliant advances, are very wasteful. Some better
methods must be invented, some more perfect apparatus devised. Modern
research has opened new possibilities for the production of an efficient
source of light, and the attention of all has been turned in the
direction indicated by able pioneers. Many have been carried away by
the enthusiasm and passion to discover, but in their zeal to reach
results, some have been misled. Starting with the idea of producing
electro-magnetic waves, they turned their attention, perhaps, too much
to the study of electro-magnetic effects, and neglected the study of
electrostatic phenomena. Naturally, nearly every investigator availed
himself of an apparatus similar to that used in earlier experiments. But
in those forms of apparatus, while the electro-magnetic inductive
effects are enormous, the electrostatic effects are excessively small.

In the Hertz experiments, for instance, a high tension induction coil is
short circuited by an arc, the resistance of which is very small, the
smaller, the more capacity is attached to the terminals; and the
difference of potential at these is enormously diminished. On the other
hand, when the discharge is not passing between the terminals, the
static effects may be considerable, but only qualitatively so, not
quantitatively, since their rise and fall is very sudden, and since
their frequency is small. In neither case, therefore, are powerful
electrostatic effects perceivable. Similar conditions exist when, as in
some interesting experiments of Dr. Lodge, Leyden jars are discharged
disruptively. It has been thought--and I believe asserted--that in such
cases most of the energy is radiated into space. In the light of the
experiments which I have described above, it will now not be thought so.
I feel safe in asserting that in such cases most of the energy is partly
taken up and converted into heat in the arc of the discharge and in the
conducting and insulating material of the jar, some energy being, of
course, given off by electrification of the air; but the amount of the
directly radiated energy is very small.

When a high tension induction coil, operated by currents alternating
only 20,000 times a second, has its terminals closed through even a very
small jar, practically all the energy passes through the dielectric of
the jar, which is heated, and the electrostatic effects manifest
themselves outwardly only to a very weak degree. Now the external
circuit of a Leyden jar, that is, the arc and the connections of the
coatings, may be looked upon as a circuit generating alternating
currents of excessively high frequency and fairly high potential, which
is closed through the coatings and the dielectric between them, and from
the above it is evident that the external electrostatic effects must be
very small, even if a recoil circuit be used. These conditions make it
appear that with the apparatus usually at hand, the observation of
powerful electrostatic effects was impossible, and what experience has
been gained in that direction is only due to the great ability of the
investigators.

But powerful electrostatic effects are a _sine qua non_ of light
production on the lines indicated by theory. Electro-magnetic effects
are primarily unavailable, for the reason that to produce the required
effects we would have to pass current impulses through a conductor,
which, long before the required frequency of the impulses could be
reached, would cease to transmit them. On the other hand,
electro-magnetic waves many times longer than those of light, and
producible by sudden discharge of a condenser, could not be utilized, it
would seem, except we avail ourselves of their effect upon conductors as
in the present methods, which are wasteful. We could not affect by means
of such waves the static molecular or atomic charges of a gas, cause
them to vibrate and to emit light. Long transverse waves cannot,
apparently, produce such effects, since excessively small
electro-magnetic disturbances may pass readily through miles of air.
Such dark waves, unless they are of the length of true light waves,
cannot, it would seem, excite luminous radiation in a Geissler tube, and
the luminous effects, which are producible by induction in a tube devoid
of electrodes, I am inclined to consider as being of an electrostatic
nature.

To produce such luminous effects, straight electrostatic thrusts are
required; these, whatever be their frequency, may disturb the molecular
charges and produce light. Since current impulses of the required
frequency cannot pass through a conductor of measurable dimensions, we
must work with a gas, and then the production of powerful electrostatic
effects becomes an imperative necessity.

It has occurred to me, however, that electrostatic effects are in many
ways available for the production of light. For instance, we may place a
body of some refractory material in a closed, and preferably more or
less exhausted, globe, connect it to a source of high, rapidly
alternating potential, causing the molecules of the gas to strike it
many times a second at enormous speeds, and in this manner, with
trillions of invisible hammers, pound it until it gets incandescent; or
we may place a body in a very highly exhausted globe, in a non-striking
vacuum, and, by employing very high frequencies and potentials,
transfer sufficient energy from it to other bodies in the vicinity, or
in general to the surroundings, to maintain it at any degree of
incandescence; or we may, by means of such rapidly alternating high
potentials, disturb the ether carried by the molecules of a gas or their
static charges, causing them to vibrate and to emit light.

But, electrostatic effects being dependent upon the potential and
frequency, to produce the most powerful action it is desirable to
increase both as far as practicable. It may be possible to obtain quite
fair results by keeping either of these factors small, provided the
other is sufficiently great; but we are limited in both directions. My
experience demonstrates that we cannot go below a certain frequency,
for, first, the potential then becomes so great that it is dangerous;
and, secondly, the light production is less efficient.

I have found that, by using the ordinary low frequencies, the
physiological effect of the current required to maintain at a certain
degree of brightness a tube four feet long, provided at the ends with
outside and inside condenser coatings, is so powerful that, I think, it
might produce serious injury to those not accustomed to such shocks;
whereas, with twenty thousand alternations per second, the tube may be
maintained at the same degree of brightness without any effect being
felt. This is due principally to the fact that a much smaller potential
is required to produce the same light effect, and also to the higher
efficiency in the light production. It is evident that the efficiency in
such cases is the greater, the higher the frequency, for the quicker the
process of charging and discharging the molecules, the less energy will
be lost in the form of dark radiation. But, unfortunately, we cannot go
beyond a certain frequency on account of the difficulty of producing and
conveying the effects.

I have stated above that a body inclosed in an unexhausted bulb may be
intensely heated by simply connecting it with a source of rapidly
alternating potential. The heating in such a case is, in all
probability, due mostly to the bombardment of the molecules of the gas
contained in the bulb. When the bulb is exhausted, the heating of the
body is much more rapid, and there is no difficulty whatever in bringing
a wire or filament to any degree of incandescence by simply connecting
it to one terminal of a coil of the proper dimensions. Thus, if the
well-known apparatus of Prof. Crookes, consisting of a bent platinum
wire with vanes mounted over it (Fig. 114), be connected to one
terminal of the coil--either one or both ends of the platinum wire being
connected--the wire is rendered almost instantly incandescent, and the
mica vanes are rotated as though a current from a battery were used. A
thin carbon filament, or, preferably, a button of some refractory
material (Fig. 115), even if it be a comparatively poor conductor,
inclosed in an exhausted globe, may be rendered highly incandescent; and
in this manner a simple lamp capable of giving any desired candle power
is provided.

The success of lamps of this kind would depend largely on the selection
of the light-giving bodies contained within the bulb. Since, under the
conditions described, refractory bodies--which are very poor conductors
and capable of withstanding for a long time excessively high degrees of
temperature--may be used, such illuminating devices may be rendered
successful.

[Illustration: FIG. 114.]

[Illustration: FIG. 115.]

It might be thought at first that if the bulb, containing the filament
or button of refractory material, be perfectly well exhausted--that is,
as far as it can be done by the use of the best apparatus--the heating
would be much less intense, and that in a perfect vacuum it could not
occur at all. This is not confirmed by my experience; quite the
contrary, the better the vacuum the more easily the bodies are brought
to incandescence. This result is interesting for many reasons.

At the outset of this work the idea presented itself to me, whether two
bodies of refractory material enclosed in a bulb exhausted to such a
degree that the discharge of a large induction coil, operated in the
usual manner, cannot pass through, could be rendered incandescent by
mere condenser action. Obviously, to reach this result enormous
potential differences and very high frequencies are required, as is
evident from a simple calculation.

But such a lamp would possess a vast advantage over an ordinary
incandescent lamp in regard to efficiency. It is well-known that the
efficiency of a lamp is to some extent a function of the degree of
incandescence, and that, could we but work a filament at many times
higher degrees of incandescence, the efficiency would be much greater.
In an ordinary lamp this is impracticable on account of the destruction
of the filament, and it has been determined by experience how far it is
advisable to push the incandescence. It is impossible to tell how much
higher efficiency could be obtained if the filament could withstand
indefinitely, as the investigation to this end obviously cannot be
carried beyond a certain stage; but there are reasons for believing that
it would be very considerably higher. An improvement might be made in
the ordinary lamp by employing a short and thick carbon; but then the
leading-in wires would have to be thick, and, besides, there are many
other considerations which render such a modification entirely
impracticable. But in a lamp as above described, the leading in wires
may be very small, the incandescent refractory material may be in the
shape of blocks offering a very small radiating surface, so that less
energy would be required to keep them at the desired incandescence; and
in addition to this, the refractory material need not be carbon, but may
be manufactured from mixtures of oxides, for instance, with carbon or
other material, or may be selected from bodies which are practically
non-conductors, and capable of withstanding enormous degrees of
temperature.

All this would point to the possibility of obtaining a much higher
efficiency with such a lamp than is obtainable in ordinary lamps. In my
experience it has been demonstrated that the blocks are brought to high
degrees of incandescence with much lower potentials than those
determined by calculation, and the blocks may be set at greater
distances from each other. We may freely assume, and it is probable,
that the molecular bombardment is an important element in the heating,
even if the globe be exhausted with the utmost care, as I have done; for
although the number of the molecules is, comparatively speaking,
insignificant, yet on account of the mean free path being very great,
there are fewer collisions, and the molecules may reach much higher
speeds, so that the heating effect due to this cause may be
considerable, as in the Crookes experiments with radiant matter.

But it is likewise possible that we have to deal here with an increased
facility of losing the charge in very high vacuum, when the potential is
rapidly alternating, in which case most of the heating would be directly
due to the surging of the charges in the heated bodies. Or else the
observed fact may be largely attributable to the effect of the points
which I have mentioned above, in consequence of which the blocks or
filaments contained in the vacuum are equivalent to condensers of many
times greater surface than that calculated from their geometrical
dimensions. Scientific men still differ in opinion as to whether a
charge should, or should not, be lost in a perfect vacuum, or in other
words, whether ether is, or is not, a conductor. If the former were the
case, then a thin filament enclosed in a perfectly exhausted globe, and
connected to a source of enormous, steady potential, would be brought to
incandescence.

[Illustration: FIG. 116.]

[Illustration: FIG. 117.]

Various forms of lamps on the above described principle, with the
refractory bodies in the form of filaments, Fig. 116, or blocks, Fig.
117, have been constructed and operated by me, and investigations are
being carried on in this line. There is no difficulty in reaching such
high degrees of incandescence that ordinary carbon is to all appearance
melted and volatilized. If the vacuum could be made absolutely perfect,
such a lamp, although inoperative with apparatus ordinarily used, would,
if operated with currents of the required character, afford an
illuminant which would never be destroyed, and which would be far more
efficient than an ordinary incandescent lamp. This perfection can, of
course, never be reached, and a very slow destruction and gradual
diminution in size always occurs, as in incandescent filaments; but
there is no possibility of a sudden and premature disabling which occurs
in the latter by the breaking of the filament, especially when the
incandescent bodies are in the shape of blocks.

With these rapidly alternating potentials there is, however, no
necessity of enclosing two blocks in a globe, but a single block, as in
Fig. 115, or filament, Fig. 118, may be used. The potential in this case
must of course be higher, but is easily obtainable, and besides it is
not necessarily dangerous.

[Illustration: FIG. 118.]

The facility with which the button or filament in such a lamp is brought
to incandescence, other things being equal, depends on the size of the
globe. If a perfect vacuum could be obtained, the size of the globe
would not be of importance, for then the heating would be wholly due to
the surging of the charges, and all the energy would be given off to the
surroundings by radiation. But this can never occur in practice. There
is always some gas left in the globe, and although the exhaustion may be
carried to the highest degree, still the space inside of the bulb must
be considered as conducting when such high potentials are used, and I
assume that, in estimating the energy that may be given off from the
filament to the surroundings, we may consider the inside surface of the
bulb as one coating of a condenser, the air and other objects
surrounding the bulb forming the other coating. When the alternations
are very low there is no doubt that a considerable portion of the energy
is given off by the electrification of the surrounding air.

In order to study this subject better, I carried on some experiments
with excessively high potentials and low frequencies. I then observed
that when the hand is approached to the bulb,--the filament being
connected with one terminal of the coil,--a powerful vibration is felt,
being due to the attraction and repulsion of the molecules of the air
which are electrified by induction through the glass. In some cases when
the action is very intense I have been able to hear a sound, which must
be due to the same cause.

[Illustration: FIG. 119.]

[Illustration: FIG. 120.]

When the alternations are low, one is apt to get an excessively powerful
shock from the bulb. In general, when one attaches bulbs or objects of
some size to the terminals of the coil, one should look out for the rise
of potential, for it may happen that by merely connecting a bulb or
plate to the terminal, the potential may rise to many times its original
value. When lamps are attached to the terminals, as illustrated in Fig.
119, then the capacity of the bulbs should be such as to give the
maximum rise of potential under the existing conditions. In this manner
one may obtain the required potential with fewer turns of wire.

The life of such lamps as described above depends, of course, largely on
the degree of exhaustion, but to some extent also on the shape of the
block of refractory material. Theoretically it would seem that a small
sphere of carbon enclosed in a sphere of glass would not suffer
deterioration from molecular bombardment, for, the matter in the globe
being radiant, the molecules would move in straight lines, and would
seldom strike the sphere obliquely. An interesting thought in connection
with such a lamp is, that in it "electricity" and electrical energy
apparently must move in the same lines.

[Illustration: FIG. 121a.]

[Illustration: FIG. 121b.]

The use of alternating currents of very high frequency makes it possible
to transfer, by electrostatic or electromagnetic induction through the
glass of a lamp, sufficient energy to keep a filament at incandescence
and so do away with the leading-in wires. Such lamps have been proposed,
but for want of proper apparatus they have not been successfully
operated. Many forms of lamps on this principle with continuous and
broken filaments have been constructed by me and experimented upon. When
using a secondary enclosed within the lamp, a condenser is
advantageously combined with the secondary. When the transference is
effected by electrostatic induction, the potentials used are, of course,
very high with frequencies obtainable from a machine. For instance, with
a condenser surface of forty square centimetres, which is not
impracticably large, and with glass of good quality 1 mm. thick, using
currents alternating twenty thousand times a second, the potential
required is approximately 9,000 volts. This may seem large, but since
each lamp may be included in the secondary of a transformer of very
small dimensions, it would not be inconvenient, and, moreover, it would
not produce fatal injury. The transformers would all be preferably in
series. The regulation would offer no difficulties, as with currents of
such frequencies it is very easy to maintain a constant current.

In the accompanying engravings some of the types of lamps of this kind
are shown. Fig. 120 is such a lamp with a broken filament, and Figs. 121
A and 121 B one with a single outside and inside coating and a single
filament. I have also made lamps with two outside and inside coatings
and a continuous loop connecting the latter. Such lamps have been
operated by me with current impulses of the enormous frequencies
obtainable by the disruptive discharge of condensers.

The disruptive discharge of a condenser is especially suited for
operating such lamps--with no outward electrical connections--by means
of electromagnetic induction, the electromagnetic inductive effects
being excessively high; and I have been able to produce the desired
incandescence with only a few short turns of wire. Incandescence may
also be produced in this manner in a simple closed filament.

Leaving now out of consideration the practicability of such lamps, I
would only say that they possess a beautiful and desirable feature,
namely, that they can be rendered, at will, more or less brilliant
simply by altering the relative position of the outside and inside
condenser coatings, or inducing and induced circuits.

When a lamp is lighted by connecting it to one terminal only of the
source, this may be facilitated by providing the globe with an outside
condenser coating, which serves at the same time as a reflector, and
connecting this to an insulated body of some size. Lamps of this kind
are illustrated in Fig. 122 and Fig. 123. Fig. 124 shows the plan of
connection. The brilliancy of the lamp may, in this case, be regulated
within wide limits by varying the size of the insulated metal plate to
which the coating is connected.

It is likewise practicable to light with one leading wire lamps such as
illustrated in Fig. 116 and Fig. 117, by connecting one terminal of the
lamp to one terminal of the source, and the other to an insulated body
of the required size. In all cases the insulated body serves to give off
the energy into the surrounding space, and is equivalent to a return
wire. Obviously, in the two last-named cases, instead of connecting the
wires to an insulated body, connections may be made to the ground.

The experiments which will prove most suggestive and of most interest to
the investigator are probably those performed with exhausted tubes. As
might be anticipated, a source of such rapidly alternating potentials is
capable of exciting the tubes at a considerable distance, and the light
effects produced are remarkable.

[Illustration: FIG. 122.]

[Illustration: FIG. 123.]

During my investigations in this line I endeavored to excite tubes,
devoid of any electrodes, by electromagnetic induction, making the tube
the secondary of the induction device, and passing through the primary
the discharges of a Leyden jar. These tubes were made of many shapes,
and I was able to obtain luminous effects which I then thought were due
wholly to electromagnetic induction. But on carefully investigating the
phenomena I found that the effects produced were more of an
electrostatic nature. It may be attributed to this circumstance that
this mode of exciting tubes is very wasteful, namely, the primary
circuit being closed, the potential, and consequently the electrostatic
inductive effect, is much diminished.

When an induction coil, operated as above described, is used, there is
no doubt that the tubes are excited by electrostatic induction, and that
electromagnetic induction has little, if anything, to do with the
phenomena.

[Illustration: FIG. 124.]

This is evident from many experiments. For instance, if a tube be taken
in one hand, the observer being near the coil, it is brilliantly lighted
and remains so no matter in what position it is held relatively to the
observer's body. Were the action electromagnetic, the tube could not be
lighted when the observer's body is interposed between it and the coil,
or at least its luminosity should be considerably diminished. When the
tube is held exactly over the centre of the coil--the latter being wound
in sections and the primary placed symmetrically to the secondary--it
may remain completely dark, whereas it is rendered intensely luminous by
moving it slightly to the right or left from the centre of the coil. It
does not light because in the middle both halves of the coil neutralize
each other, and the electric potential is zero. If the action were
electromagnetic, the tube should light best in the plane through the
centre of the coil, since the electromagnetic effect there should be a
maximum. When an arc is established between the terminals, the tubes and
lamps in the vicinity of the coil go out, but light up again when the
arc is broken, on account of the rise of potential. Yet the
electromagnetic effect should be practically the same in both cases.

By placing a tube at some distance from the coil, and nearer to one
terminal--preferably at a point on the axis of the coil--one may light
it by touching the remote terminal with an insulated body of some size
or with the hand, thereby raising the potential at that terminal nearer
to the tube. If the tube is shifted nearer to the coil so that it is
lighted by the action of the nearer terminal, it may be made to go out
by holding, on an insulated support, the end of a wire connected to the
remote terminal, in the vicinity of the nearer terminal, by this means
counteracting the action of the latter upon the tube. These effects are
evidently electrostatic. Likewise, when a tube is placed at a
considerable distance from the coil, the observer may, standing upon an
insulated support between coil and tube, light the latter by approaching
the hand to it; or he may even render it luminous by simply stepping
between it and the coil. This would be impossible with electro-magnetic
induction, for the body of the observer would act as a screen.

When the coil is energized by excessively weak currents, the
experimenter may, by touching one terminal of the coil with the tube,
extinguish the latter, and may again light it by bringing it out of
contact with the terminal and allowing a small arc to form. This is
clearly due to the respective lowering and raising of the potential at
that terminal. In the above experiment, when the tube is lighted through
a small arc, it may go out when the arc is broken, because the
electrostatic inductive effect alone is too weak, though the potential
may be much higher; but when the arc is established, the electrification
of the end of the tube is much greater, and it consequently lights.

If a tube is lighted by holding it near to the coil, and in the hand
which is remote, by grasping the tube anywhere with the other hand, the
part between the hands is rendered dark, and the singular effect of
wiping out the light of the tube may be produced by passing the hand
quickly along the tube and at the same time withdrawing it gently from
the coil, judging properly the distance so that the tube remains dark
afterwards.

If the primary coil is placed sidewise, as in Fig. 112 B for instance,
and an exhausted tube be introduced from the other side in the hollow
space, the tube is lighted most intensely because of the increased
condenser action, and in this position the striæ are most sharply
defined. In all these experiments described, and in many others, the
action is clearly electrostatic.

The effects of screening also indicate the electrostatic nature of the
phenomena and show something of the nature of electrification through
the air. For instance, if a tube is placed in the direction of the axis
of the coil, and an insulated metal plate be interposed, the tube will
generally increase in brilliancy, or if it be too far from the coil to
light, it may even be rendered luminous by interposing an insulated
metal plate. The magnitude of the effects depends to some extent on the
size of the plate. But if the metal plate be connected by a wire to the
ground, its interposition will always make the tube go out even if it be
very near the coil. In general, the interposition of a body between the
coil and tube, increases or diminishes the brilliancy of the tube, or
its facility to light up, according to whether it increases or
diminishes the electrification. When experimenting with an insulated
plate, the plate should not be taken too large, else it will generally
produce a weakening effect by reason of its great facility for giving
off energy to the surroundings.

If a tube be lighted at some distance from the coil, and a plate of hard
rubber or other insulating substance be interposed, the tube may be made
to go out. The interposition of the dielectric in this case only
slightly increases the inductive effect, but diminishes considerably the
electrification through the air.

In all cases, then, when we excite luminosity in exhausted tubes by
means of such a coil, the effect is due to the rapidly alternating
electrostatic potential; and, furthermore, it must be attributed to the
harmonic alternation produced directly by the machine, and not to any
superimposed vibration which might be thought to exist. Such
superimposed vibrations are impossible when we work with an alternate
current machine. If a spring be gradually tightened and released, it
does not perform independent vibrations; for this a sudden release is
necessary. So with the alternate currents from a dynamo machine; the
medium is harmonically strained and released, this giving rise to only
one kind of waves; a sudden contact or break, or a sudden giving way of
the dielectric, as in the disruptive discharge of a Leyden jar, are
essential for the production of superimposed waves.

In all the last described experiments, tubes devoid of any electrodes
may be used, and there is no difficulty in producing by their means
sufficient light to read by. The light effect is, however, considerably
increased by the use of phosphorescent bodies such as yttria, uranium
glass, etc. A difficulty will be found when the phosphorescent material
is used, for with these powerful effects, it is carried gradually away,
and it is preferable to use material in the form of a solid.

Instead of depending on induction at a distance to light the tube, the
same may be provided with an external--and, if desired, also with an
internal--condenser coating, and it may then be suspended anywhere in
the room from a conductor connected to one terminal of the coil, and in
this manner a soft illumination may be provided.

[Illustration: FIG. 125.]

The ideal way of lighting a hall or room would, however, be to produce
such a condition in it that an illuminating device could be moved and
put anywhere, and that it is lighted, no matter where it is put and
without being electrically connected to anything. I have been able to
produce such a condition by creating in the room a powerful, rapidly
alternating electrostatic field. For this purpose I suspend a sheet of
metal a distance from the ceiling on insulating cords and connect it to
one terminal of the induction coil, the other terminal being preferably
connected to the ground. Or else I suspend two sheets as illustrated in
Fig. 125, each sheet being connected with one of the terminals of the
coil, and their size being carefully determined. An exhausted tube may
then be carried in the hand anywhere between the sheets or placed
anywhere, even a certain distance beyond them; it remains always
luminous.

In such an electrostatic field interesting phenomena may be observed,
especially if the alternations are kept low and the potentials
excessively high. In addition to the luminous phenomena mentioned, one
may observe that any insulated conductor gives sparks when the hand or
another object is approached to it, and the sparks may often be
powerful. When a large conducting object is fastened on an insulating
support, and the hand approached to it, a vibration, due to the
rythmical motion of the air molecules is felt, and luminous streams may
be perceived when the hand is held near a pointed projection. When a
telephone receiver is made to touch with one or both of its terminals an
insulated conductor of some size, the telephone emits a loud sound; it
also emits a sound when a length of wire is attached to one or both
terminals, and with very powerful fields a sound may be perceived even
without any wire.

How far this principle is capable of practical application, the future
will tell. It might be thought that electrostatic effects are unsuited
for such action at a distance. Electromagnetic inductive effects, if
available for the production of light, might be thought better suited.
It is true the electrostatic effects diminish nearly with the cube of
the distance from the coil, whereas the electromagnetic inductive
effects diminish simply with the distance. But when we establish an
electrostatic field of force, the condition is very different, for then,
instead of the differential effect of both the terminals, we get their
conjoint effect. Besides, I would call attention to the effect, that in
an alternating electrostatic field, a conductor, such as an exhausted
tube, for instance, tends to take up most of the energy, whereas in an
electromagnetic alternating field the conductor tends to take up the
least energy, the waves being reflected with but little loss. This is
one reason why it is difficult to excite an exhausted tube, at a
distance, by electromagnetic induction. I have wound coils of very large
diameter and of many turns of wire, and connected a Geissler tube to the
ends of the coil with the object of exciting the tube at a distance; but
even with the powerful inductive effects producible by Leyden jar
discharges, the tube could not be excited unless at a very small
distance, although some judgment was used as to the dimensions of the
coil. I have also found that even the most powerful Leyden jar
discharges are capable of exciting only feeble luminous effects in a
closed exhausted tube, and even these effects upon thorough examination
I have been forced to consider of an electrostatic nature.

How then can we hope to produce the required effects at a distance by
means of electromagnetic action, when even in the closest proximity to
the source of disturbance, under the most advantageous conditions, we
can excite but faint luminosity? It is true that when acting at a
distance we have the resonance to help us out. We can connect an
exhausted tube, or whatever the illuminating device may be, with an
insulated system of the proper capacity, and so it may be possible to
increase the effect qualitatively, and only qualitatively, for we would
not get _more_ energy through the device. So we may, by resonance
effect, obtain the required electromotive force in an exhausted tube,
and excite faint luminous effects, but we cannot get enough energy to
render the light practically available, and a simple calculation, based
on experimental results, shows that even if all the energy which a tube
would receive at a certain distance from the source should be wholly
converted into light, it would hardly satisfy the practical
requirements. Hence the necessity of directing, by means of a conducting
circuit, the energy to the place of transformation. But in so doing we
cannot very sensibly depart from present methods, and all we could do
would be to improve the apparatus.

From these considerations it would seem that if this ideal way of
lighting is to be rendered practicable it will be only by the use of
electrostatic effects. In such a case the most powerful electrostatic
inductive effects are needed; the apparatus employed must, therefore, be
capable of producing high electrostatic potentials changing in value
with extreme rapidity. High frequencies are especially wanted, for
practical considerations make it desirable to keep down the potential.
By the employment of machines, or, generally speaking, of any
mechanical apparatus, but low frequencies can be reached; recourse must,
therefore, be had to some other means. The discharge of a condenser
affords us a means of obtaining frequencies by far higher than are
obtainable mechanically, and I have accordingly employed condensers in
the experiments to the above end.

When the terminals of a high tension induction coil, Fig. 126, are
connected to a Leyden jar, and the latter is discharging disruptively
into a circuit, we may look upon the arc playing between the knobs as
being a source of alternating, or generally speaking, undulating
currents, and then we have to deal with the familiar system of a
generator of such currents, a circuit connected to it, and a condenser
bridging the circuit. The condenser in such case is a veritable
transformer, and since the frequency is excessive, almost any ratio in
the strength of the currents in both the branches may be obtained. In
reality the analogy is not quite complete, for in the disruptive
discharge we have most generally a fundamental instantaneous variation
of comparatively low frequency, and a superimposed harmonic vibration,
and the laws governing the flow of currents are not the same for both.

In converting in this manner, the ratio of conversion should not be too
great, for the loss in the arc between the knobs increases with the
square of the current, and if the jar be discharged through very thick
and short conductors, with the view of obtaining a very rapid
oscillation, a very considerable portion of the energy stored is lost.
On the other hand, too small ratios are not practicable for many obvious
reasons.

As the converted currents flow in a practically closed circuit, the
electrostatic effects are necessarily small, and I therefore convert
them into currents or effects of the required character. I have effected
such conversions in several ways. The preferred plan of connections is
illustrated in Fig. 127. The manner of operating renders it easy to
obtain by means of a small and inexpensive apparatus enormous
differences of potential which have been usually obtained by means of
large and expensive coils. For this it is only necessary to take an
ordinary small coil, adjust to it a condenser and discharging circuit,
forming the primary of an auxiliary small coil, and convert upward. As
the inductive effect of the primary currents is excessively great, the
second coil need have comparatively but very few turns. By properly
adjusting the elements, remarkable results may be secured.

In endeavoring to obtain the required electrostatic effects in this
manner, I have, as might be expected, encountered many difficulties
which I have been gradually overcoming, but I am not as yet prepared to
dwell upon my experiences in this direction.

I believe that the disruptive discharge of a condenser will play an
important part in the future, for it offers vast possibilities, not only
in the way of producing light in a more efficient manner and in the line
indicated by theory, but also in many other respects.

[Illustration: FIG. 126.]

For years the efforts of inventors have been directed towards obtaining
electrical energy from heat by means of the thermopile. It might seem
invidious to remark that but few know what is the real trouble with the
thermopile. It is not the inefficiency or small output--though these are
great drawbacks--but the fact that the thermopile has its phylloxera,
that is, that by constant use it is deteriorated, which has thus far
prevented its introduction on an industrial scale. Now that all modern
research seems to point with certainty to the use of electricity of
excessively high tension, the question must present itself to many
whether it is not possible to obtain in a practicable manner this form
of energy from heat. We have been used to look upon an electrostatic
machine as a plaything, and somehow we couple with it the idea of the
inefficient and impractical. But now we must think differently, for now
we know that everywhere we have to deal with the same forces, and that
it is a mere question of inventing proper methods or apparatus for
rendering them available.

In the present systems of electrical distribution, the employment of the
iron with its wonderful magnetic properties allows us to reduce
considerably the size of the apparatus; but, in spite of this, it is
still very cumbersome. The more we progress in the study of electric and
magnetic phenomena, the more we become convinced that the present
methods will be short-lived. For the production of light, at least, such
heavy machinery would seem to be unnecessary. The energy required is
very small, and if light can be obtained as efficiently as,
theoretically, it appears possible, the apparatus need have but a very
small output. There being a strong probability that the illuminating
methods of the future will involve the use of very high potentials, it
seems very desirable to perfect a contrivance capable of converting the
energy of heat into energy of the requisite form. Nothing to speak of
has been done towards this end, for the thought that electricity of some
50,000 or 100,000 volts pressure or more, even if obtained, would be
unavailable for practical purposes, has deterred inventors from working
in this direction.

[Illustration: FIG. 127.]

In Fig. 126 a plan of connections is shown for converting currents of
high, into currents of low, tension by means of the disruptive discharge
of a condenser. This plan has been used by me frequently for operating a
few incandescent lamps required in the laboratory. Some difficulties
have been encountered in the arc of the discharge which I have been able
to overcome to a great extent; besides this, and the adjustment
necessary for the proper working, no other difficulties have been met
with, and it was easy to operate ordinary lamps, and even motors, in
this manner. The line being connected to the ground, all the wires could
be handled with perfect impunity, no matter how high the potential at
the terminals of the condenser. In these experiments a high tension
induction coil, operated from a battery or from an alternate current
machine, was employed to charge the condenser; but the induction coil
might be replaced by an apparatus of a different kind, capable of giving
electricity of such high tension. In this manner, direct or alternating
currents may be converted, and in both cases the current-impulses may be
of any desired frequency. When the currents charging the condenser are
of the same direction, and it is desired that the converted currents
should also be of one direction, the resistance of the discharging
circuit should, of course, be so chosen that there are no oscillations.

[Illustration: FIG. 128.]

In operating devices on the above plan I have observed curious phenomena
of impedance which are of interest. For instance if a thick copper bar
be bent, as indicated in Fig. 128, and shunted by ordinary incandescent
lamps, then, by passing the discharge between the knobs, the lamps may
be brought to incandescence although they are short-circuited. When a
large induction coil is employed it is easy to obtain nodes on the bar,
which are rendered evident by the different degree of brilliancy of the
lamps, as shown roughly in Fig. 128. The nodes are never clearly
defined, but they are simply maxima and minima of potentials along the
bar. This is probably due to the irregularity of the arc between the
knobs. In general when the above-described plan of conversion from high
to low tension is used, the behavior of the disruptive discharge may be
closely studied. The nodes may also be investigated by means of an
ordinary Cardew voltmeter which should be well insulated. Geissler
tubes may also be lighted across the points of the bent bar; in this
case, of course, it is better to employ smaller capacities. I have found
it practicable to light up in this manner a lamp, and even a Geissler
tube, shunted by a short, heavy block of metal, and this result seems at
first very curious. In fact, the thicker the copper bar in Fig. 128, the
better it is for the success of the experiments, as they appear more
striking. When lamps with long slender filaments are used it will be
often noted that the filaments are from time to time violently vibrated,
the vibration being smallest at the nodal points. This vibration seems
to be due to an electrostatic action between the filament and the glass
of the bulb.

[Illustration: FIG. 129.]

In some of the above experiments it is preferable to use special lamps
having a straight filament as shown in Fig. 129. When such a lamp is
used a still more curious phenomenon than those described may be
observed. The lamp may be placed across the copper bar and lighted, and
by using somewhat larger capacities, or, in other words, smaller
frequencies or smaller impulsive impedances, the filament may be brought
to any desired degree of incandescence. But when the impedance is
increased, a point is reached when comparatively little current passes
through the carbon, and most of it through the rarefied gas; or perhaps
it may be more correct to state that the current divides nearly evenly
through both, in spite of the enormous difference in the resistance, and
this would be true unless the gas and the filament behave differently.
It is then noted that the whole bulb is brilliantly illuminated, and the
ends of the leading-in wires become incandescent and often throw off
sparks in consequence of the violent bombardment, but the carbon
filament remains dark. This is illustrated in Fig. 129. Instead of the
filament a single wire extending through the whole bulb may be used,
and in this case the phenomenon would seem to be still more interesting.

From the above experiment it will be evident, that when ordinary lamps
are operated by the converted currents, those should be preferably taken
in which the platinum wires are far apart, and the frequencies used
should not be too great, else the discharge will occur at the ends of
the filament or in the base of the lamp between the leading-in wires,
and the lamp might then be damaged.

In presenting to you these results of my investigation on the subject
under consideration, I have paid only a passing notice to facts upon
which I could have dwelt at length, and among many observations I have
selected only those which I thought most likely to interest you. The
field is wide and completely unexplored, and at every step a new truth
is gleaned, a novel fact observed.

How far the results here borne out are capable of practical applications
will be decided in the future. As regards the production of light, some
results already reached are encouraging and make me confident in
asserting that the practical solution of the problem lies in the
direction I have endeavored to indicate. Still, whatever may be the
immediate outcome of these experiments I am hopeful that they will only
prove a step in further development towards the ideal and final
perfection. The possibilities which are opened by modern research are so
vast that even the most reserved must feel sanguine of the future.
Eminent scientists consider the problem of utilizing one kind of
radiation without the others a rational one. In an apparatus designed
for the production of light by conversion from any form of energy into
that of light, such a result can never be reached, for no matter what
the process of producing the required vibrations, be it electrical,
chemical or any other, it will not be possible to obtain the higher
light vibrations without going through the lower heat vibrations. It is
the problem of imparting to a body a certain velocity without passing
through all lower velocities. But there is a possibility of obtaining
energy not only in the form of light, but motive power, and energy of
any other form, in some more direct way from the medium. The time will
be when this will be accomplished, and the time has come when one may
utter such words before an enlightened audience without being considered
a visionary. We are whirling through endless space with an
inconceivable speed, all around us everything is spinning, everything is
moving, everywhere is energy. There _must_ be some way of availing
ourselves of this energy more directly. Then, with the light obtained
from the medium, with the power derived from it, with every form of
energy obtained without effort, from the store forever inexhaustible,
humanity will advance with giant strides. The mere contemplation of
these magnificent possibilities expands our minds, strengthens our hopes
and fills our hearts with supreme delight.




CHAPTER XXVII.

EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL AND HIGH
FREQUENCY.[2]

  [2] Lecture delivered before the Institution of Electrical
      Engineers, London, February, 1892.


I cannot find words to express how deeply I feel the honor of addressing
some of the foremost thinkers of the present time, and so many able
scientific men, engineers and electricians, of the country greatest in
scientific achievements.

The results which I have the honor to present before such a gathering I
cannot call my own. There are among you not a few who can lay better
claim than myself on any feature of merit which this work may contain. I
need not mention many names which are world-known--names of those among
you who are recognized as the leaders in this enchanting science; but
one, at least, I must mention--a name which could not be omitted in a
demonstration of this kind. It is a name associated with the most
beautiful invention ever made: it is Crookes!

When I was at college, a good while ago, I read, in a translation (for
then I was not familiar with your magnificent language), the description
of his experiments on radiant matter. I read it only once in my
life--that time--yet every detail about that charming work I can
remember to this day. Few are the books, let me say, which can make such
an impression upon the mind of a student.

But if, on the present occasion, I mention this name as one of many your
Institution can boast of, it is because I have more than one reason to
do so. For what I have to tell you and to show you this evening
concerns, in a large measure, that same vague world which Professor
Crookes has so ably explored; and, more than this, when I trace back the
mental process which led me to these advances--which even by myself
cannot be considered trifling, since they are so appreciated by you--I
believe that their real origin, that which started me to work in this
direction, and brought me to them, after a long period of constant
thought, was that fascinating little book which I read many years ago.

And now that I have made a feeble effort to express my homage and
acknowledge my indebtedness to him and others among you, I will make a
second effort, which I hope you will not find so feeble as the first, to
entertain you.

Give me leave to introduce the subject in a few words.

A short time ago I had the honor to bring before our American Institute
of Electrical Engineers some results then arrived at by me in a novel
line of work. I need not assure you that the many evidences which I have
received that English scientific men and engineers were interested in
this work have been for me a great reward and encouragement. I will not
dwell upon the experiments already described, except with the view of
completing, or more clearly expressing, some ideas advanced by me
before, and also with the view of rendering the study here presented
self-contained, and my remarks on the subject of this evening's lecture
consistent.

This investigation, then, it goes without saying, deals with alternating
currents, and to be more precise, with alternating currents of high
potential and high frequency. Just in how much a very high frequency is
essential for the production of the results presented is a question
which, even with my present experience, would embarrass me to answer.
Some of the experiments may be performed with low frequencies; but very
high frequencies are desirable, not only on account of the many effects
secured by their use, but also as a convenient means of obtaining, in
the induction apparatus employed, the high potentials, which in their
turn are necessary to the demonstration of most of the experiments here
contemplated.

Of the various branches of electrical investigation, perhaps the most
interesting and the most immediately promising is that dealing with
alternating currents. The progress in this branch of applied science has
been so great in recent years that it justifies the most sanguine hopes.
Hardly have we become familiar with one fact, when novel experiences are
met and new avenues of research are opened. Even at this hour
possibilities not dreamed of before are, by the use of these currents,
partly realized. As in nature all is ebb and tide, all is wave motion,
so it seems that in all branches of industry alternating
currents--electric wave motion--will have the sway.

One reason, perhaps, why this branch of science is being so rapidly
developed is to be found in the interest which is attached to its
experimental study. We wind a simple ring of iron with coils; we
establish the connections to the generator, and with wonder and delight
we note the effects of strange forces which we bring into play, which
allow us to transform, to transmit and direct energy at will. We arrange
the circuits properly, and we see the mass of iron and wires behave as
though it were endowed with life, spinning a heavy armature, through
invisible connections, with great speed and power--with the energy
possibly conveyed from a great distance. We observe how the energy of an
alternating current traversing the wire manifests itself--not so much in
the wire as in the surrounding space--in the most surprising manner,
taking the forms of heat, light, mechanical energy, and, most surprising
of all, even chemical affinity. All these observations fascinate us, and
fill us with an intense desire to know more about the nature of these
phenomena. Each day we go to our work in the hope of discovering,--in
the hope that some one, no matter who, may find a solution of one of the
pending great problems,--and each succeeding day we return to our task
with renewed ardor; and even if we _are_ unsuccessful, our work has not
been in vain, for in these strivings, in these efforts, we have found
hours of untold pleasure, and we have directed our energies to the
benefit of mankind.

We may take--at random, if you choose--any of the many experiments which
may be performed with alternating currents; a few of which only, and by
no means the most striking, form the subject of this evening's
demonstration; they are all equally interesting, equally inciting to
thought.

Here is a simple glass tube from which the air has been partially
exhausted. I take hold of it; I bring my body in contact with a wire
conveying alternating currents of high potential, and the tube in my
hand is brilliantly lighted. In whatever position I may put it, wherever
I move it in space, as far as I can reach, its soft, pleasing light
persists with undiminished brightness.

Here is an exhausted bulb suspended from a single wire. Standing on an
insulated support, I grasp it, and a platinum button mounted in it is
brought to vivid incandescence.

Here, attached to a leading wire, is another bulb, which, as I touch its
metallic socket, is filled with magnificent colors of phosphorescent
light.

Here still another, which by my fingers' touch casts a shadow--the
Crookes shadow--of the stem inside of it.

Here, again, insulated as I stand on this platform, I bring my body in
contact with one of the terminals of the secondary of this induction
coil--with the end of a wire many miles long--and you see streams of
light break forth from its distant end, which is set in violent
vibration.

Here, once more, I attach these two plates of wire gauze to the
terminals of the coil; I set them a distance apart, and I set the coil
to work. You may see a small spark pass between the plates. I insert a
thick plate of one of the best dielectrics between them, and instead of
rendering altogether impossible, as we are used to expect, I _aid_ the
passage of the discharge, which, as I insert the plate, merely changes
in appearance and assumes the form of luminous streams.

Is there, I ask, can there be, a more interesting study than that of
alternating currents?

In all these investigations, in all these experiments, which are so
very, very interesting, for many years past--ever since the greatest
experimenter who lectured in this hall discovered its principle--we have
had a steady companion, an appliance familiar to every one, a plaything
once, a thing of momentous importance now--the induction coil. There is
no dearer appliance to the electrician. From the ablest among you, I
dare say, down to the inexperienced student, to your lecturer, we all
have passed many delightful hours in experimenting with the induction
coil. We have watched its play, and thought and pondered over the
beautiful phenomena which it disclosed to our ravished eyes. So well
known is this apparatus, so familiar are these phenomena to every one,
that my courage nearly fails me when I think that I have ventured to
address so able an audience, that I have ventured to entertain you with
that same old subject. Here in reality is the same apparatus, and here
are the same phenomena, only the apparatus is operated somewhat
differently, the phenomena are presented in a different aspect. Some of
the results we find as expected, others surprise us, but all captivate
our attention, for in scientific investigation each novel result
achieved may be the centre of a new departure, each novel fact learned
may lead to important developments.

Usually in operating an induction coil we have set up a vibration of
moderate frequency in the primary, either by means of an interrupter or
break, or by the use of an alternator. Earlier English investigators, to
mention only Spottiswoode and J. E. H. Gordon, have used a rapid break
in connection with the coil. Our knowledge and experience of to-day
enables us to see clearly why these coils under the conditions of the
test did not disclose any remarkable phenomena, and why able
experimenters failed to perceive many of the curious effects which have
since been observed.

In the experiments such as performed this evening, we operate the coil
either from a specially constructed alternator capable of giving many
thousands of reversals of current per second, or, by disruptively
discharging a condenser through the primary, we set up a vibration in
the secondary circuit of a frequency of many hundred thousand or
millions per second, if we so desire; and in using either of these means
we enter a field as yet unexplored.

It is impossible to pursue an investigation in any novel line without
finally making some interesting observation or learning some useful
fact. That this statement is applicable to the subject of this lecture
the many curious and unexpected phenomena which we observe afford a
convincing proof. By way of illustration, take for instance the most
obvious phenomena, those of the discharge of the induction coil.

Here is a coil which is operated by currents vibrating with extreme
rapidity, obtained by disruptively discharging a Leyden jar. It would
not surprise a student were the lecturer to say that the secondary of
this coil consists of a small length of comparatively stout wire; it
would not surprise him were the lecturer to state that, in spite of
this, the coil is capable of giving any potential which the best
insulation of the turns is able to withstand; but although he may be
prepared, and even be indifferent as to the anticipated result, yet the
aspect of the discharge of the coil will surprise and interest him.
Every one is familiar with the discharge of an ordinary coil; it need
not be reproduced here. But, by way of contrast, here is a form of
discharge of a coil, the primary current of which is vibrating several
hundred thousand times per second. The discharge of an ordinary coil
appears as a simple line or band of light. The discharge of this coil
appears in the form of powerful brushes and luminous streams issuing
from all points of the two straight wires attached to the terminals of
the secondary. (Fig. 130.)

[Illustration: FIG. 130.]

[Illustration: FIG. 131.]

Now compare this phenomenon which you have just witnessed with the
discharge of a Holtz or Wimshurst machine--that other interesting
appliance so dear to the experimenter. What a difference there is
between these phenomena! And yet, had I made the necessary
arrangements--which could have been made easily, were it not that they
would interfere with other experiments--I could have produced with this
coil sparks which, had I the coil hidden from your view and only two
knobs exposed, even the keenest observer among you would find it
difficult, if not impossible, to distinguish from those of an influence
or friction machine. This may be done in many ways--for instance, by
operating the induction coil which charges the condenser from an
alternating-current machine of very low frequency, and preferably
adjusting the discharge circuit so that there are no oscillations set up
in it. We then obtain in the secondary circuit, if the knobs are of the
required size and properly set, a more or less rapid succession of
sparks of great intensity and small quantity, which possess the same
brilliancy, and are accompanied by the same sharp crackling sound, as
those obtained from a friction or influence machine.

Another way is to pass through two primary circuits, having a common
secondary, two currents of a slightly different period, which produce in
the secondary circuit sparks occurring at comparatively long intervals.
But, even with the means at hand this evening, I may succeed in
imitating the spark of a Holtz machine. For this purpose I establish
between the terminals of the coil which charges the condenser a long,
unsteady arc, which is periodically interrupted by the upward current of
air produced by it. To increase the current of air I place on each side
of the arc, and close to it, a large plate of mica. The condenser
charged from this coil discharges into the primary circuit of a second
coil through a small air gap, which is necessary to produce a sudden
rush of current through the primary. The scheme of connections in the
present experiment is indicated in Fig. 131.

G is an ordinarily constructed alternator, supplying the primary P of an
induction coil, the secondary S of which charges the condensers or jars
C C. The terminals of the secondary are connected to the inside coatings
of the jars, the outer coatings being connected to the ends of the
primary _p p_ of a second induction coil. This primary _p p_ has a small
air gap _a b_.

The secondary _s_ of this coil is provided with knobs or spheres K K of
the proper size and set at a distance suitable for the experiment.

A long arc is established between the terminals A B of the first
induction coil. M M are the mica plates.

Each time the arc is broken between A and B the jars are quickly charged
and discharged through the primary _p p_, producing a snapping spark
between the knobs K K. Upon the arc forming between A and B the
potential falls, and the jars cannot be charged to such high potential
as to break through the air gap _a b_ until the arc is again broken by
the draught.

In this manner sudden impulses, at long intervals, are produced in the
primary _p p_, which in the secondary _s_ give a corresponding number of
impulses of great intensity. If the secondary knobs or spheres, K K, are
of the proper size, the sparks show much resemblance to those of a Holtz
machine.

But these two effects, which to the eye appear so very different, are
only two of the many discharge phenomena. We only need to change the
conditions of the test, and again we make other observations of
interest.

When, instead of operating the induction coil as in the last two
experiments, we operate it from a high frequency alternator, as in the
next experiment, a systematic study of the phenomena is rendered much
more easy. In such case, in varying the strength and frequency of the
currents through the primary, we may observe five distinct forms of
discharge, which I have described in my former paper on the subject
before the American Institute of Electrical Engineers, May 20, 1891.

It would take too much time, and it would lead us too far from the
subject presented this evening, to reproduce all these forms, but it
seems to me desirable to show you one of them. It is a brush discharge,
which is interesting in more than one respect. Viewed from a near
position it resembles much a jet of gas escaping under great pressure.
We know that the phenomenon is due to the agitation of the molecules
near the terminal, and we anticipate that some heat must be developed by
the impact of the molecules against the terminal or against each other.
Indeed, we find that the brush is hot, and only a little thought leads
us to the conclusion that, could we but reach sufficiently high
frequencies, we could produce a brush which would give intense light and
heat, and which would resemble in every particular an ordinary flame,
save, perhaps, that both phenomena might not be due to the same
agent--save, perhaps, that chemical affinity might not be _electrical_
in its nature.

As the production of heat and light is here due to the impact of the
molecules, or atoms of air, or something else besides, and, as we can
augment the energy simply by raising the potential, we might, even with
frequencies obtained from a dynamo machine, intensify the action to such
a degree as to bring the terminal to melting heat. But with such low
frequencies we would have to deal always with something of the nature of
an electric current. If I approach a conducting object to the brush, a
thin little spark passes, yet, even with the frequencies used this
evening, the tendency to spark is not very great. So, for instance, if I
hold a metallic sphere at some distance above the terminal, you may see
the whole space between the terminal and sphere illuminated by the
streams without the spark passing; and with the much higher frequencies
obtainable by the disruptive discharge of a condenser, were it not for
the sudden impulses, which are comparatively few in number, sparking
would not occur even at very small distances. However, with incomparably
higher frequencies, which we may yet find means to produce efficiently,
and provided that electric impulses of such high frequencies could be
transmitted through a conductor, the electrical characteristics of the
brush discharge would completely vanish--no spark would pass, no shock
would be felt--yet we would still have to deal with an _electric_
phenomenon, but in the broad, modern interpretation of the word. In my
first paper, before referred to, I have pointed out the curious
properties of the brush, and described the best manner of producing it,
but I have thought it worth while to endeavor to express myself more
clearly in regard to this phenomenon, because of its absorbing interest.

When a coil is operated with currents of very high frequency, beautiful
brush effects may be produced, even if the coil be of comparatively
small dimensions. The experimenter may vary them in many ways, and, if
it were for nothing else, they afford a pleasing sight. What adds to
their interest is that they may be produced with one single terminal as
well as with two--in fact, often better with one than with two.

But of all the discharge phenomena observed, the most pleasing to the
eye, and the most instructive, are those observed with a coil which is
operated by means of the disruptive discharge of a condenser. The power
of the brushes, the abundance of the sparks, when the conditions are
patiently adjusted, is often amazing. With even a very small coil, if it
be so well insulated as to stand a difference of potential of several
thousand volts per turn, the sparks may be so abundant that the whole
coil may appear a complete mass of fire.

Curiously enough the sparks, when the terminals of the coil are set at a
considerable distance, seem to dart in every possible direction as
though the terminals were perfectly independent of each other. As the
sparks would soon destroy the insulation, it is necessary to prevent
them. This is best done by immersing the coil in a good liquid
insulator, such as boiled-out oil. Immersion in a liquid may be
considered almost an absolute necessity for the continued and successful
working of such a coil.

It is, of course, out of the question, in an experimental lecture, with
only a few minutes at disposal for the performance of each experiment,
to show these discharge phenomena to advantage, as, to produce each
phenomenon at its best, a very careful adjustment is required. But even
if imperfectly produced, as they are likely to be this evening, they are
sufficiently striking to interest an intelligent audience.

Before showing some of these curious effects I must, for the sake of
completeness, give a short description of the coil and other apparatus
used in the experiments with the disruptive discharge this evening.

[Illustration: FIG. 132.]

It is contained in a box B (Fig. 132) of thick boards of hard wood,
covered on the outside with a zinc sheet Z, which is carefully soldered
all around. It might be advisable, in a strictly scientific
investigation, when accuracy is of great importance, to do away with the
metal cover, as it might introduce many errors, principally on account
of its complex action upon the coil, as a condenser of very small
capacity and as an electrostatic and electromagnetic screen. When the
coil is used for such experiments as are here contemplated, the
employment of the metal cover offers some practical advantages, but
these are not of sufficient importance to be dwelt upon.

The coil should be placed symmetrically to the metal cover, and the
space between should, of course, not be too small, certainly not less
than, say, five centimetres, but much more if possible; especially the
two sides of the zinc box, which are at right angles to the axis of the
coil, should be sufficiently remote from the latter, as otherwise they
might impair its action and be a source of loss.

The coil consists of two spools of hard rubber R R, held apart at a
distance of 10 centimetres by bolts C and nuts _n_, likewise of hard
rubber. Each spool comprises a tube T of approximately 8 centimetres
inside diameter, and 3 millimetres thick, upon which are screwed two
flanges F F, 24 centimetres square, the space between the flanges being
about 3 centimetres. The secondary, S S, of the best gutta
percha-covered wire, has 26 layers, 10 turns in each, giving for each
half a total of 260 turns. The two halves are wound oppositely and
connected in series, the connection between both being made over the
primary. This disposition, besides being convenient, has the advantage
that when the coil is well balanced--that is, when both of its
terminals T_{1}, T_{1}, are connected to bodies or devices of equal
capacity--there is not much danger of breaking through to the primary,
and the insulation between the primary and the secondary need not be
thick. In using the coil it is advisable to attach to _both_ terminals
devices of nearly equal capacity, as, when the capacity of the terminals
is not equal, sparks will be apt to pass to the primary. To avoid this,
the middle point of the secondary may be connected to the primary, but
this is not always practicable.

The primary P P is wound in two parts, and oppositely, upon a wooden
spool w, and the four ends are led out of the oil through hard rubber
tubes _t t_. The ends of the secondary T_{1} T_{1}, are also led out of
the oil through rubber tubes t_{1} t_{1} of great thickness. The
primary and secondary layers are insulated by cotton cloth, the
thickness of the insulation, of course, bearing some proportion to the
difference of potential between the turns of the different layers. Each
half of the primary has four layers, 24 turns in each, this giving a
total of 96 turns. When both the parts are connected in series, this
gives a ratio of conversion of about 1:2.7, and with the primaries in
multiple, 1:5.4; but in operating with very rapidly alternating currents
this ratio does not convey even an approximate idea of the ratio of the
E. M. F's. in the primary and secondary circuits. The coil is held in
position in the oil on wooden supports, there being about 5 centimetres
thickness of oil all round. Where the oil is not specially needed, the
space is filled with pieces of wood, and for this purpose principally
the wooden box B surrounding the whole is used.

The construction here shown is, of course, not the best on general
principles, but I believe it is a good and convenient one for the
production of effects in which an excessive potential and a very small
current are needed.

In connection with the coil I use either the ordinary form of discharger
or a modified form. In the former I have introduced two changes which
secure some advantages, and which are obvious. If they are mentioned, it
is only in the hope that some experimenter may find them of use.

One of the changes is that the adjustable knobs A and B (Fig. 133), of
the discharger are held in jaws of brass, J J, by spring pressure, this
allowing of turning them successively into different positions, and so
doing away with the tedious process of frequent polishing up.

[Illustration: FIG. 133.]

The other change consists in the employment of a strong electromagnet
N S, which is placed with its axis at right angles to the line joining
the knobs A and B, and produces a strong magnetic field between them.
The pole pieces of the magnet are movable and properly formed so as to
protrude between the brass knobs, in order to make the field as intense
as possible; but to prevent the discharge from jumping to the magnet the
pole pieces are protected by a layer of mica, M M, of sufficient
thickness; s_{1} s_{1} and s_{2} s_{2} are screws for fastening the
wires. On each side one of the screws is for large and the other for
small wires. L L are screws for fixing in position the rods R R, which
support the knobs.

In another arrangement with the magnet I take the discharge between the
rounded pole pieces themselves, which in such case are insulated and
preferably provided with polished brass caps.

The employment of an intense magnetic field is of advantage principally
when the induction coil or transformer which charges the condenser is
operated by currents of very low frequency. In such a case the number of
the fundamental discharges between the knobs may be so small as to
render the currents produced in the secondary unsuitable for many
experiments. The intense magnetic field then serves to blow out the arc
between the knobs as soon as it is formed, and the fundamental
discharges occur in quicker succession.

[Illustration: FIG. 134.]

Instead of the magnet, a draught or blast of air may be employed with
some advantage. In this case the arc is preferably established between
the knobs A B, in Fig. 131 (the knobs _a b_ being generally joined, or
entirely done away with), as in this disposition the arc is long and
unsteady, and is easily affected by the draught.

When a magnet is employed to break the arc, it is better to choose the
connection indicated diagrammatically in Fig. 134, as in this case the
currents forming the arc are much more powerful, and the magnetic field
exercises a greater influence. The use of the magnet permits, however,
of the arc being replaced by a vacuum tube, but I have encountered great
difficulties in working with an exhausted tube.

The other form of discharger used in these and similar experiments is
indicated in Figs. 135 and 136. It consists of a number of brass pieces
_c c_ (Fig. 135), each of which comprises a spherical middle portion _m_
with an extension _e_ below--which is merely used to fasten the piece in
a lathe when polishing up the discharging surface--and a column above,
which consists of a knurled flange _f_ surmounted by a threaded stem _l_
carrying a nut _n_, by means of which a wire is fastened to the column.
The flange _f_ conveniently serves for holding the brass piece when
fastening the wire, and also for turning it in any position when it
becomes necessary to present a fresh discharging surface. Two stout
strips of hard rubber R R, with planed grooves _g g_ (Fig. 136) to fit
the middle portion of the pieces _c c_, serve to clamp the latter and
hold them firmly in position by means of two bolts C C (of which only
one is shown) passing through the ends of the strips.

[Illustration: FIG. 135.]

[Illustration: FIG. 136.]

In the use of this kind of discharger I have found three principal
advantages over the ordinary form. First, the dielectric strength of a
given total width of air space is greater when a great many small air
gaps are used instead of one, which permits of working with a smaller
length of air gap, and that means smaller loss and less deterioration of
the metal; secondly, by reason of splitting the arc up into smaller
arcs, the polished surfaces are made to last much longer; and, thirdly,
the apparatus affords some gauge in the experiments. I usually set the
pieces by putting between them sheets of uniform thickness at a certain
very small distance which is known from the experiments of Sir William
Thomson to require a certain electromotive force to be bridged by the
spark.

It should, of course, be remembered that the sparking distance is much
diminished as the frequency is increased. By taking any number of spaces
the experimenter has a rough idea of the electromotive force, and he
finds it easier to repeat an experiment, as he has not the trouble of
setting the knobs again and again. With this kind of discharger I have
been able to maintain an oscillating motion without any spark being
visible with the naked eye between the knobs, and they would not show a
very appreciable rise in temperature. This form of discharge also lends
itself to many arrangements of condensers and circuits which are often
very convenient and time-saving. I have used it preferably in a
disposition similar to that indicated in Fig. 131, when the currents
forming the arc are small.

I may here mention that I have also used dischargers with single or
multiple air gaps, in which the discharge surfaces were rotated with
great speed. No particular advantage was, however, gained by this
method, except in cases where the currents from the condenser were large
and the keeping cool of the surfaces was necessary, and in cases when,
the discharge not being oscillating of itself, the arc as soon as
established was broken by the air current, thus starting the vibration
at intervals in rapid succession. I have also used mechanical
interrupters in many ways. To avoid the difficulties with frictional
contacts, the preferred plan adopted was to establish the arc and rotate
through it at great speed a rim of mica provided with many holes and
fastened to a steel plate. It is understood, of course, that the
employment of a magnet, air current, or other interrupter, produces no
effect worth noticing, unless the self-induction, capacity and
resistance are so related that there are oscillations set up upon each
interruption.

I will now endeavor to show you some of the most noteworthy of these
discharge phenomena.

I have stretched across the room two ordinary cotton covered wires, each
about seven metres in length. They are supported on insulating cords at
a distance of about thirty centimetres. I attach now to each of the
terminals of the coil one of the wires, and set the coil in action.
Upon turning the lights off in the room you see the wires strongly
illuminated by the streams issuing abundantly from their whole surface
in spite of the cotton covering, which may even be very thick. When the
experiment is performed under good conditions, the light from the wires
is sufficiently intense to allow distinguishing the objects in a room.
To produce the best result it is, of course, necessary to adjust
carefully the capacity of the jars, the arc between the knobs and the
length of the wires. My experience is that calculation of the length of
the wires leads, in such case, to no result whatever. The experimenter
will do best to take the wires at the start very long, and then adjust
by cutting off first long pieces, and then smaller and smaller ones as
he approaches the right length.

A convenient way is to use an oil condenser of very small capacity,
consisting of two small adjustable metal plates, in connection with this
and similar experiments. In such case I take wires rather short and at
the beginning set the condenser plates at maximum distance. If the
streams from the wires increase by approach of the plates, the length of
the wires is about right; if they diminish, the wires are too long for
that frequency and potential. When a condenser is used in connection
with experiments with such a coil, it should be an oil condenser by all
means, as in using an air condenser considerable energy might be wasted.
The wires leading to the plates in the oil should be very thin, heavily
coated with some insulating compound, and provided with a conducting
covering--this preferably extending under the surface of the oil. The
conducting cover should not be too near the terminals, or ends, of the
wire, as a spark would be apt to jump from the wire to it. The
conducting coating is used to diminish the air losses, in virtue of its
action as an electrostatic screen. As to the size of the vessel
containing the oil, and the size of the plates, the experimenter gains
at once an idea from a rough trial. The size of the plates _in oil_ is,
however, calculable, as the dielectric losses are very small.

In the preceding experiment it is of considerable interest to know what
relation the quantity of the light emitted bears to the frequency and
potential of the electric impulses. My opinion is that the heat as well
as light effects produced should be proportionate, under otherwise equal
conditions of test, to the product of frequency and square of potential,
but the experimental verification of the law, whatever it may be, would
be exceedingly difficult. One thing is certain, at any rate, and that
is, that in augmenting the potential and frequency we rapidly intensify
the streams; and, though it may be very sanguine, it is surely not
altogether hopeless to expect that we may succeed in producing a
practical illuminant on these lines. We would then be simply using
burners or flames, in which there would be no chemical process, no
consumption of material, but merely a transfer of energy, and which
would, in all probability, emit more light and less heat than ordinary
flames.

[Illustration: FIG. 137.]

The luminous intensity of the streams is, of course, considerably
increased when they are focused upon a small surface. This may be shown
by the following experiment:

I attach to one of the terminals of the coil a wire _w_ (Fig. 137), bent
in a circle of about 30 centimetres in diameter, and to the other
terminal I fasten a small brass sphere _s_, the surface of the wire
being preferably equal to the surface of the sphere, and the centre of
the latter being in a line at right angles to the plane of the wire
circle and passing through its centre. When the discharge is established
under proper conditions, a luminous hollow cone is formed, and in the
dark one-half of the brass sphere is strongly illuminated, as shown in
the cut.

By some artifice or other it is easy to concentrate the streams upon
small surfaces and to produce very strong light effects. Two thin wires
may thus be rendered intensely luminous.

In order to intensify the streams the wires should be very thin and
short; but as in this case their capacity would be generally too small
for the coil--at least for such a one as the present--it is necessary to
augment the capacity to the required value, while, at the same time, the
surface of the wires remains very small. This may be done in many ways.

[Illustration: FIG. 138.]

Here, for instance, I have two plates, R R, of hard rubber (Fig. 138),
upon which I have glued two very thin wires _w w_, so as to form a name.
The wires may be bare or covered with the best insulation--it is
immaterial for the success of the experiment. Well insulated wires, if
anything, are preferable. On the back of each plate, indicated by the
shaded portion, is a tinfoil coating _t t_. The plates are placed in
line at a sufficient distance to prevent a spark passing from one wire
to the other. The two tinfoil coatings I have joined by a conductor C,
and the two wires I presently connect to the terminals of the coil. It
is now easy, by varying the strength and frequency of the currents
through the primary, to find a point at which the capacity of the system
is best suited to the conditions, and the wires become so strongly
luminous that, when the light in the room is turned off the name formed
by them appears in brilliant letters.

It is perhaps preferable to perform this experiment with a coil operated
from an alternator of high frequency, as then, owing to the harmonic
rise and fall, the streams are very uniform, though they are less
abundant than when produced with such a coil as the present one. This
experiment, however, may be performed with low frequencies, but much
less satisfactorily.

[Illustration: FIG. 139.]

When two wires, attached to the terminals of the coil, are set at the
proper distance, the streams between them may be so intense as to
produce a continuous luminous sheet. To show this phenomenon I have here
two circles, C and _c_ (Fig. 139), of rather stout wire, one being about
80 centimetres and the other 30 centimetres in diameter. To each of the
terminals of the coil I attach one of the circles. The supporting wires
are so bent that the circles may be placed in the same plane, coinciding
as nearly as possible. When the light in the room is turned off and the
coil set to work, you see the whole space between the wires uniformly
filled with streams, forming a luminous disc, which could be seen from a
considerable distance, such is the intensity of the streams. The outer
circle could have been much larger than the present one; in fact, with
this coil I have used much larger circles, and I have been able to
produce a strongly luminous sheet, covering an area of more than one
square metre, which is a remarkable effect with this very small coil. To
avoid uncertainty, the circle has been taken smaller, and the area is
now about 0.43 square metre.

The frequency of the vibration, and the quickness of succession of the
sparks between the knobs, affect to a marked degree the appearance of
the streams. When the frequency is very low, the air gives way in more
or less the same manner, as by a steady difference of potential, and the
streams consist of distinct threads, generally mingled with thin sparks,
which probably correspond to the successive discharges occurring between
the knobs. But when the frequency is extremely high, and the arc of the
discharge produces a very _loud_ and _smooth_ sound--showing both that
oscillation takes place and that the sparks succeed each other with
great rapidity--then the luminous streams formed are perfectly uniform.
To reach this result very small coils and jars of small capacity should
be used. I take two tubes of thick Bohemian glass, about 5 centimetres
in diameter and 20 centimetres long. In each of the tubes I slip a
primary of very thick copper wire. On the top of each tube I wind a
secondary of much thinner gutta-percha covered wire. The two secondaries
I connect in series, the primaries preferably in multiple arc. The tubes
are then placed in a large glass vessel, at a distance of 10 to 15
centimetres from each other, on insulating supports, and the vessel is
filled with boiled-out oil, the oil reaching about an inch above the
tubes. The free ends of the secondary are lifted out of the coil and
placed parallel to each other at a distance of about ten centimetres.
The ends which are scraped should be dipped in the oil. Two four-pint
jars joined in series may be used to discharge through the primary. When
the necessary adjustments in the length and distance of the wires above
the oil and in the arc of discharge are made, a luminous sheet is
produced between the wires which is perfectly smooth and textureless,
like the ordinary discharge through a moderately exhausted tube.

I have purposely dwelt upon this apparently insignificant experiment. In
trials of this kind the experimenter arrives at the startling conclusion
that, to pass ordinary luminous discharges through gases, no particular
degree of exhaustion is needed, but that the gas may be at ordinary or
even greater pressure. To accomplish this, a very high frequency is
essential; a high potential is likewise required, but this is merely an
incidental necessity. These experiments teach us that, in endeavoring to
discover novel methods of producing light by the agitation of atoms, or
molecules, of a gas, we need not limit our research to the vacuum tube,
but may look forward quite seriously to the possibility of obtaining the
light effects without the use of any vessel whatever, with air at
ordinary pressure.

Such discharges of very high frequency, which render luminous the air at
ordinary pressures, we have probably occasion often to witness in
Nature. I have no doubt that if, as many believe, the aurora borealis is
produced by sudden cosmic disturbances, such as eruptions at the sun's
surface, which set the electrostatic charge of the earth in an extremely
rapid vibration, the red glow observed is not confined to the upper
rarefied strata of the air, but the discharge traverses, by reason of
its very high frequency, also the dense atmosphere in the form of a
_glow_, such as we ordinarily produce in a slightly exhausted tube. If
the frequency were very low, or even more so, if the charge were not at
all vibrating, the dense air would break down as in a lightning
discharge. Indications of such breaking down of the lower dense strata
of the air have been repeatedly observed at the occurrence of this
marvelous phenomenon; but if it does occur, it can only be attributed to
the fundamental disturbances, which are few in number, for the vibration
produced by them would be far too rapid to allow a disruptive break. It
is the original and irregular impulses which affect the instruments; the
superimposed vibrations probably pass unnoticed.

When an ordinary low frequency discharge is passed through moderately
rarefied air, the air assumes a purplish hue. If by some means or other
we increase the intensity of the molecular, or atomic, vibration, the
gas changes to a white color. A similar change occurs at ordinary
pressures with electric impulses of very high frequency. If the
molecules of the air around a wire are moderately agitated, the brush
formed is reddish or violet; if the vibration is rendered sufficiently
intense, the streams become white. We may accomplish this in various
ways. In the experiment before shown with the two wires across the room,
I have endeavored to secure the result by pushing to a high value both
the frequency and potential; in the experiment with the thin wires glued
on the rubber plate I have concentrated the action upon a very small
surface--in other words, I have worked with a great electric density.

[Illustration: FIG. 140.]

A most curious form of discharge is observed with such a coil when the
frequency and potential are pushed to the extreme limit. To perform the
experiment, every part of the coil should be heavily insulated, and only
two small spheres--or, better still, two sharp-edged metal discs (_d d_,
Fig. 140) of no more than a few centimetres in diameter--should be
exposed to the air. The coil here used is immersed in oil, and the ends
of the secondary reaching out of the oil are covered with an air-tight
cover of hard rubber of great thickness. All cracks, if there are any,
should be carefully stopped up, so that the brush discharge cannot form
anywhere except on the small spheres or plates which are exposed to the
air. In this case, since there are no large plates or other bodies of
capacity attached to the terminals, the coil is capable of an extremely
rapid vibration. The potential may be raised by increasing, as far as
the experimenter judges proper, the rate of change of the primary
current. With a coil not widely differing from the present, it is best
to connect the two primaries in multiple arc; but if the secondary
should have a much greater number of turns the primaries should
preferably be used in series, as otherwise the vibration might be too
fast for the secondary. It occurs under these conditions that misty
white streams break forth from the edges of the discs and spread out
phantom-like into space. With this coil, when fairly well produced, they
are about 25 to 30 centimetres long. When the hand is held against them
no sensation is produced, and a spark, causing a shock, jumps from the
terminal only upon the hand being brought much nearer. If the
oscillation of the primary current is rendered intermittent by some
means or other, there is a corresponding throbbing of the streams, and
now the hand or other conducting object may be brought in still greater
proximity to the terminal without a spark being caused to jump.

Among the many beautiful phenomena which may be produced with such a
coil, I have here selected only those which appear to possess some
features of novelty, and lead us to some conclusions of interest. One
will not find it at all difficult to produce in the laboratory, by means
of it, many other phenomena which appeal to the eye even more than these
here shown, but present no particular feature of novelty.

Early experimenters describe the display of sparks produced by an
ordinary large induction coil upon an insulating plate separating the
terminals. Quite recently Siemens performed some experiments in which
fine effects were obtained, which were seen by many with interest. No
doubt large coils, even if operated with currents of low frequencies,
are capable of producing beautiful effects. But the largest coil ever
made could not, by far, equal the magnificent display of streams and
sparks obtained from such a disruptive discharge coil when properly
adjusted. To give an idea, a coil such as the present one will cover
easily a plate of one metre in diameter completely with the streams. The
best way to perform such experiments is to take a very thin rubber or a
glass plate and glue on one side of it a narrow ring of tinfoil of very
large diameter, and on the other a circular washer, the centre of the
latter coinciding with that of the ring, and the surfaces of both being
preferably equal, so as to keep the coil well balanced. The washer and
ring should be connected to the terminals by heavily insulated thin
wires. It is easy in observing the effect of the capacity to produce a
sheet of uniform streams, or a fine network of thin silvery threads, or
a mass of loud brilliant sparks, which completely cover the plate.

Since I have advanced the idea of the conversion by means of the
disruptive discharge, in my paper before the American Institute of
Electrical Engineers at the beginning of the past year, the interest
excited in it has been considerable. It affords us a means for producing
any potentials by the aid of inexpensive coils operated from ordinary
systems of distribution, and--what is perhaps more appreciated--it
enables us to convert currents of any frequency into currents of any
other lower or higher frequency. But its chief value will perhaps be
found in the help which it will afford us in the investigations of the
phenomena of phosphorescence, which a disruptive discharge coil is
capable of exciting in innumerable cases where ordinary coils, even the
largest, would utterly fail.

Considering its probable uses for many practical purposes, and its
possible introduction into laboratories for scientific research, a few
additional remarks as to the construction of such a coil will perhaps
not be found superfluous.

It is, of course, absolutely necessary to employ in such a coil wires
provided with the best insulation.

Good coils may be produced by employing wires covered with several
layers of cotton, boiling the coil a long time in pure wax, and cooling
under moderate pressure. The advantage of such a coil is that it can be
easily handled, but it cannot probably give as satisfactory results as a
coil immersed in pure oil. Besides, it seems that the presence of a
large body of wax affects the coil disadvantageously, whereas this does
not seem to be the case with oil. Perhaps it is because the dielectric
losses in the liquid are smaller.

I have tried at first silk and cotton covered wires with oil immersions,
but I have been gradually led to use gutta-percha covered wires, which
proved most satisfactory. Gutta-percha insulation adds, of course, to
the capacity of the coil, and this, especially if the coil be large, is
a great disadvantage when extreme frequencies are desired; but, on the
other hand, gutta-percha will withstand much more than an equal
thickness of oil, and this advantage should be secured at any price.
Once the coil has been immersed, it should never be taken out of the oil
for more than a few hours, else the gutta-percha will crack up and the
coil will not be worth half as much as before. Gutta-percha is probably
slowly attacked by the oil, but after an immersion of eight to nine
months I have found no ill effects.

I have obtained two kinds of gutta-percha wire known in commerce: in one
the insulation sticks tightly to the metal, in the other it does not.
Unless a special method is followed to expel all air, it is much safer
to use the first kind. I wind the coil within an oil tank so that all
interstices are filled up with the oil. Between the layers I use cloth
boiled out thoroughly in oil, calculating the thickness according to the
difference of potential between the turns. There seems not to be a very
great difference whatever kind of oil is used; I use paraffine or
linseed oil.

To exclude more perfectly the air, an excellent way to proceed, and
easily practicable with small coils, is the following: Construct a box
of hardwood of very thick boards which have been for a long time boiled
in oil. The boards should be so joined as to safely withstand the
external air pressure. The coil being placed and fastened in position
within the box, the latter is closed with a strong lid, and covered with
closely fitting metal sheets, the joints of which are soldered very
carefully. On the top two small holes are drilled, passing through the
metal sheet and the wood, and in these holes two small glass tubes are
inserted and the joints made air-tight. One of the tubes is connected to
a vacuum pump, and the other with a vessel containing a sufficient
quantity of boiled-out oil. The latter tube has a very small hole at the
bottom, and is provided with a stopcock. When a fairly good vacuum has
been obtained, the stopcock is opened and the oil slowly fed in.
Proceeding in this manner, it is impossible that any big bubbles, which
are the principal danger, should remain between the turns. The air is
most completely excluded, probably better than by boiling out, which,
however, when gutta-percha coated wires are used, is not practicable.

For the primaries I use ordinary line wire with a thick cotton coating.
Strands of very thin insulated wires properly interlaced would, of
course, be the best to employ for the primaries, but they are not to be
had.

In an experimental coil the size of the wires is not of great
importance. In the coil here used the primary is No. 12 and the
secondary No. 24 Brown & Sharpe gauge wire; but the sections may be
varied considerably. It would only imply different adjustments; the
results aimed at would not be materially affected.

I have dwelt at some length upon the various forms of brush discharge
because, in studying them, we not only observe phenomena which please
our eye, but also afford us food for thought, and lead us to conclusions
of practical importance. In the use of alternating currents of very high
tension, too much precaution cannot be taken to prevent the brush
discharge. In a main conveying such currents, in an induction coil or
transformer, or in a condenser, the brush discharge is a source of great
danger to the insulation. In a condenser, especially, the gaseous matter
must be most carefully expelled, for in it the charged surfaces are
near each other, and if the potentials are high, just as sure as a
weight will fall if let go, so the insulation will give way if a single
gaseous bubble of some size be present, whereas, if all gaseous matter
were carefully excluded, the condenser would safely withstand a much
higher difference of potential. A main conveying alternating currents of
very high tension may be injured merely by a blow hole or small crack in
the insulation, the more so as a blowhole is apt to contain gas at low
pressure; and as it appears almost impossible to completely obviate such
little imperfections, I am led to believe that in our future
distribution of electrical energy by currents of very high tension,
liquid insulation will be used. The cost is a great drawback, but if we
employ an oil as an insulator the distribution of electrical energy with
something like 100,000 volts, and even more, becomes, at least with
higher frequencies, so easy that it could be hardly called an
engineering feat. With oil insulation and alternate current motors,
transmissions of power can be affected with safety and upon an
industrial basis at distances of as much as a thousand miles.

A peculiar property of oils, and liquid insulation in general, when
subjected to rapidly changing electric stresses, is to disperse any
gaseous bubbles which may be present, and diffuse them through its mass,
generally long before any injurious break can occur. This feature may be
easily observed with an ordinary induction coil by taking the primary
out, plugging up the end of the tube upon which the secondary is wound,
and filling it with some fairly transparent insulator, such as paraffine
oil. A primary of a diameter something like six millimetres smaller than
the inside of the tube may be inserted in the oil. When the coil is set
to work one may see, looking from the top through the oil, many luminous
points--air bubbles which are caught by inserting the primary, and which
are rendered luminous in consequence of the violent bombardment. The
occluded air, by its impact against the oil, heats it; the oil begins to
circulate, carrying some of the air along with it, until the bubbles are
dispersed and the luminous points disappear. In this manner, unless
large bubbles are occluded in such way that circulation is rendered
impossible, a damaging break is averted, the only effect being a
moderate warming up of the oil. If, instead of the liquid, a solid
insulation, no matter how thick, were used, a breaking through and
injury of the apparatus would be inevitable.

The exclusion of gaseous matter from any apparatus in which the
dielectric is subjected to more or less rapidly changing electric forces
is, however, not only desirable in order to avoid a possible injury of
the apparatus, but also on account of economy. In a condenser, for
instance, as long as only a solid or only a liquid dielectric is used,
the loss is small; but if a gas under ordinary or small pressure be
present the loss may be very great. Whatever the nature of the force
acting in the dielectric may be, it seems that in a solid or liquid the
molecular displacement produced by the force is small: hence the product
of force and displacement is insignificant, unless the force be very
great; but in a gas the displacement, and therefore this product, is
considerable; the molecules are free to move, they reach high speeds,
and the energy of their impact is lost in heat or otherwise. If the gas
be strongly compressed, the displacement due to the force is made
smaller, and the losses are reduced.

In most of the succeeding experiments I prefer, chiefly on account of
the regular and positive action, to employ the alternator before
referred to. This is one of the several machines constructed by me for
the purpose of these investigations. It has 384 pole projections, and is
capable of giving currents of a frequency of about 10,000 per second.
This machine has been illustrated and briefly described in my first
paper before the American Institute of Electrical Engineers, May 20th,
1891, to which I have already referred. A more detailed description,
sufficient to enable any engineer to build a similar machine, will be
found in several electrical journals of that period.

The induction coils operated from the machine are rather small,
containing from 5,000 to 15,000 turns in the secondary. They are
immersed in boiled-out linseed oil, contained in wooden boxes covered
with zinc sheet.

I have found it advantageous to reverse the usual position of the wires,
and to wind, in these coils, the primaries on the top; thus allowing the
use of a much larger primary, which, of course, reduces the danger of
overheating and increases the output of the coil. I make the primary on
each side at least one centimetre shorter than the secondary, to prevent
the breaking through on the ends, which would surely occur unless the
insulation on the top of the secondary be very thick, and this, of
course, would be disadvantageous.

When the primary is made movable, which is necessary in some
experiments, and many times convenient for the purposes of adjustment, I
cover the secondary with wax, and turn it off in a lathe to a diameter
slightly smaller than the inside of the primary coil. The latter I
provide with a handle reaching out of the oil, which serves to shift it
in any position along the secondary.

I will now venture to make, in regard to the general manipulation of
induction coils, a few observations bearing upon points which have not
been fully appreciated in earlier experiments with such coils, and are
even now often overlooked.

The secondary of the coil possesses usually such a high self-induction
that the current through the wire is inappreciable, and may be so even
when the terminals are joined by a conductor of small resistance. If
capacity is added to the terminals, the self-induction is counteracted,
and a stronger current is made to flow through the secondary, though its
terminals are insulated from each other. To one entirely unacquainted
with the properties of alternating currents nothing will look more
puzzling. This feature was illustrated in the experiment performed at
the beginning with the top plates of wire gauze attached to the
terminals and the rubber plate. When the plates of wire gauze were close
together, and a small arc passed between them, the arc _prevented_ a
strong current from passing through the secondary, because it did away
with the capacity on the terminals; when the rubber plate was inserted
between, the capacity of the condenser formed counteracted the
self-induction of the secondary, a stronger current passed now, the coil
performed more work, and the discharge was by far more powerful.

The first thing, then, in operating the induction coil is to combine
capacity with the secondary to overcome the self-induction. If the
frequencies and potentials are very high, gaseous matter should be
carefully kept away from the charged surfaces. If Leyden jars are used,
they should be immersed in oil, as otherwise considerable dissipation
may occur if the jars are greatly strained. When high frequencies are
used, it is of equal importance to combine a condenser with the primary.
One may use a condenser connected to the ends of the primary or to the
terminals of the alternator, but the latter is not to be recommended, as
the machine might be injured. The best way is undoubtedly to use the
condenser in series with the primary and with the alternator, and to
adjust its capacity so as to annul the self-induction of both the
latter. The condenser should be adjustable by very small steps, and for
a finer adjustment a small oil condenser with movable plates may be used
conveniently.

I think it best at this juncture to bring before you a phenomenon,
observed by me some time ago, which to the purely scientific
investigator may perhaps appear more interesting than any of the results
which I have the privilege to present to you this evening.

It may be quite properly ranked among the brush phenomena--in fact, it
is a brush, formed at, or near, a single terminal in high vacuum.

[Illustration: FIG. 141.]

[Illustration: FIG. 142.]

In bulbs provided with a conducting terminal, though it be of aluminum,
the brush has but an ephemeral existence, and cannot, unfortunately, be
indefinitely preserved in its most sensitive state, even in a bulb
devoid of any conducting electrode. In studying the phenomenon, by all
means a bulb having no leading-in wire should be used. I have found it
best to use bulbs constructed as indicated in Figs. 141 and 142.

In Fig. 141 the bulb comprises an incandescent lamp globe _L_, in the
neck of which is sealed a barometer tube _b_, the end of which is blown
out to form a small sphere _s_. This sphere should be sealed as closely
as possible in the centre of the large globe. Before sealing, a thin
tube _t_, of aluminum sheet, may be slipped in the barometer tube, but
it is not important to employ it.

The small hollow sphere _s_ is filled with some conducting powder, and a
wire _w_ is cemented in the neck for the purpose of connecting the
conducting powder with the generator.

The construction shown in Fig. 142 was chosen in order to remove from
the brush any conducting body which might possibly affect it. The bulb
consists in this case of a lamp globe _L_, which has a neck _n_,
provided with a tube _b_ and small sphere _s_, sealed to it, so that two
entirely independent compartments are formed, as indicated in the
drawing. When the bulb is in use the neck _n_ is provided with a tinfoil
coating, which is connected to the generator and acts inductively upon
the moderately rarefied and highly conducted gas inclosed in the neck.
From there the current passes through the tube _b_ into the small sphere
_s_, to act by induction upon the gas contained in the globe _L_.

It is of advantage to make the tube _t_ very thick, the hole through it
very small, and to blow the sphere _s_ very thin. It is of the greatest
importance that the sphere _s_ be placed in the centre of the globe _L_.

[Illustration: FIG. 143.]

Figs. 143, 144 and 145 indicate different forms, or stages, of the
brush. Fig. 143 shows the brush as it first appears in a bulb provided
with a conducting terminal; but, as in such a bulb it very soon
disappears--often after a few minutes--I will confine myself to the
description of the phenomenon as seen in a bulb without conducting
electrode. It is observed under the following conditions:

When the globe _L_ (Figs. 141 and 142) is exhausted to a very high
degree, generally the bulb is not excited upon connecting the wire _w_
(Fig. 141) or the tinfoil coating of the bulb (Fig. 142) to the
terminal of the induction coil. To excite it, it is usually sufficient
to grasp the globe _L_ with the hand. An intense phosphorescence then
spreads at first over the globe, but soon gives place to a white, misty
light. Shortly afterward one may notice that the luminosity is unevenly
distributed in the globe, and after passing the current for some time
the bulb appears as in Fig. 144. From this stage the phenomenon will
gradually pass to that indicated in Fig. 145, after some minutes, hours,
days or weeks, according as the bulb is worked. Warming the bulb or
increasing the potential hastens the transit.

[Illustration: FIG. 144.]

[Illustration: FIG. 145.]

When the brush assumes the form indicated in Fig. 145, it may be brought
to a state of extreme sensitiveness to electrostatic and magnetic
influence. The bulb hanging straight down from a wire, and all objects
being remote from it, the approach of the observer at a few paces from
the bulb will cause the brush to fly to the opposite side, and if he
walks around the bulb it will always keep on the opposite side. It may
begin to spin around the terminal long before it reaches that sensitive
stage. When it begins to turn around, principally, but also before, it
is affected by a magnet, and at a certain stage it is susceptible to
magnetic influence to an astonishing degree. A small permanent magnet,
with its poles at a distance of no more than two centimetres, will
affect it visibly at a distance of two metres, slowing down or
accelerating the rotation according to how it is held relatively to the
brush. I think I have observed that at the stage when it is most
sensitive to magnetic, it is not most sensitive to electrostatic,
influence. My explanation is, that the electrostatic attraction between
the brush and the glass of the bulb, which retards the rotation, grows
much quicker than the magnetic influence when the intensity of the
stream is increased.

When the bulb hangs with the globe _L_ down, the rotation is always
clockwise. In the southern hemisphere it would occur in the opposite
direction and on the equator the brush should not turn at all. The
rotation may be reversed by a magnet kept at some distance. The brush
rotates best, seemingly, when it is at right angles to the lines of
force of the earth. It very likely rotates, when at its maximum speed,
in synchronism with the alternations, say, 10,000 times a second. The
rotation can be slowed down or accelerated by the approach or receding
of the observer, or any conducting body, but it cannot be reversed by
putting the bulb in any position. When it is in the state of the highest
sensitiveness and the potential or frequency be varied, the
sensitiveness is rapidly diminished. Changing either of these but little
will generally stop the rotation. The sensitiveness is likewise affected
by the variations of temperature. To attain great sensitiveness it is
necessary to have the small sphere _s_ in the centre of the globe _L_,
as otherwise the electrostatic action of the glass of the globe will
tend to stop the rotation. The sphere _s_ should be small and of uniform
thickness; any dissymmetry of course has the effect to diminish the
sensitiveness.

The fact that the brush rotates in a definite direction in a permanent
magnetic field seems to show that in alternating currents of very high
frequency the positive and negative impulses are not equal, but that one
always preponderates over the other.

Of course, this rotation in one direction may be due to the action of
the two elements of the same current upon each other, or to the action
of the field produced by one of the elements upon the other, as in a
series motor, without necessarily one impulse being stronger than the
other. The fact that the brush turns, as far as I could observe, in any
position, would speak for this view. In such case it would turn at any
point of the earth's surface. But, on the other hand, it is then hard to
explain why a permanent magnet should reverse the rotation, and one must
assume the preponderance of impulses of one kind.

As to the causes of the formation of the brush or stream, I think it is
due to the electrostatic action of the globe and the dissymmetry of the
parts. If the small bulb _s_ and the globe _L_ were perfect concentric
spheres, and the glass throughout of the same thickness and quality, I
think the brush would not form, as the tendency to pass would be equal
on all sides. That the formation of the stream is due to an irregularity
is apparent from the fact that it has the tendency to remain in one
position, and rotation occurs most generally only when it is brought out
of this position by electrostatic or magnetic influence. When in an
extremely sensitive state it rests in one position, most curious
experiments may be performed with it. For instance, the experimenter
may, by selecting a proper position, approach the hand at a certain
considerable distance to the bulb, and he may cause the brush to pass
off by merely stiffening the muscles of the arm. When it begins to
rotate slowly, and the hands are held at a proper distance, it is
impossible to make even the slightest motion without producing a visible
effect upon the brush. A metal plate connected to the other terminal of
the coil affects it at a great distance, slowing down the rotation often
to one turn a second.

I am firmly convinced that such a brush, when we learn how to produce it
properly, will prove a valuable aid in the investigation of the nature
of the forces acting in an electrostatic or magnetic field. If there is
any motion which is measurable going on in the space, such a brush ought
to reveal it. It is, so to speak, a beam of light, frictionless, devoid
of inertia.

I think that it may find practical applications in telegraphy. With such
a brush it would be possible to send dispatches across the Atlantic, for
instance, with any speed, since its sensitiveness may be so great that
the slightest changes will affect it. If it were possible to make the
stream more intense and very narrow, its deflections could be easily
photographed.

I have been interested to find whether there is a rotation of the stream
itself, or whether there is simply a stress traveling around the bulb.
For this purpose I mounted a light mica fan so that its vanes were in
the path of the brush. If the stream itself was rotating the fan would
be spun around. I could produce no distinct rotation of the fan,
although I tried the experiment repeatedly; but as the fan exerted a
noticeable influence on the stream, and the apparent rotation of the
latter was, in this case, never quite satisfactory, the experiment did
not appear to be conclusive.

I have been unable to produce the phenomenon with the disruptive
discharge coil, although every other of these phenomena can be well
produced by it--many, in fact, much better than with coils operated from
an alternator.

It may be possible to produce the brush by impulses of one direction, or
even by a steady potential, in which case it would be still more
sensitive to magnetic influence.

In operating an induction coil with rapidly alternating currents, we
realize with astonishment, for the first time, the great importance of
the relation of capacity, self-induction and frequency as regards the
general results. The effects of capacity are the most striking, for in
these experiments, since the self-induction and frequency both are high,
the critical capacity is very small, and need be but slightly varied to
produce a very considerable change. The experimenter may bring his body
in contact with the terminals of the secondary of the coil, or attach to
one or both terminals insulated bodies of very small bulk, such as
bulbs, and he may produce a considerable rise or fall of potential, and
greatly affect the flow of the current through the primary. In the
experiment before shown, in which a brush appears at a wire attached to
one terminal, and the wire is vibrated when the experimenter brings his
insulated body in contact with the other terminal of the coil, the
sudden rise of potential was made evident.

I may show you the behavior of the coil in another manner which
possesses a feature of some interest. I have here a little light fan of
aluminum sheet, fastened to a needle and arranged to rotate freely in a
metal piece screwed to one of the terminals of the coil. When the coil
is set to work, the molecules of the air are rhythmically attracted and
repelled. As the force with which they are repelled is greater than that
with which they are attracted, it results that there is a repulsion
exerted on the surfaces of the fan. If the fan were made simply of a
metal sheet, the repulsion would be equal on the opposite sides, and
would produce no effect. But if one of the opposing surfaces is
screened, or if, generally speaking, the bombardment on this side is
weakened in some way or other, there remains the repulsion exerted upon
the other, and the fan is set in rotation. The screening is best
effected by fastening upon one of the opposing sides of the fan
insulated conducting coatings, or, if the fan is made in the shape of an
ordinary propeller screw, by fastening on one side, and close to it, an
insulated metal plate. The static screen may, however, be omitted, and
simply a thickness of insulating material fastened to one of the sides
of the fan.

To show the behavior of the coil, the fan may be placed upon the
terminal and it will readily rotate when the coil is operated by
currents of very high frequency. With a steady potential, of course, and
even with alternating currents of very low frequency, it would not turn,
because of the very slow exchange of air and, consequently, smaller
bombardment; but in the latter case it might turn if the potential were
excessive. With a pin wheel, quite the opposite rule holds good; it
rotates best with a steady potential, and the effort is the smaller the
higher the frequency. Now, it is very easy to adjust the conditions so
that the potential is normally not sufficient to turn the fan, but that
by connecting the other terminal of the coil with an insulated body it
rises to a much greater value, so as to rotate the fan, and it is
likewise possible to stop the rotation by connecting to the terminal a
body of different size, thereby diminishing the potential.

Instead of using the fan in this experiment, we may use the "electric"
radiometer with similar effect. But in this case it will be found that
the vanes will rotate only at high exhaustion or at ordinary pressures;
they will not rotate at moderate pressures, when the air is highly
conducting. This curious observation was made conjointly by Professor
Crookes and myself. I attribute the result to the high conductivity of
the air, the molecules of which then do not act as independent carriers
of electric charges, but act all together as a single conducting body.
In such case, of course, if there is any repulsion at all of the
molecules from the vanes, it must be very small. It is possible,
however, that the result is in part due to the fact that the greater
part of the discharge passes from the leading-in wire through the highly
conducting gas, instead of passing off from the conducting vanes.

In trying the preceding experiment with the electric radiometer the
potential should not exceed a certain limit, as then the electrostatic
attraction between the vanes and the glass of the bulb may be so great
as to stop the rotation.

A most curious feature of alternate currents of high frequencies and
potentials is that they enable us to perform many experiments by the use
of one wire only. In many respects this feature is of great interest.

In a type of alternate current motor invented by me some years ago I
produced rotation by inducing, by means of a single alternating current
passed through a motor circuit, in the mass or other circuits of the
motor, secondary currents, which, jointly with the primary or inducing
current, created a moving field of force. A simple but crude form of
such a motor is obtained by winding upon an iron core a primary, and
close to it a secondary coil, joining the ends of the latter and placing
a freely movable metal disc within the influence of the field produced
by both. The iron core is employed for obvious reasons, but it is not
essential to the operation. To improve the motor, the iron core is made
to encircle the armature. Again to improve, the secondary coil is made
to partly overlap the primary, so that it cannot free itself from a
strong inductive action of the latter, repel its lines as it may. Once
more to improve, the proper difference of phase is obtained between the
primary and secondary currents by a condenser, self-induction,
resistance or equivalent windings.

I had discovered, however, that rotation is produced by means of a
single coil and core; my explanation of the phenomenon, and leading
thought in trying the experiment, being that there must be a true time
lag in the magnetization of the core. I remember the pleasure I had
when, in the writings of Professor Ayrton, which came later to my hand,
I found the idea of the time lag advocated. Whether there is a true time
lag, or whether the retardation is due to eddy currents circulating in
minute paths, must remain an open question, but the fact is that a coil
wound upon an iron core and traversed by an alternating current creates
a moving field of force, capable of setting an armature in rotation. It
is of some interest, in conjunction with the historical Arago
experiment, to mention that in lag or phase motors I have produced
rotation in the opposite direction to the moving field, which means that
in that experiment the magnet may not rotate, or may even rotate in the
opposite direction to the moving disc. Here, then, is a motor
(diagrammatically illustrated in Fig. 146), comprising a coil and iron
core, and a freely movable copper disc in proximity to the latter.

[Illustration: FIG. 146.]

To demonstrate a novel and interesting feature, I have, for a reason
which I will explain, selected this type of motor. When the ends of the
coil are connected to the terminals of an alternator the disc is set in
rotation. But it is not this experiment, now well known, which I desire
to perform. What I wish to show you is that this motor rotates with
_one single_ connection between it and the generator; that is to say,
one terminal of the motor is connected to one terminal of the
generator--in this case the secondary of a high-tension induction
coil--the other terminals of motor and generator being insulated in
space. To produce rotation it is generally (but not absolutely)
necessary to connect the free end of the motor coil to an insulated body
of some size. The experimenter's body is more than sufficient. If he
touches the free terminal with an object held in the hand, a current
passes through the coil and the copper disc is set in rotation. If an
exhausted tube is put in series with the coil, the tube lights
brilliantly, showing the passage of a strong current. Instead of the
experimenter's body, a small metal sheet suspended on a cord may be used
with the same result. In this case the plate acts as a condenser in
series with the coil. It counteracts the self-induction of the latter
and allows a strong current to pass. In such a combination, the greater
the self-induction of the coil the smaller need be the plate, and this
means that a lower frequency, or eventually a lower potential, is
required to operate the motor. A single coil wound upon a core has a
high self-induction; for this reason, principally, this type of motor
was chosen to perform the experiment. Were a secondary closed coil wound
upon the core, it would tend to diminish the self-induction, and then
it would be necessary to employ a much higher frequency and potential.
Neither would be advisable, for a higher potential would endanger the
insulation of the small primary coil, and a higher frequency would
result in a materially diminished torque.

It should be remarked that when such a motor with a closed secondary is
used, it is not at all easy to obtain rotation with excessive
frequencies, as the secondary cuts off almost completely the lines of
the primary--and this, of course, the more, the higher the
frequency--and allows the passage of but a minute current. In such a
case, unless the secondary is closed through a condenser, it is almost
essential, in order to produce rotation, to make the primary and
secondary coils overlap each other more or less.

But there is an additional feature of interest about this motor, namely,
it is not necessary to have even a single connection between the motor
and generator, except, perhaps, through the ground; for not only is an
insulated plate capable of giving off energy into space, but it is
likewise capable of deriving it from an alternating electrostatic field,
though in the latter case the available energy is much smaller. In this
instance one of the motor terminals is connected to the insulated plate
or body located within the alternating electrostatic field, and the
other terminal preferably to the ground.

It is quite possible, however, that such "no wire" motors, as they might
be called, could be operated by conduction through the rarefied air at
considerable distances. Alternate currents, especially of high
frequencies, pass with astonishing freedom through even slightly
rarefied gases. The upper strata of the air are rarefied. To reach a
number of miles out into space requires the overcoming of difficulties
of a merely mechanical nature. There is no doubt that with the enormous
potentials obtainable by the use of high frequencies and oil insulation,
luminous discharges might be passed through many miles of rarefied air,
and that, by thus directing the energy of many hundreds or thousands of
horse-power, motors or lamps might be operated at considerable distances
from stationary sources. But such schemes are mentioned merely as
possibilities. We shall have no need to transmit power in this way. We
shall have no need to _transmit_ power at all. Ere many generations
pass, our machinery will be driven by a power obtainable at any point of
the universe. This idea is not novel. Men have been led to it long ago
by instinct or reason. It has been expressed in many ways, and in many
places, in the history of old and new. We find it in the delightful myth
of Antheus, who derives power from the earth; we find it among the
subtle speculations of one of your splendid mathematicians, and in many
hints and statements of thinkers of the present time. Throughout space
there is energy. Is this energy static or kinetic? If static our hopes
are in vain; if kinetic--and this we know it is, for certain--then it is
a mere question of time when men will succeed in attaching their
machinery to the very wheelwork of nature. Of all, living or dead,
Crookes came nearest to doing it. His radiometer will turn in the light
of day and in the darkness of the night; it will turn everywhere where
there is heat, and heat is everywhere. But, unfortunately, this
beautiful little machine, while it goes down to posterity as the most
interesting, must likewise be put on record as the most inefficient
machine ever invented!

The preceding experiment is only one of many equally interesting
experiments which may be performed by the use of only one wire with
alternations of high potential and frequency. We may connect an
insulated line to a source of such currents, we may pass an
inappreciable current over the line, and on any point of the same we are
able to obtain a heavy current, capable of fusing a thick copper wire.
Or we may, by the help of some artifice, decompose a solution in any
electrolytic cell by connecting only one pole of the cell to the line or
source of energy. Or we may, by attaching to the line, or only bringing
into its vicinity, light up an incandescent lamp, an exhausted tube, or
a phosphorescent bulb.

However impracticable this plan of working may appear in many cases, it
certainly seems practicable, and even recommendable, in the production
of light. A perfected lamp would require but little energy, and if wires
were used at all we ought to be able to supply that energy without a
return wire.

It is now a fact that a body may be rendered incandescent or
phosphorescent by bringing it either in single contact or merely in the
vicinity of a source of electric impulses of the proper character, and
that in this manner a quantity of light sufficient to afford a practical
illuminant may be produced. It is, therefore, to say the least, worth
while to attempt to determine the best conditions and to invent the best
appliances for attaining this object.

Some experiences have already been gained in this direction, and I will
dwell on them briefly, in the hope that they might prove useful.

The heating of a conducting body inclosed in a bulb, and connected to a
source of rapidly alternating electric impulses, is dependent on so many
things of a different nature, that it would be difficult to give a
generally applicable rule under which the maximum heating occurs. As
regards the size of the vessel, I have lately found that at ordinary or
only slightly differing atmospheric pressures, when air is a good
insulator, and hence practically the same amount of energy by a certain
potential and frequency is given off from the body, whether the bulb be
small or large, the body is brought to a higher temperature if enclosed
in a small bulb, because of the better confinement of heat in this case.

At lower pressures, when air becomes more or less conducting, or if the
air be sufficiently warmed to become conducting, the body is rendered
more intensely incandescent in a large bulb, obviously because, under
otherwise equal conditions of test, more energy may be given off from
the body when the bulb is large.

At very high degrees of exhaustion, when the matter in the bulb becomes
"radiant," a large bulb has still an advantage, but a comparatively
slight one, over the small bulb.

Finally, at excessively high degrees of exhaustion, which cannot be
reached except by the employment of special means, there seems to be,
beyond a certain and rather small size of vessel, no perceptible
difference in the heating.

These observations were the result of a number of experiments, of which
one, showing the effect of the size of the bulb at a high degree of
exhaustion, may be described and shown here, as it presents a feature of
interest. Three spherical bulbs of 2 inches, 3 inches and 4 inches
diameter were taken, and in the centre of each was mounted an equal
length of an ordinary incandescent lamp filament of uniform thickness.
In each bulb the piece of filament was fastened to the leading-in wire
of platinum, contained in a glass stem sealed in the bulb; care being
taken, of course, to make everything as nearly alike as possible. On
each glass stem in the inside of the bulb was slipped a highly polished
tube made of aluminum sheet, which fitted the stem and was held on it by
spring pressure. The function of this aluminum tube will be explained
subsequently. In each bulb an equal length of filament protruded above
the metal tube. It is sufficient to say now that under these conditions
equal lengths of filament of the same thickness--in other words, bodies
of equal bulk--were brought to incandescence. The three bulbs were
sealed to a glass tube, which was connected to a Sprengel pump. When a
high vacuum had been reached, the glass tube carrying the bulbs was
sealed off. A current was then turned on successively on each bulb, and
it was found that the filaments came to about the same brightness, and,
if anything, the smallest bulb, which was placed midway between the two
larger ones, may have been slightly brighter. This result was expected,
for when either of the bulbs was connected to the coil the luminosity
spread through the other two, hence the three bulbs constituted really
one vessel. When all the three bulbs were connected in multiple arc to
the coil, in the largest of them the filament glowed brightest, in the
next smaller it was a little less bright, and in the smallest it only
came to redness. The bulbs were then sealed off and separately tried.
The brightness of the filaments was now such as would have been expected
on the supposition that the energy given off was proportionate to the
surface of the bulb, this surface in each case representing one of the
coatings of a condenser. Accordingly, there was less difference between
the largest and the middle sized than between the latter and the
smallest bulb.

An interesting observation was made in this experiment. The three bulbs
were suspended from a straight bare wire connected to a terminal of a
coil, the largest bulb being placed at the end of the wire, at some
distance from it the smallest bulb, and at an equal distance from the
latter the middle-sized one. The carbons glowed then in both the larger
bulbs about as expected, but the smallest did not get its share by far.
This observation led me to exchange the position of the bulbs, and I
then observed that whichever of the bulbs was in the middle was by far
less bright than it was in any other position. This mystifying result
was, of course, found to be due to the electrostatic action between the
bulbs. When they were placed at a considerable distance, or when they
were attached to the corners of an equilateral triangle of copper wire,
they glowed in about the order determined by their surfaces.

As to the shape of the vessel, it is also of some importance, especially
at high degrees of exhaustion. Of all the possible constructions, it
seems that a spherical globe with the refractory body mounted in its
centre is the best to employ. By experience it has been demonstrated
that in such a globe a refractory body of a given bulk is more easily
brought to incandescence than when differently shaped bulbs are used.
There is also an advantage in giving to the incandescent body the shape
of a sphere, for self-evident reasons. In any case the body should be
mounted in the centre, where the atoms rebounding from the glass
collide. This object is best attained in the spherical bulb; but it is
also attained in a cylindrical vessel with one or two straight filaments
coinciding with its axis, and possibly also in parabolical or spherical
bulbs with refractory body or bodies placed in the focus or foci of the
same; though the latter is not probable, as the electrified atoms should
in all cases rebound normally from the surface they strike, unless the
speed were excessive, in which case they _would_ probably follow the
general law of reflection. No matter what shape the vessel may have, if
the exhaustion be low, a filament mounted in the globe is brought to the
same degree of incandescence in all parts; but if the exhaustion be high
and the bulb be spherical or pear-shaped, as usual, focal points form
and the filament is heated to a higher degree at or near such points.

To illustrate the effect, I have here two small bulbs which are alike,
only one is exhausted to a low and the other to a very high degree. When
connected to the coil, the filament in the former glows uniformly
throughout all its length; whereas in the latter, that portion of the
filament which is in the centre of the bulb glows far more intensely
than the rest. A curious point is that the phenomenon occurs even if two
filaments are mounted in a bulb, each being connected to one terminal of
the coil, and, what is still more curious, if they be very near
together, provided the vacuum be very high. I noted in experiments with
such bulbs that the filaments would give way usually at a certain point,
and in the first trials I attributed it to a defect in the carbon. But
when the phenomenon occurred many times in succession I recognized its
real cause.

In order to bring a refractory body inclosed in a bulb to incandescence,
it is desirable, on account of economy, that all the energy supplied to
the bulb from the source should reach without loss the body to be
heated; from there, and from nowhere else, it should be radiated. It is,
of course, out of the question to reach this theoretical result, but it
is possible by a proper construction of the illuminating device to
approximate it more or less.

For many reasons, the refractory body is placed in the centre of the
bulb, and it is usually supported on a glass stem containing the
leading-in wire. As the potential of this wire is alternated, the
rarefied gas surrounding the stem is acted upon inductively, and the
glass stem is violently bombarded and heated. In this manner by far the
greater portion of the energy supplied to the bulb--especially when
exceedingly high frequencies are used--may be lost for the purpose
contemplated. To obviate this loss, or at least to reduce it to a
minimum, I usually screen the rarefied gas surrounding the stem from the
inductive action of the leading-in wire by providing the stem with a
tube or coating of conducting material. It seems beyond doubt that the
best among metals to employ for this purpose is aluminum, on account of
its many remarkable properties. Its only fault is that it is easily
fusible, and, therefore, its distance from the incandescing body should
be properly estimated. Usually, a thin tube, of a diameter somewhat
smaller than that of the glass stem, is made of the finest aluminum
sheet, and slipped on the stem. The tube is conveniently prepared by
wrapping around a rod fastened in a lathe a piece of aluminum sheet of
proper size, grasping the sheet firmly with clean chamois leather or
blotting paper, and spinning the rod very fast. The sheet is wound
tightly around the rod, and a highly polished tube of one or three
layers of the sheet is obtained. When slipped on the stem, the pressure
is generally sufficient to prevent it from slipping off, but, for
safety, the lower edge of the sheet may be turned inside. The upper
inside corner of the sheet--that is, the one which is nearest to the
refractory incandescent body--should be cut out diagonally, as it often
happens that, in consequence of the intense heat, this corner turns
toward the inside and comes very near to, or in contact with, the wire,
or filament, supporting the refractory body. The greater part of the
energy supplied to the bulb is then used up in heating the metal tube,
and the bulb is rendered useless for the purpose. The aluminum sheet
should project above the glass stem more or less--one inch or so--or
else, if the glass be too close to the incandescing body, it may be
strongly heated and become more or less conducting, whereupon it may be
ruptured, or may, by its conductivity, establish a good electrical
connection between the metal tube and the leading-in wire, in which
case, again, most of the energy will be lost in heating the former.
Perhaps the best way is to make the top of the glass tube, for about an
inch, of a much smaller diameter. To still further reduce the danger
arising from the heating of the glass stem, and also with the view of
preventing an electrical connection between the metal tube and the
electrode, I preferably wrap the stem with several layers of thin mica,
which extends at least as far as the metal tube. In some bulbs I have
also used an outside insulating cover.

The preceding remarks are only made to aid the experimenter in the first
trials, for the difficulties which he encounters he may soon find means
to overcome in his own way.

To illustrate the effect of the screen, and the advantage of using it, I
have here two bulbs of the same size, with their stems, leading-in wires
and incandescent lamp filaments tied to the latter, as nearly alike as
possible. The stem of one bulb is provided with an aluminum tube, the
stem of the other has none. Originally the two bulbs were joined by a
tube which was connected to a Sprengel pump. When a high vacuum had been
reached, first the connecting tube, and then the bulbs, were sealed off;
they are therefore of the same degree of exhaustion. When they are
separately connected to the coil giving a certain potential, the carbon
filament in the bulb provided with the aluminum screen is rendered
highly incandescent, while the filament in the other bulb may, with the
same potential, not even come to redness, although in reality the latter
bulb takes generally more energy than the former. When they are both
connected together to the terminal, the difference is even more
apparent, showing the importance of the screening. The metal tube placed
on the stem containing the leading-in wire performs really two distinct
functions: First, it acts more or less as an electrostatic screen, thus
economizing the energy supplied to the bulb; and, second, to whatever
extent it may fail to act electrostatically, it acts mechanically,
preventing the bombardment, and consequently intense heating and
possible deterioration of the slender support of the refractory
incandescent body, or of the glass stem containing the leading-in wire.
I say _slender_ support, for it is evident that in order to confine the
heat more completely to the incandescing body its support should be very
thin, so as to carry away the smallest possible amount of heat by
conduction. Of all the supports used I have found an ordinary
incandescent lamp filament to be the best, principally because among
conductors it can withstand the highest degree of heat.

The effectiveness of the metal tube as an electrostatic screen depends
largely on the degree of exhaustion.

At excessively high degrees of exhaustion--which are reached by using
great care and special means in connection with the Sprengel pump--when
the matter in the globe is in the ultra-radiant state, it acts most
perfectly. The shadow of the upper edge of the tube is then sharply
defined upon the bulb.

At a somewhat lower degree of exhaustion, which is about the ordinary
"non-striking" vacuum, and generally as long as the matter moves
predominantly in straight lines, the screen still does well. In
elucidation of the preceding remark it is necessary to state that what
is a "non-striking" vacuum for a coil operated as ordinarily, by
impulses, or currents, of low frequency, is not so, by far, when the
coil is operated by currents of very high frequency. In such case the
discharge may pass with great freedom through the rarefied gas through
which a low frequency discharge may not pass, even though the potential
be much higher. At ordinary atmospheric pressures just the reverse rule
holds good: the higher the frequency, the less the spark discharge is
able to jump between the terminals, especially if they are knobs or
spheres of some size.

Finally, at very low degrees of exhaustion, when the gas is well
conducting, the metal tube not only does not act as an electrostatic
screen, but even is a drawback, aiding to a considerable extent the
dissipation of the energy laterally from the leading-in wire. This, of
course, is to be expected. In this case, namely, the metal tube is in
good electrical connection with the leading-in wire, and most of the
bombardment is directed upon the tube. As long as the electrical
connection is not good, the conducting tube is always of some advantage,
for although it may not greatly economize energy, still it protects the
support of the refractory button, and is the means of concentrating more
energy upon the same.

To whatever extent the aluminum tube performs the function of a screen,
its usefulness is therefore limited to very high degrees of exhaustion
when it is insulated from the electrode--that is, when the gas as a
whole is non-conducting, and the molecules, or atoms, act as independent
carriers of electric charges.

In addition to acting as a more or less effective screen, in the true
meaning of the word, the conducting tube or coating may also act, by
reason of its conductivity, as a sort of equalizer or dampener of the
bombardment against the stem. To be explicit, I assume the action to be
as follows: Suppose a rhythmical bombardment to occur against the
conducting tube by reason of its imperfect action as a screen, it
certainly must happen that some molecules, or atoms, strike the tube
sooner than others. Those which come first in contact with it give up
their superfluous charge, and the tube is electrified, the
electrification instantly spreading over its surface. But this must
diminish the energy lost in the bombardment, for two reasons: first, the
charge given up by the atoms spreads over a great area, and hence the
electric density at any point is small, and the atoms are repelled with
less energy than they would be if they struck against a good insulator;
secondly, as the tube is electrified by the atoms which first come in
contact with it, the progress of the following atoms against the tube is
more or less checked by the repulsion which the electrified tube must
exert upon the similarly electrified atoms. This repulsion may perhaps
be sufficient to prevent a large portion of the atoms from striking the
tube, but at any rate it must diminish the energy of their impact. It is
clear that when the exhaustion is very low, and the rarefied gas well
conducting, neither of the above effects can occur, and, on the other
hand, the fewer the atoms, with the greater freedom they move; in other
words, the higher the degree of exhaustion, up to a limit, the more
telling will be both the effects.

[Illustration: FIG. 147.]

[Illustration: FIG. 148.]

What I have just said may afford an explanation of the phenomenon
observed by Prof. Crookes, namely, that a discharge through a bulb is
established with much greater facility when an insulator than when a
conductor is present in the same. In my opinion, the conductor acts as a
dampener of the motion of the atoms in the two ways pointed out; hence,
to cause a visible discharge to pass through the bulb, a much higher
potential is needed if a conductor, especially of much surface, be
present.

For the sake of elucidating of some of the remarks before made, I must
now refer to Figs. 147, 148 and 149, which illustrate various
arrangements with a type of bulb most generally used.

Fig. 147 is a section through a spherical bulb L, with the glass stem
_s_, contains the leading-in wire _w_, which has a lamp filament _l_
fastened to it, serving to support the refractory button _m_ in the
centre. M is a sheet of thin mica wound in several layers around the
stem _s_, and _a_ is the aluminum tube.

Fig. 148 illustrates such a bulb in a somewhat more advanced stage of
perfection. A metallic tube S is fastened by means of some cement to the
neck of the tube. In the tube is screwed a plug P, of insulating
material, in the centre of which is fastened a metallic terminal _t_,
for the connection to the leading-in wire _w_. This terminal must be
well insulated from the metal tube S; therefore, if the cement used is
conducting--and most generally it is sufficiently so--the space between
the plug P and the neck of the bulb should be filled with some good
insulating material, such as mica powder.


Fig. 149 shows a bulb made for experimental purposes. In this bulb the
aluminum tube is provided with an external connection, which serves to
investigate the effect of the tube under various conditions. It is
referred to chiefly to suggest a line of experiment followed.

Since the bombardment against the stem containing the leading-in wire is
due to the inductive action of the latter upon the rarefied gas, it is
of advantage to reduce this action as far as practicable by employing a
very thin wire, surrounded by a very thick insulation of glass or other
material, and by making the wire passing through the rarefied gas as
short as practicable. To combine these features I employ a large tube T
(Fig. 150), which protrudes into the bulb to some distance, and carries
on the top a very short glass stem _s_, into which is sealed the
leading-in wire _w_, and I protect the top of the glass stem against the
heat by a small aluminum tube _a_ and a layer of mica underneath the
same, as usual. The wire _w_, passing through the large tube to the
outside of the bulb, should be well insulated--with a glass tube, for
instance--and the space between ought to be filled out with some
excellent insulator. Among many insulating powders I have found that
mica powder is the best to employ. If this precaution is not taken, the
tube T, protruding into the bulb, will surely be cracked in consequence
of the heating by the brushes which are apt to form in the upper part of
the tube, near the exhausted globe, especially if the vacuum be
excellent, and therefore the potential necessary to operate the lamp be
very high.

[Illustration: FIG. 149.]

[Illustration: FIG. 150.]

Fig. 151 illustrates a similar arrangement, with a large tube T
protruding into the part of the bulb containing the refractory button
_m_. In this case the wire leading from the outside into the bulb is
omitted, the energy required being supplied through condenser coatings C
C. The insulating packing P should in this construction be tightly
fitting to the glass, and rather wide, or otherwise the discharge might
avoid passing through the wire _w_, which connects the inside condenser
coating to the incandescent button _m_.

The molecular bombardment against the glass stem in the bulb is a source
of great trouble. As an illustration I will cite a phenomenon only too
frequently and unwillingly observed. A bulb, preferably a large one, may
be taken, and a good conducting body, such as a piece of carbon, may be
mounted in it upon a platinum wire sealed in the glass stem. The bulb
may be exhausted to a fairly high degree, nearly to the point when
phosphorescence begins to appear. When the bulb is connected with the
coil, the piece of carbon, if small, may become highly incandescent at
first, but its brightness immediately diminishes, and then the discharge
may break through the glass somewhere in the middle of the stem, in the
form of bright sparks, in spite of the fact that the platinum wire is in
good electrical connection with the rarefied gas through the piece of
carbon or metal at the top. The first sparks are singularly bright,
recalling those drawn from a clear surface of mercury. But, as they heat
the glass rapidly, they, of course, lose their brightness, and cease
when the glass at the ruptured place becomes incandescent, or generally
sufficiently hot to conduct. When observed for the first time the
phenomenon must appear very curious, and shows in a striking manner how
radically different alternate currents, or impulses, of high frequency
behave, as compared with steady currents, or currents of low frequency.
With such currents--namely, the latter--the phenomenon would of course
not occur. When frequencies such as are obtained by mechanical means are
used, I think that the rupture of the glass is more or less the
consequence of the bombardment, which warms it up and impairs its
insulating power; but with frequencies obtainable with condensers I have
no doubt that the glass may give way without previous heating. Although
this appears most singular at first, it is in reality what we might
expect to occur. The energy supplied to the wire leading into the bulb
is given off partly by direct action through the carbon button, and
partly by inductive action through the glass surrounding the wire. The
case is thus analogous to that in which a condenser shunted by a
conductor of low resistance is connected to a source of alternating
current. As long as the frequencies are low, the conductor gets the most
and the condenser is perfectly safe; but when the frequency becomes
excessive, the _role_ of the conductor may become quite insignificant.
In the latter case the difference of potential at the terminals of the
condenser may become so great as to rupture the dielectric,
notwithstanding the fact that the terminals are joined by a conductor of
low resistance.

It is, of course, not necessary, when it is desired to produce the
incandescence of a body inclosed in a bulb by means of these currents,
that the body should be a conductor, for even a perfect non-conductor
may be quite as readily heated. For this purpose it is sufficient to
surround a conducting electrode with a non-conducting material, as, for
instance, in the bulb described before in Fig. 150, in which a thin
incandescent lamp filament is coated with a non-conductor, and supports
a button of the same material on the top. At the start the bombardment
goes on by inductive action through the non-conductor, until the same is
sufficiently heated to become conducting, when the bombardment continues
in the ordinary way.

[Illustration: FIG. 151.]

[Illustration: FIG. 152.]

A different arrangement used in some of the bulbs constructed is
illustrated in Fig. 152. In this instance a non-conductor _m_ is mounted
in a piece of common arc light carbon so as to project some small
distance above the latter. The carbon piece is connected to the
leading-in wire passing through a glass stem, which is wrapped with
several layers of mica. An aluminum tube _a_ is employed as usual for
screening. It is so arranged that it reaches very nearly as high as the
carbon and only the non-conductor _m_ projects a little above it. The
bombardment goes at first against the upper surface of carbon, the lower
parts being protected by the aluminum tube. As soon, however, as the
non-conductor _m_ is heated it is rendered good conducting, and then it
becomes the centre of the bombardment, being most exposed to the same.

I have also constructed during these experiments many such single-wire
bulbs with or without internal electrode, in which the radiant matter
was projected against, or focused upon, the body to be rendered
incandescent. Fig. 153 (page 263) illustrates one of the bulbs used. It
consists of a spherical globe L, provided with a long neck _n_, on top,
for increasing the action in some cases by the application of an
external conducting coating. The globe L is blown out on the bottom into
a very small bulb _b_, which serves to hold it firmly in a socket S of
insulating material into which it is cemented. A fine lamp filament _f_,
supported on a wire _w_, passes through the centre of the globe L. The
filament is rendered incandescent in the middle portion, where the
bombardment proceeding from the lower inside surface of the globe is
most intense. The lower portion of the globe, as far as the socket S
reaches, is rendered conducting, either by a tinfoil coating or
otherwise, and the external electrode is connected to a terminal of the
coil.

The arrangement diagrammatically indicated in Fig. 153 was found to be
an inferior one when it was desired to render incandescent a filament or
button supported in the centre of the globe, but it was convenient when
the object was to excite phosphorescence.

In many experiments in which bodies of different kind were mounted in
the bulb as, for instance, indicated in Fig. 152, some observations of
interest were made.

It was found, among other things, that in such cases, no matter where
the bombardment began, just as soon as a high temperature was reached
there was generally one of the bodies which seemed to take most of the
bombardment upon itself, the other, or others, being thereby relieved.
The quality appeared to depend principally on the point of fusion, and
on the facility with which the body was "evaporated," or, generally
speaking, disintegrated--meaning by the latter term not only the
throwing off of atoms, but likewise of large lumps. The observation made
was in accordance with generally accepted notions. In a highly exhausted
bulb, electricity is carried off from the electrode by independent
carriers, which are partly the atoms, or molecules, of the residual
atmosphere, and partly the atoms, molecules, or lumps thrown off from
the electrode. If the electrode is composed of bodies of different
character, and if one of these is more easily disintegrated than the
other, most of the electricity supplied is carried off from that body,
which is then brought to a higher temperature than the others, and this
the more, as upon an increase of the temperature the body is still more
easily disintegrated.

It seems to me quite probable that a similar process takes place in the
bulb even with a homogeneous electrode, and I think it to be the
principal cause of the disintegration. There is bound to be some
irregularity, even if the surface is highly polished, which, of course,
is impossible with most of the refractory bodies employed as electrodes.
Assume that a point of the electrode gets hotter; instantly most of the
discharge passes through that point, and a minute patch it probably
fused and evaporated. It is now possible that in consequence of the
violent disintegration the spot attacked sinks in temperature, or that a
counter force is created, as in an arc; at any rate, the local tearing
off meets with the limitations incident to the experiment, whereupon the
same process occurs on another place. To the eye the electrode appears
uniformly brilliant, but there are upon it points constantly shifting
and wandering around, of a temperature far above the mean, and this
materially hastens the process of deterioration. That some such thing
occurs, at least when the electrode is at a lower temperature,
sufficient experimental evidence can be obtained in the following
manner: Exhaust a bulb to a very high degree, so that with a fairly high
potential the discharge cannot pass--that is, not a _luminous_ one, for
a weak invisible discharge occurs always, in all probability. Now raise
slowly and carefully the potential, leaving the primary current on no
more than for an instant. At a certain point, two, three, or half a
dozen phosphorescent spots will appear on the globe. These places of the
glass are evidently more violently bombarded than others, this being due
to the unevenly distributed electric density, necessitated, of course,
by sharp projections, or, generally speaking, irregularities of the
electrode. But the luminous patches are constantly changing in position,
which is especially well observable if one manages to produce very few,
and this indicates that the configuration of the electrode is rapidly
changing.

From experiences of this kind I am led to infer that, in order to be
most durable, the refractory button in the bulb should be in the form of
a sphere with a highly polished surface. Such a small sphere could be
manufactured from a diamond or some other crystal, but a better way
would be to fuse, by the employment of extreme degrees of temperature,
some oxide--as, for instance, zirconia--into a small drop, and then keep
it in the bulb at a temperature somewhat below its point of fusion.

Interesting and useful results can, no doubt, be reached in the
direction of extreme degrees of heat. How can such high temperatures be
arrived at? How are the highest degrees of heat reached in nature? By
the impact of stars, by high speeds and collisions. In a collision any
rate of heat generation may be attained. In a chemical process we are
limited. When oxygen and hydrogen combine, they fall, metaphorically
speaking, from a definite height. We cannot go very far with a blast,
nor by confining heat in a furnace, but in an exhausted bulb we can
concentrate any amount of energy upon a minute button. Leaving
practicability out of consideration, this, then, would be the means
which, in my opinion, would enable us to reach the highest temperature.
But a great difficulty when proceeding in this way is encountered,
namely, in most cases the body is carried off before it can fuse and
form a drop. This difficulty exists principally with an oxide, such as
zirconia, because it cannot be compressed in so hard a cake that it
would not be carried off quickly. I have endeavored repeatedly to fuse
zirconia, placing it in a cup of arc light carbon, as indicated in Fig.
152. It glowed with a most intense light, and the stream of the
particles projected out of the carbon cup was of a vivid white; but
whether it was compressed in a cake or made into a paste with carbon, it
was carried off before it could be fused. The carbon cup, containing
zirconia, had to be mounted very low in the neck of a large bulb, as the
heating of the glass by the projected particles of the oxide was so
rapid that in the first trial the bulb was cracked almost in an instant,
when the current was turned on. The heating of the glass by the
projected particles was found to be always greater when the carbon cup
contained a body which was rapidly carried off--I presume, because in
such cases, with the same potential, higher speeds were reached, and
also because, per unit of time, more matter was projected--that is, more
particles would strike the glass.

The before-mentioned difficulty did not exist, however, when the body
mounted in the carbon cup offered great resistance to deterioration. For
instance, when an oxide was first fused in an oxygen blast, and then
mounted in the bulb, it melted very readily into a drop.

Generally, during the process of fusion, magnificent light effects were
noted, of which it would be difficult to give an adequate idea. Fig. 152
is intended to illustrate the effect observed with a ruby drop. At first
one may see a narrow funnel of white light projected against the top of
the globe, where it produces an irregularly outlined phosphorescent
patch. When the point of the ruby fuses, the phosphorescence becomes
very powerful; but as the atoms are projected with much greater speed
from the surface of the drop, soon the glass gets hot and "tired," and
now only the outer edge of the patch glows. In this manner an intensely
phosphorescent, sharply defined line, _l_, corresponding to the outline
of the drop, is produced, which spreads slowly over the globe as the
drop gets larger. When the mass begins to boil, small bubbles and
cavities are formed, which cause dark colored spots to sweep across the
globe. The bulb may be turned downward without fear of the drop falling
off, as the mass possesses considerable viscosity.

I may mention here another feature of some interest, which I believe to
have noted in the course of these experiments, though the observations
do not amount to a certitude. It _appeared_ that under the molecular
impact caused by the rapidly alternating potential, the body was fused
and maintained in that state at a lower temperature in a highly
exhausted bulb than was the case at normal pressure and application of
heat in the ordinary way--that is, at least, judging from the quantity
of the light emitted. One of the experiments performed may be mentioned
here by way of illustration. A small piece of pumice stone was stuck on
a platinum wire, and first melted to it in a gas burner. The wire was
next placed between two pieces of charcoal, and a burner applied, so as
to produce an intense heat, sufficient to melt down the pumice stone
into a small glass-like button. The platinum wire had to be taken of
sufficient thickness, to prevent its melting in the fire. While in the
charcoal fire, or when held in a burner to get a better idea of the
degree of heat, the button glowed with great brilliancy. The wire with
the button was then mounted in a bulb, and upon exhausting the same to a
high degree, the current was turned on slowly, so as to prevent the
cracking of the button. The button was heated to the point of fusion,
and when it melted, it did not, apparently, glow with the same
brilliancy as before, and this would indicate a lower temperature.
Leaving out of consideration the observer's possible, and even probable,
error, the question is, can a body under these conditions be brought
from a solid to a liquid state with the evolution of _less_ light?

When the potential of a body is rapidly alternated, it is certain that
the structure is jarred. When the potential is very high, although the
vibrations may be few--say 20,000 per second--the effect upon the
structure may be considerable. Suppose, for example, that a ruby is
melted into a drop by a steady application of energy. When it forms a
drop, it will emit visible and invisible waves, which will be in a
definite ratio, and to the eye the drop will appear to be of a certain
brilliancy. Next, suppose we diminish to any degree we choose the energy
steadily supplied, and, instead, supply energy which rises and falls
according to a certain law. Now, when the drop is formed, there will be
emitted from it three different kinds of vibrations--the ordinary
visible, and two kinds of invisible waves: that is, the ordinary dark
waves of all lengths, and, in addition, waves of a well defined
character. The latter would not exist by a steady supply of the energy;
still they help to jar and loosen the structure. If this really be the
case, then the ruby drop will emit relatively less visible and more
invisible waves than before. Thus it would seem that when a platinum
wire, for instance, is fused by currents alternating with extreme
rapidity, it emits at the point of fusion less light and more visible
radiation than it does when melted by a steady current, though the total
energy used up in the process of fusion is the same in both cases. Or,
to cite another example, a lamp filament is not capable of withstanding
as long with currents of extreme frequency as it does with steady
currents, assuming that it be worked at the same luminous intensity.
This means that for rapidly alternating currents the filament should be
shorter and thicker. The higher the frequency--that is, the greater the
departure from the steady flow--the worse it would be for the filament.
But if the truth of this remark were demonstrated, it would be erroneous
to conclude that such a refractory button as used in these bulbs would
be deteriorated quicker by currents of extremely high frequency than by
steady or low frequency currents. From experience I may say that just
the opposite holds good: the button withstands the bombardment better
with currents of very high frequency. But this is due to the fact that a
high frequency discharge passes through a rarefied gas with much greater
freedom than a steady or low frequency discharge, and this will mean
that with the former we can work with a lower potential or with a less
violent impact. As long, then, as the gas is of no consequence, a steady
or low frequency current is better; but as soon as the action of the gas
is desired and important, high frequencies are preferable.

In the course of these experiments a great many trials were made with
all kinds of carbon buttons. Electrodes made of ordinary carbon buttons
were decidedly more durable when the buttons were obtained by the
application of enormous pressure. Electrodes prepared by depositing
carbon in well known ways did not show up well; they blackened the globe
very quickly. From many experiences I conclude that lamp filaments
obtained in this manner can be advantageously used only with low
potentials and low frequency currents. Some kinds of carbon withstand so
well that, in order to bring them to the point of fusion, it is
necessary to employ very small buttons. In this case the observation is
rendered very difficult on account of the intense heat produced.
Nevertheless there can be no doubt that all kinds of carbon are fused
under the molecular bombardment, but the liquid state must be one of
great instability. Of all the bodies tried there were two which
withstood best--diamond and carborundum. These two showed up about
equally, but the latter was preferable for many reasons. As it is more
than likely that this body is not yet generally known, I will venture to
call your attention to it.

It has been recently produced by Mr. E. G. Acheson, of Monongahela City,
Pa., U. S. A. It is intended to replace ordinary diamond powder for
polishing precious stones, etc., and I have been informed that it
accomplishes this object quite successfully. I do not know why the name
"carborundum" has been given to it, unless there is something in the
process of its manufacture which justifies this selection. Through the
kindness of the inventor, I obtained a short while ago some samples
which I desired to test in regard to their qualities of phosphorescence
and capability of withstanding high degrees of heat.

Carborundum can be obtained in two forms--in the form of "crystals" and
of powder. The former appear to the naked eye dark colored, but are very
brilliant; the latter is of nearly the same color as ordinary diamond
powder, but very much finer. When viewed under a microscope the samples
of crystals given to me did not appear to have any definite form, but
rather resembled pieces of broken up egg coal of fine quality. The
majority were opaque, but there were some which were transparent and
colored. The crystals are a kind of carbon containing some impurities;
they are extremely hard, and withstand for a long time even an oxygen
blast. When the blast is directed against them they at first form a
cake of some compactness, probably in consequence of the fusion of
impurities they contain. The mass withstands for a very long time the
blast without further fusion; but a slow carrying off, or burning,
occurs, and, finally, a small quantity of a glass-like residue is left,
which, I suppose, is melted alumina. When compressed strongly they
conduct very well, but not as well as ordinary carbon. The powder, which
is obtained from the crystals in some way, is practically
non-conducting. It affords a magnificent polishing material for stones.

The time has been too short to make a satisfactory study of the
properties of this product, but enough experience has been gained in a
few weeks I have experimented upon it to say that it does possess some
remarkable properties in many respects. It withstands excessively high
degrees of heat, it is little deteriorated by molecular bombardment, and
it does not blacken the globe as ordinary carbon does. The only
difficulty which I have experienced in its use in connection with these
experiments was to find some binding material which would resist the
heat and the effect of the bombardment as successfully as carborundum
itself does.

I have here a number of bulbs which I have provided with buttons of
carborundum. To make such a button of carborundum crystals I proceed in
the following manner: I take an ordinary lamp filament and dip its point
in tar, or some other thick substance or paint which may be readily
carbonized. I next pass the point of the filament through the crystals,
and then hold it vertically over a hot plate. The tar softens and forms
a drop on the point of the filament, the crystals adhering to the
surface of the drop. By regulating the distance from the plate the tar
is slowly dried out and the button becomes solid. I then once more dip
the button in tar and hold it again over a plate until the tar is
evaporated, leaving only a hard mass which firmly binds the crystals.
When a larger button is required I repeat the process several times, and
I generally also cover the filament a certain distance below the button
with crystals. The button being mounted in a bulb, when a good vacuum
has been reached, first a weak and then a strong discharge is passed
through the bulb to carbonize the tar and expel all gases, and later it
is brought to a very intense incandescence.

When the powder is used I have found it best to proceed as follows: I
make a thick paint of carborundum and tar, and pass a lamp filament
through the paint. Taking then most of the paint off by rubbing the
filament against a piece of chamois leather, I hold it over a hot plate
until the tar evaporates and the coating becomes firm. I repeat this
process as many times as it is necessary to obtain a certain thickness
of coating. On the point of the coated filament I form a button in the
same manner.

There is no doubt that such a button--properly prepared under great
pressure--of carborundum, especially of powder of the best quality, will
withstand the effect of the bombardment fully as well as anything we
know. The difficulty is that the binding material gives way, and the
carborundum is slowly thrown off after some time. As it does not seem to
blacken the globe in the least, it might be found useful for coating the
filaments of ordinary incandescent lamps, and I think that it is even
possible to produce thin threads or sticks of carborundum which will
replace the ordinary filaments in an incandescent lamp. A carborundum
coating seems to be more durable than other coatings, not only because
the carborundum can withstand high degrees of heat, but also because it
seems to unite with the carbon better than any other material I have
tried. A coating of zirconia or any other oxide, for instance, is far
more quickly destroyed. I prepared buttons of diamond dust in the same
manner as of carborundum, and these came in durability nearest to those
prepared of carborundum, but the binding paste gave way much more
quickly in the diamond buttons; this, however, I attributed to the size
and irregularity of the grains of the diamond.

It was of interest to find whether carborundum possesses the quality of
phosphorescence. One is, of course, prepared to encounter two
difficulties: first, as regards the rough product, the "crystals," they
are good conducting, and it is a fact that conductors do not
phosphoresce; second, the powder, being exceedingly fine, would not be
apt to exhibit very prominently this quality, since we know that when
crystals, even such as diamond or ruby, are finely powdered, they lose
the property of phosphorescence to a considerable degree.

The question presents itself here, can a conductor phosphoresce? What is
there in such a body as a metal, for instance, that would deprive it of
the quality of phosphoresence, unless it is that property which
characterizes it as a conductor? For it is a fact that most of the
phosphorescent bodies lose that quality when they are sufficiently
heated to become more or less conducting. Then, if a metal be in a
large measure, or perhaps entirely, deprived of that property, it should
be capable of phosphoresence. Therefore it is quite possible that at
some extremely high frequency, when behaving practically as a
non-conductor, a metal or any other conductor might exhibit the quality
of phosphoresence, even though it be entirely incapable of
phosphorescing under the impact of a low-frequency discharge. There is,
however, another possible way how a conductor might at least _appear_ to
phosphoresce.

Considerable doubt still exists as to what really is phosphorescence,
and as to whether the various phenomena comprised under this head are
due to the same causes. Suppose that in an exhausted bulb, under the
molecular impact, the surface of a piece of metal or other conductor is
rendered strongly luminous, but at the same time it is found that it
remains comparatively cool, would not this luminosity be called
phosphorescence? Now such a result, theoretically at least, is possible,
for it is a mere question of potential or speed. Assume the potential of
the electrode, and consequently the speed of the projected atoms, to be
sufficiently high, the surface of the metal piece, against which the
atoms are projected, would be rendered highly incandescent, since the
process of heat generation would be incomparably faster than that of
radiating or conducting away from the surface of the collision. In the
eye of the observer a single impact of the atoms would cause an
instantaneous flash, but if the impacts were repeated with sufficient
rapidity, they would produce a continuous impression upon his retina. To
him then the surface of the metal would appear continuously incandescent
and of constant luminous intensity, while in reality the light would be
either intermittent, or at least changing periodically in intensity. The
metal piece would rise in temperature until equilibrium was
attained--that is, until the energy continuously radiated would equal
that intermittently supplied. But the supplied energy might under such
conditions not be sufficient to bring the body to any more than a very
moderate mean temperature, especially if the frequency of the atomic
impacts be very low--just enough that the fluctuation of the intensity
of the light emitted could not be detected by the eye. The body would
now, owing to the manner in which the energy is supplied, emit a strong
light, and yet be at a comparatively very low mean temperature. How
should the observer name the luminosity thus produced? Even if the
analysis of the light would teach him something definite, still he would
probably rank it under the phenomena of phosphorescence. It is
conceivable that in such a way both conducting and non-conducting bodies
may be maintained at a certain luminous intensity, but the energy
required would very greatly vary with the nature and properties of the
bodies.

These and some foregoing remarks of a speculative nature were made
merely to bring out curious features of alternate currents or electric
impulses. By their help we may cause a body to emit _more_ light, while
at a certain mean temperature, than it would emit if brought to that
temperature by a steady supply; and, again, we may bring a body to the
point of fusion, and cause it to emit _less_ light than when fused by
the application of energy in ordinary ways. It all depends on how we
supply the energy, and what kind of vibrations we set up; in one case
the vibrations are more, in the other less, adapted to affect our sense
of vision.

Some effects, which I had not observed before, obtained with carborundum
in the first trials, I attributed to phosphorescence, but in subsequent
experiments it appeared that it was devoid of that quality. The crystals
possess a noteworthy feature. In a bulb provided with a single electrode
in the shape of a small circular metal disc, for instance, at a certain
degree of exhaustion the electrode is covered with a milky film, which
is separated by a dark space from the glow filling the bulb. When the
metal disc is covered with carborundum crystals, the film is far more
intense, and snow-white. This I found later to be merely an effect of
the bright surface of the crystals, for when an aluminum electrode was
highly polished, it exhibited more or less the same phenomenon. I made a
number of experiments with the samples of crystals obtained, principally
because it would have been of special interest to find that they are
capable of phosphorescence, on account of their being conducting. I
could not produce phosphorescence distinctly, but I must remark that a
decisive opinion cannot be formed until other experimenters have gone
over the same ground.

The powder behaved in some experiments as though it contained alumina,
but it did not exhibit with sufficient distinctness the red of the
latter. Its dead color brightens considerably under the molecular
impact, but I am now convinced it does not phosphoresce. Still, the
tests with the powder are not conclusive, because powdered carborundum
probably does not behave like a phosphorescent sulphide, for example,
which could be finely powdered without impairing the phosphorescence,
but rather like powdered ruby or diamond, and therefore it would be
necessary, in order to make a decisive test, to obtain it in a large
lump and polish up the surface.

If the carborundum proves useful in connection with these and similar
experiments, its chief value will be found in the production of
coatings, thin conductors, buttons, or other electrodes capable of
withstanding extremely high degrees of heat.

The production of a small electrode, capable of withstanding enormous
temperatures, I regard as of the greatest importance in the manufacture
of light. It would enable us to obtain, by means of currents of very
high frequencies, certainly 20 times, if not more, the quantity of light
which is obtained in the present incandescent lamp by the same
expenditure of energy. This estimate may appear to many exaggerated, but
in reality I think it is far from being so. As this statement might be
misunderstood, I think it is necessary to expose clearly the problem
with which, in this line of work, we are confronted, and the manner in
which, in my opinion, a solution will be arrived at.

Any one who begins a study of the problem will be apt to think that what
is wanted in a lamp with an electrode is a very high degree of
incandescence of the electrode. There he will be mistaken. The high
incandescence of the button is a necessary evil, but what is really
wanted is the high incandescence of the gas surrounding the button. In
other words, the problem in such a lamp is to bring a mass of gas to the
highest possible incandescence. The higher the incandescence, the
quicker the mean vibration, the greater is the economy of the light
production. But to maintain a mass of gas at a high degree of
incandescence in a glass vessel, it will always be necessary to keep the
incandescent mass away from the glass; that is, to confine it as much as
possible to the central portion of the globe.

In one of the experiments this evening a brush was produced at the end
of a wire. The brush was a flame, a source of heat and light. It did not
emit much perceptible heat, nor did it glow with an intense light; but
is it the less a flame because it does not scorch my hand? Is it the
less a flame because it does not hurt my eyes by its brilliancy? The
problem is precisely to produce in the bulb such a flame, much smaller
in size, but incomparably more powerful. Were there means at hand for
producing electric impulses of a sufficiently high frequency, and for
transmitting them, the bulb could be done away with, unless it were used
to protect the electrode, or to economize the energy by confining the
heat. But as such means are not at disposal, it becomes necessary to
place the terminal in the bulb and rarefy the air in the same. This is
done merely to enable the apparatus to perform the work which it is not
capable of performing at ordinary air pressure. In the bulb we are able
to intensify the action to any degree--so far that the brush emits a
powerful light.

The intensity of the light emitted depends principally on the frequency
and potential of the impulses, and on the electric density on the
surface of the electrode. It is of the greatest importance to employ the
smallest possible button, in order to push the density very far. Under
the violent impact of the molecules of the gas surrounding it, the small
electrode is of course brought to an extremely high temperature, but
around it is a mass of highly incandescent gas, a flame photosphere,
many hundred times the volume of the electrode. With a diamond,
carborundum or zirconia button the photosphere can be as much as one
thousand times the volume of the button. Without much reflection one
would think that in pushing so far the incandescence of the electrode it
would be instantly volatilized. But after a careful consideration one
would find that, theoretically, it should not occur, and in this
fact--which, moreover, is experimentally demonstrated--lies principally
the future value of such a lamp.

At first, when the bombardment begins, most of the work is performed on
the surface of the button, but when a highly conducting photosphere is
formed the button is comparatively relieved. The higher the
incandescence of the photosphere, the more it approaches in conductivity
to that of the electrode, and the more, therefore, the solid and the gas
form one conducting body. The consequence is that the further the
incandescence is forced the more work, comparatively, is performed on
the gas, and the less on the electrode. The formation of a powerful
photosphere is consequently the very means for protecting the electrode.
This protection, of course, is a relative one, and it should not be
thought that by pushing the incandescence higher the electrode is
actually less deteriorated. Still, theoretically, with extreme
frequencies, this result must be reached, but probably at a temperature
too high for most of the refractory bodies known. Given, then, an
electrode which can withstand to a very high limit the effect of the
bombardment and outward strain, it would be safe, no matter how much it
was forced beyond that limit. In an incandescent lamp quite different
considerations apply. There the gas is not at all concerned; the whole
of the work is performed on the filament; and the life of the lamp
diminishes so rapidly with the increase of the degree of incandescence
that economical reasons compel us to work it at a low incandescence. But
if an incandescent lamp is operated with currents of very high
frequency, the action of the gas cannot be neglected, and the rules for
the most economical working must be considerably modified.

In order to bring such a lamp with one or two electrodes to a great
perfection, it is necessary to employ impulses of very high frequency.
The high frequency secures, among others, two chief advantages, which
have a most important bearing upon the economy of the light production.
First, the deterioration of the electrode is reduced by reason of the
fact that we employ a great many small impacts, instead of a few violent
ones, which quickly shatter the structure; secondly, the formation of a
large photosphere is facilitated.

In order to reduce the deterioration of the electrode to the minimum, it
is desirable that the vibration be harmonic, for any suddenness hastens
the process of destruction. An electrode lasts much longer when kept at
incandescence by currents, or impulses, obtained from a high frequency
alternator, which rise and fall more or less harmonically, than by
impulses obtained from a disruptive discharge coil. In the latter case
there is no doubt that most of the damage is done by the fundamental
sudden discharges.

One of the elements of loss in such a lamp is the bombardment of the
globe. As the potential is very high, the molecules are projected with
great speed; they strike the glass, and usually excite a strong
phosphorescence. The effect produced is very pretty, but for economical
reasons it would be perhaps preferable to prevent, or at least reduce to
a minimum, the bombardment against the globe, as in such case it is, as
a rule, not the object to excite phosphorescence, and as some loss of
energy results from the bombardment. This loss in the bulb is
principally dependent on the potential of the impulses and on the
electric density on the surface of the electrode. In employing very high
frequencies the loss of energy by the bombardment is greatly reduced,
for, first, the potential needed to perform a given amount of work is
much smaller; and, secondly, by producing a highly conducting
photosphere around the electrode, the same result is obtained as though
the electrode were much larger, which is equivalent to a smaller
electric density. But be it by the diminution of the maximum potential
or of the density, the gain is effected in the same manner, namely, by
avoiding violent shocks, which strain the glass much beyond its limit of
elasticity. If the frequency could be brought high enough, the loss due
to the imperfect elasticity of the glass would be entirely negligible.
The loss due to bombardment of the globe may, however, be reduced by
using two electrodes instead of one. In such case each of the electrodes
may be connected to one of the terminals; or else, if it is preferable
to use only one wire, one electrode may be connected to one terminal and
the other to the ground or to an insulated body of some surface, as, for
instance, a shade on the lamp. In the latter case, unless some judgment
is used, one of the electrodes might glow more intensely than the other.

But on the whole I find it preferable, when using such high frequencies,
to employ only one electrode and one connecting wire. I am convinced
that the illuminating device of the near future will not require for its
operation more than one lead, and, at any rate, it will have no
leading-in wire, since the energy required can be as well transmitted
through the glass. In experimental bulbs the leading-in wire is not
generally used on account of convenience, as in employing condenser
coatings in the manner indicated in Fig. 151, for example, there is some
difficulty in fitting the parts, but these difficulties would not exist
if a great many bulbs were manufactured; otherwise the energy can be
conveyed through the glass as well as through a wire, and with these
high frequencies the losses are very small. Such illustrating devices
will necessarily involve the use of very high potentials, and this, in
the eyes of practical men, might be an objectionable feature. Yet, in
reality, high potentials are not objectionable--certainly not in the
least so far as the safety of the devices is concerned.

There are two ways of rendering an electric appliance safe. One is to
use low potentials, the other is to determine the dimensions of the
apparatus so that it is safe, no matter how high a potential is used. Of
the two, the latter seems to me the better way, for then the safety is
absolute, unaffected by any possible combination of circumstances which
might render even a low-potential appliance dangerous to life and
property. But the practical conditions require not only the judicious
determination of the dimensions of the apparatus; they likewise
necessitate the employment of energy of the proper kind. It is easy, for
instance, to construct a transformer capable of giving, when operated
from an ordinary alternate current machine of low tension, say 50,000
volts, which might be required to light a highly exhausted
phosphorescent tube, so that, in spite of the high potential, it is
perfectly safe, the shock from it producing no inconvenience. Still such
a transformer would be expensive, and in itself inefficient; and,
besides, what energy was obtained from it would not be economically used
for the production of light. The economy demands the employment of
energy in the form of extremely rapid vibrations. The problem of
producing light has been likened to that of maintaining a certain
high-pitch note by means of a bell. It should be said a _barely audible_
note; and even these words would not express it, so wonderful is the
sensitiveness of the eye. We may deliver powerful blows at long
intervals, waste a good deal of energy, and still not get what we want;
or we may keep up the note by delivering frequent taps, and get nearer
to the object sought by the expenditure of much less energy. In the
production of light, as far as the illuminating device is concerned,
there can be only one rule--that is, to use as high frequencies as can
be obtained; but the means for the production and conveyance of impulses
of such character impose, at present at least, great limitations. Once
it is decided to use very high frequencies, the return wire becomes
unnecessary, and all the appliances are simplified. By the use of
obvious means the same result is obtained as though the return wire were
used. It is sufficient for this purpose to bring in contact with the
bulb, or merely in the vicinity of the same, an insulated body of some
surface. The surface need, of course, be the smaller, the higher the
frequency and potential used, and necessarily, also, the higher the
economy of the lamp or other device.

This plan of working has been resorted to on several occasions this
evening. So, for instance, when the incandescence of a button was
produced by grasping the bulb with the hand, the body of the
experimenter merely served to intensify the action. The bulb used was
similar to that illustrated in Fig. 148, and the coil was excited to a
small potential, not sufficient to bring the button to incandescence
when the bulb was hanging from the wire; and incidentally, in order to
perform the experiment in a more suitable manner, the button was taken
so large that a perceptible time had to elapse before, upon grasping the
bulb, it could be rendered incandescent. The contact with the bulb was,
of course, quite unnecessary. It is easy, by using a rather large bulb
with an exceedingly small electrode, to adjust the conditions so that
the latter is brought to bright incandescence by the mere approach of
the experimenter within a few feet of the bulb, and that the
incandescence subsides upon his receding.

[Illustration: FIG. 153.]

[Illustration: FIG. 154.]

In another experiment, when phosphorescence was excited, a similar bulb
was used. Here again, originally, the potential was not sufficient to
excite phosphorescence until the action was intensified--in this case,
however, to present a different feature, by touching the socket with a
metallic object held in the hand. The electrode in the bulb was a carbon
button so large that it could not be brought to incandescence, and
thereby spoil the effect produced by phosphorescence.

Again, in another of the early experiments, a bulb was used, as
illustrated in Fig. 141. In this instance, by touching the bulb with one
or two fingers, one or two shadows of the stem inside were projected
against the glass, the touch of the finger producing the same results as
the application of an external negative electrode under ordinary
circumstances.

In all these experiments the action was intensified by augmenting the
capacity at the end of the lead connected to the terminal. As a rule, it
is not necessary to resort to such means, and would be quite unnecessary
with still higher frequencies; but when it _is_ desired, the bulb, or
tube, can be easily adapted to the purpose.

In Fig. 153, for example, an experimental bulb, L, is shown, which
is provided with a neck, _n_, on the top, for the application of an
external tinfoil coating, which may be connected to a body of larger
surface. Such a lamp as illustrated in Fig. 154 may also be lighted by
connecting the tinfoil coating on the neck _n_ to the terminal, and the
leading-in wire, _w_, to an insulated plate. If the bulb stands in a
socket upright, as shown in the cut, a shade of conducting material may
be slipped in the neck, _n_, and the action thus magnified.

A more perfected arrangement used in some of these bulbs is illustrated
in Fig. 155. In this case the construction of the bulb is as shown and
described before, when reference was made to Fig. 148. A zinc sheet, Z,
with a tubular extension, T, is applied over the metallic socket, S.
The bulb hangs downward from the terminal, _t_, the zinc sheet, Z,
performing the double office of intensifier and reflector. The reflector
is separated from the terminal, _t_, by an extension of the insulating
plug, P.

A similar disposition with a phosphorescent tube is illustrated in
Fig. 156. The tube, T, is prepared from two short tubes of different
diameter, which are sealed on the ends. On the lower end is placed an
inside conducting coating, C, which connects to the wire _w_. The wire
has a hook on the upper end for suspension, and passes through the
centre of the inside tube, which is filled with some good and tightly
packed insulator. On the outside of the upper end of the tube, T, is
another conducting coating, C_{1}, upon which is slipped a metallic
reflector Z, which should be separated by a thick insulation from the
end of wire _w_.

The economical use of such a reflector or intensifier would require that
all energy supplied to an air condenser should be recoverable, or, in
other words, that there should not be any losses, neither in the
gaseous medium nor through its action elsewhere. This is far from being
so, but, fortunately, the losses may be reduced to anything desired. A
few remarks are necessary on this subject, in order to make the
experiences gathered in the course of these investigations perfectly
clear.

[Illustration: FIG. 155.]

Suppose a small helix with many well insulated turns, as in experiment
Fig. 146, has one of its ends connected to one of the terminals of the
induction coil, and the other to a metal plate, or, for the sake of
simplicity, a sphere, insulated in space. When the coil is set to work,
the potential of the sphere is alternated, and a small helix now behaves
as though its free end were connected to the other terminal of the
induction coil. If an iron rod be held within a small helix, it is
quickly brought to a high temperature, indicating the passage of a
strong current through the helix. How does the insulated sphere act in
this case? It can be a condenser, storing and returning the energy
supplied to it, or it can be a mere sink of energy, and the conditions
of the experiment determine whether it is rather one than the other. The
sphere being charged to a high potential, it acts inductively upon the
surrounding air, or whatever gaseous medium there might be. The
molecules, or atoms, which are near the sphere, are of course more
attracted, and move through a greater distance than the farther ones.
When the nearest molecules strike the sphere, they are repelled, and
collisions occur at all distances within the inductive action of the
sphere. It is now clear that, if the potential be steady, but little
loss of energy can be caused in this way, for the molecules which are
nearest to the sphere, having had an additional charge imparted to them
by contact, are not attracted until they have parted, if not with all,
at least with most of the additional charge, which can be accomplished
only after a great many collisions. From the fact, that with a steady
potential there is but little loss in dry air, one must come to such a
conclusion. When the potential of a sphere, instead of being steady, is
alternating, the conditions are entirely different. In this case a
rhythmical bombardment occurs, no matter whether the molecules, after
coming in contact with the sphere, lose the imparted charge or not; what
is more, if the charge is not lost, the impacts are only the more
violent. Still, if the frequency of the impulses be very small, the loss
caused by the impacts and collisions would not be serious, unless the
potential were excessive. But when extremely high frequencies and more
or less high potentials are used, the loss may very great. The total
energy lost per unit of time is proportionate to the product of the
number of impacts per second, or the frequency and the energy lost in
each impact. But the energy of an impact must be proportionate to the
square of the electric density of the sphere, since the charge imparted
to the molecule is proportionate to that density. I conclude from this
that the total energy lost must be proportionate to the product of the
frequency and the square of the electric density; but this law needs
experimental confirmation. Assuming the preceding considerations to be
true, then, by rapidly alternating the potential of a body immersed in
an insulating gaseous medium, any amount of energy may be dissipated
into space. Most of that energy then, I believe, is not dissipated in
the form of long ether waves, propagated to considerable distance, as is
thought most generally, but is consumed--in the case of an insulated
sphere, for example--in impact and collisional losses--that is, heat
vibrations--on the surface and in the vicinity of the sphere. To reduce
the dissipation, it is necessary to work with a small electric
density--the smaller, the higher the frequency.

[Illustration: FIG. 156.]

But since, on the assumption before made, the loss is diminished with
the square of the density, and since currents of very high frequencies
involve considerable waste when transmitted through conductors, it
follows that, on the whole, it is better to employ one wire than two.
Therefore, if motors, lamps, or devices of any kind are perfected,
capable of being advantageously operated by currents of extremely high
frequency, economical reasons will make it advisable to use only one
wire, especially if the distances are great.

When energy is absorbed in a condenser, the same behaves as though its
capacity were increased. Absorption always exists more or less, but
generally it is small and of no consequence as long as the frequencies
are not very great. In using extremely high frequencies, and,
necessarily in such case, also high potentials, the absorption--or, what
is here meant more particularly by this term, the loss of energy due to
the presence of a gaseous medium--is an important factor to be
considered, as the energy absorbed in the air condenser may be any
fraction of the supplied energy. This would seem to make it very
difficult to tell from the measured or computed capacity of an air
condenser its actual capacity or vibration period, especially if the
condenser is of very small surface and is charged to a very high
potential. As many important results are dependent upon the correctness
of the estimation of the vibration period, this subject demands the most
careful scrutiny of other investigators. To reduce the probable error as
much as possible in experiments of the kind alluded to, it is advisable
to use spheres or plates of large surface, so as to make the density
exceedingly small. Otherwise, when it is practicable, an oil condenser
should be used in preference. In oil or other liquid dielectrics there
are seemingly no such losses as in gaseous media. It being impossible to
exclude entirely the gas in condensers with solid dielectrics, such
condensers should be immersed in oil, for economical reasons, if nothing
else; they can then be strained to the utmost, and will remain cool. In
Leyden jars the loss due to air is comparatively small, as the tinfoil
coatings are large, close together, and the charged surfaces not
directly exposed; but when the potentials are very high, the loss may be
more or less considerable at, or near, the upper edge of the foil, where
the air is principally acted upon. If the jar be immersed in boiled-out
oil, it will be capable of performing four times the amount of work
which it can for any length of time when used in the ordinary way, and
the loss will be inappreciable.

It should not be thought that the loss in heat in an air condenser is
necessarily associated with the formation of _visible_ streams or
brushes. If a small electrode, inclosed in an unexhausted bulb, is
connected to one of the terminals of the coil, streams can be seen to
issue from the electrode, and the air in the bulb is heated; if instead
of a small electrode a large sphere is inclosed in the bulb, no streams
are observed, still the air is heated.

Nor should it be thought that the temperature of an air condenser would
give even an approximate idea of the loss in heat incurred, as in such
case heat must be given off much more quickly, since there is, in
addition to the ordinary radiation, a very active carrying away of heat
by independent carriers going on, and since not only the apparatus, but
the air at some distance from it is heated in consequence of the
collisions which must occur.

Owing to this, in experiments with such a coil, a rise of temperature
can be distinctly observed only when the body connected to the coil is
very small. But with apparatus on a larger scale, even a body of
considerable bulk would be heated, as, for instance, the body of a
person; and I think that skilled physicians might make observations of
utility in such experiments, which, if the apparatus were judiciously
designed, would not present the slightest danger.

A question of some interest, principally to meteorologists, presents
itself here. How does the earth behave? The earth is an air condenser,
but is it a perfect or a very imperfect one--a mere sink of energy?
There can be little doubt that to such small disturbance as might be
caused in an experiment, the earth behaves as an almost perfect
condenser. But it might be different when its charge is set in vibration
by some sudden disturbance occurring in the heavens. In such case, as
before stated, probably only little of the energy of the vibrations set
up would be lost into space in the form of long ether radiations, but
most of the energy, I think, would spend itself in molecular impacts and
collisions, and pass off into space in the form of short heat, and
possibly light, waves. As both the frequency of the vibrations of the
charge and the potential are in all probability excessive, the energy
converted into heat may be considerable. Since the density must be
unevenly distributed, either in consequence of the irregularity of the
earth's surface, or on account of the condition of the atmosphere in
various places, the effect produced would accordingly vary from place to
place. Considerable variations in the temperature and pressure of the
atmosphere may in this manner be caused at any point of the surface of
the earth. The variations may be gradual or very sudden, according to
the nature of the general disturbance, and may produce rain and storms,
or locally modify the weather in any way.

From the remarks before made, one may see what an important factor of
loss the air in the neighborhood of a charged surface becomes when the
electric density is great and the frequency of the impulses excessive.
But the action, as explained, implies that the air is insulating--that
is, that it is composed of independent carriers immersed in an
insulating medium. This is the case only when the air is at something
like ordinary or greater, or at extremely small, pressure. When the air
is slightly rarefied and conducting, then true conduction losses occur
also. In such case, of course, considerable energy may be dissipated
into space even with a steady potential, or with impulses of low
frequency, if the density is very great.

When the gas is at very low pressure, an electrode is heated more
because higher speeds can be reached. If the gas around the electrode is
strongly compressed, the displacements, and consequently the speeds, are
very small, and the heating is insignificant. But if in such case the
frequency could be sufficiently increased, the electrode would be
brought to a high temperature as well as if the gas were at very low
pressure; in fact, exhausting the bulb is only necessary because we
cannot produce, (and possibly not convey) currents of the required
frequency.

Returning to the subject of electrode lamps, it is obviously of
advantage in such a lamp to confine as much as possible the heat to the
electrode by preventing the circulation of the gas in the bulb. If a
very small bulb be taken, it would confine the heat better than a large
one, but it might not be of sufficient capacity to be operated from the
coil, or, if so, the glass might get too hot. A simple way to improve in
this direction is to employ a globe of the required size, but to place a
small bulb, the diameter of which is properly estimated, over the
refractory button contained in the globe. This arrangement is
illustrated in Fig. 157.

[Illustration: FIG. 157.]

[Illustration: FIG. 158.]

The globe L has in this case a large neck _n_, allowing the small bulb
_b_ to slip through. Otherwise the construction is the same as shown in
Fig. 147, for example. The small bulb is conveniently supported upon the
stem _s_, carrying the refractory button _m_. It is separated from the
aluminum tube _a_ by several layers of mica M, in order to prevent the
cracking of the neck by the rapid heating of the aluminum tube upon a
sudden turning on of the current. The inside bulb should be as small as
possible when it is desired to obtain light only by incandescence of the
electrode. If it is desired to produce phosphorescence, the bulb should
be larger, else it would be apt to get too hot, and the phosphorescence
would cease. In this arrangement usually only the small bulb shows
phosphorescence, as there is practically no bombardment against the
outer globe. In some of these bulbs constructed as illustrated in Fig.
157, the small tube was coated with phosphorescent paint, and beautiful
effects were obtained. Instead of making the inside bulb large, in order
to avoid undue heating, it answers the purpose to make the electrode _m_
larger. In this case the bombardment is weakened by reason of the
smaller electric density.

Many bulbs were constructed on the plan illustrated in Fig. 158. Here a
small bulb _b_, containing the refractory button _m_, upon being
exhausted to a very high degree was sealed in a large globe L, which was
then moderately exhausted and sealed off. The principal advantage of
this construction was that it allowed of reaching extremely high vacua,
and, at the same time of using a large bulb. It was found, in the course
of experiments with bulbs such as illustrated in Fig. 158, that it was
well to make the stem _s_, near the seal at _e_, very thick, and the
leading-in wire _w_ thin, as it occurred sometimes that the stem at _e_
was heated and the bulb was cracked. Often the outer globe L was
exhausted only just enough to allow the discharge to pass through, and
the space between the bulbs appeared crimson, producing a curious
effect. In some cases, when the exhaustion in globe L was very low, and
the air good conducting, it was found necessary, in order to bring the
button _m_ to high incandescence, to place, preferably on the upper part
of the neck of the globe, a tinfoil coating which was connected to an
insulated body, to the ground, or to the other terminal of the coil, as
the highly conducting air weakened the effect somewhat, probably by
being acted upon inductively from the wire _w_, where it entered the
bulb at _e_. Another difficulty--which, however, is always present when
the refractory button is mounted in a very small bulb--existed in the
construction illustrated in Fig. 158, namely, the vacuum in the bulb _b_
would be impaired in a comparatively short time.

The chief idea in the two last described constructions was to confine
the heat to the central portion of the globe by preventing the exchange
of air. An advantage is secured, but owing to the heating of the inside
bulb and slow evaporation of the glass, the vacuum is hard to maintain,
even if the construction illustrated in Fig. 157 be chosen, in which
both bulbs communicate.

But by far the better way--the ideal way--would be to reach sufficiently
high frequencies. The higher the frequency, the slower would be the
exchange of the air, and I think that a frequency may be reached, at
which there would be no exchange whatever of the air molecules around
the terminal. We would then produce a flame in which there would be no
carrying away of material, and a queer flame it would be, for it would
be rigid! With such high frequencies the inertia of the particles would
come into play. As the brush, or flame, would gain rigidity in virtue of
the inertia of the particles, the exchange of the latter would be
prevented. This would necessarily occur, for, the number of impulses
being augmented, the potential energy of each would diminish, so that
finally only atomic vibrations could be set up, and the motion of
translation through measurable space would cease. Thus an ordinary gas
burner connected to a source of rapidly alternating potential might have
its efficiency augmented to a certain limit, and this for two
reasons--because of the additional vibration imparted, and because of a
slowing down of the process of carrying off. But the renewal being
rendered difficult, a renewal being necessary to maintain the _burner_,
a continued increase of the frequency of the impulses, assuming they
could be transmitted to and impressed upon the flame, would result in
the "extinction" of the latter, meaning by this term only the cessation
of the chemical process.

I think, however, that in the case of an electrode immersed in a fluid
insulating medium, and surrounded by independent carriers of electric
charges, which can be acted upon inductively, a sufficient high
frequency of the impulses would probably result in a gravitation of the
gas all around toward the electrode. For this it would be only necessary
to assume that the independent bodies are irregularly shaped; they would
then turn toward the electrode their side of the greatest electric
density, and this would be a position in which the fluid resistance to
approach would be smaller than that offered to the receding.

The general opinion, I do not doubt, is that it is out of the question
to reach any such frequencies as might--assuming some of the views
before expressed to be true--produce any of the results which I have
pointed out as mere possibilities. This may be so, but in the course of
these investigations, from the observation of many phenomena, I have
gained the conviction that these frequencies would be much lower than
one is apt to estimate at first. In a flame we set up light vibrations
by causing molecules, or atoms, to collide. But what is the ratio of the
frequency of the collisions and that of the vibrations set up? Certainly
it must be incomparably smaller than that of the strokes of the bell and
the sound vibrations, or that of the discharges and the oscillations of
the condenser. We may cause the molecules of the gas to collide by the
use of alternate electric impulses of high frequency, and so we may
imitate the process in a flame; and from experiments with frequencies
which we are now able to obtain, I think that the result is producible
with impulses which are transmissible through a conductor.

In connection with thoughts of a similar nature, it appeared to me of
great interest to demonstrate the rigidity of a vibrating gaseous
column. Although with such low frequencies as, say 10,000 per second,
which I was able to obtain without difficulty from a specially
constructed alternator, the task looked discouraging at first, I made a
series of experiments. The trials with air at ordinary pressure led to
no result, but with air moderately rarefied I obtain what I think to be
an unmistakable experimental evidence of the property sought for. As a
result of this kind might lead able investigators to conclusions of
importance, I will describe one of the experiments performed.

It is well known that when a tube is slightly exhausted, the discharge
may be passed through it in the form of a thin luminous thread. When
produced with currents of low frequency, obtained from a coil operated
as usual, this thread is inert. If a magnet be approached to it, the
part near the same is attracted or repelled, according to the direction
of the lines of force of the magnet. It occurred to me that if such a
thread would be produced with currents of very high frequency, it should
be more or less rigid, and as it was visible it could be easily studied.
Accordingly I prepared a tube about one inch in diameter and one metre
long, with outside coating at each end. The tube was exhausted to a
point at which, by a little working, the thread discharge could be
obtained. It must be remarked here that the general aspect of the tube,
and the degree of exhaustion, are quite other than when ordinary low
frequency currents are used. As it was found preferable to work with one
terminal, the tube prepared was suspended from the end of a wire
connected to the terminal, the tinfoil coating being connected to the
wire, and to the lower coating sometimes a small insulated plate was
attached. When the thread was formed, it extended through the upper part
of the tube and lost itself in the lower end. If it possessed rigidity
it resembled, not exactly an elastic cord stretched tight between two
supports, but a cord suspended from a height with a small weight
attached at the end. When the finger or a small magnet was approached to
the upper end of the luminous thread, it could be brought locally out of
position by electrostatic or magnetic action; and when the disturbing
object was very quickly removed, an analogous result was produced, as
though a suspended cord would be displaced and quickly released near the
point of suspension. In doing this the luminous thread was set in
vibration, and two very sharply marked nodes, and a third indistinct
one, were formed. The vibration, once set up, continued for fully eight
minutes, dying gradually out. The speed of the vibration often varied
perceptibly, and it could be observed that the electrostatic attraction
of the glass affected the vibrating thread; but it was clear that the
electrostatic action was not the cause of the vibration, for the thread
was most generally stationary, and could always be set in vibration by
passing the finger quickly near the upper part of the tube. With a
magnet the thread could be split in two and both parts vibrated. By
approaching the hand to the lower coating of the tube, or insulation
plate if attached, the vibration was quickened; also, as far as I could
see, by raising the potential or frequency. Thus, either increasing the
frequency or passing a stronger discharge of the same frequency
corresponded to a tightening of the cord. I did not obtain any
experimental evidence with condenser discharges. A luminous band excited
in the bulb by repeated discharges of a Leyden jar must possess
rigidity, and if deformed and suddenly released, should vibrate. But
probably the amount of vibrating matter is so small that in spite of the
extreme speed, the inertia cannot prominently assert itself. Besides,
the observation in such a case is rendered extremely difficult on
account of the fundamental vibration.

The demonstration of the fact--which still needs better experimental
confirmation--that a vibrating gaseous column possesses rigidity, might
greatly modify the views of thinkers. When with low frequencies and
insignificant potentials indications of that property may be noted, how
must a gaseous medium behave under the influence of enormous
electrostatic stresses which may be active in the interstellar space,
and which may alternate with inconceivable rapidity? The existence of
such an electrostatic, rhythmically throbbing force--of a vibrating
electrostatic field--would show a possible way how solids might have
formed from the ultra-gaseous uterus, and how transverse and all kinds
of vibrations may be transmitted through a gaseous medium filling all
space. Then, ether might be a true fluid, devoid of rigidity, and at
rest, it being merely necessary as a connecting link to enable
interaction. What determines the rigidity of a body? It must be the
speed and the amount of motive matter. In a gas the speed maybe
considerable, but the density is exceedingly small; in a liquid the
speed would be likely to be small, though the density may be
considerable; and in both cases the inertia resistance offered to
displacement is practically _nil_. But place a gaseous (or liquid)
column in an intense, rapidly alternating electrostatic field, set the
particles vibrating with enormous speeds, then the inertia resistance
asserts itself. A body might move with more or less freedom through the
vibrating mass, but as a whole it would be rigid.

There is a subject which I must mention in connection with these
experiments: it is that of high vacua. This is a subject, the study of
which is not only interesting, but useful, for it may lead to results of
great practical importance. In commercial apparatus, such as
incandescent lamps, operated from ordinary systems of distribution, a
much higher vacuum than is obtained at present would not secure a very
great advantage. In such a case the work is performed on the filament,
and the gas is little concerned; the improvement, therefore, would be
but trifling. But when we begin to use very high frequencies and
potentials, the action of the gas becomes all important, and the degree
of exhaustion materially modifies the results. As long as ordinary
coils, even very large ones, were used, the study of the subject was
limited, because just at a point when it became most interesting it had
to be interrupted on account of the "non-striking" vacuum being reached.
But at present we are able to obtain from a small disruptive discharge
coil potentials much higher than even the largest coil was capable of
giving, and, what is more, we can make the potential alternate with
great rapidity. Both of these results enable us now to pass a luminous
discharge through almost any vacua obtainable, and the field of our
investigations is greatly extended. Think we as we may, of all the
possible directions to develop a practical illuminant, the line of high
vacua seems to be the most promising at present. But to reach extreme
vacua the appliances must be much more improved, and ultimate perfection
will not be attained until we shall have discharged the mechanical and
perfected an _electrical_ vacuum pump. Molecules and atoms can be thrown
out of a bulb under the action of an enormous potential: _this_ will be
the principle of the vacuum pump of the future. For the present, we must
secure the best results we can with mechanical appliances. In this
respect, it might not be out of the way to say a few words about the
method of, and apparatus for, producing excessively high degrees of
exhaustion of which I have availed myself in the course of these
investigations. It is very probable that other experimenters have used
similar arrangements; but as it is possible that there may be an item of
interest in their description, a few remarks, which will render this
investigation more complete, might be permitted.

[Illustration: FIG. 159.]

The apparatus is illustrated in a drawing shown in Fig. 159. S
represents a Sprengel pump, which has been specially constructed to
better suit the work required. The stop-cock which is usually employed
has been omitted, and instead of it a hollow stopper _s_ has been fitted
in the neck of the reservoir R. This stopper has a small hole _h_,
through which the mercury descends; the size of the outlet _o_ being
properly determined with respect to the section of the fall tube _t_,
which is sealed to the reservoir instead of being connected to it in the
usual manner. This arrangement overcomes the imperfections and troubles
which often arise from the use of the stopcock on the reservoir and the
connections of the latter with the fall tube.

The pump is connected through a U-shaped tube _t_ to a very large
reservoir R_{1}. Especial care was taken in fitting the grinding
surfaces of the stoppers p and p_{1}, and both of these and the mercury
caps above them were made exceptionally long. After the U-shaped tube
was fitted and put in place, it was heated, so as to soften and take
off the strain resulting from imperfect fitting. The U-shaped tube was
provided with a stopcock C, and two ground connections g and g_{1},--one
for a small bulb _b_, usually containing caustic potash, and the other
for the receiver _r_, to be exhausted.

The reservoir R_{1}, was connected by means of a rubber tube to a
slightly larger reservoir R_{2}, each of the two reservoirs being
provided with a stopcock C_{1} and C_{2}, respectively. The reservoir
R_{2} could be raised and lowered by a wheel and rack, and the range of
its motion was so determined that when it was filled with mercury and
the stopcock C_{2} closed, so as to form a Torricellian vacuum in it
when raised, it could be lifted so high that the reservoir R_{1} would
stand a little above stopcock C_{1}; and when this stopcock was closed
and the reservoir R_{2} descended, so as to form a Torricellian vacuum
in reservoir R_{1}, it could be lowered so far as to completely empty
the latter, the mercury filling the reservoir R_{2} up to a little above
stopcock C_{2}.

The capacity of the pump and of the connections was taken as small as
possible relatively to the volume of reservoir, R_{1}, since, of course,
the degree of exhaustion depended upon the ratio of these quantities.

With this apparatus I combined the usual means indicated by former
experiments for the production of very high vacua. In most of the
experiments it was most convenient to use caustic potash. I may venture
to say, in regard to its use, that much time is saved and a more perfect
action of the pump insured by fusing and boiling the potash as soon as,
or even before, the pump settles down. If this course is not followed,
the sticks, as ordinarily employed, may give off moisture at a certain
very slow rate, and the pump may work for many hours without reaching a
very high vacuum. The potash was heated either by a spirit lamp or by
passing a discharge through it, or by passing a current through a wire
contained in it. The advantage in the latter case was that the heating
could be more rapidly repeated.

Generally the process of exhaustion was the following:--At the start,
the stop-cocks C and C_{1} being open, and all other connections closed,
the reservoir R_{2} was raised so far that the mercury filled the
reservoir R_{1} and a part of the narrow connecting U-shaped tube.
When the pump was set to work, the mercury would, of course, quickly
rise in the tube, and reservoir R_{2} was lowered, the experimenter
keeping the mercury at about the same level. The reservoir R_{2} was
balanced by a long spring which facilitated the operation, and the
friction of the parts was generally sufficient to keep it in almost any
position. When the Sprengel pump had done its work, the reservoir R_{2}
was further lowered and the mercury descended in R_{1} and filled R_{2},
whereupon stopcock C_{2} was closed. The air adhering to the walls of
R_{1} and that absorbed by the mercury was carried off, and to free the
mercury of all air the reservoir R_{2} was for a long time worked up and
down. During this process some air, which would gather below stopcock
C_{2}, was expelled from R_{2} by lowering it far enough and opening the
stopcock, closing the latter again before raising the reservoir. When
all the air had been expelled from the mercury, and no air would gather
in R_{2} when it was lowered, the caustic potash was resorted to. The
reservoir R_{2} was now again raised until the mercury in R_{1}, stood
above stopcock C_{1}. The caustic potash was fused and boiled, and
moisture partly carried off by the pump and partly re-absorbed; and this
process of heating and cooling was repeated many times, and each time,
upon the moisture being absorbed or carried off, the reservoir R_{2} was
for a long time raised and lowered. In this manner all the moisture was
carried off from the mercury, and both the reservoirs were in proper
condition to be used. The reservoir R_{2} was then again raised to the
top, and the pump was kept working for a long time. When the highest
vacuum obtainable with the pump had been reached, the potash bulb was
usually wrapped with cotton which was sprinkled with ether so as to keep
the potash at a very low temperature, then the reservoir R_{2} was
lowered, and upon reservoir R_{1} being emptied the receiver was quickly
sealed up.

When a new bulb was put on, the mercury was always raised above stopcock
C_{1}, which was closed, so as to always keep the mercury and both the
reservoirs in fine condition, and the mercury was never withdrawn from
R_{1} except when the pump had reached the highest degree of exhaustion.
It is necessary to observe this rule if it is desired to use the
apparatus to advantage.

By means of this arrangement I was able to proceed very quickly, and
when the apparatus was in perfect order it was possible to reach the
phosphorescent stage in a small bulb in less than fifteen minutes, which
is certainly very quick work for a small laboratory arrangement
requiring all in all about 100 pounds of mercury. With ordinary small
bulbs the ratio of the capacity of the pump, receiver, and connections,
and that of reservoir R was about 1 to 20, and the degrees of exhaustion
reached were necessarily very high, though I am unable to make a precise
and reliable statement how far the exhaustion was carried.

What impresses the investigator most in the course of these experiences
is the behavior of gases when subjected to great rapidly alternating
electrostatic stresses. But he must remain in doubt as to whether the
effects observed are due wholly to the molecules, or atoms, of the gas
which chemical analysis discloses to us, or whether there enters into
play another medium of a gaseous nature, comprising atoms, or molecules,
immersed in a fluid pervading the space. Such a medium surely must
exist, and I am convinced that, for instance, even if air were absent,
the surface and neighborhood of a body in space would be heated by
rapidly alternating the potential of the body; but no such heating of
the surface or neighborhood could occur if all free atoms were removed
and only a homogeneous, incompressible, and elastic fluid--such as ether
is supposed to be--would remain, for then there would be no impacts, no
collisions. In such a case, as far as the body itself is concerned, only
frictional losses in the inside could occur.

It is a striking fact that the discharge through a gas is established
with ever-increasing freedom as the frequency of the impulses is
augmented. It behaves in this respect quite contrarily to a metallic
conductor. In the latter the impedance enters prominently into play as
the frequency is increased, but the gas acts much as a series of
condensers would; the facility with which the discharge passes through,
seems to depend on the rate of change of potential. If it acts so, then
in a vacuum tube even of great length, and no matter how strong the
current, self-induction could not assert itself to any appreciable
degree. We have, then, as far as we can now see, in the gas a conductor
which is capable of transmitting electric impulses of any frequency
which we may be able to produce. Could the frequency be brought high
enough, then a queer system of electric distribution, which would be
likely to interest gas companies, might be realized: metal pipes
filled with gas--the metal being the insulator, the gas the
conductor--supplying phosphorescent bulbs, or perhaps devices as yet
uninvented. It is certainly possible to take a hollow core of copper,
rarefy the gas in the same, and by passing impulses of sufficiently high
frequency through a circuit around it, bring the gas inside to a high
degree of incandescence; but as to the nature of the forces there would
be considerable uncertainty, for it would be doubtful whether with such
impulses the copper core would act as a static screen. Such paradoxes
and apparent impossibilities we encounter at every step in this line of
work, and therein lies, to a great extent, the charm of the study.

I have here a short and wide tube which is exhausted to a high degree
and covered with a substantial coating of bronze, the coating barely
allowing the light to shine through. A metallic cap, with a hook for
suspending the tube, is fastened around the middle portion of the
latter, the clasp being in contact with the bronze coating. I now want
to light the gas inside by suspending the tube on a wire connected to
the coil. Any one who would try the experiment for the first time, not
having any previous experience, would probably take care to be quite
alone when making the trial, for fear that he might become the joke of
his assistants. Still, the bulb lights in spite of the metal coating,
and the light can be distinctly perceived through the latter. A long
tube covered with aluminum bronze lights when held in one hand--the
other touching the terminal of the coil--quite powerfully. It might be
objected that the coatings are not sufficiently conducting; still, even
if they were highly resistant, they ought to screen the gas. They
certainly screen it perfectly in a condition of rest, but far from
perfectly when the charge is surging in the coating. But the loss of
energy which occurs within the tube, notwithstanding the screen, is
occasioned principally by the presence of the gas. Were we to take a
large hollow metallic sphere and fill it with a perfect, incompressible,
fluid dielectric, there would be no loss inside of the sphere, and
consequently the inside might be considered as perfectly screened,
though the potential be very rapidly alternating. Even were the sphere
filled with oil, the loss would be incomparably smaller than when the
fluid is replaced by a gas, for in the latter case the force produces
displacements; that means impact and collisions in the inside.

No matter what the pressure of the gas may be, it becomes an important
factor in the heating of a conductor when the electric density is great
and the frequency very high. That in the heating of conductors by
lightning discharges, air is an element of great importance, is almost
as certain as an experimental fact. I may illustrate the action of the
air by the following experiment: I take a short tube which is exhausted
to a moderate degree and has a platinum wire running through the middle
from one end to the other. I pass a steady or low frequency current
through the wire, and it is heated uniformly in all parts. The heating
here is due to conduction, or frictional losses, and the gas around the
wire has--as far as we can see--no function to perform. But now let me
pass sudden discharges, or high frequency currents, through the wire.
Again the wire is heated, this time principally on the ends and least in
the middle portion; and if the frequency of the impulses, or the rate of
change, is high enough, the wire might as well be cut in the middle as
not, for practically all heating is due to the rarefied gas. Here the
gas might only act as a conductor of no impedance diverting the current
from the wire as the impedance of the latter is enormously increased,
and merely heating the ends of the wire by reason of their resistance to
the passage of the discharge. But it is not at all necessary that the
gas in the tube should be conducting; it might be at an extremely low
pressure, still the ends of the wire would be heated--as, however, is
ascertained by experience--only the two ends would in such case not be
electrically connected through the gaseous medium. Now what with these
frequencies and potentials occurs in an exhausted tube, occurs in the
lightning discharges at ordinary pressure. We only need remember one of
the facts arrived at in the course of these investigations, namely, that
to impulses of very high frequency the gas at ordinary pressure behaves
much in the same manner as though it were at moderately low pressure. I
think that in lightning discharges frequently wires or conducting
objects are volatilized merely because air is present, and that, were
the conductor immersed in an insulating liquid, it would be safe, for
then the energy would have to spend itself somewhere else. From the
behavior of gases under sudden impulses of high potential, I am led to
conclude that there can be no surer way of diverting a lightning
discharge than by affording it a passage through a volume of gas, if
such a thing can be done in a practical manner.

There are two more features upon which I think it necessary to dwell in
connection with these experiments--the "radiant state" and the
"non-striking vacuum."

Any one who has studied Crookes' work must have received the impression
that the "radiant state" is a property of the gas inseparably connected
with an extremely high degree of exhaustion. But it should be remembered
that the phenomena observed in an exhausted vessel are limited to the
character and capacity of the apparatus which is made use of. I think
that in a bulb a molecule, or atom, does not precisely move in a
straight line because it meets no obstacle, but because the velocity
imparted to it is sufficient to propel it in a sensibly straight line.
The mean free path is one thing, but the velocity--the energy associated
with the moving body--is another, and under ordinary circumstances I
believe that it is a mere question of potential or speed. A disruptive
discharge coil, when the potential is pushed very far, excites
phosphorescence and projects shadows, at comparatively low degrees of
exhaustion. In a lightning discharge, matter moves in straight lines at
ordinary pressure when the mean free path is exceedingly small, and
frequently images of wires or other metallic objects have been produced
by the particles thrown off in straight lines.

I have prepared a bulb to illustrate by an experiment the correctness of
these assertions. In a globe L, Fig. 160, I have mounted upon a lamp
filament _f_ a piece of lime _l_. The lamp filament is connected with a
wire which leads into the bulb, and the general construction of the
latter is as indicated in Fig. 148, before described. The bulb being
suspended from a wire connected to the terminal of the coil, and the
latter being set to work, the lime piece _l_ and the projecting parts of
the filament _f_ are bombarded. The degree of exhaustion is just such
that with the potential the coil is capable of giving, phosphorescence
of the glass is produced, but disappears as soon as the vacuum is
impaired. The lime containing moisture, and moisture being given off as
soon as heating occurs, the phosphorescence lasts only for a few
moments. When the lime has been sufficiently heated, enough moisture has
been given off to impair materially the vacuum of the bulb. As the
bombardment goes on, one point of the lime piece is more heated than
other points, and the result is that finally practically all the
discharge passes through that point which is intensely heated, and a
white stream of lime particles (Fig. 160) then breaks forth from that
point. This stream is composed of "radiant" matter, yet the degree of
exhaustion is low. But the particles move in straight lines because the
velocity imparted to them is great, and this is due to three causes--to
the great electric density, the high temperature of the small point, and
the fact that the particles of the lime are easily torn and thrown
off--far more easily than those of carbon. With frequencies such as we
are able to obtain, the particles are bodily thrown off and projected to
a considerable distance; but with sufficiently high frequencies no such
thing would occur; in such case only a stress would spread or a
vibration would be propagated through the bulb. It would be out of the
question to reach any such frequency on the assumption that the atoms
move with the speed of light; but I believe that such a thing is
impossible; for this an enormous potential would be required. With
potentials which we are able to obtain, even with a disruptive discharge
coil, the speed must be quite insignificant.

[Illustration: FIG. 160.]

As to the "non-striking vacuum," the point to be noted is, that it can
occur only with low frequency impulses, and it is necessitated by the
impossibility of carrying off enough energy with such impulses in high
vacuum, since the few atoms which are around the terminal upon coming in
contact with the same, are repelled and kept at a distance for a
comparatively long period of time, and not enough work can be performed
to render the effect perceptible to the eye. If the difference of
potential between the terminals is raised, the dielectric breaks down.
But with very high frequency impulses there is no necessity for such
breaking down, since any amount of work can be performed by continually
agitating the atoms in the exhausted vessel, provided the frequency is
high enough. It is easy to reach--even with frequencies obtained from an
alternator as here used--a stage at which the discharge does not pass
between two electrodes in a narrow tube, each of these being connected
to one of the terminals of the coil, but it is difficult to reach a
point at which a luminous discharge would not occur around each
electrode.

[Illustration: FIG. 161.]

[Illustration: FIG. 162.]

A thought which naturally presents itself in connection with high
frequency currents, is to make use of their powerful electrodynamic
inductive action to produce light effects in a sealed glass globe. The
leading-in wire is one of the defects of the present incandescent lamp,
and if no other improvement were made, that imperfection at least should
be done away with. Following this thought, I have carried on
experiments in various directions, of which some were indicated in my
former paper. I may here mention one or two more lines of experiment
which have been followed up.

Many bulbs were constructed as shown in Fig. 161 and Fig. 162.

In Fig. 161, a wide tube, T, was sealed to a smaller W
shaped tube U, of phosphorescent glass. In the tube T, was placed a coil
C, of aluminum wire, the ends of which were provided with small spheres,
t and t_{1}, of aluminum, and reached into the U tube. The tube T
was slipped into a socket containing a primary coil, through which
usually the discharges of Leyden jars were directed, and the rarefied
gas in the small U tube was excited to strong luminosity by the
high-tension current induced in the coil C. When Leyden jar discharges
were used to induce currents in the coil C, it was found necessary to
pack the tube T tightly with insulating powder, as a discharge would
occur frequently between the turns of the coil, especially when the
primary was thick and the air gap, through which the jars discharged,
large, and no little trouble was experienced in this way.

In Fig. 162 is illustrated another form of the bulb constructed. In this
case a tube T is sealed to a globe L. The tube contains a coil C, the
ends of which pass through two small glass tubes t and t_{1}, which
are sealed to the tube T. Two refractory buttons m and m_{1}, are
mounted on lamp filaments which are fastened to the ends of the wires
passing through the glass tubes t and t_{1}. Generally in bulbs made
on this plan the globe L communicated with the tube T. For this purpose
the ends of the small tubes t and t_{1} were heated just a trifle in
the burner, merely to hold the wires, but not to interfere with the
communication. The tube T, with the small tubes, wires through the same,
and the refractory buttons m and m_{1}, were first prepared, and
then sealed to globe L, whereupon the coil C was slipped in and the
connections made to its ends. The tube was then packed with insulating
powder, jamming the latter as tight as possible up to very nearly the
end; then it was closed and only a small hole left through which the
remainder of the powder was introduced, and finally the end of the tube
was closed. Usually in bulbs constructed as shown in Fig. 162 an
aluminum tube _a_ was fastened to the upper end _s_ of each of the tubes
t and t_{1} in order to protect that end against the heat. The
buttons m and m_{1} could be brought to any degree of incandescence
by passing the discharges of Leyden jars around the coil C. In such
bulbs with two buttons a very curious effect is produced by the
formation of the shadows of each of the two buttons.

Another line of experiment, which has been assiduously followed, was to
induce by electro-dynamic induction a current or luminous discharge in
an exhausted tube or bulb. This matter has received such able treatment
at the hands of Prof. J. J. Thomson, that I could add but little to what
he has made known, even had I made it the special subject of this
lecture. Still, since experiments in this line have gradually led me to
the present views and results, a few words must be devoted here to this
subject.

It has occurred, no doubt, to many that as a vacuum tube is made longer,
the electromotive force per unit length of the tube, necessary to pass a
luminous discharge through the latter, becomes continually smaller;
therefore, if the exhausted tube be made long enough, even with low
frequencies a luminous discharge could be induced in such a tube closed
upon itself. Such a tube might be placed around a hall or on a ceiling,
and at once a simple appliance capable of giving considerable light
would be obtained. But this would be an appliance hard to manufacture
and extremely unmanageable. It would not do to make the tube up of small
lengths, because there would be with ordinary frequencies considerable
loss in the coatings, and besides, if coatings were used, it would be
better to supply the current directly to the tube by connecting the
coatings to a transformer. But even if all objections of such nature
were removed, with low frequencies the light conversion itself would be
inefficient, as I have before stated. In using extremely high
frequencies the length of the secondary--in other words, the size of the
vessel--can be reduced as much as desired, and the efficiency of the
light conversion is increased, provided that means are invented for
efficiently obtaining such high frequencies. Thus one is led, from
theoretical and practical considerations, to the use of high
frequencies, and this means high electromotive forces and small currents
in the primary. When one works with condenser charges--and they are the
only means up to the present known for reaching these extreme
frequencies--one gets to electromotive forces of several thousands of
volts per turn of the primary. We cannot multiply the electro-dynamic
inductive effect by taking more turns in the primary, for we arrive at
the conclusion that the best way is to work with one single turn--though
we must sometimes depart from this rule--and we must get along with
whatever inductive effect we can obtain with one turn. But before one
has long experimented with the extreme frequencies required to set up in
a small bulb an electromotive force of several thousands of volts, one
realizes the great importance of electrostatic effects, and these
effects grow relatively to the electro-dynamic in significance as the
frequency is increased.

Now, if anything is desirable in this case, it is to increase the
frequency, and this would make it still worse for the electrodynamic
effects. On the other hand, it is easy to exalt the electrostatic action
as far as one likes by taking more turns on the secondary, or combining
self-induction and capacity to raise the potential. It should also be
remembered that, in reducing the current to the smallest value and
increasing the potential, the electric impulses of high frequency can be
more easily transmitted through a conductor.

These and similar thoughts determined me to devote more attention to the
electrostatic phenomena, and to endeavor to produce potentials as high
as possible, and alternating as fast as they could be made to alternate.
I then found that I could excite vacuum tubes at considerable distance
from a conductor connected to a properly constructed coil, and that I
could, by converting the oscillatory current of a conductor to a higher
potential, establish electrostatic alternating fields which acted
through the whole extent of the room, lighting up a tube no matter where
it was held in space. I thought I recognized that I had made a step in
advance, and I have persevered in this line; but I wish to say that I
share with all lovers of science and progress the one and only
desire--to reach a result of utility to men in any direction to which
thought or experiment may lead me. I think that this departure is the
right one, for I cannot see, from the observation of the phenomena which
manifest themselves as the frequency is increased, what there would
remain to act between two circuits conveying, for instance, impulses of
several hundred millions per second, except electrostatic forces. Even
with such trifling frequencies the energy would be practically all
potential, and my conviction has grown strong that, to whatever kind of
motion light may be due, it is produced by tremendous electrostatic
stresses vibrating with extreme rapidity.

[Illustration: FIG. 163.]

[Illustration: FIG. 164.]

Of all these phenomena observed with currents, or electric impulses, of
high frequency, the most fascinating for an audience are certainly those
which are noted in an electrostatic field acting through considerable
distance; and the best an unskilled lecturer can do is to begin and
finish with the exhibition of these singular effects. I take a tube in
my hand and move it about, and it is lighted wherever I may hold it;
throughout space the invisible forces act. But I may take another tube
and it might not light, the vacuum being very high. I excite it by means
of a disruptive discharge coil, and now it will light in the
electrostatic field. I may put it away for a few weeks or months, still
it retains the faculty of being excited. What change have I produced in
the tube in the act of exciting it? If a motion imparted to atoms, it is
difficult to perceive how it can persist so long without being arrested
by frictional losses; and if a strain exerted in the dielectric, such as
a simple electrification would produce, it is easy to see how it may
persist indefinitely, but very difficult to understand why such a
condition should aid the excitation when we have to deal with potentials
which are rapidly alternating.

Since I have exhibited these phenomena for the first time, I have
obtained some other interesting effects. For instance, I have produced
the incandescence of a button, filament, or wire enclosed in a tube. To
get to this result it was necessary to economize the energy which is
obtained from the field, and direct most of it on the small body to be
rendered incandescent. At the beginning the task appeared difficult, but
the experiences gathered permitted me to reach the result easily. In
Fig. 163 and Fig. 164, two such tubes are illustrated, which are
prepared for the occasion. In Fig. 163 a short tube T_{1}, sealed to
another long tube T, is provided with a stem _s_, with a platinum wire
sealed in the latter. A very thin lamp filament _l_, is fastened to this
wire and connection to the outside is made through a thin copper wire
_w_. The tube is provided with outside and inside coatings, C and C_{1},
respectively, and is filled as far as the coatings reach with
conducting, and the space above with insulating, powder. These coatings
are merely used to enable me to perform two experiments with the
tube--namely, to produce the effect desired either by direct connection
of the body of the experimenter or of another body to the wire _w_, or
by acting inductively through the glass. The stem _s_ is provided with
an aluminum tube _a_, for purposes before explained, and only a small
part of the filament reaches out of this tube. By holding the tube T_{1}
anywhere in the electrostatic field, the filament is rendered
incandescent.

A more interesting piece of apparatus is illustrated in Fig. 164. The
construction is the same as before, only instead of the lamp filament a
small platinum wire _p_, sealed in a stem _s_, and bent above it in a
circle, is connected to the copper wire _w_, which is joined to an
inside coating C. A small stem s_{1} is provided with a needle, on the
point of which is arranged, to rotate very freely, a very light fan of
mica _v_. To prevent the fan from falling out, a thin stem of glass _g_,
is bent properly and fastened to the aluminum tube. When the glass tube
is held anywhere in the electrostatic field the platinum wire becomes
incandescent, and the mica vanes are rotated very fast.

Intense phosphorescence may be excited in a bulb by merely connecting it
to a plate within the field, and the plate need not be any larger than
an ordinary lamp shade. The phosphorescence excited with these currents
is incomparably more powerful than with ordinary apparatus. A small
phosphorescent bulb, when attached to a wire connected to a coil, emits
sufficient light to allow reading ordinary print at a distance of five
to six paces. It was of interest to see how some of the phosphorescent
bulbs of Professor Crookes would behave with these currents, and he has
had the kindness to lend me a few for the occasion. The effects produced
are magnificent, especially by the sulphide of calcium and sulphide of
zinc. With the disruptive discharge coil they glow intensely merely by
holding them in the hand and connecting the body to the terminal of the
coil.

To whatever results investigations of this kind may lead, the chief
interest lies, for the present, in the possibilities they offer for the
production of an efficient illuminating device. In no branch of electric
industry is an advance more desired than in the manufacture of light.
Every thinker, when considering the barbarous methods employed, the
deplorable losses incurred in our best systems of light production, must
have asked himself, What is likely to be the light of the future? Is it
to be an incandescent solid, as in the present lamp, or an incandescent
gas, or a phosphorescent body, or something like a burner, but
incomparably more efficient?

There is little chance to perfect a gas burner; not, perhaps, because
human ingenuity has been bent upon that problem for centuries without a
radical departure having been made--though the argument is not devoid of
force--but because in a burner the highest vibrations can never be
reached, except by passing through all the low ones. For how is a flame
to proceed unless by a fall of lifted weights? Such process cannot be
maintained without renewal, and renewal is repeated passing from low to
high vibrations. One way only seems to be open to improve a burner, and
that is by trying to reach higher degrees of incandescence. Higher
incandescence is equivalent to a quicker vibration: that means more
light from the same material, and that again, means more economy. In
this direction some improvements have been made, but the progress is
hampered by many limitations. Discarding, then, the burner, there
remains the three ways first mentioned, which are essentially
electrical.

Suppose the light of the immediate future to be a solid, rendered
incandescent by electricity. Would it not seem that it is better to
employ a small button than a frail filament? From many considerations it
certainly must be concluded that a button is capable of a higher
economy, assuming, of course, the difficulties connected with the
operation of such a lamp to be effectively overcome. But to light such
a lamp we require a high potential; and to get this economically, we
must use high frequencies.

Such considerations apply even more to the production of light by the
incandescence of a gas, or by phosphorescence. In all cases we require
high frequencies and high potentials. These thoughts occurred to me a
long time ago.

Incidentally we gain, by the use of high frequencies, many advantages,
such as higher economy in the light production, the possibility of
working with one lead, the possibility of doing away with the leading-in
wire, etc.

The question is, how far can we go with frequencies? Ordinary conductors
rapidly lose the facility of transmitting electric impulses when the
frequency is greatly increased. Assume the means for the production of
impulses of very great frequency brought to the utmost perfection, every
one will naturally ask how to transmit them when the necessity arises.
In transmitting such impulses through conductors we must remember that
we have to deal with _pressure_ and _flow_, in the ordinary
interpretation of these terms. Let the pressure increase to an enormous
value, and let the flow correspondingly diminish, then such
impulses--variations merely of pressure, as it were--can no doubt be
transmitted through a wire even if their frequency be many hundreds of
millions per second. It would, of course, be out of question to transmit
such impulses through a wire immersed in a gaseous medium, even if the
wire were provided with a thick and excellent insulation, for most of
the energy would be lost in molecular bombardment and consequent
heating. The end of the wire connected to the source would be heated,
and the remote end would receive but a trifling part of the energy
supplied. The prime necessity, then, if such electric impulses are to be
used, is to find means to reduce as much as possible the dissipation.

The first thought is, to employ the thinnest possible wire surrounded by
the thickest practicable insulation. The next thought is to employ
electrostatic screens. The insulation of the wire may be covered with a
thin conducting coating and the latter connected to the ground. But this
would not do, as then all the energy would pass through the conducting
coating to the ground and nothing would get to the end of the wire. If a
ground connection is made it can only be made through a conductor
offering an enormous impedance, or through a condenser of extremely
small capacity. This, however, does not do away with other difficulties.

If the wave length of the impulses is much smaller than the length of
the wire, then corresponding short waves will be set up in the
conducting coating, and it will be more or less the same as though the
coating were directly connected to earth. It is therefore necessary to
cut up the coating in sections much shorter than the wave length. Such
an arrangement does not still afford a perfect screen, but it is ten
thousand times better than none. I think it preferable to cut up the
conducting coating in small sections, even if the current waves be much
longer than the coating.

If a wire were provided with a perfect electrostatic screen, it would be
the same as though all objects were removed from it at infinite
distance. The capacity would then be reduced to the capacity of the wire
itself, which would be very small. It would then be possible to send
over the wire current vibrations of very high frequencies at enormous
distances, without affecting greatly the character of the vibrations. A
perfect screen is of course out of the question, but I believe that with
a screen such as I have just described telephony could be rendered
practicable across the Atlantic. According to my ideas, the gutta-percha
covered wire should be provided with a third conducting coating
subdivided in sections. On the top of this should be again placed a
layer of gutta-percha and other insulation, and on the top of the whole
the armor. But such cables will not be constructed, for ere long
intelligence--transmitted without wires--will throb through the earth
like a pulse through a living organism. The wonder is that, with the
present state of knowledge and the experiences gained, no attempt is
being made to disturb the electrostatic or magnetic condition of the
earth, and transmit, if nothing else, intelligence.

It has been my chief aim in presenting these results to point out
phenomena or features of novelty, and to advance ideas which I am
hopeful will serve as starting points of new departures. It has been my
chief desire this evening to entertain you with some novel experiments.
Your applause, so frequently and generously accorded, has told me that I
have succeeded.

In conclusion, let me thank you most heartily for your kindness and
attention, and assure you that the honor I have had in addressing such
a distinguished audience, the pleasure I have had in presenting these
results to a gathering of so many able men--and among them also some of
those in whose work for many years past I have found enlightenment and
constant pleasure--I shall never forget.




CHAPTER XXVIII.

ON LIGHT AND OTHER HIGH FREQUENCY PHENOMENA.[3]

  [3] A lecture delivered before the Franklin Institute, Philadelphia,
      February, 1893, and before the National Electric Light
      Association, St. Louis, March, 1893.


INTRODUCTORY.--SOME THOUGHTS ON THE EYE.

When we look at the world around us, on Nature, we are impressed with
its beauty and grandeur. Each thing we perceive, though it may be
vanishingly small, is in itself a world, that is, like the whole of the
universe, matter and force governed by law,--a world, the contemplation
of which fills us with feelings of wonder and irresistibly urges us to
ceaseless thought and inquiry. But in all this vast world, of all
objects our senses reveal to us, the most marvellous, the most appealing
to our imagination, appears no doubt a highly developed organism, a
thinking being. If there is anything fitted to make us admire Nature's
handiwork, it is certainly this inconceivable structure, which performs
its innumerable motions of obedience to external influence. To
understand its workings, to get a deeper insight into this Nature's
masterpiece, has ever been for thinkers a fascinating aim, and after
many centuries of arduous research men have arrived at a fair
understanding of the functions of its organs and senses. Again, in all
the perfect harmony of its parts, of the parts which constitute the
material or tangible of our being, of all its organs and senses, the eye
is the most wonderful. It is the most precious, the most indispensable
of our perceptive or directive organs, it is the great gateway through
which all knowledge enters the mind. Of all our organs, it is the one,
which is in the most intimate relation with that which we call
intellect. So intimate is this relation, that it is often said, the very
soul shows itself in the eye.

It can be taken as a fact, which the theory of the action of the eye
implies, that for each external impression, that is, for each image
produced upon the retina, the ends of the visual nerves, concerned in
the conveyance of the impression to the mind, must be under a peculiar
stress or in a vibratory state. It now does not seem improbable that,
when by the power of thought an image is evoked, a distinct reflex
action, no matter how weak, is exerted upon certain ends of the visual
nerves, and therefore upon the retina. Will it ever be within human
power to analyze the condition of the retina when disturbed by thought
or reflex action, by the help of some optical or other means of such
sensitiveness, that a clear idea of its state might be gained at any
time? If this were possible, then the problem of reading one's thoughts
with precision, like the characters of an open book, might be much
easier to solve than many problems belonging to the domain of positive
physical science, in the solution of which many, if not the majority, of
scientific men implicitly believe. Helmholtz, has shown that the fundi
of the eye are themselves, luminous, and he was able to _see_, in total
darkness, the movement of his arm by the light of his own eyes. This is
one of the most remarkable experiments recorded in the history of
science, and probably only a few men could satisfactorily repeat it, for
it is very likely, that the luminosity of the eyes is associated with
uncommon activity of the brain and great imaginative power. It is
fluorescence of brain action, as it were.

Another fact having a bearing on this subject which has probably been
noted by many, since it is stated in popular expressions, but which I
cannot recollect to have found chronicled as a positive result of
observation is, that at times, when a sudden idea or image presents
itself to the intellect, there is a distinct and sometimes painful
sensation of luminosity produced in the eye, observable even in broad
daylight.

The saying then, that the soul shows itself in the eye, is deeply
founded, and we feel that it expresses a great truth. It has a profound
meaning even for one who, like a poet or artist, only following his
inborn instinct or love for Nature, finds delight in aimless thoughts
and in the mere contemplation of natural phenomena, but a still more
profound meaning for one who, in the spirit of positive scientific
investigation, seeks to ascertain the causes of the effects. It is
principally the natural philosopher, the physicist, for whom the eye is
the subject of the most intense admiration.

Two facts about the eye must forcibly impress the mind of the physicist,
notwithstanding he may think or say that it is an imperfect optical
instrument, forgetting, that the very conception of that which is
perfect or seems so to him, has been gained through this same
instrument. First, the eye is, as far as our positive knowledge goes,
the only organ which is _directly_ affected by that subtile medium,
which as science teaches us, must fill all space; secondly, it is the
most sensitive of our organs, incomparably more sensitive to external
impressions than any other.

The organ of hearing implies the impact of ponderable bodies, the organ
of smell the transference of detached material particles, and the organs
of taste, and of touch or force, the direct contact, or at least some
interference of ponderable matter, and this is true even in those
instances of animal organisms, in which some of these organs are
developed to a degree of truly marvelous perfection. This being so, it
seems wonderful that the organ of sight solely should be capable of
being stirred by that, which all our other organs are powerless to
detect, yet which plays an essential part in all natural phenomena,
which transmits all energy and sustains all motion and, that most
intricate of all, life, but which has properties such that even a
scientifically trained mind cannot help drawing a distinction between it
and all that is called matter. Considering merely this, and the fact
that the eye, by its marvelous power, widens our otherwise very narrow
range of perception far beyond the limits of the small world which is
our own, to embrace myriads of other worlds, suns and stars in the
infinite depths of the universe, would make it justifiable to assert,
that it is an organ of a higher order. Its performances are beyond
comprehension. Nature as far as we know never produced anything more
wonderful. We can get barely a faint idea of its prodigious power by
analyzing what it does and by comparing. When ether waves impinge upon
the human body, they produce the sensations of warmth or cold, pleasure
or pain, or perhaps other sensations of which we are not aware, and any
degree or intensity of these sensations, which degrees are infinite in
number, hence an infinite number of distinct sensations. But our sense
of touch, or our sense of force, cannot reveal to us these differences
in degree or intensity, unless they are very great. Now we can readily
conceive how an organism, such as the human, in the eternal process of
evolution, or more philosophically speaking, adaptation to Nature, being
constrained to the use of only the sense of touch or force, for
instance, might develop this sense to such a degree of sensitiveness or
perfection, that it would be capable of distinguishing the minutest
differences in the temperature of a body even at some distance, to a
hundredth, or thousandth, or millionth part of a degree. Yet, even this
apparently impossible performance would not begin to compare with that
of the eye, which is capable of distinguishing and conveying to the mind
in a single instant innumerable peculiarities of the body, be it in
form, or color, or other respects. This power of the eye rests upon two
things, namely, the rectilinear propagation of the disturbance by which
it is effected, and upon its sensitiveness. To say that the eye is
sensitive is not saying anything. Compared with it, all other organs are
monstrously crude. The organ of smell which guides a dog on the trail of
a deer, the organ of touch or force which guides an insect in its
wanderings, the organ of hearing, which is affected by the slightest
disturbances of the air, are sensitive organs, to be sure, but what are
they compared with the human eye! No doubt it responds to the faintest
echoes or reverberations of the medium; no doubt, it brings us tidings
from other worlds, infinitely remote, but in a language we cannot as yet
always understand. And why not? Because we live in a medium filled with
air and other gases, vapors and a dense mass of solid particles flying
about. These play an important part in many phenomena; they fritter away
the energy of the vibrations before they can reach the eye; they too,
are the carriers of germs of destruction, they get into our lungs and
other organs, clog up the channels and imperceptibly, yet inevitably,
arrest the stream of life. Could we but do away with all ponderable
matter in the line of sight of the telescope, it would reveal to us
undreamt of marvels. Even the unaided eye, I think, would be capable of
distinguishing in the pure medium, small objects at distances measured
probably by hundreds or perhaps thousands of miles.

But there is something else about the eye which impresses us still more
than these wonderful features which we observed, viewing it from the
standpoint of a physicist, merely as an optical instrument,--something
which appeals to us more than its marvelous faculty of being directly
affected by the vibrations of the medium, without interference of gross
matter, and more than its inconceivable sensitiveness and discerning
power. It is its significance in the processes of life. No matter what
one's views on nature and life may be, he must stand amazed when, for
the first time in his thoughts, he realizes the importance of the eye in
the physical processes and mental performances of the human organism.
And how could it be otherwise, when he realizes, that the eye is the
means through which the human race has acquired the entire knowledge it
possesses, that it controls all our motions, more still, all our
actions.

There is no way of acquiring knowledge except through the eye. What is
the foundation of all philosophical systems of ancient and modern times,
in fact, of all the philosophy of man? _I am, I think; I think,
therefore I am._ But how could I think and how would I know that I
exist, if I had not the eye? For knowledge involves consciousness;
consciousness involves ideas, conceptions; conceptions involve pictures
or images, and images the sense of vision, and therefore the organ of
sight. But how about blind men, will be asked? Yes, a blind man may
depict in magnificent poems, forms and scenes from real life, from a
world he physically does not see. A blind man may touch the keys of an
instrument with unerring precision, may model the fastest boat, may
discover and invent, calculate and construct, may do still greater
wonders--but all the blind men who have done such things have descended
from those who had seeing eyes. Nature may reach the same result in many
ways. Like a wave in the physical world, in the infinite ocean of the
medium which pervades all, so in the world of organisms, in life, an
impulse started proceeds onward, at times, may be, with the speed of
light, at times, again, so slowly that for ages and ages it seems to
stay, passing through processes of a complexity inconceivable to men,
but in all its forms, in all its stages, its energy ever and ever
integrally present. A single ray of light from a distant star falling
upon the eye of a tyrant in bygone times, may have altered the course of
his life, may have changed the destiny of nations, may have transformed
the surface of the globe, so intricate, so inconceivably complex are the
processes in Nature. In no way can we get such an overwhelming idea of
the grandeur of Nature, as when we consider, that in accordance with the
law of the conservation of energy, throughout the infinite, the forces
are in a perfect balance, and hence the energy of a single thought may
determine the motion of a Universe. It is not necessary that every
individual, not even that every generation or many generations, should
have the physical instrument of sight, in order to be able to form
images and to think, that is, form ideas or conceptions; but sometime or
other, during the process of evolution, the eye certainly must have
existed, else thought, as we understand it, would be impossible; else
conceptions, like spirit, intellect, mind, call it as you may, could not
exist. It is conceivable, that in some other world, in some other
beings, the eye is replaced by a different organ, equally or more
perfect, but these beings cannot be men.

Now what prompts us all to voluntary motions and actions of any kind?
Again the eye. If I am conscious of the motion, I must have an idea or
conception, that is, an image, therefore the eye. If I am not precisely
conscious of the motion, it is, because the images are vague or
indistinct, being blurred by the superimposition of many. But when I
perform the motion, does the impulse which prompts me to the action come
from within or from without? The greatest physicists have not disdained
to endeavor to answer this and similar questions and have at times
abandoned themselves to the delights of pure and unrestrained thought.
Such questions are generally considered not to belong to the realm of
positive physical science, but will before long be annexed to its
domain. Helmholtz has probably thought more on life than any modern
scientist. Lord Kelvin expressed his belief that life's process is
electrical and that there is a force inherent to the organism and
determining its motions. Just as much as I am convinced of any physical
truth I am convinced that the motive impulse must come from the outside.
For, consider the lowest organism we know--and there are probably many
lower ones--an aggregation of a few cells only. If it is capable of
voluntary motion it can perform an infinite number of motions, all
definite and precise. But now a mechanism consisting of a finite number
of parts and few at that, cannot perform an infinite number of definite
motions, hence the impulses which govern its movements must come from
the environment. So, the atom, the ulterior element of the Universe's
structure, is tossed about in space, eternally, a play to external
influences, like a boat in a troubled sea. Were it to stop its motion
_it would die_. Matter at rest, if such a thing could exist, would be
matter dead. Death of matter! Never has a sentence of deeper
philosophical meaning been uttered. This is the way in which Prof.
Dewar forcibly expresses it in the description of his admirable
experiments, in which liquid oxygen is handled as one handles water, and
air at ordinary pressure is made to condense and even to solidify by the
intense cold. Experiments, which serve to illustrate, in his language,
the last feeble manifestations of life, the last quiverings of matter
about to die. But human eyes shall not witness such death. There is no
death of matter, for throughout the infinite universe, all has to move,
to vibrate, that is, to live.

I have made the preceding statements at the peril of treading upon
metaphysical ground, in my desire to introduce the subject of this
lecture in a manner not altogether uninteresting, I may hope, to an
audience such as I have the honor to address. But now, then, returning
to the subject, this divine organ of sight, this indispensable
instrument for thought and all intellectual enjoyment, which lays open
to us the marvels of this universe, through which we have acquired what
knowledge we possess, and which prompts us to, and controls, all our
physical and mental activity. By what is it affected? By light! What is
light?

We have witnessed the great strides which have been made in all
departments of science in recent years. So great have been the advances
that we cannot refrain from asking ourselves, Is this all true, or is it
but a dream? Centuries ago men have lived, have thought, discovered,
invented, and have believed that they were soaring, while they were
merely proceeding at a snail's pace. So we too may be mistaken. But
taking the truth of the observed events as one of the implied facts of
science, we must rejoice in the immense progress already made and still
more in the anticipation of what must come, judging from the
possibilities opened up by modern research. There is, however, an
advance which we have been witnessing, which must be particularly
gratifying to every lover of progress. It is not a discovery, or an
invention, or an achievement in any particular direction. It is an
advance in all directions of scientific thought and experiment. I mean
the generalization of the natural forces and phenomena, the looming up
of a certain broad idea on the scientific horizon. It is this idea which
has, however, long ago taken possession of the most advanced minds, to
which I desire to call your attention, and which I intend to illustrate
in a general way, in these experiments, as the first step in answering
the question "What is light?" and to realize the modern meaning of this
word.

It is beyond the scope of my lecture to dwell upon the subject of light
in general, my object being merely to bring presently to your notice a
certain class of light effects and a number of phenomena observed in
pursuing the study of these effects. But to be consistent in my remarks
it is necessary to state that, according to that idea, now accepted by
the majority of scientific men as a positive result of theoretical and
experimental investigation, the various forms or manifestations of
energy which were generally designated as "electric" or more precisely
"electromagnetic" are energy manifestations of the same nature as those
of radiant heat and light. Therefore the phenomena of light and heat and
others besides these, may be called electrical phenomena. Thus
electrical science has become the mother science of all and its study
has become all important. The day when we shall know exactly what
"electricity" is, will chronicle an event probably greater, more
important than any other recorded in the history of the human race. The
time will come when the comfort, the very existence, perhaps, of man
will depend upon that wonderful agent. For our existence and comfort we
require heat, light and mechanical power. How do we now get all these?
We get them from fuel, we get them by consuming material. What will man
do when the forests disappear, when the coal fields are exhausted? Only
one thing, according to our present knowledge will remain; that is, to
transmit power at great distances. Men will go to the waterfalls, to the
tides, which are the stores of an infinitesimal part of Nature's
immeasurable energy. There will they harness the energy and transmit the
same to their settlements, to warm their homes by, to give them light,
and to keep their obedient slaves, the machines, toiling. But how will
they transmit this energy if not by electricity? Judge then, if the
comfort, nay, the very existence, of man will not depend on electricity.
I am aware that this view is not that of a practical engineer, but
neither is it that of an illusionist, for it is certain, that power
transmission, which at present is merely a stimulus to enterprise, will
some day be a dire necessity.

It is more important for the student, who takes up the study of light
phenomena, to make himself thoroughly acquainted with certain modern
views, than to peruse entire books on the subject of light itself, as
disconnected from these views. Were I therefore to make these
demonstrations before students seeking information--and for the sake of
the few of those who may be present, give me leave to so assume--it
would be my principal endeavor to impress these views upon their minds
in this series of experiments.

It might be sufficient for this purpose to perform a simple and
well-known experiment. I might take a familiar appliance, a Leyden jar,
charge it from a frictional machine, and then discharge it. In
explaining to you its permanent state when charged, and its transitory
condition when discharging, calling your attention to the forces which
enter into play and to the various phenomena they produce, and pointing
out the relation of the forces and phenomena, I might fully succeed in
illustrating that modern idea. No doubt, to the thinker, this simple
experiment would appeal as much as the most magnificent display. But
this is to be an experimental demonstration, and one which should
possess, besides instructive, also entertaining features and as such, a
simple experiment, such as the one cited, would not go very far towards
the attainment of the lecturer's aim. I must therefore choose another
way of illustrating, more spectacular certainly, but perhaps also more
instructive. Instead of the frictional machine and Leyden jar, I shall
avail myself in these experiments, of an induction coil of peculiar
properties, which was described in detail by me in a lecture before the
London Institution of Electrical Engineers, in Feb., 1892. This
induction coil is capable of yielding currents of enormous potential
differences, alternating with extreme rapidity. With this apparatus I
shall endeavor to show you three distinct classes of effects, or
phenomena, and it is my desire that each experiment, while serving for
the purposes of illustration, should at the same time teach us some
novel truth, or show us some novel aspect of this fascinating science.
But before doing this, it seems proper and useful to dwell upon the
apparatus employed, and method of obtaining the high potentials and
high-frequency currents which are made use of in these experiments.


[Illustration: FIG. 165.]

ON THE APPARATUS AND METHOD OF CONVERSION.

These high-frequency currents are obtained in a peculiar manner. The
method employed was advanced by me about two years ago in an
experimental lecture before the American Institute of Electrical
Engineers. A number of ways, as practiced in the laboratory, of
obtaining these currents either from continuous or low frequency
alternating currents, is diagramatically indicated in Fig. 165, which
will be later described in detail. The general plan is to charge
condensers, from a direct or alternate-current source, preferably of
high-tension, and to discharge them disruptively while observing
well-known conditions necessary to maintain the oscillations of the
current. In view of the general interest taken in high-frequency
currents and effects producible by them, it seems to me advisable to
dwell at some length upon this method of conversion. In order to give
you a clear idea of the action, I will suppose that a continuous-current
generator is employed, which is often very convenient. It is desirable
that the generator should possess such high tension as to be able to
break through a small air space. If this is not the case, then auxiliary
means have to be resorted to, some of which will be indicated
subsequently. When the condensers are charged to a certain potential,
the air, or insulating space, gives way and a disruptive discharge
occurs. There is then a sudden rush of current and generally a large
portion of accumulated electrical energy spends itself. The condensers
are thereupon quickly charged and the same process is repeated in more
or less rapid succession. To produce such sudden rushes of current it is
necessary to observe certain conditions. If the rate at which the
condensers are discharged is the same as that at which they are charged,
then, clearly, in the assumed case the condensers do not come into play.
If the rate of discharge be smaller than the rate of charging, then,
again, the condensers cannot play an important part. But if, on the
contrary, the rate of discharging is greater than that of charging, then
a succession of rushes of current is obtained. It is evident that, if
the rate at which the energy is dissipated by the discharge is very much
greater than the rate of supply to the condensers, the sudden rushes
will be comparatively few, with long-time intervals between. This always
occurs when a condenser of considerable capacity is charged by means of
a comparatively small machine. If the rates of supply and dissipation
are not widely different, then the rushes of current will be in quicker
succession, and this the more, the more nearly equal both the rates are,
until limitations incident to each case and depending upon a number of
causes are reached. Thus we are able to obtain from a continuous-current
generator as rapid a succession of discharges as we like. Of course, the
higher the tension of the generator, the smaller need be the capacity of
the condensers, and for this reason, principally, it is of advantage to
employ a generator of very high tension. Besides, such a generator
permits the attaining of greater rates of vibration.

The rushes of current may be of the same direction under the conditions
before assumed, but most generally there is an oscillation superimposed
upon the fundamental vibration of the current. When the conditions are
so determined that there are no oscillations, the current impulses are
unidirectional and thus a means is provided of transforming a continuous
current of high tension, into a direct current of lower tension, which I
think may find employment in the arts.

This method of conversion is exceedingly interesting and I was much
impressed by its beauty when I first conceived it. It is ideal in
certain respects. It involves the employment of no mechanical devices of
any kind, and it allows of obtaining currents of any desired frequency
from an ordinary circuit, direct or alternating. The frequency of the
fundamental discharges depending on the relative rates of supply and
dissipation can be readily varied within wide limits, by simple
adjustments of these quantities, and the frequency of the superimposed
vibration by the determination of the capacity, self-induction and
resistance of the circuit. The potential of the currents, again, may be
raised as high as any insulation is capable of withstanding safely by
combining capacity and self-induction or by induction in a secondary,
which need have but comparatively few turns.

As the conditions are often such that the intermittence or oscillation
does not readily establish itself, especially when a direct current
source is employed, it is of advantage to associate an interrupter with
the arc, as I have, some time ago, indicated the use of an air-blast or
magnet, or other such device readily at hand. The magnet is employed
with special advantage in the conversion of direct currents, as it is
then very effective. If the primary source is an alternate current
generator, it is desirable, as I have stated on another occasion, that
the frequency should be low, and that the current forming the arc be
large, in order to render the magnet more effective.

A form of such discharger with a magnet which has been found convenient,
and adopted after some trials, in the conversion of direct currents
particularly, is illustrated in Fig. 166. N S are the pole pieces of a
very strong magnet which is excited by a coil C. The pole pieces are
slotted for adjustment and can be fastened in any position by screws s
s_{1}. The discharge rods d d_{1}, thinned down on the ends in order
to allow a closer approach of the magnetic pole pieces, pass through the
columns of brass b b_{1} and are fastened in position by screws s_{2}
s_{2}. Springs r r_{1} and collars c c_{1} are slipped on the
rods, the latter serving to set the points of the rods at a certain
suitable distance by means of screws s_{3} s_{3}, and the former to
draw the points apart. When it is desired to start the arc, one of the
large rubber handles h h_{1} is tapped quickly with the hand, whereby
the points of the rods are brought in contact but are instantly
separated by the springs r r_{1}. Such an arrangement has been found
to be often necessary, namely in cases when the E. M. F. was not large
enough to cause the discharge to break through the gap, and also when it
was desirable to avoid short circuiting of the generator by the metallic
contact of the rods. The rapidity of the interruptions of the current
with a magnet depends on the intensity of the magnetic field and on the
potential difference at the end of the arc. The interruptions are
generally in such quick succession as to produce a musical sound. Years
ago it was observed that when a powerful induction coil is discharged
between the poles of a strong magnet, the discharge produces a loud
noise, not unlike a small pistol shot. It was vaguely stated that the
spark was intensified by the presence of the magnetic field. It is now
clear that the discharge current, flowing for some time, was interrupted
a great number of times by the magnet, thus producing the sound. The
phenomenon is especially marked when the field circuit of a large magnet
or dynamo is broken in a powerful magnetic field.

[Illustration: FIG. 166.]

When the current through the gap is comparatively large, it is of
advantage to slip on the points of the discharge rods pieces of very
hard carbon and let the arc play between the carbon pieces. This
preserves the rods, and besides has the advantage of keeping the air
space hotter, as the heat is not conducted away as quickly through the
carbons, and the result is that a smaller E. M. F. in the arc gap is
required to maintain a succession of discharges.

[Illustration: FIG. 167.]

Another form of discharger, which may be employed with advantage in
some cases, is illustrated in Fig. 167. In this form the discharge rods
d d_{1} pass through perforations in a wooden box B, which is thickly
coated with mica on the inside, as indicated by the heavy lines. The
perforations are provided with mica tubes m m_{1} of some thickness,
which are preferably not in contact with the rods d d_{1}. The box has
a cover C which is a little larger and descends on the outside of the
box. The spark gap is warmed by a small lamp _l_ contained in the box. A
plate _p_ above the lamp allows the draught to pass only through the
chimney _e_ of the lamp, the air entering through holes _o o_ in or near
the bottom of the box and following the path indicated by the arrows.
When the discharger is in operation, the door of the box is closed so
that the light of the arc is not visible outside. It is desirable to
exclude the light as perfectly as possible, as it interferes with some
experiments. This form of discharger is simple and very effective when
properly manipulated. The air being warmed to a certain temperature, has
its insulating power impaired; it becomes dielectrically weak, as it
were, and the consequence is that the arc can be established at much
greater distance. The arc should, of course, be sufficiently insulating
to allow the discharge to pass through the gap _disruptively_. The arc
formed under such conditions, when long, may be made extremely
sensitive, and the weak draught through the lamp chimney _c_ is quite
sufficient to produce rapid interruptions. The adjustment is made by
regulating the temperature and velocity of the draught. Instead of using
the lamp, it answers the purpose to provide for a draught of warm air in
other ways. A very simple way which has been practiced is to enclose the
arc in a long vertical tube, with plates on the top and bottom for
regulating the temperature and velocity of the air current. Some
provision had to be made for deadening the sound.

The air may be rendered dielectrically weak also by rarefaction.
Dischargers of this kind have likewise been used by me in connection
with a magnet. A large tube is for this purpose provided with heavy
electrodes of carbon or metal, between which the discharge is made to
pass, the tube being placed in a powerful magnetic field. The exhaustion
of the tube is carried to a point at which the discharge breaks through
easily, but the pressure should be more than 75 millimetres, at which
the ordinary thread discharge occurs. In another form of discharger,
combining the features before mentioned, the discharge was made to pass
between two adjustable magnetic pole pieces, the space between them
being kept at an elevated temperature.

It should be remarked here that when such, or interrupting devices of
any kind, are used and the currents are passed through the primary of a
disruptive discharge coil, it is not, as a rule, of advantage to produce
a number of interruptions of the current per second greater than the
natural frequency of vibration of the dynamo supply circuit, which is
ordinarily small. It should also be pointed out here, that while the
devices mentioned in connection with the disruptive discharge are
advantageous under certain conditions, they may be sometimes a source of
trouble, as they produce intermittences and other irregularities in the
vibration which it would be very desirable to overcome.

There is, I regret to say, in this beautiful method of conversion a
defect, which fortunately is not vital, and which I have been gradually
overcoming. I will best call attention to this defect and indicate a
fruitful line of work, by comparing the electrical process with its
mechanical analogue. The process may be illustrated in this manner.
Imagine a tank with a wide opening at the bottom, which is kept closed
by spring pressure, but so that it snaps off _suddenly_ when the liquid
in the tank has reached a certain height. Let the fluid be supplied to
the tank by means of a pipe feeding at a certain rate. When the critical
height of the liquid is reached, the spring gives way and the bottom of
the tank drops out. Instantly the liquid falls through the wide opening,
and the spring, reasserting itself, closes the bottom again. The tank is
now filled, and after a certain time interval the same process is
repeated. It is clear, that if the pipe feeds the fluid quicker than the
bottom outlet is capable of letting it pass through, the bottom will
remain off and the tank will still overflow. If the rates of supply are
exactly equal, then the bottom lid will remain partially open and no
vibration of the same and of the liquid column will generally occur,
though it might, if started by some means. But if the inlet pipe does
not feed the fluid fast enough for the outlet, then there will be always
vibration. Again, in such case, each time the bottom flaps up or down,
the spring and the liquid column, if the pliability of the spring and
the inertia of the moving parts are properly chosen, will perform
independent vibrations. In this analogue the fluid may be likened to
electricity or electrical energy, the tank to the condenser, the spring
to the dielectric, and the pipe to the conductor through which
electricity is supplied to the condenser. To make this analogy quite
complete it is necessary to make the assumption, that the bottom, each
time it gives way, is knocked violently against a non-elastic stop, this
impact involving some loss of energy; and that, besides, some
dissipation of energy results due to frictional losses. In the preceding
analogue the liquid is supposed to be under a steady pressure. If the
presence of the fluid be assumed to vary rhythmically, this may be taken
as corresponding to the case of an alternating current. The process is
then not quite as simple to consider, but the action is the same in
principle.

It is desirable, in order to maintain the vibration economically, to
reduce the impact and frictional losses as much as possible. As regards
the latter, which in the electrical analogue correspond to the losses
due to the resistance of the circuits, it is impossible to obviate them
entirely, but they can be reduced to a minimum by a proper selection of
the dimensions of the circuits and by the employment of thin conductors
in the form of strands. But the loss of energy caused by the first
breaking through of the dielectric--which in the above example
corresponds to the violent knock of the bottom against the inelastic
stop--would be more important to overcome. At the moment of the breaking
through, the air space has a very high resistance, which is probably
reduced to a very small value when the current has reached some
strength, and the space is brought to a high temperature. It would
materially diminish the loss of energy if the space were always kept at
an extremely high temperature, but then there would be no disruptive
break. By warming the space moderately by means of a lamp or otherwise,
the economy as far as the arc is concerned is sensibly increased. But
the magnet or other interrupting device does not diminish the loss in
the arc. Likewise, a jet of air only facilitates the carrying off of the
energy. Air, or a gas in general, behaves curiously in this respect.
When two bodies charged to a very high potential, discharge disruptively
through an air space, any amount of energy may be carried off by the
air. This energy is evidently dissipated by bodily carriers, in impact
and collisional losses of the molecules. The exchange of the molecules
in the space occurs with inconceivable rapidity. A powerful discharge
taking place between two electrodes, they may remain entirely cool, and
yet the loss in the air may represent any amount of energy. It is
perfectly practicable, with very great potential differences in the gap,
to dissipate several horse-power in the arc of the discharge without
even noticing a small increase in the temperature of the electrodes. All
the frictional losses occur then practically in the air. If the exchange
of the air molecules is prevented, as by enclosing the air hermetically,
the gas inside of the vessel is brought quickly to a high temperature,
even with a very small discharge. It is difficult to estimate how much
of the energy is lost in sound waves, audible or not, in a powerful
discharge. When the currents through the gap are large, the electrodes
may become rapidly heated, but this is not a reliable measure of the
energy wasted in the arc, as the loss through the gap itself may be
comparatively small. The air or a gas in general is, at ordinary
pressure at least, clearly not the best medium through which a
disruptive discharge should occur. Air or other gas under great pressure
is of course a much more suitable medium for the discharge gap. I have
carried on long-continued experiments in this direction, unfortunately
less practicable on account of the difficulties and expense in getting
air under great pressure. But even if the medium in the discharge space
is solid or liquid, still the same losses take place, though they are
generally smaller, for just as soon as the arc is established, the solid
or liquid is volatilized. Indeed, there is no body known which would not
be disintegrated by the arc, and it is an open question among scientific
men, whether an arc discharge could occur at all in the air itself
without the particles of the electrodes being torn off. When the current
through the gap is very small and the arc very long, I believe that a
relatively considerable amount of heat is taken up in the disintegration
of the electrodes, which partially on this account may remain quite
cold.

The ideal medium for a discharge gap should only _crack_, and the ideal
electrode should be of some material which cannot be disintegrated. With
small currents through the gap it is best to employ aluminum, but not
when the currents are large. The disruptive break in the air, or more or
less in any ordinary medium, is not of the nature of a crack, but it is
rather comparable to the piercing of innumerable bullets through a mass
offering great frictional resistances to the motion of the bullets, this
involving considerable loss of energy. A medium which would merely crack
when strained electrostatically--and this possibly might be the case
with a perfect vacuum, that is, pure ether--would involve a very small
loss in the gap, so small as to be entirely negligible, at least
theoretically, because a crack may be produced by an infinitely small
displacement. In exhausting an oblong bulb provided with two aluminum
terminals, with the greatest care, I have succeeded in producing such a
vacuum that the secondary discharge of a disruptive discharge coil would
break disruptively through the bulb in the form of fine spark streams.
The curious point was that the discharge would completely ignore the
terminals and start far behind the two aluminum plates which served as
electrodes. This extraordinary high vacuum could only be maintained for
a very short while. To return to the ideal medium, think, for the sake
of illustration, of a piece of glass or similar body clamped in a vice,
and the latter tightened more and more. At a certain point a minute
increase of the pressure will cause the glass to crack. The loss of
energy involved in splitting the glass may be practically nothing, for
though the force is great, the displacement need be but extremely small.
Now imagine that the glass would possess the property of closing again
perfectly the crack upon a minute diminution of the pressure. This is
the way the dielectric in the discharge space should behave. But
inasmuch as there would be always some loss in the gap, the medium,
which should be continuous, should exchange through the gap at a rapid
rate. In the preceding example, the glass being perfectly closed, it
would mean that the dielectric in the discharge space possesses a great
insulating power; the glass being cracked, it would signify that the
medium in the space is a good conductor. The dielectric should vary
enormously in resistance by minute variations of the E. M. F. across the
discharge space. This condition is attained, but in an extremely
imperfect manner, by warming the air space to a certain critical
temperature, dependent on the E. M. F. across the gap, or by otherwise
impairing the insulating power of the air. But as a matter of fact the
air does never break down _disruptively_, if this term be rigorously
interpreted, for before the sudden rush of the current occurs, there is
always a weak current preceding it, which rises first gradually and then
with comparative suddenness. That is the reason why the rate of change
is very much greater when glass, for instance, is broken through, than
when the break takes place through an air space of equivalent dielectric
strength. As a medium for the discharge space, a solid, or even a
liquid, would be preferable therefor. It is somewhat difficult to
conceive of a solid body which would possess the property of closing
instantly after it has been cracked. But a liquid, especially under
great pressure, behaves practically like a solid, while it possesses the
property of closing the crack. Hence it was thought that a liquid
insulator might be more suitable as a dielectric than air. Following out
this idea, a number of different forms of dischargers in which a variety
of such insulators, sometimes under great pressure, were employed, have
been experimented upon. It is thought sufficient to dwell in a few words
upon one of the forms experimented upon. One of these dischargers is
illustrated in Figs. 168_a_ and 168_b_.

[Illustration: FIG. 168a.]

[Illustration: FIG. 168b.]

A hollow metal pulley P (Fig. 168_a_), was fastened upon an arbor _a_,
which by suitable means was rotated at a considerable speed. On the
inside of the pulley, but disconnected from the same, was supported a
thin disc _h_ (which is shown thick for the sake of clearness), of hard
rubber in which there were embedded two metal segments _s s_ with
metallic extensions _e e_ into which were screwed conducting terminals
_t t_ covered with thick tubes of hard rubber _t t_. The rubber disc _h_
with its metallic segments _s s_, was finished in a lathe, and its
entire surface highly polished so as to offer the smallest possible
frictional resistance to the motion through a fluid. In the hollow of
the pulley an insulating liquid such as a thin oil was poured so as to
reach very nearly to the opening left in the flange _f_, which was
screwed tightly on the front side of the pulley. The terminals _t t_,
were connected to the opposite coatings of a battery of condensers so
that the discharge occurred through the liquid. When the pulley was
rotated, the liquid was forced against the rim of the pulley and
considerable fluid pressure resulted. In this simple way the discharge
gap was filled with a medium which behaved practically like a solid,
which possessed the quality of closing instantly upon the occurrence of
the break, and which moreover was circulating through the gap at a rapid
rate. Very powerful effects were produced by discharges of this kind
with liquid interrupters, of which a number of different forms were
made. It was found that, as expected, a longer spark for a given length
of wire was obtainable in this way than by using air as an interrupting
device. Generally the speed, and therefore also the fluid pressure, was
limited by reason of the fluid friction, in the form of discharger
described, but the practically obtainable speed was more than sufficient
to produce a number of breaks suitable for the circuits ordinarily used.
In such instances the metal pulley P was provided with a few projections
inwardly, and a definite number of breaks was then produced which could
be computed from the speed of rotation of the pulley. Experiments were
also carried on with liquids of different insulating power with the view
of reducing the loss in the arc. When an insulating liquid is moderately
warmed, the loss in the arc is diminished.

A point of some importance was noted in experiments with various
discharges of this kind. It was found, for instance, that whereas the
conditions maintained in these forms were favorable for the production
of a great spark length, the current so obtained was not best suited to
the production of light effects. Experience undoubtedly has shown, that
for such purposes a harmonic rise and fall of the potential is
preferable. Be it that a solid is rendered incandescent, or
phosphorescent, or be it that energy is transmitted by condenser coating
through the glass, it is quite certain that a harmonically rising and
falling potential produces less destructive action, and that the vacuum
is more permanently maintained. This would be easily explained if it
were ascertained that the process going on in an exhausted vessel is of
an electrolytic nature.

In the diagrammatical sketch, Fig. 165, which has been already referred
to, the cases which are most likely to be met with in practice are
illustrated. One has at his disposal either direct or alternating
currents from a supply station. It is convenient for an experimenter in
an isolated laboratory to employ a machine G, such as illustrated,
capable of giving both kinds of currents. In such case it is also
preferable to use a machine with multiple circuits, as in many
experiments it is useful and convenient to have at one's disposal
currents of different phases. In the sketch, D represents the direct and
A the alternating circuit. In each of these, three branch circuits are
shown, all of which are provided with double line switches _s s s s s
s_. Consider first the direct current conversion; I_a_ represents the
simplest case. If the E. M. F. of the generator is sufficient to break
through a small air space, at least when the latter is warmed or
otherwise rendered poorly insulating, there is no difficulty in
maintaining a vibration with fair economy by judicious adjustment of the
capacity, self-induction and resistance of the circuit L containing the
devices _l l m_. The magnet N, S, can be in this case advantageously
combined with the air space. The discharger _d d_ with the magnet may be
placed either way, as indicated by the full or by the dotted lines. The
circuit I_a_ with the connections and devices is supposed to possess
dimensions such as are suitable for the maintenance of a vibration. But
usually the E. M. F. on the circuit or branch I_a_ will be something
like a 100 volts or so, and in this case it is not sufficient to break
through the gap. Many different means may be used to remedy this by
raising the E. M. F. across the gap. The simplest is probably to insert
a large self-induction coil in series with the circuit L. When the arc
is established, as by the discharger illustrated in Fig. 166, the magnet
blows the arc out the instant it is formed. Now the extra current of the
break, being of high E. M. F., breaks through the gap, and a path of low
resistance for the dynamo current being again provided, there is a
sudden rush of current from the dynamo upon the weakening or subsidence
of the extra current. This process is repeated in rapid succession, and
in this manner I have maintained oscillation with as low as 50 volts, or
even less, across the gap. But conversion on this plan is not to be
recommended on account of the too heavy currents through the gap and
consequent heating of the electrodes; besides, the frequencies obtained
in this way are low, owing to the high self-induction necessarily
associated with the circuit. It is very desirable to have the E. M. F.
as high as possible, first, in order to increase the economy of the
conversion, and, secondly, to obtain high frequencies. The difference of
potential in this electric oscillation is, of course, the equivalent of
the stretching force in the mechanical vibration of the spring. To
obtain very rapid vibration in a circuit of some inertia, a great
stretching force or difference of potential is necessary. Incidentally,
when the E. M. F. is very great, the condenser which is usually employed
in connection with the circuit need but have a small capacity, and many
other advantages are gained. With a view of raising the E. M. F. to a
many times greater value than obtainable from ordinary distribution
circuits, a rotating transformer _g_ is used, as indicated at II_a_,
Fig. 165, or else a separate high potential machine is driven by means
of a motor operated from the generator G. The latter plan is in fact
preferable, as changes are easier made. The connections from the high
tension winding are quite similar to those in branch I_a_ with the
exception that a condenser C, which should be adjustable, is connected
to the high tension circuit. Usually, also, an adjustable self-induction
coil in series with the circuit has been employed in these experiments.
When the tension of the currents is very high, the magnet ordinarily
used in connection with the discharger is of comparatively small value,
as it is quite easy to adjust the dimensions of the circuit so that
oscillation is maintained. The employment of a steady E. M. F. in the
high frequency conversion affords some advantages over the employment of
alternating E. M. F., as the adjustments are much simpler and the action
can be easier controlled. But unfortunately one is limited by the
obtainable potential difference. The winding also breaks down easily in
consequence of the sparks which form between the sections of the
armature or commutator when a vigorous oscillation takes place. Besides,
these transformers are expensive to build. It has been found by
experience that it is best to follow the plan illustrated at III_a_. In
this arrangement a rotating transformer _g_, is employed to convert the
low tension direct currents into low frequency alternating currents,
preferably also of small tension. The tension of the currents is then
raised in a stationary transformer T. The secondary S of this
transformer is connected to an adjustable condenser C which discharges
through the gap or discharger _d d_, placed in either of the ways
indicated, through the primary P of a disruptive discharge coil, the
high frequency current being obtained from the secondary S of this coil,
as described on previous occasions. This will undoubtedly be found the
cheapest and most convenient way of converting direct currents.

The three branches of the circuit A represent the usual cases met in
practice when alternating currents are converted. In Fig. 1_b_ a
condenser C, generally of large capacity, is connected to the circuit L
containing the devices _l l_, _m m_. The devices _m m_ are supposed to
be of high self-induction so as to bring the frequency of the circuit
more or less to that of the dynamo. In this instance the discharger _d
d_ should best have a number of makes and breaks per second equal to
twice the frequency of the dynamo. If not so, then it should have at
least a number equal to a multiple or even fraction of the dynamo
frequency. It should be observed, referring to I_b_, that the conversion
to a high potential is also effected when the discharger _d d_, which is
shown in the sketch, is omitted. But the effects which are produced by
currents which rise instantly to high values, as in a disruptive
discharge, are entirely different from those produced by dynamo currents
which rise and fall harmonically. So, for instance, there might be in a
given case a number of makes and breaks at _d d_ equal to just twice the
frequency of the dynamo, or in other words, there may be the same number
of fundamental oscillations as would be produced without the discharge
gap, and there might even not be any quicker superimposed vibration; yet
the differences of potential at the various points of the circuit, the
impedance and other phenomena, dependent upon the rate of change, will
bear no similarity in the two cases. Thus, when working with currents
discharging disruptively, the element chiefly to be considered is not
the frequency, as a student might be apt to believe, but the rate of
change per unit of time. With low frequencies in a certain measure the
same effects may be obtained as with high frequencies, provided the rate
of change is sufficiently great. So if a low frequency current is raised
to a potential of, say, 75,000 volts, and the high tension current
passed through a series of high resistance lamp filaments, the
importance of the rarefied gas surrounding the filament is clearly
noted, as will be seen later; or, if a low frequency current of several
thousand amperes is passed through a metal bar, striking phenomena of
impedance are observed, just as with currents of high frequencies. But
it is, of course, evident that with low frequency currents it is
impossible to obtain such rates of change per unit of time as with high
frequencies, hence the effects produced by the latter are much more
prominent. It is deemed advisable to make the preceding remarks,
inasmuch as many more recently described effects have been unwittingly
identified with high frequencies. Frequency alone in reality does not
mean anything, except when an undisturbed harmonic oscillation is
considered.

In the branch III_b_ a similar disposition to that in I_b_ is
illustrated, with the difference that the currents discharging through
the gap _d d_ are used to induce currents in the secondary S of a
transformer T. In such case the secondary should be provided with an
adjustable condenser for the purpose of tuning it to the primary.

II_b_ illustrates a plan of alternate current high frequency conversion
which is most frequently used and which is found to be most convenient.
This plan has been dwelt upon in detail on previous occasions and need
not be described here.

Some of these results were obtained by the use of a high frequency
alternator. A description of such machines will be found in my original
paper before the American Institute of Electrical Engineers, and in
periodicals of that period, notably in THE ELECTRICAL ENGINEER of March
18, 1891.

I will now proceed with the experiments.


ON PHENOMENA PRODUCED BY ELECTROSTATIC FORCE.

The first class of effects I intend to show you are effects produced by
electrostatic force. It is the force which governs the the motion of the
atoms, which causes them to collide and develop the life-sustaining
energy of heat and light, and which causes them to aggregate in an
infinite variety of ways, according to Nature's fanciful designs, and to
form all these wondrous structures we perceive around us; it is, in
fact, if our present views be true, the most important force for us to
consider in Nature. As the term _electrostatic_ might imply a steady
electric condition, it should be remarked, that in these experiments the
force is not constant, but varies at a rate which may be considered
moderate, about one million times a second, or thereabouts. This enables
me to produce many effects which are not producible with an unvarying
force.

When two conducting bodies are insulated and electrified, we say that an
electrostatic force is acting between them. This force manifests itself
in attractions, repulsions and stresses in the bodies and space or
medium without. So great may be the strain exerted in the air, or
whatever separates the two conducting bodies, that it may break down,
and we observe sparks or bundles of light or streamers, as they are
called. These streamers form abundantly when the force through the air
is rapidly varying. I will illustrate this action of electrostatic force
in a novel experiment in which I will employ the induction coil before
referred to. The coil is contained in a trough filled with oil, and
placed under the table. The two ends of the secondary wire pass through
the two thick columns of hard rubber which protrude to some height above
the table. It is necessary to insulate the ends or terminals of the
secondary heavily with hard rubber, because even dry wood is by far too
poor an insulator for these currents of enormous potential differences.
On one of the terminals of the coil, I have placed a large sphere of
sheet brass, which is connected to a larger insulated brass plate, in
order to enable me to perform the experiments under conditions, which,
as you will see, are more suitable for this experiment. I now set the
coil to work and approach the free terminal with a metallic object held
in my hand, this simply to avoid burns. As I approach the metallic
object to a distance of eight or ten inches, a torrent of furious sparks
breaks forth from the end of the secondary wire, which passes through
the rubber column. The sparks cease when the metal in my hand touches
the wire. My arm is now traversed by a powerful electric current,
vibrating at about the rate of one million times a second. All around me
the electrostatic force makes itself felt, and the air molecules and
particles of dust flying about are acted upon and are hammering
violently against my body. So great is this agitation of the particles,
that when the lights are turned out you may see streams of feeble light
appear on some parts of my body. When such a streamer breaks out on any
part of the body, it produces a sensation like the pricking of a needle.
Were the potentials sufficiently high and the frequency of the vibration
rather low, the skin would probably be ruptured under the tremendous
strain, and the blood would rush out with great force in the form of
fine spray or jet so thin as to be invisible, just as oil will when
placed on the positive terminal of a Holtz machine. The breaking through
of the skin though it may seem impossible at first, would perhaps occur,
by reason of the tissues under the skin being incomparably better
conducting. This, at least, appears plausible, judging from some
observations.

[Illustration: FIG. 169.]

I can make these streams of light visible to all, by touching with the
metallic object one of the terminals as before, and approaching my free
hand to the brass sphere, which is connected to the second terminal of
the coil. As the hand is approached, the air between it and the sphere,
or in the immediate neighborhood, is more violently agitated, and you
see streams of light now break forth from my finger tips and from the
whole hand (Fig. 169). Were I to approach the hand closer, powerful
sparks would jump from the brass sphere to my hand, which might be
injurious. The streamers offer no particular inconvenience, except that
in the ends of the finger tips a burning sensation is felt. They should
not be confounded with those produced by an influence machine, because
in many respects they behave differently. I have attached the brass
sphere and plate to one of the terminals in order to prevent the
formation of visible streamers on that terminal, also in order to
prevent sparks from jumping at a considerable distance. Besides, the
attachment is favorable for the working of the coil.

The streams of light which you have observed issuing from my hand are
due to a potential of about 200,000 volts, alternating in rather
irregular intervals, sometimes like a million times a second. A
vibration of the same amplitude, but four times as fast, to maintain
which over 3,000,000 volts would be required, would be more than
sufficient to envelop my body in a complete sheet of flame. But this
flame would not burn me up; quite contrarily, the probability is that I
would not be injured in the least. Yet a hundredth part of that energy,
otherwise directed, would be amply sufficient to kill a person.

The amount of energy which may thus be passed into the body of a person
depends on the frequency and potential of the currents, and by making
both of these very great, a vast amount of energy may be passed into the
body without causing any discomfort, except perhaps, in the arm, which
is traversed by a true conduction current. The reason why no pain in the
body is felt, and no injurious effect noted, is that everywhere, if a
current be imagined to flow through the body, the direction of its flow
would be at right angles to the surface; hence the body of the
experimenter offers an enormous section to the current, and the density
is very small, with the exception of the arm, perhaps, where the density
may be considerable. But if only a small fraction of that energy would
be applied in such a way that a current would traverse the body in the
same manner as a low frequency current, a shock would be received which
might be fatal. A direct or low frequency alternating current is fatal,
I think, principally because its distribution through the body is not
uniform, as it must divide itself in minute streamlets of great density,
whereby some organs are vitally injured. That such a process occurs I
have not the least doubt, though no evidence might apparently exist, or
be found upon examination. The surest to injure and destroy life, is a
continuous current, but the most painful is an alternating current of
very low frequency. The expression of these views, which are the result
of long continued experiment and observation, both with steady and
varying currents, is elicited by the interest which is at present taken
in this subject, and by the manifestly erroneous ideas which are daily
propounded in journals on this subject.

I may illustrate an effect of the electrostatic force by another
striking experiment, but before, I must call your attention to one or
two facts. I have said before, that when the medium between two
oppositely electrified bodies is strained beyond a certain limit it
gives way and, stated in popular language, the opposite electric charges
unite and neutralize each other. This breaking down of the medium occurs
principally when the force acting between the bodies is steady, or
varies at a moderate rate. Were the variation sufficiently rapid, such a
destructive break would not occur, no matter how great the force, for
all the energy would be spent in radiation, convection and mechanical
and chemical action. Thus the _spark_ length, or greatest distance which
a _spark_ will jump between the electrified bodies is the smaller, the
greater the variation or time rate of change. But this rule may be taken
to be true only in a general way, when comparing rates which are widely
different.

[Illustration: FIG. 170a.]

[Illustration: FIG. 170b.]

I will show you by an experiment the difference in the effect produced
by a rapidly varying and a steady or moderately varying force. I have
here two large circular brass plates _p p_ (Fig. 170_a_ and Fig.
170_b_), supported on movable insulating stands on the table, connected
to the ends of the secondary of a coil similar to the one used before. I
place the plates ten or twelve inches apart and set the coil to work.
You see the whole space between the plates, nearly two cubic feet,
filled with uniform light, Fig. 170_a_. This light is due to the
streamers you have seen in the first experiment, which are now much more
intense. I have already pointed out the importance of these streamers in
commercial apparatus and their still greater importance in some purely
scientific investigations. Often they are too weak to be visible, but
they always exist, consuming energy and modifying the action of the
apparatus. When intense, as they are at present, they produce ozone in
great quantity, and also, as Professor Crookes has pointed out, nitrous
acid. So quick is the chemical action that if a coil, such as this one,
is worked for a very long time it will make the atmosphere of a small
room unbearable, for the eyes and throat are attacked. But when
moderately produced, the streamers refresh the atmosphere wonderfully,
like a thunder-storm, and exercises unquestionably a beneficial effect.

In this experiment the force acting between the plates changes in
intensity and direction at a very rapid rate. I will now make the rate
of change per unit time much smaller. This I effect by rendering the
discharges through the primary of the induction coil less frequent, and
also by diminishing the rapidity of the vibration in the secondary. The
former result is conveniently secured by lowering the E. M. F. over the
air gap in the primary circuit, the latter by approaching the two brass
plates to a distance of about three or four inches. When the coil is set
to work, you see no streamers or light between the plates, yet the
medium between them is under a tremendous strain. I still further
augment the strain by raising the E. M. F. in the primary circuit, and
soon you see the air give way and the hall is illuminated by a shower of
brilliant and noisy sparks, Fig. 170_b_. These sparks could be produced
also with unvarying force; they have been for many years a familiar
phenomenon, though they were usually obtained from an entirely different
apparatus. In describing these two phenomena so radically different in
appearance, I have advisedly spoken of a "force" acting between the
plates. It would be in accordance with accepted views to say, that there
was an "alternating E. M. F," acting between the plates. This term is
quite proper and applicable in all cases where there is evidence of at
least a possibility of an essential inter-dependence of the electric
state of the plates, or electric action in their neighborhood. But if
the plates were removed to an infinite distance, or if at a finite
distance, there is no probability or necessity whatever for such
dependence. I prefer to use the term "electrostatic force," and to say
that such a force is acting around each plate or electrified insulated
body in general. There is an inconvenience in using this expression as
the term incidentally means a steady electric condition; but a proper
nomenclature will eventually settle this difficulty.

I now return to the experiment to which I have already alluded, and with
which I desire to illustrate a striking effect produced by a rapidly
varying electrostatic force. I attach to the end of the wire, _l_ (Fig.
171), which is in connection with one of the terminals of the secondary
of the induction coil, an exhausted bulb _b_. This bulb contains a thin
carbon filament _f_, which is fastened to a platinum wire _w_, sealed in
the glass and leading outside of the bulb, where it connects to the wire
_l_. The bulb may be exhausted to any degree attainable with ordinary
apparatus. Just a moment before, you have witnessed the breaking down of
the air between the charged brass plates. You know that a plate of
glass, or any other insulating material, would break down in like
manner. Had I therefore a metallic coating attached to the outside of
the bulb, or placed near the same, and were this coating connected to
the other terminal of the coil, you would be prepared to see the glass
give way if the strain were sufficiently increased. Even were the
coating not connected to the other terminal, but to an insulated plate,
still, if you have followed recent developments, you would naturally
expect a rupture of the glass.

[Illustration: FIG. 171.]

[Illustration: FIG. 172a.]

[Illustration: FIG. 172b.]

But it will certainly surprise you to note that under the action of the
varying electrostatic force, the glass gives way when all other bodies
are removed from the bulb. In fact, all the surrounding bodies we
perceive might be removed to an infinite distance without affecting the
result in the slightest. When the coil is set to work, the glass is
invariably broken through at the seal, or other narrow channel, and the
vacuum is quickly impaired. Such a damaging break would not occur with
a steady force, even if the same were many times greater. The break is
due to the agitation of the molecules of the gas within the bulb, and
outside of the same. This agitation, which is generally most violent in
the narrow pointed channel near the seal, causes a heating and rupture
of the glass. This rupture, would, however, not occur, not even with a
varying force, if the medium filling the inside of the bulb, and that
surrounding it, were perfectly homogeneous. The break occurs much
quicker if the top of the bulb is drawn out into a fine fibre. In bulbs
used with these coils such narrow, pointed channels must therefore be
avoided.

When a conducting body is immersed in air, or similar insulating medium,
consisting of, or containing, small freely movable particles capable of
being electrified, and when the electrification of the body is made to
undergo a very rapid change--which is equivalent to saying that the
electrostatic force acting around the body is varying in intensity,--the
small particles are attracted and repelled, and their violent impacts
against the body may cause a mechanical motion of the latter. Phenomena
of this kind are noteworthy, inasmuch as they have not been observed
before with apparatus such as has been commonly in use. If a very light
conducting sphere be suspended on an exceedingly fine wire, and charged
to a steady potential, however high, the sphere will remain at rest.
Even if the potential would be rapidly varying, provided that the small
particles of matter, molecules or atoms, are evenly distributed, no
motion of the sphere should result. But if one side of the conducting
sphere is covered with a thick insulating layer, the impacts of the
particles will cause the sphere to move about, generally in irregular
curves, Fig. 172_a_. In like manner, as I have shown on a previous
occasion, a fan of sheet metal, Fig. 172_b_, covered partially with
insulating material as indicated, and placed upon the terminal of the
coil so as to turn freely on it, is spun around.

All these phenomena you have witnessed and others which will be shown
later, are due to the presence of a medium like air, and would not occur
in a continuous medium. The action of the air may be illustrated still
better by the following experiment. I take a glass tube _t_, Fig. 173,
of about an inch in diameter, which has a platinum wire _w_ sealed in
the lower end, and to which is attached a thin lamp filament _f_. I
connect the wire with the terminal of the coil and set the coil to work.
The platinum wire is now electrified positively and negatively in rapid
succession and the wire and air inside of the tube is rapidly heated by
the impacts of the particles, which may be so violent as to render the
filament incandescent. But if I pour oil in the tube, just as soon as
the wire is covered with the oil, all action apparently ceases and there
is no marked evidence of heating. The reason of this is that the oil is
a practically continuous medium. The displacements in such a continuous
medium are, with these frequencies, to all appearance incomparably
smaller than in air, hence the work performed in such a medium is
insignificant. But oil would behave very differently with frequencies
many times as great, for even though the displacements be small, if the
frequency were much greater, considerable work might be performed in the
oil.

[Illustration: FIG. 173.]

[Illustration: FIG. 174.]

The electrostatic attractions and repulsions between bodies of
measurable dimensions are, of all the manifestations of this force, the
first so-called _electrical_ phenomena noted. But though they have been
known to us for many centuries, the precise nature of the mechanism
concerned in these actions is still unknown to us, and has not been even
quite satisfactorily explained. What kind of mechanism must that be? We
cannot help wondering when we observe two magnets attracting and
repelling each other with a force of hundreds of pounds with apparently
nothing between them. We have in our commercial dynamos magnets capable
of sustaining in mid-air tons of weight. But what are even these forces
acting between magnets when compared with the tremendous attractions and
repulsions produced by electrostatic force, to which there is apparently
no limit as to intensity. In lightning discharges bodies are often
charged to so high a potential that they are thrown away with
inconceivable force and torn asunder or shattered into fragments. Still
even such effects cannot compare with the attractions and repulsions
which exist between charged molecules or atoms, and which are sufficient
to project them with speeds of many kilometres a second, so that under
their violent impact bodies are rendered highly incandescent and are
volatilized. It is of special interest for the thinker who inquires into
the nature of these forces to note that whereas the actions between
individual molecules or atoms occur seemingly under any conditions, the
attractions and repulsions of bodies of measurable dimensions imply a
medium possessing insulating properties. So, if air, either by being
rarefied or heated, is rendered more or less conducting, these actions
between two electrified bodies practically cease, while the actions
between the individual atoms continue to manifest themselves.

An experiment may serve as an illustration and as a means of bringing
out other features of interest. Some time ago I showed that a lamp
filament or wire mounted in a bulb and connected to one of the terminals
of a high tension secondary coil is set spinning, the top of the
filament generally describing a circle. This vibration was very
energetic when the air in the bulb was at ordinary pressure and became
less energetic when the air in the bulb was strongly compressed. It
ceased altogether when the air was exhausted so as to become
comparatively good conducting. I found at that time that no vibration
took place when the bulb was very highly exhausted. But I conjectured
that the vibration which I ascribed to the electrostatic action between
the walls of the bulb and the filament should take place also in a
highly exhausted bulb. To test this under conditions which were more
favorable, a bulb like the one in Fig. 174, was constructed. It
comprised a globe _b_, in the neck of which was sealed a platinum wire
_w_ carrying a thin lamp filament _f_. In the lower part of the globe a
tube _t_ was sealed so as to surround the filament. The exhaustion was
carried as far as it was practicable with the apparatus employed.

This bulb verified my expectation, for the filament was set spinning
when the current was turned on, and became incandescent. It also showed
another interesting feature, bearing upon the preceding remarks, namely,
when the filament had been kept incandescent some time, the narrow tube
and the space inside were brought to an elevated temperature, and as the
gas in the tube then became conducting, the electrostatic attraction
between the glass and the filament became very weak or ceased, and the
filament came to rest. When it came to rest it would glow far more
intensely. This was probably due to its assuming the position in the
centre of the tube where the molecular bombardment was most intense, and
also partly to the fact that the individual impacts were more violent
and that no part of the supplied energy was converted into mechanical
movement. Since, in accordance with accepted views, in this experiment
the incandescence must be attributed to the impacts of the particles,
molecules or atoms in the heated space, these particles must therefore,
in order to explain such action, be assumed to behave as independent
carriers of electric charges immersed in an insulating medium; yet there
is no attractive force between the glass tube and the filament because
the space in the tube is, as a whole, conducting.

It is of some interest to observe in this connection that whereas the
attraction between two electrified bodies may cease owing to the
impairing of the insulating power of the medium in which they are
immersed, the repulsion between the bodies may still be observed. This
may be explained in a plausible way. When the bodies are placed at some
distance in a poorly conducting medium, such as slightly warmed or
rarefied air, and are suddenly electrified, opposite electric charges
being imparted to them, these charges equalize more or less by leakage
through the air. But if the bodies are similarly electrified, there is
less opportunity afforded for such dissipation, hence the repulsion
observed in such case is greater than the attraction. Repulsive actions
in a gaseous medium are however, as Prof. Crookes has shown, enhanced by
molecular bombardment.


ON CURRENT OR DYNAMIC ELECTRICITY PHENOMENA.

So far, I have considered principally effects produced by a varying
electrostatic force in an insulating medium, such as air. When such a
force is acting upon a conducting body of measurable dimensions, it
causes within the same, or on its surface, displacements of the
electricity and gives rise to electric currents, and these produce
another kind of phenomena, some of which I shall presently endeavor to
illustrate. In presenting this second class of electrical effects, I
will avail myself principally of such as are producible without any
return circuit, hoping to interest you the more by presenting these
phenomena in a more or less novel aspect.

It has been a long time customary, owing to the limited experience with
vibratory currents, to consider an electric current as something
circulating in a closed conducting path. It was astonishing at first to
realize that a current may flow through the conducting path even if the
latter be interrupted, and it was still more surprising to learn, that
sometimes it may be even easier to make a current flow under such
conditions than through a closed path. But that old idea is gradually
disappearing, even among practical men, and will soon be entirely
forgotten.

[Illustration: FIG. 175.]

If I connect an insulated metal plate P, Fig. 175, to one of the
terminals T of the induction coil by means of a wire, though this plate
be very well insulated, a current passes through the wire when the coil
is set to work. First I wish to give you evidence that there _is_ a
current passing through the connecting wire. An obvious way of
demonstrating this is to insert between the terminal of the coil and the
insulated plate a very thin platinum or german silver wire _w_ and bring
the latter to incandescence or fusion by the current. This requires a
rather large plate or else current impulses of very high potential and
frequency. Another way is to take a coil C, Fig. 175, containing many
turns of thin insulated wire and to insert the same in the path of the
current to the plate. When I connect one of the ends of the coil to the
wire leading to another insulated plate P_{1}, and its other end to the
terminal T_{1} of the induction coil, and set the latter to work, a
current passes through the inserted coil C and the existence of the
current may be made manifest in various ways. For instance, I insert an
iron core _i_ within the coil. The current being one of very high
frequency, will, if it be of some strength, soon bring the iron core to
a noticeably higher temperature, as the hysteresis and current losses
are great with such high frequencies. One might take a core of some
size, laminated or not, it would matter little; but ordinary iron wire
1/16th or 1/8th of an inch thick is suitable for the purpose. While the
induction coil is working, a current traverses the inserted coil and
only a few moments are sufficient to bring the iron wire _i_ to an
elevated temperature sufficient to soften the sealing-wax _s_, and cause
a paper washer _p_ fastened by it to the iron wire to fall off. But with
the apparatus such as I have here, other, much more interesting,
demonstrations of this kind can be made. I have a secondary S, Fig 176,
of coarse wire, wound upon a coil similar to the first. In the preceding
experiment the current through the coil C, Fig. 175, was very small, but
there being many turns a strong heating effect was, nevertheless,
produced in the iron wire. Had I passed that current through a conductor
in order to show the heating of the latter, the current might have been
too small to produce the effect desired. But with this coil provided
with a secondary winding, I can now transform the feeble current of high
tension which passes through the primary P into a strong secondary
current of low tension, and this current will quite certainly do what I
expect. In a small glass tube (_t_, Fig. 176), I have enclosed a coiled
platinum wire, _w_, this merely in order to protect the wire. On each
end of the glass tube is sealed a terminal of stout wire to which one of
the ends of the platinum wire _w_, is connected. I join the terminals of
the secondary coil to these terminals and insert the primary _p_,
between the insulated plate P_{1}, and the terminal T_{1}, of the
induction coil as before. The latter being set to work, instantly the
platinum wire _w_ is rendered incandescent and can be fused, even if it
be very thick.

[Illustration: FIG. 176.]

Instead of the platinum wire I now take an ordinary 50-volt 16 C. P.
lamp. When I set the induction coil in operation the lamp filament is
brought to high incandescence. It is, however, not necessary to use the
insulated plate, for the lamp (_l_, Fig. 177) is rendered incandescent
even if the plate P_{1} be disconnected. The secondary may also be
connected to the primary as indicated by the dotted line in Fig. 177, to
do away more or less with the electrostatic induction or to modify the
action otherwise.

[Illustration: FIG. 177.]

I may here call attention to a number of interesting observations with
the lamp. First, I disconnect one of the terminals of the lamp from the
secondary S. When the induction coil plays, a glow is noted which fills
the whole bulb. This glow is due to electrostatic induction. It
increases when the bulb is grasped with the hand, and the capacity of
the experimenter's body thus added to the secondary circuit. The
secondary, in effect, is equivalent to a metallic coating, which would
be placed near the primary. If the secondary, or its equivalent, the
coating, were placed symmetrically to the primary, the electrostatic
induction would be nil under ordinary conditions, that is, when a
primary return circuit is used, as both halves would neutralize each
other. The secondary _is_ in fact placed symmetrically to the primary,
but the action of both halves of the latter, when only one of its ends
is connected to the induction coil, is not exactly equal; hence
electrostatic induction takes place, and hence the glow in the bulb. I
can nearly equalize the action of both halves of the primary by
connecting the other, free end of the same to the insulated plate, as in
the preceding experiment. When the plate is connected, the glow
disappears. With a smaller plate it would not entirely disappear and
then it would contribute to the brightness of the filament when the
secondary is closed, by warming the air in the bulb.

[Illustration: FIG. 178a.]

[Illustration: FIG. 178b.]

[Illustration: FIG. 179a.]

[Illustration: FIG. 179b.]

To demonstrate another interesting feature, I have adjusted the coils
used in a certain way. I first connect both the terminals of the lamp to
the secondary, one end of the primary being connected to the terminal
T_{1} of the induction coil and the other to the insulated plate P_{1}
as before. When the current is turned on, the lamp glows brightly, as
shown in Fig. 178_b_, in which C is a fine wire coil and S a coarse wire
secondary wound upon it. If the insulated plate P_{1} is disconnected,
leaving one of the ends _a_ of the primary insulated, the filament
becomes dark or generally it diminishes in brightness (Fig. 178_a_).
Connecting again the plate P_{1} and raising the frequency of the
current, I make the filament quite dark or barely red (Fig. 179_b_).
Once more I will disconnect the plate. One will of course infer that
when the plate is disconnected, the current through the primary will be
weakened, that therefore the E. M. F. will fall in the secondary S, and
that the brightness of the lamp will diminish. This might be the case
and the result can be secured by an easy adjustment of the coils; also
by varying the frequency and potential of the currents. But it is
perhaps of greater interest to note, that the lamp increases in
brightness when the plate is disconnected (Fig. 179_a_). In this case
all the energy the primary receives is now sunk into it, like the charge
of a battery in an ocean cable, but most of that energy is recovered
through the secondary and used to light the lamp. The current traversing
the primary is strongest at the end _b_ which is connected to the
terminal T_{1} of the induction coil, and diminishes in strength towards
the remote end _a_. But the dynamic inductive effect exerted upon the
secondary S is now greater than before, when the suspended plate was
connected to the primary. These results might have been produced by a
number of causes. For instance, the plate P_{1} being connected, the
reaction from the coil C may be such as to diminish the potential at the
terminal T_{1} of the induction coil, and therefore weaken the current
through the primary of the coil C. Or the disconnecting of the plate
may diminish the capacity effect with relation to the primary of the
latter coil to such an extent that the current through it is diminished,
though the potential at the terminal T_{1} of the induction coil may be
the same or even higher. Or the result might have been produced by the
change of phase of the primary and secondary currents and consequent
reaction. But the chief determining factor is the relation of the
self-induction and capacity of coil C and plate P_{1} and the frequency
of the currents. The greater brightness of the filament in Fig. 179_a_,
is, however, in part due to the heating of the rarefied gas in the lamp
by electrostatic induction, which, as before remarked, is greater when
the suspended plate is disconnected.

Still another feature of some interest I may here bring to your
attention. When the insulated plate is disconnected and the secondary of
the coil opened, by approaching a small object to the secondary, but
very small sparks can be drawn from it, showing that the electrostatic
induction is small in this case. But upon the secondary being closed
upon itself or through the lamp, the filament glowing brightly, strong
sparks are obtained from the secondary. The electrostatic induction is
now much greater, because the closed secondary determines a greater flow
of current through the primary and principally through that half of it
which is connected to the induction coil. If now the bulb be grasped
with the hand, the capacity of the secondary with reference to the
primary is augmented by the experimenter's body and the luminosity of
the filament is increased, the incandescence now being due partly to the
flow of current through the filament and partly to the molecular
bombardment of the rarefied gas in the bulb.

The preceding experiments will have prepared one for the next following
results of interest, obtained in the course of these investigations.
Since I can pass a current through an insulated wire merely by
connecting one of its ends to the source of electrical energy, since I
can induce by it another current, magnetize an iron core, and, in short,
perform all operations as though a return circuit were used, clearly I
can also drive a motor by the aid of only one wire. On a former occasion
I have described a simple form of motor comprising a single exciting
coil, an iron core and disc. Fig. 180 illustrates a modified way of
operating such an alternate current motor by currents induced in a
transformer connected to one lead, and several other arrangements of
circuits for operating a certain class of alternating motors founded on
the action of currents of differing phase. In view of the present state
of the art it is thought sufficient to describe these arrangements in a
few words only. The diagram, Fig. 180 II., shows a primary coil P,
connected with one of its ends to the line L leading from a high tension
transformer terminal T_{1}. In inductive relation to this primary P is a
secondary S of coarse wire in the circuit of which is a coil _c_. The
currents induced in the secondary energize the iron core _i_, which is
preferably, but not necessarily, subdivided, and set the metal disc _d_
in rotation. Such a motor M_{2} as diagramatically shown in Fig. 180
II., has been called a "magnetic lag motor," but this expression may be
objected to by those who attribute the rotation of the disc to eddy
currents circulating in minute paths when the core _i_ is finally
subdivided. In order to operate such a motor effectively on the plan
indicated, the frequencies should not be too high, not more than four or
five thousand, though the rotation is produced even with ten thousand
per second, or more.

In Fig. 180 I., a motor M_{1} having two energizing circuits, A and B,
is diagrammatically indicated. The circuit A is connected to the line L
and in series with it is a primary P, which may have its free end
connected to an insulated plate P_{1}, such connection being indicated
by the dotted lines. The other motor circuit B is connected to the
secondary S which is in inductive relation to the primary P. When the
transformer terminal T_{1} is alternately electrified, currents traverse
the open line L and also circuit A and primary P. The currents through
the latter induce secondary currents in the circuit S, which pass
through the energizing coil B of the motor. The currents through the
secondary S and those through the primary P differ in phase 90 degrees,
or nearly so, and are capable of rotating an armature placed in
inductive relation to the circuits A and B.

In Fig. 180 III., a similar motor M_{3} with two energizing circuits
A_{1} and B_{1} is illustrated. A primary P, connected with one of its
ends to the line L has a secondary S, which is preferably wound for a
tolerably high E. M. F., and to which the two energizing circuits of the
motor are connected, one directly to the ends of the secondary and the
other through a condenser C, by the action of which the currents
traversing the circuit A_{1} and B_{1} are made to differ in phase.

[Illustration: FIG. 180.]

[Illustration: FIG. 181.]

[Illustration: FIG. 182.]

In Fig. 180 IV., still another arrangement is shown. In this case two
primaries P_{1} and P_{2} are connected to the line L, one through a
condenser C of small capacity, and the other directly. The primaries are
provided with secondaries S_{1} and S_{2} which are in series with the
energizing circuits, A_{2} and B_{2} and a motor M_{3}, the condenser C
again serving to produce the requisite difference in the phase of the
currents traversing the motor circuits. As such phase motors with two or
more circuits are now well known in the art, they have been here
illustrated diagrammatically. No difficulty whatever is found in
operating a motor in the manner indicated, or in similar ways; and
although such experiments up to this day present only scientific
interest, they may at a period not far distant, be carried out with
practical objects in view.

It is thought useful to devote here a few remarks to the subject of
operating devices of all kinds by means of only one leading wire. It is
quite obvious, that when high-frequency currents are made use of, ground
connections are--at least when the E. M. F. of the currents is
great--better than a return wire. Such ground connections are
objectionable with steady or low frequency currents on account of
destructive chemical actions of the former and disturbing influences
exerted by both on the neighboring circuits; but with high frequencies
these actions practically do not exist. Still, even ground connections
become superfluous when the E. M. F. is very high, for soon a condition
is reached, when the current may be passed more economically through
open, than through closed, conductors. Remote as might seem an
industrial application of such single wire transmission of energy to one
not experienced in such lines of experiment, it will not seem so to
anyone who for some time has carried on investigations of such nature.
Indeed I cannot see why such a plan should not be practicable. Nor
should it be thought that for carrying out such a plan currents of very
high frequency are expressly required, for just as soon as potentials of
say 30,000 volts are used, the single wire transmission may be effected
with low frequencies, and experiments have been made by me from which
these inferences are made.

When the frequencies are very high it has been found in laboratory
practice quite easy to regulate the effects in the manner shown in
diagram Fig. 181. Here two primaries P and P_{1} are shown, each
connected with one of its ends to the line L and with the other end to
the condenser plates C and C, respectively. Near these are placed other
condenser plates C_{1} and C_{1}, the former being connected to the line
L and the latter to an insulated larger plate P_{2}. On the primaries
are wound secondaries S and S_{1}, of coarse wire, connected to the
devices _d_ and _l_ respectively. By varying the distances of the
condenser plates C and C_{1}, and C and C_{1} the currents through the
secondaries S and S_{1} are varied in intensity. The curious feature is
the great sensitiveness, the slightest change in the distance of the
plates producing considerable variations in the intensity or strength of
the currents. The sensitiveness may be rendered extreme by making the
frequency such, that the primary itself, without any plate attached to
its free end, satisfies, in conjunction with the closed secondary, the
condition of resonance. In such condition an extremely small change in
the capacity of the free terminal produces great variations. For
instance, I have been able to adjust the conditions so that the mere
approach of a person to the coil produces a considerable change in the
brightness of the lamps attached to the secondary. Such observations and
experiments possess, of course, at present, chiefly scientific interest,
but they may soon become of practical importance.

Very high frequencies are of course not practicable with motors on
account of the necessity of employing iron cores. But one may use sudden
discharges of low frequency and thus obtain certain advantages of
high-frequency currents without rendering the iron core entirely
incapable of following the changes and without entailing a very great
expenditure of energy in the core. I have found it quite practicable to
operate with such low frequency disruptive discharges of condensers,
alternating-current motors. A certain class of such motors which I
advanced a few years ago, which contain closed secondary circuits, will
rotate quite vigorously when the discharges are directed through the
exciting coils. One reason that such a motor operates so well with these
discharges is that the difference of phase between the primary and
secondary currents is 90 degrees, which is generally not the case with
harmonically rising and falling currents of low frequency. It might not
be without interest to show an experiment with a simple motor of this
kind, inasmuch as it is commonly thought that disruptive discharges are
unsuitable for such purposes. The motor is illustrated in Fig. 182. It
comprises a rather large iron core _i_ with slots on the top into which
are embedded thick copper washers _c c_. In proximity to the core is a
freely-movable metal disc D. The core is provided with a primary
exciting coil C_{1} the ends _a_ and _b_ of which are connected to the
terminals of the secondary S of an ordinary transformer, the primary P
of the latter being connected to an alternating distribution circuit or
generator G of low or moderate frequency. The terminals of the secondary
S are attached to a condenser C which discharges through an air gap _d
d_ which may be placed in series or shunt to the coil C_{1}. When the
conditions are properly chosen the disc D rotates with considerable
effort and the iron core _i_ does not get very perceptibly hot. With
currents from a high-frequency alternator, on the contrary, the core
gets rapidly hot and the disc rotates with a much smaller effort. To
perform the experiment properly it should be first ascertained that the
disc D is not set in rotation when the discharge is not occurring at _d
d_. It is preferable to use a large iron core and a condenser of large
capacity so as to bring the superimposed quicker oscillation to a very
low pitch or to do away with it entirely. By observing certain
elementary rules I have also found it practicable to operate ordinary
series or shunt direct-current motors with such disruptive discharges,
and this can be done with or without a return wire.


IMPEDANCE PHENOMENA.

Among the various current phenomena observed, perhaps the most
interesting are those of impedance presented by conductors to currents
varying at a rapid rate. In my first paper before the American Institute
of Electrical Engineers, I have described a few striking observations of
this kind. Thus I showed that when such currents or sudden discharges
are passed through a thick metal bar there may be points on the bar only
a few inches apart, which have a sufficient potential difference between
them to maintain at bright incandescence an ordinary filament lamp. I
have also described the curious behavior of rarefied gas surrounding a
conductor, due to such sudden rushes of current. These phenomena have
since been more carefully studied and one or two novel experiments of
this kind are deemed of sufficient interest to be described here.

Referring to Fig. 183_a_, B and B_{1} are very stout copper bars
connected at their lower ends to plates C and C_{1}, respectively, of a
condenser, the opposite plates of the latter being connected to the
terminals of the secondary S of a high-tension transformer, the primary
P of which is supplied with alternating currents from an ordinary
low-frequency dynamo G or distribution circuit. The condenser
discharges through an adjustable gap _d d_ as usual. By establishing a
rapid vibration it was found quite easy to perform the following curious
experiment. The bars B and B_{1} were joined at the top by a low-voltage
lamp l_{3}; a little lower was placed by means of clamps _c c_, a
50-volt lamp l_{2}; and still lower another 100-volt lamp l_{1}; and
finally, at a certain distance below the latter lamp, an exhausted tube
T. By carefully determining the positions of these devices it was found
practicable to maintain them all at their proper illuminating power. Yet
they were all connected in multiple arc to the two stout copper bars and
required widely different pressures. This experiment requires of course
some time for adjustment but is quite easily performed.

[Illustration: FIGS. 183a, 183b and 183c.]

In Figs. 183_b_ and 183_c_, two other experiments are illustrated which,
unlike the previous experiment, do not require very careful adjustments.
In Fig. 183_b_, two lamps, l_{1} and l_{2}, the former a 100-volt
and the latter a 50-volt are placed in certain positions as indicated,
the 100-volt lamp being below the 50-volt lamp. When the arc is playing
at _d d_ and the sudden discharges are passed through the bars B B_{1},
the 50-volt lamp will, as a rule, burn brightly, or at least this result
is easily secured, while the 100-volt lamp will burn very low or remain
quite dark, Fig. 183_b_. Now the bars B B_{1} may be joined at the top
by a thick cross bar B_{2} and it is quite easy to maintain the 100-volt
lamp at full candle-power while the 50-volt lamp remains dark, Fig.
183_c_. These results, as I have pointed out previously, should not be
considered to be due exactly to frequency but rather to the time rate of
change which may be great, even with low frequencies. A great many other
results of the same kind, equally interesting, especially to those who
are only used to manipulate steady currents, may be obtained and they
afford precious clues in investigating the nature of electric currents.

In the preceding experiments I have already had occasion to show some
light phenomena and it would now be proper to study these in particular;
but to make this investigation more complete I think it necessary to
make first a few remarks on the subject of electrical resonance which
has to be always observed in carrying out these experiments.


ON ELECTRICAL RESONANCE.

The effects of resonance are being more and more noted by engineers and
are becoming of great importance in the practical operation of apparatus
of all kinds with alternating currents. A few general remarks may
therefore be made concerning these effects. It is clear, that if we
succeed in employing the effects of resonance practically in the
operation of electric devices the return wire will, as a matter of
course, become unnecessary, for the electric vibration may be conveyed
with one wire just as well as, and sometimes even better than, with two.
The question first to answer is, then, whether pure resonance effects
are producible. Theory and experiment both show that such is impossible
in Nature, for as the oscillation becomes more and more vigorous, the
losses in the vibrating bodies and environing media rapidly increase and
necessarily check the vibration which otherwise would go on increasing
forever. It is a fortunate circumstance that pure resonance is not
producible, for if it were there is no telling what dangers might not
lie in wait for the innocent experimenter. But to a certain degree
resonance is producible, the magnitude of the effects being limited by
the imperfect conductivity and imperfect elasticity of the media or,
generally stated, by frictional losses. The smaller these losses, the
more striking are the effects. The same is the case in mechanical
vibration. A stout steel bar may be set in vibration by drops of water
falling upon it at proper intervals; and with glass, which is more
perfectly elastic, the resonance effect is still more remarkable, for a
goblet may be burst by singing into it a note of the proper pitch. The
electrical resonance is the more perfectly attained, the smaller the
resistance or the impedance of the conducting path and the more perfect
the dielectric. In a Leyden jar discharging through a short stranded
cable of thin wires these requirements are probably best fulfilled, and
the resonance effects are therefore very prominent. Such is not the case
with dynamo machines, transformers and their circuits, or with
commercial apparatus in general in which the presence of iron cores
complicates the action or renders it impossible. In regard to Leyden
jars with which resonance effects are frequently demonstrated, I would
say that the effects observed are often _attributed_ but are seldom
_due_ to true resonance, for an error is quite easily made in this
respect. This may be undoubtedly demonstrated by the following
experiment. Take, for instance, two large insulated metallic plates or
spheres which I shall designate A and B; place them at a certain small
distance apart and charge them from a frictional or influence machine to
a potential so high that just a slight increase of the difference of
potential between them will cause the small air or insulating space to
break down. This is easily reached by making a few preliminary trials.
If now another plate--fastened on an insulating handle and connected by
a wire to one of the terminals of a high tension secondary of an
induction coil, which is maintained in action by an alternator
(preferably high frequency)--is approached to one of the charged bodies
A or B, so as to be nearer to either one of them, the discharge will
invariably occur between them; at least it will, if the potential of the
coil in connection with the plate is sufficiently high. But the
explanation of this will soon be found in the fact that the approached
plate acts inductively upon the bodies A and B and causes a spark to
pass between them. When this spark occurs, the charges which were
previously imparted to these bodies from the influence machine, must
needs be lost, since the bodies are brought in electrical connection
through the arc formed. Now this arc is formed whether there be
resonance or not. But even if the spark would not be produced, still
there is an alternating E. M. F. set up between the bodies when the
plate is brought near one of them; therefore the approach of the plate,
if it _does_ not always actually, will, at any rate, _tend_ to break
down the air space by inductive action. Instead of the spheres or plates
A and B we may take the coatings of a Leyden jar with the same result,
and in place of the machine,--which is a high frequency alternator
preferably, because it is more suitable for the experiment and also for
the argument,--we may take another Leyden jar or battery of jars. When
such jars are discharging through a circuit of low resistance the same
is traversed by currents of very high frequency. The plate may now be
connected to one of the coatings of the second jar, and when it is
brought near to the first jar just previously charged to a high
potential from an influence machine, the result is the same as before,
and the first jar will discharge through a small air space upon the
second being caused to discharge. But both jars and their circuits need
not be tuned any closer than a basso profundo is to the note produced by
a mosquito, as small sparks will be produced through the air space, or
at least the latter will be considerably more strained owing to the
setting up of an alternating E. M. F. by induction, which takes place
when one of the jars begins to discharge. Again another error of a
similar nature is quite easily made. If the circuits of the two jars are
run parallel and close together, and the experiment has been performed
of discharging one by the other, and now a coil of wire be added to one
of the circuits whereupon the experiment does not succeed, the
conclusion that this is due to the fact that the circuits are now not
tuned, would be far from being safe. For the two circuits act as
condenser coatings and the addition of the coil to one of them is
equivalent to bridging them, at the point where the coil is placed, by a
small condenser, and the effect of the latter might be to prevent the
spark from jumping through the discharge space by diminishing the
alternating E. M. F. acting across the same. All these remarks, and many
more which might be added but for fear of wandering too far from the
subject, are made with the pardonable intention of cautioning the
unsuspecting student, who might gain an entirely unwarranted opinion of
his skill at seeing every experiment succeed; but they are in no way
thrust upon the experienced as novel observations.

In order to make reliable observations of electric resonance effects it
is very desirable, if not necessary, to employ an alternator giving
currents which rise and fall harmonically, as in working with make and
break currents the observations are not always trustworthy, since many
phenomena, which depend on the rate of change, may be produced with
widely different frequencies. Even when making such observations with an
alternator one is apt to be mistaken. When a circuit is connected to an
alternator there are an indefinite number of values for capacity and
self-induction which, in conjunction, will satisfy the condition of
resonance. So there are in mechanics an infinite number of tuning forks
which will respond to a note of a certain pitch, or loaded springs which
have a definite period of vibration. But the resonance will be most
perfectly attained in that case in which the motion is effected with the
greatest freedom. Now in mechanics, considering the vibration in the
common medium--that is, air--it is of comparatively little importance
whether one tuning fork be somewhat larger than another, because the
losses in the air are not very considerable. One may, of course, enclose
a tuning fork in an exhausted vessel and by thus reducing the air
resistance to a minimum obtain better resonant action. Still the
difference would not be very great. But it would make a great difference
if the tuning fork were immersed in mercury. In the electrical vibration
it is of enormous importance to arrange the conditions so that the
vibration is effected with the greatest freedom. The magnitude of the
resonance effect depends, under otherwise equal conditions, on the
quantity of electricity set in motion or on the strength of the current
driven through the circuit. But the circuit opposes the passage of the
currents by reason of its impedance and therefore, to secure the best
action it is necessary to reduce the impedance to a minimum. It is
impossible to overcome it entirely, but merely in part, for the ohmic
resistance cannot be overcome. But when the frequency of the impulses is
very great, the flow of the current is practically determined by
self-induction. Now self-induction can be overcome by combining it with
capacity. If the relation between these is such, that at the frequency
used they annul each other, that is, have such values as to satisfy the
condition of resonance, and the greatest quantity of electricity is made
to flow through the external circuit, then the best result is obtained.
It is simpler and safer to join the condenser in series with the
self-induction. It is clear that in such combinations there will be,
for a given frequency, and considering only the fundamental vibration,
values which will give the best result, with the condenser in shunt to
the self-induction coil; of course more such values than with the
condenser in series. But practical conditions determine the selection.
In the latter case in performing the experiments one may take a small
self-induction and a large capacity or a small capacity and a large
self-induction, but the latter is preferable, because it is inconvenient
to adjust a large capacity by small steps. By taking a coil with a very
large self-induction the critical capacity is reduced to a very small
value, and the capacity of the coil itself may be sufficient. It is
easy, especially by observing certain artifices, to wind a coil through
which the impedance will be reduced to the value of the ohmic resistance
only; and for any coil there is, of course, a frequency at which the
maximum current will be made to pass through the coil. The observation
of the relation between self-induction, capacity and frequency is
becoming important in the operation of alternate current apparatus, such
as transformers or motors, because by a judicious determination of the
elements the employment of an expensive condenser becomes unnecessary.
Thus it is possible to pass through the coils of an alternating current
motor under the normal working conditions the required current with a
low E. M. F. and do away entirely with the false current, and the larger
the motor, the easier such a plan becomes practicable; but it is
necessary for this to employ currents of very high potential or high
frequency.

[Illustration: FIG. 184.]

In Fig. 184 I. is shown a plan which has been followed in the study of
the resonance effects by means of a high frequency alternator. C_{1} is
a coil of many turns, which is divided into small separate sections for
the purpose of adjustment. The final adjustment was made sometimes with
a few thin iron wires (though this is not always advisable) or with a
closed secondary. The coil C_{1} is connected with one of its ends to
the line L from the alternator G and with the other end to one of the
plates _c_ of a condenser c c_{1}, the plate (c_{1}) of the latter
being connected to a much larger plate P_{1}. In this manner both
capacity and self-induction were adjusted to suit the dynamo frequency.

As regards the rise of potential through resonant action, of course,
theoretically, it may amount to anything since it depends on
self-induction and resistance and since these may have any value. But in
practice one is limited in the selection of these values and besides
these, there are other limiting causes. One may start with, say, 1,000
volts and raise the E. M. F. to 50 times that value, but one cannot
start with 100,000 and raise it to ten times that value because of the
losses in the media which are great, especially if the frequency is
high. It should be possible to start with, for instance, two volts from
a high or low frequency circuit of a dynamo and raise the E. M. F. to
many hundred times that value. Thus coils of the proper dimensions might
be connected each with only one of its ends to the mains from a machine
of low E. M. F., and though the circuit of the machine would not be
closed in the ordinary acceptance of the term, yet the machine might be
burned out if a proper resonance effect would be obtained. I have not
been able to produce, nor have I observed with currents from a dynamo
machine, such great rises of potential. It is possible, if not probable,
that with currents obtained from apparatus containing iron the
disturbing influence of the latter is the cause that these theoretical
possibilities cannot be realized. But if such is the case I attribute it
solely to the hysteresis and Foucault current losses in the core.
Generally it was necessary to transform upward, when the E. M. F. was
very low, and usually an ordinary form of induction coil was employed,
but sometimes the arrangement illustrated in Fig. 184 II., has been
found to be convenient. In this case a coil C is made in a great many
sections, a few of these being used as a primary. In this manner both
primary and secondary are adjustable. One end of the coil is connected
to the line L_{1} from the alternator, and the other line L is connected
to the intermediate point of the coil. Such a coil with adjustable
primary and secondary will be found also convenient in experiments with
the disruptive discharge. When true resonance is obtained the top of the
wave must of course be on the free end of the coil as, for instance, at
the terminal of the phosphorescence bulb B. This is easily recognized
by observing the potential of a point on the wire _w_ near to the coil.

In connection with resonance effects and the problem of transmission of
energy over a single conductor which was previously considered, I would
say a few words on a subject which constantly fills my thoughts and
which concerns the welfare of all. I mean the transmission of
intelligible signals or perhaps even power to any distance without the
use of wires. I am becoming daily more convinced of the practicability
of the scheme; and though I know full well that the great majority of
scientific men will not believe that such results can be practically and
immediately realized, yet I think that all consider the developments in
recent years by a number of workers to have been such as to encourage
thought and experiment in this direction. My conviction has grown so
strong, that I no longer look upon this plan of energy or intelligence
transmission as a mere theoretical possibility, but as a serious problem
in electrical engineering, which must be carried out some day. The idea
of transmitting intelligence without wires is the natural outcome of the
most recent results of electrical investigations. Some enthusiasts have
expressed their belief that telephony to any distance by induction
through the air is possible. I cannot stretch my imagination so far, but
I do firmly believe that it is practicable to disturb by means of
powerful machines the electrostatic condition of the earth and thus
transmit intelligible signals and perhaps power. In fact, what is there
against the carrying out of such a scheme? We now know that electric
vibration may be transmitted through a single conductor. Why then not
try to avail ourselves of the earth for this purpose? We need not be
frightened by the idea of distance. To the weary wanderer counting the
mile-posts the earth may appear very large, but to that happiest of all
men, the astronomer, who gazes at the heavens and by their standard
judges the magnitude of our globe, it appears very small. And so I think
it must seem to the electrician, for when he considers the speed with
which an electric disturbance is propagated through the earth all his
ideas of distance must completely vanish.

A point of great importance would be first to know what is the capacity
of the earth? and what charge does it contain if electrified? Though we
have no positive evidence of a charged body existing in space without
other oppositely electrified bodies being near, there is a fair
probability that the earth is such a body, for by whatever process it
was separated from other bodies--and this is the accepted view of its
origin--it must have retained a charge, as occurs in all processes of
mechanical separation. If it be a charged body insulated in space its
capacity should be extremely small, less than one-thousandth of a farad.
But the upper strata of the air are conducting, and so, perhaps, is the
medium in free space beyond the atmosphere, and these may contain an
opposite charge. Then the capacity might be incomparably greater. In any
case it is of the greatest importance to get an idea of what quantity of
electricity the earth contains. It is difficult to say whether we shall
ever acquire this necessary knowledge, but there is hope that we may,
and that is, by means of electrical resonance. If ever we can ascertain
at what period the earth's charge, when disturbed, oscillates with
respect to an oppositely electrified system or known circuit, we shall
know a fact possibly of the greatest importance to the welfare of the
human race. I propose to seek for the period by means of an electrical
oscillator, or a source of alternating electric currents. One of the
terminals of the source would be connected to earth as, for instance, to
the city water mains, the other to an insulated body of large surface.
It is possible that the outer conducting air strata, or free space,
contain an opposite charge and that, together with the earth, they form
a condenser of very large capacity. In such case the period of vibration
may be very low and an alternating dynamo machine might serve for the
purpose of the experiment. I would then transform the current to a
potential as high as it would be found possible and connect the ends of
the high tension secondary to the ground and to the insulated body. By
varying the frequency of the currents and carefully observing the
potential of the insulated body and watching for the disturbance at
various neighboring points of the earth's surface resonance might be
detected. Should, as the majority of scientific men in all probability
believe, the period be extremely small, then a dynamo machine would not
do and a proper electrical oscillator would have to be produced and
perhaps it might not be possible to obtain such rapid vibrations. But
whether this be possible or not, and whether the earth contains a charge
or not, and whatever may be its period of vibration, it certainly is
possible--for of this we have daily evidence--to produce some electrical
disturbance sufficiently powerful to be perceptible by suitable
instruments at any point of the earth's surface.

[Illustration: FIG. 185.]

Assume that a source of alternating current S be connected, as in Fig.
185, with one of its terminals to earth (conveniently to the water
mains) and with the other to a body of large surface P. When the
electric oscillation is set up there will be a movement of electricity
in and out of P, and alternating currents will pass through the earth,
converging to, or diverging from, the point C where the ground
connection is made. In this manner neighboring points on the earth's
surface within a certain radius will be disturbed. But the disturbance
will diminish with the distance, and the distance at which the effect
will still be perceptible will depend on the quantity of electricity set
in motion. Since the body P is insulated, in order to displace a
considerable quantity, the potential of the source must be excessive,
since there would be limitations as to the surface of P. The conditions
might be adjusted so that the generator or source S will set up the same
electrical movement as though its circuit were closed. Thus it is
certainly practicable to impress an electric vibration at least of a
certain low period upon the earth by means of proper machinery. At what
distance such a vibration might be made perceptible can only be
conjectured. I have on another occasion considered the question how the
earth might behave to electric disturbances. There is no doubt that,
since in such an experiment the electrical density at the surface could
be but extremely small considering the size of the earth, the air would
not act as a very disturbing factor, and there would be not much energy
lost through the action of the air, which would be the case if the
density were great. Theoretically, then, it could not require a great
amount of energy to produce a disturbance perceptible at great distance,
or even all over the surface of the globe. Now, it is quite certain that
at any point within a certain radius of the source S a properly adjusted
self-induction and capacity device can be set in action by resonance.
But not only can this be done, but another source S_{1}, Fig. 185,
similar to S, or any number of such sources, can be set to work in
synchronism with the latter, and the vibration thus intensified and
spread over a large area, or a flow of electricity produced to or from
the source S_{1} if the same be of opposite phase to the source S. I
think that beyond doubt it is possible to operate electrical devices in
a city through the ground or pipe system by resonance from an electrical
oscillator located at a central point. But the practical solution of
this problem would be of incomparably smaller benefit to man than the
realization of the scheme of transmitting intelligence, or perhaps
power, to any distance through the earth or environing medium. If this
is at all possible, distance does not mean anything. Proper apparatus
must first be produced by means of which the problem can be attacked and
I have devoted much thought to this subject. I am firmly convinced that
it can be done and hope that we shall live to see it done.


ON THE LIGHT PHENOMENA PRODUCED BY HIGH-FREQUENCY CURRENTS OF HIGH
POTENTIAL AND GENERAL REMARKS RELATING TO THE SUBJECT.

Returning now to the light effects which it has been the chief object to
investigate, it is thought proper to divide these effects into four
classes: 1. Incandescence of a solid. 2. Phosphorescence. 3.
Incandescence or phosphorescence of a rarefied gas; and 4. Luminosity
produced in a gas at ordinary pressure. The first question is: How are
these luminous effects produced? In order to answer this question as
satisfactorily as I am able to do in the light of accepted views and
with the experience acquired, and to add some interest to this
demonstration, I shall dwell here upon a feature which I consider of
great importance, inasmuch as it promises, besides, to throw a better
light upon the nature of most of the phenomena produced by
high-frequency electric currents. I have on other occasions pointed out
the great importance of the presence of the rarefied gas, or atomic
medium in general, around the conductor through which alternate currents
of high frequency are passed, as regards the heating of the conductor by
the currents. My experiments, described some time ago, have shown that,
the higher the frequency and potential difference of the currents, the
more important becomes the rarefied gas in which the conductor is
immersed, as a factor of the heating. The potential difference, however,
is, as I then pointed out, a more important element than the frequency.
When both of these are sufficiently high, the heating may be almost
entirely due to the presence of the rarefied gas. The experiments to
follow will show the importance of the rarefied gas, or, generally, of
gas at ordinary or other pressure as regards the incandescence or other
luminous effects produced by currents of this kind.

I take two ordinary 50-volt 16 C. P. lamps which are in every respect
alike, with the exception, that one has been opened at the top and the
air has filled the bulb, while the other is at the ordinary degree of
exhaustion of commercial lamps. When I attach the lamp which is
exhausted to the terminal of the secondary of the coil, which I have
already used, as in experiments illustrated in Fig. 179_a_ for instance,
and turn on the current, the filament, as you have before seen, comes to
high incandescence. When I attach the second lamp, which is filled with
air, instead of the former, the filament still glows, but much less
brightly. This experiment illustrates only in part the truth of the
statements before made. The importance of the filament's being immersed
in rarefied gas is plainly noticeable but not to such a degree as might
be desirable. The reason is that the secondary of this coil is wound for
low tension, having only 150 turns, and the potential difference at the
terminals of the lamp is therefore small. Were I to take another coil
with many more turns in the secondary, the effect would be increased,
since it depends partially on the potential difference, as before
remarked. But since the effect likewise depends on the frequency, it
maybe properly stated that it depends on the time rate of the variation
of the potential difference. The greater this variation, the more
important becomes the gas as an element of heating. I can produce a much
greater rate of variation in another way, which, besides, has the
advantage of doing away with the objections, which might be made in the
experiment just shown, even if both the lamps were connected in series
or multiple arc to the coil, namely, that in consequence of the
reactions existing between the primary and secondary coil the
conclusions are rendered uncertain. This result I secure by charging,
from an ordinary transformer which is fed from the alternating current
supply station, a battery of condensers, and discharging the latter
directly through a circuit of small self-induction, as before
illustrated in Figs. 183_a_, 183_b_, and 183_c_.

[Illustration: FIG. 186a.]

[Illustration: FIG. 186b.]

[Illustration: FIG. 186c.]

In Figs. 186_a_, 186_b_ and 186_c_, the heavy copper bars B B_{1}, are
connected to the opposite coatings of a battery of condensers, or
generally in such way, that the high frequency or sudden discharges are
made to traverse them. I connect first an ordinary 50-volt incandescent
lamp to the bars by means of the clamps _c c_. The discharges being
passed through the lamp, the filament is rendered incandescent, though
the current through it is very small, and would not be nearly sufficient
to produce a visible effect under the conditions of ordinary use of the
lamp. Instead of this I now attach to the bars another lamp exactly like
the first, but with the seal broken off, the bulb being therefore filled
with air at ordinary pressure. When the discharges are directed through
the filament, as before, it does not become incandescent. But the result
might still be attributed to one of the many possible reactions. I
therefore connect both the lamps in multiple arc as illustrated in Fig.
186_a_. Passing the discharges through both the lamps, again the
filament in the exhausted lamp _l_ glows very brightly while that in the
non-exhausted lamp l_{1} remains dark, as previously. But it should
not be thought that the latter lamp is taking only a small fraction of
the energy supplied to both the lamps; on the contrary, it may consume a
considerable portion of the energy and it may become even hotter than
the one which burns brightly. In this experiment the potential
difference at the terminals of the lamps varies in sign theoretically
three to four million times a second. The ends of the filaments are
correspondingly electrified, and the gas in the bulbs is violently
agitated and a large portion of the supplied energy is thus converted
into heat. In the non-exhausted bulb, there being a few million times
more gas molecules than in the exhausted one, the bombardment, which is
most violent at the ends of the filament, in the neck of the bulb,
consumes a large portion of the energy without producing any visible
effect. The reason is that, there being many molecules, the bombardment
is quantitatively considerable, but the individual impacts are not very
violent, as the speeds of the molecules are comparatively small owing to
the small free path. In the exhausted bulb, on the contrary, the speeds
are very great, and the individual impacts are violent and therefore
better adapted to produce a visible effect. Besides, the convection of
heat is greater in the former bulb. In both the bulbs the current
traversing the filaments is very small, incomparably smaller than that
which they require on an ordinary low-frequency circuit. The potential
difference, however, at the ends of the filaments is very great and
might be possibly 20,000 volts or more, if the filaments were straight
and their ends far apart. In the ordinary lamp a spark generally occurs
between the ends of the filament or between the platinum wires outside,
before such a difference of potential can be reached.

It might be objected that in the experiment before shown the lamps,
being in multiple arc, the exhausted lamp might take a much larger
current and that the effect observed might not be exactly attributable
to the action of the gas in the bulbs. Such objections will lose much
weight if I connect the lamps in series, with the same result. When this
is done and the discharges are directed through the filaments, it is
again noted that the filament in the non-exhausted bulb l_{1}, remains
dark, while that in the exhausted one (_l_) glows even more intensely
than under its normal conditions of working, Fig. 186_b_. According to
general ideas the current through the filaments should now be the same,
were it not modified by the presence of the gas around the filaments.

At this juncture I may point out another interesting feature, which
illustrates the effect of the rate of change of potential of the
currents. I will leave the two lamps connected in series to the bars
B B_{1}, as in the previous experiment, Fig. 186_b_, but will presently
reduce considerably the frequency of the currents, which was excessive
in the experiment just before shown. This I may do by inserting a
self-induction coil in the path of the discharges, or by augmenting the
capacity of the condensers. When I now pass these low-frequency
discharges through the lamps, the exhausted lamp _l_ again is as bright
as before, but it is noted also that the non-exhausted lamp l_{1}
glows, though not quite as intensely as the other. Reducing the current
through the lamps, I may bring the filament in the latter lamp to
redness, and, though the filament in the exhausted lamp _l_ is bright,
Fig. 186_c_, the degree of its incandescence is much smaller than in
Fig. 186_b_, when the currents were of a much higher frequency.

In these experiments the gas acts in two opposite ways in determining
the degree of the incandescence of the filaments, that is, by convection
and bombardment. The higher the frequency and potential of the currents,
the more important becomes the bombardment. The convection on the
contrary should be the smaller, the higher the frequency. When the
currents are steady there is practically no bombardment, and convection
may therefore with such currents also considerably modify the degree of
incandescence and produce results similar to those just before shown.
Thus, if two lamps exactly alike, one exhausted and one not exhausted,
are connected in multiple arc or series to a direct-current machine, the
filament in the non-exhausted lamp will require a considerably greater
current to be rendered incandescent. This result is entirely due to
convection, and the effect is the more prominent the thinner the
filament. Professor Ayrton and Mr. Kilgour some time ago published
quantitative results concerning the thermal emissivity by radiation and
convection in which the effect with thin wires was clearly shown. This
effect may be strikingly illustrated by preparing a number of small,
short, glass tubes, each containing through its axis the thinnest
obtainable platinum wire. If these tubes be highly exhausted, a number
of them may be connected in multiple arc to a direct-current machine and
all of the wires may be kept at incandescence with a smaller current
than that required to render incandescent a single one of the wires if
the tube be not exhausted. Could the tubes be so highly exhausted that
convection would be nil, then the relative amounts of heat given off by
convection and radiation could be determined without the difficulties
attending thermal quantitative measurements. If a source of electric
impulses of high frequency and very high potential is employed, a still
greater number of the tubes may be taken and the wires rendered
incandescent by a current not capable of warming perceptibly a wire of
the same size immersed in air at ordinary pressure, and conveying the
energy to all of them.

I may here describe a result which is still more interesting, and to
which I have been led by the observation of these phenomena. I noted
that small differences in the density of the air produced a considerable
difference in the degree of incandescence of the wires, and I thought
that, since in a tube, through which a luminous discharge is passed, the
gas is generally not of uniform density, a very thin wire contained in
the tube might be rendered incandescent at certain places of smaller
density of the gas, while it would remain dark at the places of greater
density, where the convection would be greater and the bombardment less
intense. Accordingly a tube _t_ was prepared, as illustrated in Fig.
187, which contained through the middle a very fine platinum wire _w_.
The tube was exhausted to a moderate degree and it was found that when
it was attached to the terminal of a high-frequency coil the platinum
wire _w_ would indeed, become incandescent in patches, as illustrated in
Fig. 187. Later a number of these tubes with one or more wires were
prepared, each showing this result. The effect was best noted when the
striated discharge occurred in the tube, but was also produced when the
striæ were not visible, showing that, even then, the gas in the tube was
not of uniform density. The position of the striæ was generally such,
that the rarefactions corresponded to the places of incandescence or
greater brightness on the wire _w_. But in a few instances it was noted,
that the bright spots on the wire were covered by the dense parts of the
striated discharge as indicated by _l_ in Fig. 187, though the effect
was barely perceptible. This was explained in a plausible way by
assuming that the convection was not widely different in the dense and
rarefied places, and that the bombardment was greater on the dense
places of the striated discharge. It is, in fact, often observed in
bulbs, that under certain conditions a thin wire is brought to higher
incandescence when the air is not too highly rarefied. This is the case
when the potential of the coil is not high enough for the vacuum, but
the result may be attributed to many different causes. In all cases this
curious phenomenon of incandescence disappears when the tube, or rather
the wire, acquires throughout a uniform temperature.

[Illustration: FIG. 187.]

[Illustration: FIG. 188.]

Disregarding now the modifying effect of convection there are then two
distinct causes which determine the incandescence of a wire or filament
with varying currents, that is, conduction current and bombardment. With
steady currents we have to deal only with the former of these two
causes, and the heating effect is a minimum, since the resistance is
least to steady flow. When the current is a varying one the resistance
is greater, and hence the heating effect is increased. Thus if the rate
of change of the current is very great, the resistance may increase to
such an extent that the filament is brought to incandescence with
inappreciable currents, and we are able to take a short and thick block
of carbon or other material and bring it to bright incandescence with a
current incomparably smaller than that required to bring to the same
degree of incandescence an ordinary thin lamp filament with a steady or
low frequency current. This result is important, and illustrates how
rapidly our views on these subjects are changing, and how quickly our
field of knowledge is extending. In the art of incandescent lighting, to
view this result in one aspect only, it has been commonly considered as
an essential requirement for practical success, that the lamp filament
should be thin and of high resistance. But now we know that the
resistance of the filament to the steady flow does not mean anything;
the filament might as well be short and thick; for if it be immersed in
rarefied gas it will become incandescent by the passage of a small
current. It all depends on the frequency and potential of the currents.
We may conclude from this, that it would be of advantage, so far as the
lamp is considered, to employ high frequencies for lighting, as they
allow the use of short and thick filaments and smaller currents.

If a wire or filament be immersed in a homogeneous medium, all the
heating is due to true conduction current, but if it be enclosed in an
exhausted vessel the conditions are entirely different. Here the gas
begins to act and the heating effect of the conduction current, as is
shown in many experiments, may be very small compared with that of the
bombardment. This is especially the case if the circuit is not closed
and the potentials are of course very high. Suppose that a fine filament
enclosed in an exhausted vessel be connected with one of its ends to the
terminal of a high tension coil and with its other end to a large
insulated plate. Though the circuit is not closed, the filament, as I
have before shown, is brought to incandescence. If the frequency and
potential be comparatively low, the filament is heated by the current
passing _through it_. If the frequency and potential, and principally
the latter, be increased, the insulated plate need be but very small, or
may be done away with entirely; still the filament will become
incandescent, practically all the heating being then due to the
bombardment. A practical way of combining both the effects of conduction
currents and bombardment is illustrated in Fig. 188, in which an
ordinary lamp is shown provided with a very thin filament which has one
of the ends of the latter connected to a shade serving the purpose of
the insulated plate, and the other end to the terminal of a high tension
source. It should not be thought that only rarefied gas is an important
factor in the heating of a conductor by varying currents, but gas at
ordinary pressure may become important, if the potential difference and
frequency of the currents is excessive. On this subject I have already
stated, that when a conductor is fused by a stroke of lightning, the
current through it may be exceedingly small, not even sufficient to heat
the conductor perceptibly, were the latter immersed in a homogeneous
medium.

From the preceding it is clear that when a conductor of high resistance
is connected to the terminals of a source of high frequency currents of
high potential, there may occur considerable dissipation of energy,
principally at the ends of the conductor, in consequence of the action
of the gas surrounding the conductor. Owing to this, the current through
a section of the conductor at a point midway between its ends may be
much smaller than through a section near the ends. Furthermore, the
current passes principally through the outer portions of the conductor,
but this effect is to be distinguished from the skin effect as
ordinarily interpreted, for the latter would, or should, occur also in a
continuous incompressible medium. If a great many incandescent lamps are
connected in series to a source of such currents, the lamps at the ends
may burn brightly, whereas those in the middle may remain entirely dark.
This is due principally to bombardment, as before stated. But even if
the currents be steady, provided the difference of potential is very
great, the lamps at the end will burn more brightly than those in the
middle. In such case there is no rhythmical bombardment, and the result
is produced entirely by leakage. This leakage or dissipation into space
when the tension is high, is considerable when incandescent lamps are
used, and still more considerable with arcs, for the latter act like
flames. Generally, of course, the dissipation is much smaller with
steady, than with varying, currents.

I have contrived an experiment which illustrates in an interesting
manner the effect of lateral diffusion. If a very long tube is attached
to the terminal of a high frequency coil, the luminosity is greatest
near the terminal and falls off gradually towards the remote end. This
is more marked if the tube is narrow.

A small tube about one-half inch in diameter and twelve inches long
(Fig. 189), has one of its ends drawn out into a fine fibre _f_ nearly
three feet long. The tube is placed in a brass socket T which can be
screwed on the terminal T_{1} of the induction coil. The discharge
passing through the tube first illuminates the bottom of the same, which
is of comparatively large section; but through the long glass fibre the
discharge cannot pass. But gradually the rarefied gas inside becomes
warmed and more conducting and the discharge spreads into the glass
fibre. This spreading is so slow, that it may take half a minute or more
until the discharge has worked through up to the top of the glass fibre,
then presenting the appearance of a strongly luminous thin thread. By
adjusting the potential at the terminal the light may be made to travel
upwards at any speed. Once, however, the glass fibre is heated, the
discharge breaks through its entire length instantly. The interesting
point to be noted is that, the higher the frequency of the currents, or
in other words, the greater relatively the lateral dissipation, at a
slower rate may the light be made to propagate through the fibre. This
experiment is best performed with a highly exhausted and freshly made
tube. When the tube has been used for some time the experiment often
fails. It is possible that the gradual and slow impairment of the vacuum
is the cause. This slow propagation of the discharge through a very
narrow glass tube corresponds exactly to the propagation of heat through
a bar warmed at one end. The quicker the heat is carried away laterally
the longer time it will take for the heat to warm the remote end. When
the current of a low frequency coil is passed through the fibre from end
to end, then the lateral dissipation is small and the discharge
instantly breaks through almost without exception.

[Illustration: FIG. 189.]

[Illustration: FIG. 190.]

After these experiments and observations which have shown the importance
of the discontinuity or atomic structure of the medium and which will
serve to explain, in a measure at least, the nature of the four kinds of
light effects producible with these currents, I may now give you an
illustration of these effects. For the sake of interest I may do this in
a manner which to many of you might be novel. You have seen before that
we may now convey the electric vibration to a body by means of a single
wire or conductor of any kind. Since the human frame is conducting I
may convey the vibration through my body.

First, as in some previous experiments, I connect my body with one of
the terminals of a high-tension transformer and take in my hand an
exhausted bulb which contains a small carbon button mounted upon a
platinum wire leading to the outside of the bulb, and the button is
rendered incandescent as soon as the transformer is set to work (Fig.
190). I may place a conducting shade on the bulb which serves to
intensify the action, but is not necessary. Nor is it required that the
button should be in conducting connection with the hand through a wire
leading through the glass, for sufficient energy may be transmitted
through the glass itself by inductive action to render the button
incandescent.

[Illustration: FIG. 191.]

[Illustration: FIG. 192.]

Next I take a highly exhausted bulb containing a strongly phosphorescent
body, above which is mounted a small plate of aluminum on a platinum
wire leading to the outside, and the currents flowing through my body
excite intense phosphorescence in the bulb (Fig. 191). Next again I take
in my hand a simple exhausted tube, and in the same manner the gas
inside the tube is rendered highly incandescent or phosphorescent (Fig.
192). Finally, I may take in my hand a wire, bare or covered with thick
insulation, it is quite immaterial; the electrical vibration is so
intense as to cover the wire with a luminous film (Fig. 193).

[Illustration: FIG. 193.]

[Illustration: FIG. 194.]

[Illustration: FIG. 195.]

A few words must now be devoted to each of these phenomena. In the first
place, I will consider the incandescence of a button or of a solid in
general, and dwell upon some facts which apply equally to all these
phenomena. It was pointed out before that when a thin conductor, such as
a lamp filament, for instance, is connected with one of its ends to the
terminal of a transformer of high tension the filament is brought to
incandescence partly by a conduction current and partly by bombardment.
The shorter and thicker the filament the more important becomes the
latter, and finally, reducing the filament to a mere button, all the
heating must practically be attributed to the bombardment. So in the
experiment before shown, the button is rendered incandescent by the
rhythmical impact of freely movable small bodies in the bulb. These
bodies may be the molecules of the residual gas, particles of dust or
lumps torn from the electrode; whatever they are, it is certain that the
heating of the button is essentially connected with the pressure of such
freely movable particles, or of atomic matter in general in the bulb.
The heating is the more intense the greater the number of impacts per
second and the greater the energy of each impact. Yet the button would
be heated also if it were connected to a source of a steady potential.
In such a case electricity would be carried away from the button by the
freely movable carriers or particles flying about, and the quantity of
electricity thus carried away might be sufficient to bring the button to
incandescence by its passage through the latter. But the bombardment
could not be of great importance in such case. For this reason it would
require a comparatively very great supply of energy to the button to
maintain it at incandescence with a steady potential. The higher the
frequency of the electric impulses the more economically can the button
be maintained at incandescence. One of the chief reasons why this is so,
is, I believe, that with impulses of very high frequency there is less
exchange of the freely movable carriers around the electrode and this
means, that in the bulb the heated matter is better confined to the
neighborhood of the button. If a double bulb, as illustrated in Fig. 194
be made, comprising a large globe B and a small one _b_, each containing
as usual a filament _f_ mounted on a platinum wire w and w_{1}, it
is found, that if the filaments _f f_ be exactly alike, it requires less
energy to keep the filament in the globe _b_ at a certain degree of
incandescence, than that in the globe B. This is due to the confinement
of the movable particles around the button. In this case it is also
ascertained, that the filament in the small globe _b_ is less
deteriorated when maintained a certain length of time at incandescence.
This is a necessary consequence of the fact that the gas in the small
bulb becomes strongly heated and therefore a very good conductor, and
less work is then performed on the button, since the bombardment becomes
less intense as the conductivity of the gas increases. In this
construction, of course, the small bulb becomes very hot and when it
reaches an elevated temperature the convection and radiation on the
outside increase. On another occasion I have shown bulbs in which this
drawback was largely avoided. In these instances a very small bulb,
containing a refractory button, was mounted in a large globe and the
space between the walls of both was highly exhausted. The outer large
globe remained comparatively cool in such constructions. When the large
globe was on the pump and the vacuum between the walls maintained
permanent by the continuous action of the pump, the outer globe would
remain quite cold, while the button in the small bulb was kept at
incandescence. But when the seal was made, and the button in the small
bulb maintained incandescent some length of time, the large globe too
would become warmed. From this I conjecture that if vacuous space (as
Prof. Dewar finds) cannot convey heat, it is so merely in virtue of our
rapid motion through space or, generally speaking, by the motion of the
medium relatively to us, for a permanent condition could not be
maintained without the medium being constantly renewed. A vacuum cannot,
according to all evidence, be permanently maintained around a hot body.

In these constructions, before mentioned, the small bulb inside would,
at least in the first stages, prevent all bombardment against the outer
large globe. It occurred to me then to ascertain how a metal sieve would
behave in this respect, and several bulbs, as illustrated in Fig. 195,
were prepared for this purpose. In a globe _b_, was mounted a thin
filament _f_ (or button) upon a platinum wire _w_ passing through a
glass stem and leading to the outside of the globe. The filament _f_ was
surrounded by a metal sieve _s_. It was found in experiments with such
bulbs that a sieve with wide meshes apparently did not in the slightest
affect the bombardment against the globe _b_. When the vacuum was high,
the shadow of the sieve was clearly projected against the globe and the
latter would get hot in a short while. In some bulbs the sieve _s_ was
connected to a platinum wire sealed in the glass. When this wire was
connected to the other terminal of the induction coil (the E. M. F.
being kept low in this case), or to an insulated plate, the bombardment
against the outer globe _b_ was diminished. By taking a sieve with fine
meshes the bombardment against the globe _b_ was always diminished, but
even then if the exhaustion was carried very far, and when the potential
of the transformer was very high, the globe _b_ would be bombarded and
heated quickly, though no shadow of the sieve was visible, owing to the
smallness of the meshes. But a glass tube or other continuous body
mounted so as to surround the filament, did entirely cut off the
bombardment and for a while the outer globe _b_ would remain perfectly
cold. Of course when the glass tube was sufficiently heated the
bombardment against the outer globe could be noted at once. The
experiments with these bulbs seemed to show that the speeds of the
projected molecules or particles must be considerable (though quite
insignificant when compared with that of light), otherwise it would be
difficult to understand how they could traverse a fine metal sieve
without being affected, unless it were found that such small particles
or atoms cannot be acted upon directly at measurable distances. In
regard to the speed of the projected atoms, Lord Kelvin has recently
estimated it at about one kilometre a second or thereabouts in an
ordinary Crookes bulb. As the potentials obtainable with a disruptive
discharge coil are much higher than with ordinary coils, the speeds
must, of course, be much greater when the bulbs are lighted from such a
coil. Assuming the speed to be as high as five kilometres and uniform
through the whole trajectory, as it should be in a very highly exhausted
vessel, then if the alternate electrifications of the electrode would be
of a frequency of five million, the greatest distance a particle could
get away from the electrode would be one millimetre, and if it could be
acted upon directly at that distance, the exchange of electrode matter
or of the atoms would be very slow and there would be practically no
bombardment against the bulb. This at least should be so, if the action
of an electrode upon the atoms of the residual gas would be such as upon
electrified bodies which we can perceive. A hot body enclosed in an
exhausted bulb produces always atomic bombardment, but a hot body has no
definite rhythm, for its molecules perform vibrations of all kinds.

If a bulb containing a button or filament be exhausted as high as is
possible with the greatest care and by the use of the best artifices, it
is often observed that the discharge cannot, at first, break through,
but after some time, probably in consequence of some changes within the
bulb, the discharge finally passes through and the button is rendered
incandescent. In fact, it appears that the higher the degree of
exhaustion the easier is the incandescence produced. There seem to be no
other causes to which the incandescence might be attributed in such case
except to the bombardment or similar action of the residual gas, or of
particles of matter in general. But if the bulb be exhausted with the
greatest care can these play an important part? Assume the vacuum in the
bulb to be tolerably perfect, the great interest then centres in the
question: Is the medium which pervades all space continuous or atomic?
If atomic, then the heating of a conducting button or filament in an
exhausted vessel might be due largely to ether bombardment, and then the
heating of a conductor in general through which currents of high
frequency or high potential are passed must be modified by the behavior
of such medium; then also the skin effect, the apparent increase of the
ohmic resistance, etc., admit, partially at least, of a different
explanation.

It is certainly more in accordance with many phenomena observed with
high-frequency currents to hold that all space is pervaded with free
atoms, rather than to assume that it is devoid of these, and dark and
cold, for so it must be, if filled with a continuous medium, since in
such there can be neither heat nor light. Is then energy transmitted by
independent carriers or by the vibration of a continuous medium? This
important question is by no means as yet positively answered. But most
of the effects which are here considered, especially the light effects,
incandescence, or phosphorescence, involve the presence of free atoms
and would be impossible without these.

In regard to the incandescence of a refractory button (or filament) in
an exhausted receiver, which has been one of the subjects of this
investigation, the chief experiences, which may serve as a guide in
constructing such bulbs, may be summed up as follows: 1. The button
should be as small as possible, spherical, of a smooth or polished
surface, and of refractory material which withstands evaporation best.
2. The support of the button should be very thin and screened by an
aluminum and mica sheet, as I have described on another occasion. 3. The
exhaustion of the bulb should be as high as possible. 4. The frequency
of the currents should be as high as practicable. 5. The currents should
be of a harmonic rise and fall, without sudden interruptions. 6. The
heat should be confined to the button by inclosing the same in a small
bulb or otherwise. 7. The space between the walls of the small bulb and
the outer globe should be highly exhausted.

Most of the considerations which apply to the incandescence of a solid
just considered may likewise be applied to phosphorescence. Indeed, in
an exhausted vessel the phosphorescence is, as a rule, primarily excited
by the powerful beating of the electrode stream of atoms against the
phosphorescent body. Even in many cases, where there is no evidence of
such a bombardment, I think that phosphorescence is excited by violent
impacts of atoms, which are not necessarily thrown off from the
electrode but are acted upon from the same inductively through the
medium or through chains of other atoms. That mechanical shocks play an
important part in exciting phosphorescence in a bulb may be seen from
the following experiment. If a bulb, constructed as that illustrated in
Fig. 174, be taken and exhausted with the greatest care so that the
discharge cannot pass, the filament _f_ acts by electrostatic induction
upon the tube _t_ and the latter is set in vibration. If the tube _o_ be
rather wide, about an inch or so, the filament may be so powerfully
vibrated that whenever it hits the glass tube it excites
phosphorescence. But the phosphorescence ceases when the filament comes
to rest. The vibration can be arrested and again started by varying the
frequency of the currents. Now the filament has its own period of
vibration, and if the frequency of the currents is such that there is
resonance, it is easily set vibrating, though the potential of the
currents be small. I have often observed that the filament in the bulb
is destroyed by such mechanical resonance. The filament vibrates as a
rule so rapidly that it cannot be seen and the experimenter may at first
be mystified. When such an experiment as the one described is carefully
performed, the potential of the currents need be extremely small, and
for this reason I infer that the phosphorescence is then due to the
mechanical shock of the filament against the glass, just as it is
produced by striking a loaf of sugar with a knife. The mechanical shock
produced by the projected atoms is easily noted when a bulb containing a
button is grasped in the hand and the current turned on suddenly. I
believe that a bulb could be shattered by observing the conditions of
resonance.

In the experiment before cited it is, of course, open to say, that the
glass tube, upon coming in contact with the filament, retains a charge
of a certain sign upon the point of contact. If now the filament again
touches the glass at the same point while it is oppositely charged, the
charges equalize under evolution of light. But nothing of importance
would be gained by such an explanation. It is unquestionable that the
initial charges given to the atoms or to the glass play some part in
exciting phosphorescence. So, for instance, if a phosphorescent bulb be
first excited by a high frequency coil by connecting it to one of the
terminals of the latter and the degree of luminosity be noted, and then
the bulb be highly charged from a Holtz machine by attaching it
preferably to the positive terminal of the machine, it is found that
when the bulb is again connected to the terminal of the high frequency
coil, the phosphorescence is far more intense. On another occasion I
have considered the possibility of some phosphorescent phenomena in
bulbs being produced by the incandescence of an infinitesimal layer on
the surface of the phosphorescent body. Certainly the impact of the
atoms is powerful enough to produce intense incandescence by the
collisions, since they bring quickly to a high temperature a body of
considerable bulk. If any such effect exists, then the best appliance
for producing phosphorescence in a bulb, which we know so far, is a
disruptive discharge coil giving an enormous potential with but few
fundamental discharges, say 25-30 per second, just enough to produce a
continuous impression upon the eye. It is a fact that such a coil
excites phosphorescence under almost any condition and at all degrees of
exhaustion, and I have observed effects which appear to be due to
phosphorescence even at ordinary pressures of the atmosphere, when the
potentials are extremely high. But if phosphorescent light is produced
by the equalization of charges of electrified atoms (whatever this may
mean ultimately), then the higher the frequency of the impulses or
alternate electrifications, the more economical will be the light
production. It is a long known and noteworthy fact that all the
phosphorescent bodies are poor conductors of electricity and heat, and
that all bodies cease to emit phosphorescent light when they are brought
to a certain temperature. Conductors on the contrary do not possess this
quality. There are but few exceptions to the rule. Carbon is one of
them. Becquerel noted that carbon phosphoresces at a certain elevated
temperature preceding the dark red. This phenomenon may be easily
observed in bulbs provided with a rather large carbon electrode (say, a
sphere of six millimetres diameter). If the current is turned on after a
few seconds, a snow white film covers the electrode, just before it gets
dark red. Similar effects are noted with other conducting bodies, but
many scientific men will probably not attribute them to true
phosphorescence. Whether true incandescence has anything to do with
phosphorescence excited by atomic impact or mechanical shocks still
remains to be decided, but it is a fact that all conditions, which tend
to localize and increase the heating effect at the point of impact, are
almost invariably the most favorable for the production of
phosphorescence. So, if the electrode be very small, which is equivalent
to saying in general, that the electric density is great; if the
potential be high, and if the gas be highly rarefied, all of which
things imply high speed of the projected atoms, or matter, and
consequently violent impacts--the phosphorescence is very intense. If a
bulb provided with a large and small electrode be attached to the
terminal of an induction coil, the small electrode excites
phosphorescence while the large one may not do so, because of the
smaller electric density and hence smaller speed of the atoms. A bulb
provided with a large electrode may be grasped with the hand while the
electrode is connected to the terminal of the coil and it may not
phosphoresce; but if instead of grasping the bulb with the hand, the
same be touched with a pointed wire, the phosphorescence at once
spreads through the bulb, because of the great density at the point of
contact. With low frequencies it seems that gases of great atomic weight
excite more intense phosphorescence than those of smaller weight, as for
instance, hydrogen. With high frequencies the observations are not
sufficiently reliable to draw a conclusion. Oxygen, as is well-known,
produces exceptionally strong effects, which may be in part due to
chemical action. A bulb with hydrogen residue seems to be most easily
excited. Electrodes which are most easily deteriorated produce more
intense phosphorescence in bulbs, but the condition is not permanent
because of the impairment of the vacuum and the deposition of the
electrode matter upon the phosphorescent surfaces. Some liquids, as
oils, for instance, produce magnificent effects of phosphorescence (or
fluorescence?), but they last only a few seconds. So if a bulb has a
trace of oil on the walls and the current is turned on, the
phosphorescence only persists for a few moments until the oil is carried
away. Of all bodies so far tried, sulphide of zinc seems to be the most
susceptible to phosphorescence. Some samples, obtained through the
kindness of Prof. Henry in Paris, were employed in many of these bulbs.
One of the defects of this sulphide is, that it loses its quality of
emitting light when brought to a temperature which is by no means high.
It can therefore, be used only for feeble intensities. An observation
which might deserve notice is, that when violently bombarded from an
aluminum electrode it assumes a black color, but singularly enough, it
returns to the original condition when it cools down.

The most important fact arrived at in pursuing investigations in this
direction is, that in all cases it is necessary, in order to excite
phosphorescence with a minimum amount of energy, to observe certain
conditions. Namely, there is always, no matter what the frequency of the
currents, degree of exhaustion and character of the bodies in the bulb,
a certain potential (assuming the bulb excited from one terminal) or
potential difference (assuming the bulb to be excited with both
terminals) which produces the most economical result. If the potential
be increased, considerable energy may be wasted without producing any
more light, and if it be diminished, then again the light production is
not as economical. The exact condition under which the best result is
obtained seems to depend on many things of a different nature, and it is
to be yet investigated by other experimenters, but it will certainly
have to be observed when such phosphorescent bulbs are operated, if the
best results are to be obtained.

Coming now to the most interesting of these phenomena, the incandescence
or phosphorescence of gases, at low pressures or at the ordinary
pressure of the atmosphere, we must seek the explanation of these
phenomena in the same primary causes, that is, in shocks or impacts of
the atoms. Just as molecules or atoms beating upon a solid body excite
phosphorescence in the same or render it incandescent, so when colliding
among themselves they produce similar phenomena. But this is a very
insufficient explanation and concerns only the crude mechanism. Light is
produced by vibrations which go on at a rate almost inconceivable. If we
compute, from the energy contained in the form of known radiations in a
definite space the force which is necessary to set up such rapid
vibrations, we find, that though the density of the ether be
incomparably smaller than that of any body we know, even hydrogen, the
force is something surpassing comprehension. What is this force, which
in mechanical measure may amount to thousands of tons per square inch?
It is electrostatic force in the light of modern views. It is impossible
to conceive how a body of measurable dimensions could be charged to so
high a potential that the force would be sufficient to produce these
vibrations. Long before any such charge could be imparted to the body it
would be shattered into atoms. The sun emits light and heat, and so does
an ordinary flame or incandescent filament, but in neither of these can
the force be accounted for if it be assumed that it is associated with
the body as a whole. Only in one way may we account for it, namely, by
identifying it with the atom. An atom is so small, that if it be charged
by coming in contact with an electrified body and the charge be assumed
to follow the same law as in the case of bodies of measurable
dimensions, it must retain a quantity of electricity which is fully
capable of accounting for these forces and tremendous rates of
vibration. But the atom behaves singularly in this respect--it always
takes the same "charge."

It is very likely that resonant vibration plays a most important part in
all manifestations of energy in nature. Throughout space all matter is
vibrating, and all rates of vibration are represented, from the lowest
musical note to the highest pitch of the chemical rays, hence an atom,
or complex of atoms, no matter what its period, must find a vibration
with which it is in resonance. When we consider the enormous rapidity
of the light vibrations, we realize the impossibility of producing such
vibrations directly with any apparatus of measurable dimensions, and we
are driven to the only possible means of attaining the object of setting
up waves of light by electrical means and economically, that is, to
affect the molecules or atoms of a gas, to cause them to collide and
vibrate. We then must ask ourselves--How can free molecules or atoms be
affected?

[Illustration: FIG. 196.]

[Illustration: FIG. 197.]

It is a fact that they can be affected by electrostatic force, as is
apparent in many of these experiments. By varying the electrostatic
force we can agitate the atoms, and cause them to collide accompanied by
evolution of heat and light. It is not demonstrated beyond doubt that we
can affect them otherwise. If a luminous discharge is produced in a
closed exhausted tube, do the atoms arrange themselves in obedience to
any other but to electrostatic force acting in straight lines from atom
to atom? Only recently I investigated the mutual action between two
circuits with extreme rates of vibration. When a battery of a few jars
(_c c c c_, Fig. 196) is discharged through a primary P of low
resistance (the connections being as illustrated in Figs. 183_a_, 183_b_
and 183_c_), and the frequency of vibration is many millions there are
great differences of potential between points on the primary not more
than a few inches apart. These differences may be 10,000 volts per inch,
if not more, taking the maximum value of the E. M. F. The secondary _s_
is therefore acted upon by electrostatic induction, which is in such
extreme cases of much greater importance than the electro-dynamic. To
such sudden impulses the primary as well as the secondary are poor
conductors, and therefore great differences of potential may be produced
by electrostatic induction between adjacent points on the secondary.
Then sparks may jump between the wires and streamers become visible in
the dark if the light of the discharge through the spark gap _d d_ be
carefully excluded. If now we substitute a closed vacuum tube for the
metallic secondary _s_, the differences of potential produced in the
tube by electrostatic induction from the primary are fully sufficient to
excite portions of it; but as the points of certain differences of
potential on the primary are not fixed, but are generally constantly
changing in position, a luminous band is produced in the tube,
apparently not touching the glass, as it should, if the points of
maximum and minimum differences of potential were fixed on the primary.
I do not exclude the possibility of such a tube being excited only by
electro-dynamic induction, for very able physicists hold this view; but
in my opinion, there is as yet no positive proof given that atoms of a
gas in a closed tube may arrange themselves in chains under the action
of an electromotive impulse produced by electro-dynamic induction in the
tube. I have been unable so far to produce striæ in a tube, however
long, and at whatever degree of exhaustion, that is, striæ at right
angles to the supposed direction of the discharge or the axis of the
tube; but I have distinctly observed in a large bulb, in which a wide
luminous band was produced by passing a discharge of a battery through a
wire surrounding the bulb, a circle of feeble luminosity between two
luminous bands, one of which was more intense than the other.
Furthermore, with my present experience I do not think that such a gas
discharge in a closed tube can vibrate, that is, vibrate as a whole. I
am convinced that no discharge through a gas can vibrate. The atoms of a
gas behave very curiously in respect to sudden electric impulses. The
gas does not seem to possess any appreciable inertia to such impulses,
for it is a fact, that the higher the frequency of the impulses, with
the greater freedom does the discharge pass through the gas. If the gas
possesses no inertia then it cannot vibrate, for some inertia is
necessary for the free vibration. I conclude from this that if a
lightning discharge occurs between two clouds, there can be no
oscillation, such as would be expected, considering the capacity of the
clouds. But if the lightning discharge strike the earth, there is always
vibration--in the earth, but not in the cloud. In a gas discharge each
atom vibrates at its own rate, but there is no vibration of the
conducting gaseous mass as a whole. This is an important consideration
in the great problem of producing light economically, for it teaches us
that to reach this result we must use impulses of very high frequency
and necessarily also of high potential. It is a fact that oxygen
produces a more intense light in a tube. Is it because oxygen atoms
possess some inertia and the vibration does not die out instantly? But
then nitrogen should be as good, and chlorine and vapors of many other
bodies much better than oxygen, unless the magnetic properties of the
latter enter prominently into play. Or, is the process in the tube of an
electrolytic nature? Many observations certainly speak for it, the most
important being that matter is always carried away from the electrodes
and the vacuum in a bulb cannot be permanently maintained. If such
process takes place in reality, then again must we take refuge in high
frequencies, for, with such, electrolytic action should be reduced to a
minimum, if not rendered entirely impossible. It is an undeniable fact
that with very high frequencies, provided the impulses be of harmonic
nature, like those obtained from an alternator, there is less
deterioration and the vacua are more permanent. With disruptive
discharge coils there are sudden rises of potential and the vacua are
more quickly impaired, for the electrodes are deteriorated in a very
short time. It was observed in some large tubes, which were provided
with heavy carbon blocks B B_{1}, connected to platinum wires w w_{1}
(as illustrated in Fig. 197), and which were employed in experiments
with the disruptive discharge instead of the ordinary air gap, that the
carbon particles under the action of the powerful magnetic field in
which the tube was placed, were deposited in regular fine lines in the
middle of the tube, as illustrated. These lines were attributed to the
deflection or distortion of the discharge by the magnetic field, but why
the deposit occurred principally where the field was most intense did
not appear quite clear. A fact of interest, likewise noted, was that the
presence of a strong magnetic field increases the deterioration of the
electrodes, probably by reason of the rapid interruptions it produces,
whereby there is actually a higher E. M. F. maintained between the
electrodes.

Much would remain to be said about the luminous effects produced in
gases at low or ordinary pressures. With the present experiences before
us we cannot say that the essential nature of these charming phenomena
is sufficiently known. But investigations in this direction are being
pushed with exceptional ardor. Every line of scientific pursuit has its
fascinations, but electrical investigation appears to possess a
peculiar attraction, for there is no experiment or observation of any
kind in the domain of this wonderful science which would not forcibly
appeal to us. Yet to me it seems, that of all the many marvelous things
we observe, a vacuum tube, excited by an electric impulse from a distant
source, bursting forth out of the darkness and illuminating the room
with its beautiful light, is as lovely a phenomenon as can greet our
eyes. More interesting still it appears when, reducing the fundamental
discharges across the gap to a very small number and waving the tube
about we produce all kinds of designs in luminous lines. So by way of
amusement I take a straight long tube, or a square one, or a square
attached to a straight tube, and by whirling them about in the hand, I
imitate the spokes of a wheel, a Gramme winding, a drum winding, an
alternate current motor winding, etc. (Fig. 198). Viewed from a distance
the effect is weak and much of its beauty is lost, but being near or
holding the tube in the hand, one cannot resist its charm.

[Illustration: FIG. 198.]

In presenting these insignificant results I have not attempted to
arrange and co-ordinate them, as would be proper in a strictly
scientific investigation, in which every succeeding result should be a
logical sequence of the preceding, so that it might be guessed in
advance by the careful reader or attentive listener. I have preferred to
concentrate my energies chiefly upon advancing novel facts or ideas
which might serve as suggestions to others, and this may serve as an
excuse for the lack of harmony. The explanations of the phenomena have
been given in good faith and in the spirit of a student prepared to find
that they admit of a better interpretation. There can be no great harm
in a student taking an erroneous view, but when great minds err, the
world must dearly pay for their mistakes.




CHAPTER XXIX.

TESLA ALTERNATING CURRENT GENERATORS FOR HIGH FREQUENCY, IN DETAIL.


It has become a common practice to operate arc lamps by alternating or
pulsating, as distinguished from continuous, currents; but an objection
which has been raised to such systems exists in the fact that the arcs
emit a pronounced sound, varying with the rate of the alternations or
pulsations of current. This noise is due to the rapidly alternating
heating and cooling, and consequent expansion and contraction, of the
gaseous matter forming the arc, which corresponds with the periods or
impulses of the current. Another disadvantageous feature is found in the
difficulty of maintaining an alternating current arc in consequence of
the periodical increase in resistance corresponding to the periodical
working of the current. This feature entails a further disadvantage,
namely, that small arcs are impracticable.

Theoretical considerations have led Mr. Tesla to the belief that these
disadvantageous features could be obviated by employing currents of a
sufficiently high number of alternations, and his anticipations have
been confirmed in practice. These rapidly alternating currents render it
possible to maintain small arcs which, besides, possess the advantages
of silence and persistency. The latter quality is due to the necessarily
rapid alternations, in consequence of which the arc has no time to cool,
and is always maintained at a high temperature and low resistance.

At the outset of his experiments Mr. Tesla encountered great
difficulties in the construction of high frequency machines. A generator
of this kind is described here, which, though constructed quite some
time ago, is well worthy of a detailed description. It may be mentioned,
in passing, that dynamos of this type have been used by Mr. Tesla in his
lighting researches and experiments with currents of high potential and
high frequency, and reference to them will be found in his lectures
elsewhere printed in this volume.[4]

  [4] See pages 153-4 5.

In the accompanying engravings, Figs. 199 and 200 show the machine,
respectively, in side elevation and vertical cross-section; Figs. 201,
202 and 203 showing enlarged details of construction. As will be seen, A
is an annular magnetic frame, the interior of which is provided with a
large number of pole-pieces D.

Owing to the very large number and small size of the poles and the
spaces between them, the field coils are applied by winding an insulated
conductor F zigzag through the grooves, as shown in Fig. 203, carrying
the wire around the annulus to form as many layers as is desired. In
this way the pole-pieces D will be energized with alternately opposite
polarity around the entire ring.

For the armature, Mr. Tesla employs a spider carrying a ring J, turned
down, except at its edges, to form a trough-like receptacle for a mass
of fine annealed iron wires K, which are wound in the groove to form the
core proper for the armature-coils. Pins L are set in the sides of the
ring J and the coils M are wound over the periphery of the
armature-structure and around the pins. The coils M are connected
together in series, and these terminals N carried through the hollow
shaft H to contact-rings P P, from which the currents are taken off by
brushes O.

[Illustration: FIG. 199.]

In this way a machine with a very large number of poles may be
constructed. It is easy, for instance, to obtain in this manner three
hundred and seventy-five to four hundred poles in a machine that may be
safely driven at a speed of fifteen hundred or sixteen hundred
revolutions per minute, which will produce ten thousand or eleven
thousand alternations of current per second. Arc lamps R R are shown in
the diagram as connected up in series with the machine in Fig. 200. If
such a current be applied to running arc lamps, the sound produced by or
in the arc becomes practically inaudible, for, by increasing the rate of
change in the current, and consequently the number of vibrations per
unit of time of the gaseous material of the arc up to, or beyond, ten
thousand or eleven thousand per second, or to what is regarded as the
limit of audition, the sound due to such vibrations will not be audible.
The exact number of changes or undulations necessary to produce this
result will vary somewhat according to the size of the arc--that is to
say, the smaller the arc, the greater the number of changes that will be
required to render it inaudible within certain limits. It should also be
stated that the arc should not exceed a certain length.

[Illustration: FIGS. 200, 201, 202 and 203.]

The difficulties encountered in the construction of these machines are
of a mechanical as well as an electrical nature. The machines may be
designed on two plans: the field may be formed either of alternating
poles, or of polar projections of the same polarity. Up to about 15,000
alternations per second in an experimental machine, the former plan may
be followed, but a more efficient machine is obtained on the second
plan.

In the machine above described, which was capable of running two arcs of
normal candle power, the field was composed of a ring of wrought iron
32 inches outside diameter, and about 1 inch thick. The inside diameter
was 30 inches. There were 384 polar projections. The wire was wound in
zigzag form, but two wires were wound so as to completely envelop the
projections. The distance between the projections is about 3/16 inch,
and they are a little over 1/16 inch thick. The field magnet was made
relatively small so as to adapt the machine for a constant current.
There are 384 coils connected in two series. It was found impracticable
to use any wire much thicker than No. 26 B. and S. gauge on account of
the local effects. In such a machine the clearance should be as small as
possible; for this reason the machine was made only 1-1/4 inch wide, so
that the binding wires might be obviated. The armature wires must be
wound with great care, as they are apt to fly off in consequence of the
great peripheral speed. In various experiments this machine has been run
as high as 3,000 revolutions per minute. Owing to the great speed it was
possible to obtain as high as 10 amperes out of the machine. The
electromotive force was regulated by means of an adjustable condenser
within very wide limits, the limits being the greater, the greater the
speed. This machine was frequently used to run Mr. Tesla's laboratory
lights.

[Illustration: FIG. 204.]

The machine above described was only one of many such types constructed.
It serves well for an experimental machine, but if still higher
alternations are required and higher efficiency is necessary, then a
machine on a plan shown in Figs. 204 to 207, is preferable. The
principal advantage of this type of machine is that there is not much
magnetic leakage, and that a field may be produced, varying greatly in
intensity in places not much distant from each other.

In these engravings, Figs. 204 and 205 illustrate a machine in which the
armature conductor and field coils are stationary, while the field
magnet core revolves. Fig. 206 shows a machine embodying the same plan
of construction, but having a stationary field magnet and rotary
armature.

The conductor in which the currents are induced may be arranged in
various ways; but Mr. Tesla prefers the following method: He employs an
annular plate of copper D, and by means of a saw cuts in it radial slots
from one edge nearly through to the other, beginning alternately from
opposite edges. In this way a continuous zigzag conductor is formed.
When the polar projections are 1/8 inch wide, the width of the conductor
should not, under any circumstances, be more than 1/32 inch wide; even
then the eddy effect is considerable.

[Illustration: FIG. 205.]

To the inner edge of this plate are secured two rings of non-magnetic
metal E, which are insulated from the copper conductor, but held firmly
thereto by means of the bolts F. Within the rings E is then placed an
annular coil G, which is the energizing coil for the field magnet. The
conductor D and the parts attached thereto are supported by means of the
cylindrical shell or casting A A, the two parts of which are brought
together and clamped to the outer edge of the conductor D.

[Illustration: FIG. 206.]

The core for the field magnet is built up of two circular parts H H,
formed with annular grooves I, which, when the two parts are brought
together, form a space for the reception of the energizing coil G. The
hubs of the cores are trued off, so as to fit closely against one
another, while the outer portions or flanges which form the polar faces
J J, are reduced somewhat in thickness to make room for the conductor D,
and are serrated on their faces. The number of serrations in the polar
faces is arbitrary; but there must exist between them and the radial
portions of the conductor D certain relation, which will be understood
by reference to Fig. 207 in which N N represent the projections or
points on one face of the core of the field, and S S the points of the
other face. The conductor D is shown in this figure in section _a a'_
designating the radial portions of the conductor, and _b_ the insulating
divisions between them. The relative width of the parts _a a'_ and the
space between any two adjacent points N N or S S is such that when the
radial portions _a_ of the conductor are passing between the opposite
points N S where the field is strongest, the intermediate radial
portions _a'_ are passing through the widest spaces midway between such
points and where the field is weakest. Since the core on one side is of
opposite polarity to the part facing it, all the projections of one
polar face will be of opposite polarity to those of the other face.
Hence, although the space between any two adjacent points on the same
face may be extremely small, there will be no leakage of the magnetic
lines between any two points of the same name, but the lines of force
will pass across from one set of points to the other. The construction
followed obviates to a great degree the distortion of the magnetic lines
by the action of the current in the conductor D, in which it will be
observed the current is flowing at any given time from the centre toward
the periphery in one set of radial parts _a_ and in the opposite
direction in the adjacent parts _a'_.

In order to connect the energizing coil G, Fig. 204, with a source of
continuous current, Mr. Tesla utilizes two adjacent radial portions of
the conductor D for connecting the terminals of the coil G with two
binding posts M. For this purpose the plate D is cut entirely through,
as shown, and the break thus made is bridged over by a short conductor
C. The plate D is cut through to form two terminals _d_, which are
connected to binding posts N. The core H H, when rotated by the driving
pulley, generates in the conductors D an alternating current, which is
taken off from the binding posts N.

[Illustration: FIG. 207.]

When it is desired to rotate the conductor between the faces of a
stationary field magnet, the construction shown in Fig. 206, is adopted.
The conductor D in this case is or may be made in substantially the same
manner as above described by slotting an annular conducting-plate and
supporting it between two heads O, held together by bolts _o_ and fixed
to the driving-shaft K. The inner edge of the plate or conductor D is
preferably flanged to secure a firmer union between it and the heads O.
It is insulated from the head. The field-magnet in this case consists of
two annular parts H H, provided with annular grooves I for the reception
of the coils. The flanges or faces surrounding the annular groove are
brought together, while the inner flanges are serrated, as in the
previous case, and form the polar faces. The two parts H H are formed
with a base R, upon which the machine rests. S S are non-magnetic
bushings secured or set in the central opening of the cores. The
conductor D is cut entirely through at one point to form terminals, from
which insulated conductors T are led through the shaft to
collecting-rings V.

In one type of machine of this kind constructed by Mr. Tesla, the field
had 480 polar projections on each side, and from this machine it was
possible to obtain 30,000 alternations per second. As the polar
projections must necessarily be very narrow, very thin wires or sheets
must be used to avoid the eddy current effects. Mr. Tesla has thus
constructed machines with a stationary armature and rotating field, in
which case also the field-coil was supported so that the revolving part
consisted only of a wrought iron body devoid of any wire and also
machines with a rotating armature and stationary field. The machines may
be either drum or disc, but Mr. Tesla's experience shows the latter to
be preferable.

       *       *       *       *       *

In the course of a very interesting article contributed to the
_Electrical World_ in February, 1891, Mr. Tesla makes some suggestive
remarks on these high frequency machines and his experiences with them,
as well as with other parts of the high frequency apparatus. Part of it
is quoted here and is as follows:--

The writer will incidentally mention that any one who attempts for the
first time to construct such a machine will have a tale of woe to tell.
He will first start out, as a matter of course, by making an armature
with the required number of polar projections. He will then get the
satisfaction of having produced an apparatus which is fit to accompany a
thoroughly Wagnerian opera. It may besides possess the virtue of
converting mechanical energy into heat in a nearly perfect manner. If
there is a reversal in the polarity of the projections, he will get heat
out of the machine; if there is no reversal, the heating will be less,
but the output will be next to nothing. He will then abandon the iron in
the armature, and he will get from the Scylla to the Charybdis. He will
look for one difficulty and will find another, but, after a few trials,
he may get nearly what he wanted.

Among the many experiments which may be performed with such a machine,
of not the least interest are those performed with a high-tension
induction coil. The character of the discharge is completely changed.
The arc is established at much greater distances, and it is so easily
affected by the slightest current of air that it often wriggles around
in the most singular manner. It usually emits the rhythmical sound
peculiar to the alternate current arcs, but the curious point is that
the sound may be heard with a number of alternations far above ten
thousand per second, which by many is considered to be about the limit
of audition. In many respects the coil behaves like a static machine.
Points impair considerably the sparking interval, electricity escaping
from them freely, and from a wire attached to one of the terminals
streams of light issue, as though it were connected to a pole of a
powerful Toepler machine. All these phenomena are, of course, mostly due
to the enormous differences of potential obtained. As a consequence of
the self-induction of the coil and the high frequency, the current is
minute while there is a corresponding rise of pressure. A current
impulse of some strength started in such a coil should persist to flow
no less than four ten-thousandths of a second. As this time is greater
than half the period, it occurs that an opposing electromotive force
begins to act while the current is still flowing. As a consequence, the
pressure rises as in a tube filled with liquid and vibrated rapidly
around its axis. The current is so small that, in the opinion and
involuntary experience of the writer, the discharge of even a very large
coil cannot produce seriously injurious effects, whereas, if the same
coil were operated with a current of lower frequency, though the
electromotive force would be much smaller, the discharge would be most
certainly injurious. This result, however, is due in part to the high
frequency. The writer's experiences tend to show that the higher the
frequency the greater the amount of electrical energy which may be
passed through the body without serious discomfort; whence it seems
certain that human tissues act as condensers.

One is not quite prepared for the behavior of the coil when connected to
a Leyden jar. One, of course, anticipates that since the frequency is
high the capacity of the jar should be small. He therefore takes a very
small jar, about the size of a small wine glass, but he finds that even
with this jar the coil is practically short-circuited. He then reduces
the capacity until he comes to about the capacity of two spheres, say,
ten centimetres in diameter and two to four centimetres apart. The
discharge then assumes the form of a serrated band exactly like a
succession of sparks viewed in a rapidly revolving mirror; the
serrations, of course, corresponding to the condenser discharges. In
this case one may observe a queer phenomenon. The discharge starts at
the nearest points, works gradually up, breaks somewhere near the top of
the spheres, begins again at the bottom, and so on. This goes on so fast
that several serrated bands are seen at once. One may be puzzled for a
few minutes, but the explanation is simple enough. The discharge begins
at the nearest points, the air is heated and carries the arc upward
until it breaks, when it is re-established at the nearest points, etc.
Since the current passes easily through a condenser of even small
capacity, it will be found quite natural that connecting only one
terminal to a body of the same size, no matter how well insulated,
impairs considerably the striking distance of the arc.

Experiments with Geissler tubes are of special interest. An exhausted
tube, devoid of electrodes of any kind, will light up at some distance
from the coil. If a tube from a vacuum pump is near the coil the whole
of the pump is brilliantly lighted. An incandescent lamp approached to
the coil lights up and gets perceptibly hot. If a lamp have the
terminals connected to one of the binding posts of the coil and the hand
is approached to the bulb, a very curious and rather unpleasant
discharge from the glass to the hand takes place, and the filament may
become incandescent. The discharge resembles to some extent the stream
issuing from the plates of a powerful Toepler machine, but is of
incomparably greater quantity. The lamp in this case acts as a
condenser, the rarefied gas being one coating, the operator's hand the
other. By taking the globe of a lamp in the hand, and by bringing the
metallic terminals near to or in contact with a conductor connected to
the coil, the carbon is brought to bright incandescence and the glass is
rapidly heated. With a 100-volt 10 C. P. lamp one may without great
discomfort stand as much current as will bring the lamp to a
considerable brilliancy; but it can be held in the hand only for a few
minutes, as the glass is heated in an incredibly short time. When a tube
is lighted by bringing it near to the coil it may be made to go out by
interposing a metal plate on the hand between the coil and tube; but if
the metal plate be fastened to a glass rod or otherwise insulated, the
tube may remain lighted if the plate be interposed, or may even
increase in luminosity. The effect depends on the position of the plate
and tube relatively to the coil, and may be always easily foretold by
_assuming_ that conduction takes place from one terminal of the coil to
the other. According to the position of the plate, it may either divert
from or direct the current to the tube.

In another line of work the writer has in frequent experiments
maintained incandescent lamps of 50 or 100 volts burning at any desired
candle power with both the terminals of each lamp connected to a stout
copper wire of no more than a few feet in length. These experiments seem
interesting enough, but they are not more so than the queer experiment
of Faraday, which has been revived and made much of by recent
investigators, and in which a discharge is made to jump between two
points of a bent copper wire. An experiment may be cited here which may
seem equally interesting. If a Geissler tube, the terminals of which are
joined by a copper wire, be approached to the coil, certainly no one
would be prepared to see the tube light up. Curiously enough, it does
light up, and, what is more, the wire does not seem to make much
difference. Now one is apt to think in the first moment that the
impedance of the wire might have something to do with the phenomenon.
But this is of course immediately rejected, as for this an enormous
frequency would be required. This result, however, seems puzzling only
at first; for upon reflection it is quite clear that the wire can make
but little difference. It may be explained in more than one way, but it
agrees perhaps best with observation to assume that conduction takes
place from the terminals of the coil through the space. On this
assumption, if the tube with the wire be held in any position, the wire
can divert little more than the current which passes through the space
occupied by the wire and the metallic terminals of the tube; through the
adjacent space the current passes practically undisturbed. For this
reason, if the tube be held in any position at right angles to the line
joining the binding posts of the coil, the wire makes hardly any
difference, but in a position more or less parallel with that line it
impairs to a certain extent the brilliancy of the tube and its facility
to light up. Numerous other phenomena may be explained on the same
assumption. For instance, if the ends of the tube be provided with
washers of sufficient size and held in the line joining the terminals of
the coil, it will not light up, and then nearly the whole of the
current, which would otherwise pass uniformly through the space between
the washers, is diverted through the wire. But if the tube be inclined
sufficiently to that line, it will light up in spite of the washers.
Also, if a metal plate be fastened upon a glass rod and held at right
angles to the line joining the binding posts, and nearer to one of them,
a tube held more or less parallel with the line will light up instantly
when one of the terminals touches the plate, and will go out when
separated from the plate. The greater the surface of the plate, up to a
certain limit, the easier the tube will light up. When a tube is placed
at right angles to the straight line joining the binding posts, and then
rotated, its luminosity steadily increases until it is parallel with
that line. The writer must state, however, that he does not favor the
idea of a leakage or current through the space any more than as a
suitable explanation, for he is convinced that all these experiments
could not be performed with a static machine yielding a constant
difference of potential, and that condenser action is largely concerned
in these phenomena.

It is well to take certain precautions when operating a Ruhmkorff coil
with very rapidly alternating currents. The primary current should not
be turned on too long, else the core may get so hot as to melt the
gutta-percha or paraffin, or otherwise injure the insulation, and this
may occur in a surprisingly short time, considering the current's
strength. The primary current being turned on, the fine wire terminals
may be joined without great risk, the impedance being so great that it
is difficult to force enough current through the fine wire so as to
injure it, and in fact the coil may be on the whole much safer when the
terminals of the fine wire are connected than when they are insulated;
but special care should be taken when the terminals are connected to the
coatings of a Leyden jar, for with anywhere near the critical capacity,
which just counteracts the self-induction at the existing frequency, the
coil might meet the fate of St. Polycarpus. If an expensive vacuum pump
is lighted up by being near to the coil or touched with a wire connected
to one of the terminals, the current should be left on no more than a
few moments, else the glass will be cracked by the heating of the
rarefied gas in one of the narrow passages--in the writer's own
experience _quod erat demonstrandum_.[5]

  [5] It is thought necessary to remark that, although the induction
      coil may give quite a good result when operated with such
      rapidly alternating currents, yet its construction, quite
      irrespective of the iron core, makes it very unfit for such
      high frequencies, and to obtain the best results the
      construction should be greatly modified.

There are a good many other points of interest which may be observed in
connection with such a machine. Experiments with the telephone, a
conductor in a strong field or with a condenser or arc, seem to afford
certain proof that sounds far above the usual accepted limit of hearing
would be perceived. A telephone will emit notes of twelve to thirteen
thousand vibrations per second; then the inability of the core to follow
such rapid alternations begins to tell. If, however, the magnet and core
be replaced by a condenser and the terminals connected to the
high-tension secondary of a transformer, higher notes may still be
heard. If the current be sent around a finely laminated core and a small
piece of thin sheet iron be held gently against the core, a sound may be
still heard with thirteen to fourteen thousand alternations per second,
provided the current is sufficiently strong. A small coil, however,
tightly packed between the poles of a powerful magnet, will emit a sound
with the above number of alternations, and arcs may be audible with a
still higher frequency. The limit of audition is variously estimated. In
Sir William Thomson's writings it is stated somewhere that ten thousand
per second, or nearly so, is the limit. Other, but less reliable,
sources give it as high as twenty-four thousand per second. The above
experiments have convinced the writer that notes of an incomparably
higher number of vibrations per second would be perceived provided they
could be produced with sufficient power. There is no reason why it
should not be so. The condensations and rarefactions of the air would
necessarily set the diaphragm in a corresponding vibration and some
sensation would be produced, whatever--within certain limits--the
velocity of transmission to their nerve centres, though it is probable
that for want of exercise the ear would not be able to distinguish any
such high note. With the eye it is different; if the sense of vision is
based upon some resonance effect, as many believe, no amount of increase
in the intensity of the ethereal vibration could extend our range of
vision on either side of the visible spectrum.

The limit of audition of an arc depends on its size. The greater the
surface by a given heating effect in the arc, the higher the limit of
audition. The highest notes are emitted by the high-tension discharges
of an induction coil in which the arc is, so to speak, all surface. If
_R_ be the resistance of an arc, and _C_ the current, and the linear
dimensions be _n_ times increased, then the resistance is _R_/_n_, and
with the same current density the current would be _n_^2_C_; hence the
heating effect is _n_^3 times greater, while the surface is only _n_^2
times as great. For this reason very large arcs would not emit any
rhythmical sound even with a very low frequency. It must be observed,
however, that the sound emitted depends to some extent also on the
composition of the carbon. If the carbon contain highly refractory
material, this, when heated, tends to maintain the temperature of the
arc uniform and the sound is lessened; for this reason it would seem
that an alternating arc requires such carbons.

With currents of such high frequencies it is possible to obtain
noiseless arcs, but the regulation of the lamp is rendered extremely
difficult on account of the excessively small attractions or repulsions
between conductors conveying these currents.

An interesting feature of the arc produced by these rapidly alternating
currents is its persistency. There are two causes for it, one of which
is always present, the other sometimes only. One is due to the character
of the current and the other to a property of the machine. The first
cause is the more important one, and is due directly to the rapidity of
the alternations. When an arc is formed by a periodically undulating
current, there is a corresponding undulation in the temperature of the
gaseous column, and, therefore, a corresponding undulation in the
resistance of the arc. But the resistance of the arc varies enormously
with the temperature of the gaseous column, being practically infinite
when the gas between the electrodes is cold. The persistence of the arc,
therefore, depends on the inability of the column to cool. It is for
this reason impossible to maintain an arc with the current alternating
only a few times a second. On the other hand, with a practically
continuous current, the arc is easily maintained, the column being
constantly kept at a high temperature and low resistance. The higher the
frequency the smaller the time interval during which the arc may cool
and increase considerably in resistance. With a frequency of 10,000 per
second or more in an arc of equal size excessively small variations of
temperature are superimposed upon a steady temperature, like ripples on
the surface of a deep sea. The heating effect is practically continuous
and the arc behaves like one produced by a continuous current, with the
exception, however, that it may not be quite as easily started, and that
the electrodes are equally consumed; though the writer has observed
some irregularities in this respect.

The second cause alluded to, which possibly may not be present, is due
to the tendency of a machine of such high frequency to maintain a
practically constant current. When the arc is lengthened, the
electromotive force rises in proportion and the arc appears to be more
persistent.

Such a machine is eminently adapted to maintain a constant current, but
it is very unfit for a constant potential. As a matter of fact, in
certain types of such machines a nearly constant current is an almost
unavoidable result. As the number of poles or polar projections is
greatly increased, the clearance becomes of great importance. One has
really to do with a great number of very small machines. Then there is
the impedance in the armature, enormously augmented by the high
frequency. Then, again, the magnetic leakage is facilitated. If there
are three or four hundred alternate poles, the leakage is so great that
it is virtually the same as connecting, in a two-pole machine, the poles
by a piece of iron. This disadvantage, it is true, may be obviated more
or less by using a field throughout of the same polarity, but then one
encounters difficulties of a different nature. All these things tend to
maintain a constant current in the armature circuit.

In this connection it is interesting to notice that even to-day
engineers are astonished at the performance of a constant current
machine, just as, some years ago, they used to consider it an
extraordinary performance if a machine was capable of maintaining a
constant potential difference between the terminals. Yet one result is
just as easily secured as the other. It must only be remembered that in
an inductive apparatus of any kind, if constant potential is required,
the inductive relation between the primary or exciting and secondary or
armature circuit must be the closest possible; whereas, in an apparatus
for constant current just the opposite is required. Furthermore, the
opposition to the current's flow in the induced circuit must be as small
as possible in the former and as great as possible in the latter case.
But opposition to a current's flow may be caused in more than one way.
It may be caused by ohmic resistance or self-induction. One may make the
induced circuit of a dynamo machine or transformer of such high
resistance that when operating devices of considerably smaller
resistance within very wide limits a nearly constant current is
maintained. But such high resistance involves a great loss in power,
hence it is not practicable. Not so self-induction. Self-induction does
not necessarily mean loss of power. The moral is, use self-induction
instead of resistance. There is, however, a circumstance which favors
the adoption of this plan, and this is, that a very high self-induction
may be obtained cheaply by surrounding a comparatively small length of
wire more or less completely with iron, and, furthermore, the effect may
be exalted at will by causing a rapid undulation of the current. To sum
up, the requirements for constant current are: Weak magnetic connection
between the induced and inducing circuits, greatest possible
self-induction with the least resistance, greatest practicable rate of
change of the current. Constant potential, on the other hand, requires:
Closest magnetic connection between the circuits, steady induced
current, and, if possible, no reaction. If the latter conditions could
be fully satisfied in a constant potential machine, its output would
surpass many times that of a machine primarily designed to give constant
current. Unfortunately, the type of machine in which these conditions
may be satisfied is of little practical value, owing to the small
electromotive force obtainable and the difficulties in taking off the
current.

With their keen inventor's instinct, the now successful arc-light men
have early recognized the desiderata of a constant current machine.
Their arc light machines have weak fields, large armatures, with a great
length of copper wire and few commutator segments to produce great
variations in the current's strength and to bring self-induction into
play. Such machines may maintain within considerable limits of variation
in the resistance of the circuit a practically constant current. Their
output is of course correspondingly diminished, and, perhaps with the
object in view not to cut down the output too much, a simple device
compensating exceptional variations is employed. The undulation of the
current is almost essential to the commercial success of an arc-light
system. It introduces in the circuit a steadying element taking the
place of a large ohmic resistance, without involving a great loss in
power, and, what is more important, it allows the use of simple clutch
lamps, which with a current of a certain number of impulses per second,
best suitable for each particular lamp, will, if properly attended to,
regulate even better than the finest clock-work lamps. This discovery
has been made by the writer--several years too late.

It has been asserted by competent English electricians that in a
constant-current machine or transformer the regulation is effected by
varying the phase of the secondary current. That this view is erroneous
may be easily proved by using, instead of lamps, devices each possessing
self-induction and capacity or self-induction and resistance--that is,
retarding and accelerating components--in such proportions as to not
affect materially the phase of the secondary current. Any number of such
devices may be inserted or cut out, still it will be found that the
regulation occurs, a constant current being maintained, while the
electromotive force is varied with the number of the devices. The change
of phase of the secondary current is simply a result following from the
changes in resistance, and, though secondary reaction is always of more
or less importance, yet the real cause of the regulation lies in the
existence of the conditions above enumerated. It should be stated,
however, that in the case of a machine the above remarks are to be
restricted to the cases in which the machine is independently excited.
If the excitation be effected by commutating the armature current, then
the fixed position of the brushes makes any shifting of the neutral line
of the utmost importance, and it may not be thought immodest of the
writer to mention that, as far as records go, he seems to have been the
first who has successfully regulated machines by providing a bridge
connection between a point of the external circuit and the commutator by
means of a third brush. The armature and field being properly
proportioned and the brushes placed in their determined positions, a
constant current or constant potential resulted from the shifting of the
diameter of commutation by the varying loads.

In connection with machines of such high frequencies, the condenser
affords an especially interesting study. It is easy to raise the
electromotive force of such a machine to four or five times the value by
simply connecting the condenser to the circuit, and the writer has
continually used the condenser for the the purposes of regulation, as
suggested by Blakesley in his book on alternate currents, in which he
has treated the most frequently occurring condenser problems with
exquisite simplicity and clearness. The high frequency allows the use of
small capacities and renders investigation easy. But, although in most
of the experiments the result may be foretold, some phenomena observed
seem at first curious. One experiment performed three or four months ago
with such a machine and a condenser may serve as an illustration. A
machine was used giving about 20,000 alternations per second. Two bare
wires about twenty feet long and two millimetres in diameter, in close
proximity to each other, were connected to the terminals of the machine
at the one end, and to a condenser at the other. A small transformer
without an iron core, of course, was used to bring the reading within
range of a Cardew voltmeter by connecting the voltmeter to the
secondary. On the terminals of the condenser the electromotive force was
about 120 volts, and from there inch by inch it gradually fell until at
the terminals of the machine it was about 65 volts. It was virtually as
though the condenser were a generator, and the line and armature circuit
simply a resistance connected to it. The writer looked for a case of
resonance, but he was unable to augment the effect by varying the
capacity very carefully and gradually or by changing the speed of the
machine. A case of pure resonance he was unable to obtain. When a
condenser was connected to the terminals of the machine--the
self-induction of the armature being first determined in the maximum and
minimum position and the mean value taken--the capacity which gave the
highest electromotive force corresponded most nearly to that which just
counteracted the self-induction with the existing frequency. If the
capacity was increased or diminished, the electromotive force fell as
expected.

With frequencies as high as the above mentioned, the condenser effects
are of enormous importance. The condenser becomes a highly efficient
apparatus capable of transferring considerable energy.

       *       *       *       *       *

In an appendix to this book will be found a description of the Tesla
oscillator, which its inventor believes will among other great
advantages give him the necessary high frequency conditions, while
relieving him of the inconveniences that attach to generators of the
type described at the beginning of this chapter.




CHAPTER XXX.

ALTERNATE CURRENT ELECTROSTATIC INDUCTION APPARATUS.[6]


  [6] Article by Mr. Tesla in _The Electrical Engineer_, N. Y.,
      May 6, 1891.

About a year and a half ago while engaged in the study of alternate
currents of short period, it occurred to me that such currents could be
obtained by rotating charged surfaces in close proximity to conductors.
Accordingly I devised various forms of experimental apparatus of which
two are illustrated in the accompanying engravings.

[Illustration: FIG. 208.]

In the apparatus shown in Fig. 208, A is a ring of dry shellacked hard
wood provided on its inside with two sets of tin-foil coatings, _a_ and
_b_, all the _a_ coatings and all the _b_ coatings being connected
together, respectively, but independent from each other. These two sets
of coatings are connected to two terminals, T. For the sake of
clearness only a few coatings are shown. Inside of the ring A, and in
close proximity to it there is arranged to rotate a cylinder B, likewise
of dry, shellacked hard wood, and provided with two similar sets of
coatings, _a^1_ and _b^1_, all the coatings _a^1_ being connected to one
ring and all the others, _b^1_, to another marked + and -. These two
sets, _a^1_ and _b^1_ are charged to a high potential by a Holtz or
Wimshurst machine, and may be connected to a jar of some capacity. The
inside of ring A is coated with mica in order to increase the induction
and also to allow higher potentials to be used.

[Illustration: FIG. 209.]

When the cylinder B with the charged coatings is rotated, a circuit
connected to the terminals T is traversed by alternating currents.
Another form of apparatus is illustrated in Fig. 209. In this apparatus
the two sets of tin-foil coatings are glued on a plate of ebonite, and a
similar plate which is rotated, and the coatings of which are charged as
in Fig. 208, is provided.

The output of such an apparatus is very small, but some of the effects
peculiar to alternating currents of short periods may be observed. The
effects, however, cannot be compared with those obtainable with an
induction coil which is operated by an alternate current machine of high
frequency, some of which were described by me a short while ago.




CHAPTER XXXI.

"MASSAGE" WITH CURRENTS OF HIGH FREQUENCY.[7]

  [7] Article by Mr. Tesla in _The Electrical Engineer_ of Dec. 23d,
      1891.

I trust that the present brief communication will not be interpreted as
an effort on my part to put myself on record as a "patent medicine" man,
for a serious worker cannot despise anything more than the misuse and
abuse of electricity which we have frequent occasion to witness. My
remarks are elicited by the lively interest which prominent medical
practitioners evince at every real advance in electrical investigation.
The progress in recent years has been so great that every electrician
and electrical engineer is confident that electricity will become the
means of accomplishing many things that have been heretofore, with our
existing knowledge, deemed impossible. No wonder then that progressive
physicians also should expect to find in it a powerful tool and help in
new curative processes. Since I had the honor to bring before the
American Institute of Electrical Engineers some results in utilizing
alternating currents of high tension, I have received many letters from
noted physicians inquiring as to the physical effects of such currents
of high frequency. It may be remembered that I then demonstrated that a
body perfectly well insulated in air can be heated by simply connecting
it with a source of rapidly alternating high potential. The heating in
this case is due in all probability to the bombardment of the body by
air, or possibly by some other medium, which is molecular or atomic in
construction, and the presence of which has so far escaped our
analysis--for according to my ideas, the true ether radiation with such
frequencies as even a few millions per second must be very small. This
body may be a good conductor or it may be a very poor conductor of
electricity with little change in the result. The human body is, in such
a case, a fine conductor, and if a person insulated in a room, or no
matter where, is brought into contact with such a source of rapidly
alternating high potential, the skin is heated by bombardment. It is a
mere question of the dimensions and character of the apparatus to
produce any degree of heating desired.

It has occurred to me whether, with such apparatus properly prepared, it
would not be possible for a skilled physician to find in it a means for
the effective treatment of various types of disease. The heating will,
of course, be superficial, that is, on the skin, and would result,
whether the person operated on were in bed or walking around a room,
whether dressed in thick clothes or whether reduced to nakedness. In
fact, to put it broadly, it is conceivable that a person entirely nude
at the North Pole might keep himself comfortably warm in this manner.

Without vouching for all the results, which must, of course, be
determined by experience and observation, I can at least warrant the
fact that heating would occur by the use of this method of subjecting
the human body to bombardment by alternating currents of high potential
and frequency such I have long worked with. It is only reasonable to
expect that some of the novel effects will be wholly different from
those obtainable with the old familiar therapeutic methods generally
used. Whether they would all be beneficial or not remains to be proved.




CHAPTER XXXII.

ELECTRIC DISCHARGE IN VACUUM TUBES.[8]

  [8] Article by Mr. Tesla in _The Electrical Engineer_. N. Y.,
      July 1, 1891.


In _The Electrical Engineer_ of June 10 I have noted the description of
some experiments of Prof. J. J. Thomson, on the "Electric Discharge in
Vacuum Tubes," and in your issue of June 24 Prof. Elihu Thomson
describes an experiment of the same kind. The fundamental idea in these
experiments is to set up an electromotive force in a vacuum
tube---preferably devoid of any electrodes--by means of electro-magnetic
induction, and to excite the tube in this manner.

As I view the subject I should, think that to any experimenter who had
carefully studied the problem confronting us and who attempted to find a
solution of it, this idea must present itself as naturally as, for
instance, the idea of replacing the tinfoil coatings of a Leyden jar by
rarefied gas and exciting luminosity in the condenser thus obtained by
repeatedly charging and discharging it. The idea being obvious, whatever
merit there is in this line of investigation must depend upon the
completeness of the study of the subject and the correctness of the
observations. The following lines are not penned with any desire on my
part to put myself on record as one who has performed similar
experiments, but with a desire to assist other experimenters by pointing
out certain peculiarities of the phenomena observed, which, to all
appearances, have not been noted by Prof. J. J. Thomson, who, however,
seems to have gone about systematically in his investigations, and who
has been the first to make his results known. These peculiarities noted
by me would seem to be at variance with the views of Prof. J. J.
Thomson, and present the phenomena in a different light.

My investigations in this line occupied me principally during the winter
and spring of the past year. During this time many different experiments
were performed, and in my exchanges of ideas on this subject with Mr.
Alfred S. Brown, of the Western Union Telegraph Company, various
different dispositions were suggested which were carried out by me in
practice. Fig. 210 may serve as an example of one of the many forms of
apparatus used. This consisted of a large glass tube sealed at one end
and projecting into an ordinary incandescent lamp bulb. The primary,
usually consisting of a few turns of thick, well-insulated copper sheet
was inserted within the tube, the inside space of the bulb furnishing
the secondary. This form of apparatus was arrived at after some
experimenting, and was used principally with the view of enabling me to
place a polished reflecting surface on the inside of the tube, and for
this purpose the last turn of the primary was covered with a thin silver
sheet. In all forms of apparatus used there was no special difficulty in
exciting a luminous circle or cylinder in proximity to the primary.

[Illustration: FIG. 210.]

As to the number of turns, I cannot quite understand why Prof. J. J.
Thomson should think that a few turns were "quite sufficient," but lest
I should impute to him an opinion he may not have, I will add that I
have gained this impression from the reading of the published abstracts
of his lecture. Clearly, the number of turns which gives the best result
in any case, is dependent on the dimensions of the apparatus, and, were
it not for various considerations, one turn would always give the best
result.

I have found that it is preferable to use in these experiments an
alternate current machine giving a moderate number of alternations per
second to excite the induction coil for charging the Leyden jar which
discharges through the primary--shown diagrammatically in Fig. 211,--as
in such case, before the disruptive discharge takes place, the tube or
bulb is slightly excited and the formation of the luminous circle is
decidedly facilitated. But I have also used a Wimshurst machine in some
experiments.

[Illustration: FIG. 211.]

Prof. J. J. Thomson's view of the phenomena under consideration seems to
be that they are wholly due to electro-magnetic action. I was, at one
time, of the same opinion, but upon carefully investigating the subject
I was led to the conviction that they are more of an electrostatic
nature. It must be remembered that in these experiments we have to deal
with primary currents of an enormous frequency or rate of change and of
high potential, and that the secondary conductor consists of a rarefied
gas, and that under such conditions electrostatic effects must play an
important part.

[Illustration: FIG. 212.]

In support of my view I will describe a few experiments made by me. To
excite luminosity in the tube it is not absolutely necessary that the
conductor should be closed. For instance, if an ordinary exhausted tube
(preferably of large diameter) be surrounded by a spiral of thick copper
wire serving as the primary, a feebly luminous spiral may be induced in
the tube, roughly shown in Fig. 212. In one of these experiments a
curious phenomenon was observed; namely, two intensely luminous circles,
each of them close to a turn of the primary spiral, were formed inside
of the tube, and I attributed this phenomenon to the existence of nodes
on the primary. The circles were connected by a faint luminous spiral
parallel to the primary and in close proximity to it. To produce this
effect I have found it necessary to strain the jar to the utmost. The
turns of the spiral tend to close and form circles, but this, of course,
would be expected, and does not necessarily indicate an electro-magnetic
effect; Whereas the fact that a glow can be produced along the primary
in the form of an open spiral argues for an electrostatic effect.

[Illustration: FIG. 213.]

In using Dr. Lodge's recoil circuit, the electrostatic action is
likewise apparent. The arrangement is illustrated in Fig. 213. In his
experiment two hollow exhausted tubes H H were slipped over the wires of
the recoil circuit and upon discharging the jar in the usual manner
luminosity was excited in the tubes.

Another experiment performed is illustrated in Fig. 214. In this case an
ordinary lamp-bulb was surrounded by one or two turns of thick copper
wire P and the luminous circle L excited in the bulb by discharging the
jar through the primary. The lamp-bulb was provided with a tinfoil
coating on the side opposite to the primary and each time the tinfoil
coating was connected to the ground or to a large object the luminosity
of the circle was considerably increased. This was evidently due to
electrostatic action.

In other experiments I have noted that when the primary touches the
glass the luminous circle is easier produced and is more sharply
defined; but I have not noted that, generally speaking, the circles
induced were very sharply defined, as Prof. J. J. Thomson has observed;
on the contrary, in my experiments they were broad and often the whole
of the bulb or tube was illuminated; and in one case I have observed an
intensely purplish glow, to which Prof. J. J. Thomson refers. But the
circles were always in close proximity to the primary and were
considerably easier produced when the latter was very close to the
glass, much more so than would be expected assuming the action to be
electromagnetic and considering the distance; and these facts speak for
an electrostatic effect.

[Illustration: FIG. 214.]

[Illustration: FIG. 215.]

Furthermore I have observed that there is a molecular bombardment in the
plane of the luminous circle at right angles to the glass--supposing the
circle to be in the plane of the primary--this bombardment being
evident from the rapid heating of the glass near the primary. Were the
bombardment not at right angles to the glass the heating could not be so
rapid. If there is a circumferential movement of the molecules
constituting the luminous circle, I have thought that it might be
rendered manifest by placing within the tube or bulb, radially to the
circle, a thin plate of mica coated with some phosphorescent material
and another such plate tangentially to the circle. If the molecules
would move circumferentially, the former plate would be rendered more
intensely phosphorescent. For want of time I have, however, not been
able to perform the experiment.

Another observation made by me was that when the specific inductive
capacity of the medium between the primary and secondary is increased,
the inductive effect is augmented. This is roughly illustrated in Fig.
215. In this case luminosity was excited in an exhausted tube or bulb B
and a glass tube T slipped between the primary and the bulb, when the
effect pointed out was noted. Were the action wholly electromagnetic no
change could possibly have been observed.

I have likewise noted that when a bulb is surrounded by a wire closed
upon itself and in the plane of the primary, the formation of the
luminous circle within the bulb is not prevented. But if instead of the
wire a broad strip of tinfoil is glued upon the bulb, the formation of
the luminous band was prevented, because then the action was distributed
over a greater surface. The effect of the closed tinfoil was no doubt of
an electrostatic nature, for it presented a much greater resistance than
the closed wire and produced therefore a much smaller electromagnetic
effect.

Some of the experiments of Prof. J. J. Thomson also would seem to show
some electrostatic action. For instance, in the experiment with the bulb
enclosed in a bell jar, I should think that when the latter is exhausted
so far that the gas enclosed reaches the maximum conductivity, the
formation of the circle in the bulb and jar is prevented because of the
space surrounding the primary being highly conducting; when the jar is
further exhausted, the conductivity of the space around the primary
diminishes and the circles appear necessarily first in the bell jar, as
the rarefied gas is nearer to the primary. But were the inductive effect
very powerful, they would probably appear in the bulb also. If, however,
the bell jar were exhausted to the highest degree they would very likely
show themselves in the bulb only, that is, supposing the vacuous space
to be non-conducting. On the assumption that in these phenomena
electrostatic actions are concerned we find it easily explicable why the
introduction of mercury or the heating of the bulb prevents the
formation of the luminous band or shortens the after-glow; and also why
in some cases a platinum wire may prevent the excitation of the tube.
Nevertheless some of the experiments of Prof. J. J. Thomson would seem
to indicate an electromagnetic effect. I may add that in one of my
experiments in which a vacuum was produced by the Torricellian method, I
was unable to produce the luminous band, but this may have been due to
the weak exciting current employed.

My principal argument is the following: I have experimentally proved
that if the same discharge which is barely sufficient to excite a
luminous band in the bulb when passed through the primary circuit be so
directed as to exalt the electrostatic inductive effect--namely, by
converting upwards--an exhausted tube, devoid of electrodes, may be
excited at a distance of several feet.


SOME EXPERIMENTS ON THE ELECTRIC DISCHARGE IN VACUUM TUBES.[9]

BY PROF. J. J. THOMSON, M.A., F.R.S.

  [9] Abstract of a paper read before Physical Society of London.

    [Illustration: FIG. 216.]

    [Illustration: FIG. 217.]

    [Illustration: FIG. 218.]

    [Illustration: FIG. 219.]

    The phenomena of vacuum discharges were, Prof. Thomson said,
    greatly simplified when their path was wholly gaseous, the
    complication of the dark space surrounding the negative electrode,
    and the stratifications so commonly observed in ordinary vacuum
    tubes, being absent. To produce discharges in tubes devoid of
    electrodes was, however, not easy to accomplish, for the only
    available means of producing an electromotive force in the
    discharge circuit was by electro-magnetic induction. Ordinary
    methods of producing variable induction were valueless, and
    recourse was had to the oscillatory discharge of a Leyden jar,
    which combines the two essentials of a current whose maximum value
    is enormous, and whose rapidity of alternation is immensely great.
    The discharge circuits, which may take the shape of bulbs, or of
    tubes bent in the form of coils, were placed in close proximity to
    glass tubes filled with mercury, which formed the path of the
    oscillatory discharge. The parts thus corresponded to the windings
    of an induction coil, the vacuum tubes being the secondary, and the
    tubes filled with mercury the primary. In such an apparatus the
    Leyden jar need not be large, and neither primary nor secondary
    need have many turns, for this would increase the self-induction of
    the former, and lengthen the discharge path in the latter.
    Increasing the self-induction of the primary reduces the E. M. F.
    induced in the secondary, whilst lengthening the secondary does not
    increase the E. M. F. per unit length. The two or three turns, as
    shown in Fig. 216, in each, were found to be quite sufficient, and,
    on discharging the Leyden jar between two highly polished knobs in
    the primary circuit, a plain uniform band of light was seen to pass
    round the secondary. An exhausted bulb, Fig. 217, containing traces
    of oxygen was placed within a primary spiral of three turns, and,
    on passing the jar discharge, a circle of light was seen within the
    bulb in close proximity to the primary circuit, accompanied by a
    purplish glow, which lasted for a second or more. On heating the
    bulb, the duration of the glow was greatly diminished, and it could
    be instantly extinguished by the presence of an electro-magnet.
    Another exhausted bulb, Fig. 218, surrounded by a primary spiral,
    was contained in a bell-jar, and when the pressure of air in the
    jar was about that of the atmosphere, the secondary discharge
    occurred in the bulb, as is ordinarily the case. On exhausting the
    jar, however, the luminous discharge grew fainter, and a point was
    reached at which no secondary discharge was visible. Further
    exhaustion of the jar caused the secondary discharge to appear
    outside of the bulb. The fact of obtaining no luminous discharge,
    either in the bulb or jar, the author could only explain on two
    suppositions, viz.: that under the conditions then existing the
    specific inductive capacity of the gas was very great, or that a
    discharge could pass without being luminous. The author had also
    observed that the conductivity of a vacuum tube without electrodes
    increased as the pressure diminished, until a certain point was
    reached, and afterwards diminished again, thus showing that the
    high resistance of a nearly perfect vacuum is in no way due to the
    presence of the electrodes. One peculiarity of the discharges was
    their local nature, the rings of light being much more sharply
    defined than was to be expected. They were also found to be most
    easily produced when the chain of molecules in the discharge were
    all of the same kind. For example, a discharge could be easily sent
    through a tube many feet long, but the introduction of a small
    pellet of mercury in the tube stopped the discharge, although the
    conductivity of the mercury was much greater than that of the
    vacuum. In some cases he had noticed that a very fine wire placed
    within a tube, on the side remote from the primary circuit, would
    prevent a luminous discharge in that tube.

    Fig. 219 shows an exhausted secondary coil of one loop containing
    bulbs; the discharge passed along the inner side of the bulbs, the
    primary coils being placed within the secondary.


[9]In _The Electrical Engineer_ of August 12, I find some remarks of
Prof. J. J. Thomson, which appeared originally in the London
_Electrician_ and which have a bearing upon some experiments described
by me in your issue of July 1.

  [9] Article by Mr. Tesla in _The Electrical Engineer_, N. Y.,
      August 26, 1891.

I did not, as Prof. J. J. Thomson seems to believe, misunderstand his
position in regard to the cause of the phenomena considered, but I
thought that in his experiments, as well as in my own, electrostatic
effects were of great importance. It did not appear, from the meagre
description of his experiments, that all possible precautions had been
taken to exclude these effects. I did not doubt that luminosity could be
excited in a closed tube when electrostatic action is completely
excluded. In fact, at the outset, I myself looked for a purely
electrodynamic effect and believed that I had obtained it. But many
experiments performed at that time proved to me that the electrostatic
effects were generally of far greater importance, and admitted of a more
satisfactory explanation of most of the phenomena observed.

In using the term _electrostatic_ I had reference rather to the nature
of the action than to a stationary condition, which is the usual
acceptance of the term. To express myself more clearly, I will suppose
that near a closed exhausted tube be placed a small sphere charged to a
very high potential. The sphere would act inductively upon the tube, and
by distributing electricity over the same would undoubtedly produce
luminosity (if the potential be sufficiently high), until a permanent
condition would be reached. Assuming the tube to be perfectly well
insulated, there would be only one instantaneous flash during the act of
distribution. This would be due to the electrostatic action simply.

But now, suppose the charged sphere to be moved at short intervals with
great speed along the exhausted tube. The tube would now be permanently
excited, as the moving sphere would cause a constant redistribution of
electricity and collisions of the molecules of the rarefied gas. We
would still have to deal with an electrostatic effect, and in addition
an electrodynamic effect would be observed. But if it were found that,
for instance, the effect produced depended more on the specific
inductive capacity than on the magnetic permeability of the
medium--which would certainly be the case for speeds incomparably lower
than that of light--then I believe I would be justified in saying that
the effect produced was more of an electrostatic nature. I do not mean
to say, however, that any similar condition prevails in the case of the
discharge of a Leyden jar through the primary, but I think that such an
action would be desirable.

It is in the spirit of the above example that I used the terms "more of
an electrostatic nature," and have investigated the influence of bodies
of high specific inductive capacity, and observed, for instance, the
importance of the quality of glass of which the tube is made. I also
endeavored to ascertain the influence of a medium of high permeability
by using oxygen. It appeared from rough estimation that an oxygen tube
when excited under similar conditions--that is, as far as could be
determined--gives more light; but this, of course, may be due to many
causes.

Without doubting in the least that, with the care and precautions taken
by Prof. J. J. Thomson, the luminosity excited was due solely to
electrodynamic action, I would say that in many experiments I have
observed curious instances of the ineffectiveness of the screening, and
I have also found that the electrification through the air is often of
very great importance, and may, in some cases, determine the excitation
of the tube.

In his original communication to the _Electrician_, Prof. J. J. Thomson
refers to the fact that the luminosity in a tube near a wire through
which a Leyden jar was discharged was noted by Hittorf. I think that the
feeble luminous effect referred to has been noted by many
experimenters, but in my experiments the effects were much more powerful
than those usually noted.

The following is the communication[10] referred to:--

  [10] Note by Prof. J. J. Thomson in the London _Electrician_,
       July 24, 1891.

    "Mr. Tesla seems to ascribe the effects he observed to
    electrostatic action, and I have no doubt, from the description he
    gives of his method of conducting his experiments, that in them
    electrostatic action plays a very important part. He seems,
    however, to have misunderstood my position with respect to the
    cause of these discharges, which is not, as he implies, that
    luminosity in tubes without electrodes cannot be produced by
    electrostatic action, but that it can also be produced when this
    action is excluded. As a matter of fact, it is very much easier to
    get the luminosity when these electrostatic effects are operative
    than when they are not. As an illustration of this I may mention
    that the first experiment I tried with the discharge of a Leyden
    jar produced luminosity in the tube, but it was not until after six
    weeks' continuous experimenting that I was able to get a discharge
    in the exhausted tube which I was satisfied was due to what is
    ordinarily called electrodynamic action. It is advisable to have a
    clear idea of what we mean by electrostatic action. If, previous to
    the discharge of the jar, the primary coil is raised to a high
    potential, it will induce over the glass of the tube a distribution
    of electricity. When the potential of the primary suddenly falls,
    this electrification will redistribute itself, and may pass through
    the rarefied gas and produce luminosity in doing so. Whilst the
    discharge of the jar is going on, it is difficult, and, from a
    theoretical point of view, undesirable, to separate the effect into
    parts, one of which is called electrostatic, the other
    electromagnetic; what we can prove is that in this case the
    discharge is not such as would be produced by electromotive forces
    derived from a potential function. In my experiments the primary
    coil was connected to earth, and, as a further precaution, the
    primary was separated from the discharge tube by a screen of
    blotting paper, moistened with dilute sulphuric acid, and connected
    to earth. Wet blotting paper is a sufficiently good conductor to
    screen off a stationary electrostatic effect, though it is not a
    good enough one to stop waves of alternating electromotive
    intensity. When showing the experiments to the Physical Society I
    could not, of course, keep the tubes covered up, but, unless my
    memory deceives me, I stated the precautions which had been taken
    against the electrostatic effect. To correct misapprehension I may
    state that I did not read a formal paper to the Society, my object
    being to exhibit a few of the most typical experiments. The account
    of the experiments in the _Electrician_ was from a reporter's note,
    and was not written, or even read, by me. I have now almost
    finished writing out, and hope very shortly to publish, an account
    of these and a large number of allied experiments, including some
    analogous to those mentioned by Mr. Tesla on the effect of
    conductors placed near the discharge tube, which I find, in some
    cases, to produce a diminution, in others an increase, in the
    brightness of the discharge, as well as some on the effect of the
    presence of substances of large specific inductive capacity. These
    seem to me to admit of a satisfactory explanation, for which,
    however, I must refer to my paper."




PART III.

MISCELLANEOUS INVENTIONS AND WRITINGS.




CHAPTER XXXIII.

METHOD OF OBTAINING DRIECT FROM ALTERNATING CURRENTS.


This method consists in obtaining direct from alternating currents, or
in directing the waves of an alternating current so as to produce direct
or substantially direct currents by developing or producing in the
branches of a circuit including a source of alternating currents, either
permanently or periodically, and by electric, electro-magnetic, or
magnetic agencies, manifestations of energy, or what may be termed
active resistances of opposite electrical character, whereby the
currents or current waves of opposite sign will be diverted through
different circuits, those of one sign passing over one branch and those
of opposite sign over the other.

We may consider herein only the case of a circuit divided into two
paths, inasmuch as any further subdivision involves merely an extension
of the general principle. Selecting, then, any circuit through which is
flowing an alternating current, Mr. Tesla divides such circuit at any
desired point into two branches or paths. In one of these paths he
inserts some device to create an electromotive force counter to the
waves or impulses of current of one sign and a similar device in the
other branch which opposes the waves of opposite sign. Assume, for
example, that these devices are batteries, primary or secondary, or
continuous current dynamo machines. The waves or impulses of opposite
direction composing the main current have a natural tendency to divide
between the two branches; but by reason of the opposite electrical
character or effect of the two branches, one will offer an easy passage
to a current of a certain direction, while the other will offer a
relatively high resistance to the passage of the same current. The
result of this disposition is, that the waves of current of one sign
will, partly or wholly, pass over one of the paths or branches, while
those of the opposite sign pass over the other. There may thus be
obtained from an alternating current two or more direct currents without
the employment of any commutator such as it has been heretofore
regarded as necessary to use. The current in either branch may be
used in the same way and for the same purposes as any other direct
current--that is, it may be made to charge secondary batteries, energize
electro-magnets, or for any other analogous purpose.

Fig. 220 represents a plan of directing the alternating currents by
means of devices purely electrical in character. Figs. 221, 222, 223,
224, 225, and 226 are diagrams illustrative of other ways of carrying
out the invention.

[Illustration: FIG. 220.]

In Fig. 220, A designates a generator of alternating currents, and B B
the main or line circuit therefrom. At any given point in this circuit
at or near which it is desired to obtain direct currents, the circuit B
is divided into two paths or branches C D. In each of these branches is
placed an electrical generator, which for the present we will assume
produces direct or continuous currents. The direction of the current
thus produced is opposite in one branch to that of the current in the
other branch, or, considering the two branches as forming a closed
circuit, the generators E F are connected up in series therein, one
generator in each part or half of the circuit. The electromotive force
of the current sources E and F may be equal to or higher or lower than
the electromotive forces in the branches C D, or between the points X
and Y of the circuit B B. If equal, it is evident that current waves of
one sign will be opposed in one branch and assisted in the other to such
an extent that all the waves of one sign will pass over one branch and
those of opposite sign over the other. If, on the other hand, the
electromotive force of the sources E F be lower than that between X and
Y, the currents in both branches will be alternating, but the waves of
one sign will preponderate. One of the generators or sources of current
E or F may be dispensed with; but it is preferable to employ both, if
they offer an appreciable resistance, as the two branches will be
thereby better balanced. The translating or other devices to be acted
upon by the current are designated by the letters G, and they are
inserted in the branches C D in any desired manner; but in order to
better preserve an even balance between the branches due regard should,
of course, be had to the number and character of the devices.

[Illustration: FIG. 221.]

Figs. 221, 222, 223, and 224 illustrate what may termed
"electro-magnetic" devices for accomplishing a similar result--that is
to say, instead of producing directly by a generator an electromotive
force in each branch of the circuit, Mr. Tesla establishes a field or
fields of force and leads the branches through the same in such manner
that an active opposition of opposite effect or direction will be
developed therein by the passage, or tendency to pass, of the
alternations of current. In Fig. 221, for example, A is the generator of
alternating currents, B B the line circuit, and C D the branches over
which the alternating currents are directed. In each branch is included
the secondary of a transformer or induction coil, which, since they
correspond in their functions to the batteries of the previous figure,
are designated by the letters E F. The primaries H H' of the induction
coils or transformers are connected either in parallel or series with a
source of direct or continuous currents I, and the number of
convolutions is so calculated for the strength of the current from I
that the cores J J' will be saturated. The connections are such that the
conditions in the two transformers are of opposite character--that is to
say, the arrangement is such that a current wave or impulse
corresponding in direction with that of the direct current in one
primary, as H, is of opposite direction to that in the other primary H'.
It thus results that while one secondary offers a resistance or
opposition to the passage through it of a wave of one sign, the other
secondary similarly opposes a wave of opposite sign. In consequence, the
waves of one sign will, to a greater or less extent, pass by way of one
branch, while those of opposite sign in like manner pass over the other
branch.

In lieu of saturating the primaries by a source of continuous current,
we may include the primaries in the branches C D, respectively, and
periodically short-circuit by any suitable mechanical devices--such as
an ordinary revolving commutator--their secondaries. It will be
understood, of course, that the rotation and action of the commutator
must be in synchronism or in proper accord with the periods of the
alternations in order to secure the desired results. Such a disposition
is represented diagrammatically in Fig. 222. Corresponding to the
previous figures, A is the generator of alternating currents, B B the
line, and C D the two branches for the direct currents. In branch C are
included two primary coils E E', and in branch D are two similar
primaries F F' The corresponding secondaries for these coils and which
are on the same subdivided cores J or J', are in circuits the terminals
of which connect to opposite segments K K', and L L', respectively, of a
commutator. Brushes _b b_ bear upon the commutator and alternately
short-circuit the plates K and K', and L and L', through a connection
_c_. It is obvious that either the magnets and commutator, or the
brushes, may revolve.

[Illustration: FIG. 222.]

The operation will be understood from a consideration of the effects of
closing or short-circuiting the secondaries. For example, if at the
instant when a given wave of current passes, one set of secondaries be
short-circuited, nearly all the current flows through the corresponding
primaries; but the secondaries of the other branch being open-circuited,
the self-induction in the primaries is highest, and hence little or no
current will pass through that branch. If, as the current alternates,
the secondaries of the two branches are alternately short-circuited, the
result will be that the currents of one sign pass over one branch and
those of the opposite sign over the other. The disadvantages of this
arrangement, which would seem to result from the employment of sliding
contacts, are in reality very slight, inasmuch as the electromotive
force of the secondaries may be made exceedingly low, so that sparking
at the brushes is avoided.

[Illustration: FIG. 223.]

Fig. 223 is a diagram, partly in section, of another plan of carrying
out the invention. The circuit B in this case is divided, as before, and
each branch includes the coils of both the fields and revolving
armatures of two induction devices. The armatures O P are preferably
mounted on the same shaft, and are adjusted relatively to one another in
such manner that when the self-induction in one branch, as C, is
maximum, in the other branch D it is minimum. The armatures are rotated
in synchronism with the alternations from the source A. The winding or
position of the armature coils is such that a current in a given
direction passed through both armatures would establish in one, poles
similar to those in the adjacent poles of the field, and in the other,
poles unlike the adjacent field poles, as indicated by _n n s s_ in the
diagram. If the like poles are presented, as shown in circuit D, the
condition is that of a closed secondary upon a primary, or the position
of least inductive resistance; hence a given alternation of current will
pass mainly through D. A half revolution of the armatures produces an
opposite effect and the succeeding current impulse passes through C.
Using this figure as an illustration, it is evident that the fields N M
may be permanent magnets or independently excited and the armatures O P
driven, as in the present case, so as to produce alternate currents,
which will set up alternately impulses of opposite direction in the two
branches D C, which in such case would include the armature circuits and
translating devices only.

In Fig. 224 a plan alternative with that shown in Fig. 222 is
illustrated. In the previous case illustrated, each branch C and D
contained one or more primary coils, the secondaries of which were
periodically short circuited in synchronism with the alternations of
current from the main source A, and for this purpose a commutator was
employed. The latter may, however, be dispensed with and an armature
with a closed coil substituted.

[Illustration: FIG. 224.]

Referring to Fig. 224 in one of the branches, as C, are two coils M',
wound on laminated cores, and in the other branches D are similar coils
N'. A subdivided or laminated armature O', carrying a closed coil R', is
rotatably supported between the coils M' N', as shown. In the position
shown--that is, with the coil R' parallel with the convolutions of the
primaries N' M'--practically the whole current will pass through branch
D, because the self-induction in coils M' M' is maximum. If, therefore,
the armature and coil be rotated at a proper speed relatively to the
periods or alternations of the source A, the same results are obtained
as in the case of Fig. 222.

Fig. 225 is an instance of what may be called, in distinction to the
others, a "magnetic" means of securing the result. V and W are two
strong permanent magnets provided with armatures V' W', respectively.
The armatures are made of thin laminæ of soft iron or steel, and the
amount of magnetic metal which they contain is so calculated that they
will be fully or nearly saturated by the magnets. Around the armatures
are coils E F, contained, respectively, in the circuits C and D. The
connections and electrical conditions in this case are similar to those
in Fig. 221, except that the current source of I, Fig. 221, is dispensed
with and the saturation of the core of coils E F obtained from the
permanent magnets.

[Illustration: FIG. 225.]

The previous illustrations have all shown the two branches or paths
containing the translating or induction devices as in derivation one to
the other; but this is not always necessary. For example, in Fig. 226, A
is an alternating-current generator; B B, the line wires or circuit. At
any given point in the circuit let us form two paths, as D D', and at
another point two paths, as C C'. Either pair or group of paths is
similar to the previous dispositions with the electrical source or
induction device in one branch only, while the two groups taken together
form the obvious equivalent of the cases in which an induction device or
generator is included in both branches. In one of the paths, as D, are
included the devices to be operated by the current. In the other branch,
as D', is an induction device that opposes the current impulses of one
direction and directs them through the branch D. So, also, in branch C
are translating devices G, and in branch C' an induction device or its
equivalent that diverts through C impulses of opposite direction to
those diverted by the device in branch D'. The diagram shows a special
form of induction device for this purpose. J J' are the cores, formed
with pole-pieces, upon which are wound the coils M N. Between these
pole-pieces are mounted at right angles to one another the magnetic
armatures O P, preferably mounted on the same shaft and designed to be
rotated in synchronism with the alternations of current. When one of the
armatures is in line with the poles or in the position occupied by
armature P, the magnetic circuit of the induction device is practically
closed; hence there will be the greatest opposition to the passage of a
current through coils N N. The alternation will therefore pass by way of
branch D. At the same time, the magnetic circuit of the other induction
device being broken by the position of the armature O, there will be
less opposition to the current in coils M, which will shunt the current
from branch C. A reversal of the current being attended by a shifting of
the armatures, the opposite effect is produced.

[Illustration: FIG. 226.]

Other modifications of these methods are possible, but need not be
pointed out. In all these plans, it will be observed, there is developed
in one or all of these branches of a circuit from a source of
alternating currents, an active (as distinguished from a dead)
resistance or opposition to the currents of one sign, for the purpose of
diverting the currents of that sign through the other or another path,
but permitting the currents of opposite sign to pass without substantial
opposition.

Whether the division of the currents or waves of current of opposite
sign be effected with absolute precision or not is immaterial, since it
will be sufficient if the waves are only partially diverted or directed,
for in such case the preponderating influence in each branch of the
circuit of the waves of one sign secures the same practical results in
many if not all respects as though the current were direct and
continuous.

An alternating and a direct current have been combined so that the waves
of one direction or sign were partially or wholly overcome by the direct
current; but by this plan only one set of alternations are utilized,
whereas by the system just described the entire current is rendered
available. By obvious applications of this discovery Mr. Tesla is
enabled to produce a self-exciting alternating dynamo, or to operate
direct current meters on alternating-current circuits or to run various
devices--such as arc lamps--by direct currents in the same circuit with
incandescent lamps or other devices operated by alternating currents.

It will be observed that if an intermittent counter or opposing force be
developed in the branches of the circuit and of higher electromotive
force than that of the generator, an alternating current will result in
each branch, with the waves of one sign preponderating, while a
constantly or uniformly acting opposition in the branches of higher
electromotive force than the generator would produce a pulsating
current, which conditions would be, under some circumstances, the
equivalent of those described.




CHAPTER XXXIV.

CONDENSERS WITH PLATES IN OIL.


[Illustration: FIG. 227.]

[Illustration: FIG. 228.]

In experimenting with currents of high frequency and high potential, Mr.
Tesla has found that insulating materials such as glass, mica, and in
general those bodies which possess the highest specific inductive
capacity, are inferior as insulators in such devices when currents of
the kind described are employed compared with those possessing high
insulating power, together with a smaller specific inductive capacity;
and he has also found that it is very desirable to exclude all gaseous
matter from the apparatus, or any access of the same to the electrified
surfaces, in order to prevent heating by molecular bombardment and the
loss or injury consequent thereon. He has therefore devised a method to
accomplish these results and produce highly efficient and reliable
condensers, by using oil as the dielectric[11]. The plan admits of a
particular construction of condenser, in which the distance between the
plates is adjustable, and of which he takes advantage.

  [11] Mr. Tesla's experiments, as the careful reader of his three
    lectures will perceive, have revealed a very important fact which
    is taken advantage of in this invention. Namely, he has shown that
    in a condenser a considerable amount of energy may be wasted, and
    the condenser may break down merely because gaseous matter is
    present between the surfaces. A number of experiments are described
    in the lectures, which bring out this fact forcibly and serve as a
    guide in the operation of high tension apparatus. But besides
    bearing upon this point, these experiments also throw a light upon
    investigations of a purely scientific nature and explain now the
    lack of harmony among the observations of various investigators.
    Mr. Tesla shows that in a fluid such as oil the losses are very
    small as compared with those incurred in a gas.

In the accompanying illustrations, Fig. 227 is a section of a condenser
constructed in accordance with this principle and having stationary
plates; and Fig. 228 is a similar view of a condenser with adjustable
plates.

Any suitable box or receptacle A may be used to contain the plates or
armatures. These latter are designated by B and C and are connected,
respectively, to terminals D and E, which pass out through the sides of
the case. The plates ordinarily are separated by strips of porous
insulating material F, which are used merely for the purpose of
maintaining them in position. The space within the can is filled with
oil G. Such a condenser will prove highly efficient and will not become
heated or permanently injured.

In many cases it is desirable to vary or adjust the capacity of a
condenser, and this is provided for by securing the plates to adjustable
supports--as, for example, to rods H--passing through stuffing boxes K
in the sides of case A and furnished with nuts L, the ends of the rods
being threaded for engagement with the nuts.

It is well known that oils possess insulating properties, and it has
been a common practice to interpose a body of oil between two conductors
for purposes of insulation; but Mr. Tesla believes he has discovered
peculiar properties in oils which render them very valuable in this
particular form of device.




CHAPTER XXXV.

ELECTROLYTIC REGISTERING METER.


An ingenious form of electrolytic meter attributable to Mr. Tesla is one
in which a conductor is immersed in a solution, so arranged that metal
may be deposited from the solution or taken away in such a manner that
the electrical resistance of the conductor is varied in a definite
proportion to the strength of the current the energy of which is to be
computed, whereby this variation in resistance serves as a measure of
the energy and also may actuate registering mechanism, whenever the
resistance rises above or falls below certain limits.

In carrying out this idea Mr. Tesla employs an electrolytic cell,
through which extend two conductors parallel and in close proximity to
each other. These conductors he connects in series through a resistance,
but in such manner that there is an equal difference of potential
between them throughout their entire extent. The free ends or terminals
of the conductors are connected either in series in the circuit
supplying the current to the lamps or other devices, or in parallel to a
resistance in the circuit and in series with the current consuming
devices. Under such circumstances a current passing through the
conductors establishes a difference of potential between them which is
proportional to the strength of the current, in consequence of which
there is a leakage of current from one conductor to the other across the
solution. The strength of this leakage current is proportional to the
difference of potential, and, therefore, in proportion to the strength
of the current passing through the conductors. Moreover, as there is a
constant difference of potential between the two conductors throughout
the entire extent that is exposed to the solution, the current density
through such solution is the same at all corresponding points, and hence
the deposit is uniform along the whole of one of the conductors, while
the metal is taken away uniformly from the other. The resistance of one
conductor is by this means diminished, while that of the other is
increased, both in proportion to the strength of the current passing
through the conductors. From such variation in the resistance of either
or both of the conductors forming the positive and negative electrodes
of the cell, the current energy expended may be readily computed. Figs.
229 and 230 illustrate two forms of such a meter.

[Illustration: FIG. 229.]

In Fig. 229 G designates a direct-current generator. L L are the
conductors of the circuit extending therefrom. A is a tube of glass, the
ends of which are sealed, as by means of insulating plugs or caps B B. C
C' are two conductors extending through the tube A, their ends passing
out through the plugs B to terminals thereon. These conductors may be
corrugated or formed in other proper ways to offer the desired
electrical resistance. R is a resistance connected in series with the
two conductors C C', which by their free terminals are connected up in
circuit with one of the conductors L.

The method of using this device and computing by means thereof the
energy of the current will be readily understood. First, the resistances
of the two conductors C C', respectively, are accurately measured and
noted. Then a known current is passed through the instrument for a given
time, and by a second measurement the increase and diminution of the
resistances of the two conductors are respectively taken. From these
data the constant is obtained--that is to say, for example, the
increase of resistance of one conductor or the diminution of the
resistance of the other per lamp hour. These two measurements evidently
serve as a check, since the gain of one conductor should equal the loss
of the other. A further check is afforded by measuring both wires in
series with the resistance, in which case the resistance of the whole
should remain constant.

[Illustration: FIG. 230.]

In Fig. 230 the conductors C C' are connected in parallel, the current
device at X passing in one branch first through a resistance R' and then
through conductor C, while on the other branch it passes first through
conductor C', and then through resistance R''. The resistances R' R''
are equal, as also are the resistances of the conductors C C'. It is,
moreover, preferable that the respective resistances of the conductors C
C' should be a known and convenient fraction of the coils or resistances
R' R''. It will be observed that in the arrangement shown in Fig. 230
there is a constant potential difference between the two conductors C C'
throughout their entire length.

It will be seen that in both cases illustrated, the proportionality of
the increase or decrease of resistance to the current strength will
always be preserved, for what one conductor gains the other loses, and
the resistances of the conductors C C' being small as compared with the
resistances in series with them. It will be understood that after each
measurement or registration of a given variation of resistance in one or
both conductors, the direction of the current should be changed or the
instrument reversed, so that the deposit will be taken from the
conductor which has gained and added to that which has lost. This
principle is capable of many modifications. For instance, since there is
a section of the circuit--to wit, the conductor C or C'--that varies in
resistance in proportion to the current strength, such variation may be
utilized, as is done in many analogous cases, to effect the operation of
various automatic devices, such as registers. It is better, however, for
the sake of simplicity to compute the energy by measurements of
resistance.

The chief advantages of this arrangement are, first, that it is possible
to read off directly the amount of the energy expended by means of a
properly constructed ohm-meter and without resorting to weighing the
deposit; secondly it is not necessary to employ shunts, for the whole of
the current to be measured may be passed through the instrument; third,
the accuracy of the instrument and correctness of the indications are
but slightly affected by changes in temperature. It is also said that
such meters have the merit of superior economy and compactness, as well
as of cheapness in construction. Electrolytic meters seem to need every
auxiliary advantage to make them permanently popular and successful, no
matter how much ingenuity may be shown in their design.




CHAPTER XXXVI.

THERMO-MAGNETIC MOTORS AND PYRO-MAGNETIC GENERATORS.


No electrical inventor of the present day dealing with the problems of
light and power considers that he has done himself or his opportunities
justice until he has attacked the subject of thermo-magnetism. As far
back as the beginning of the seventeenth century it was shown by Dr.
William Gilbert, the father of modern electricity, that a loadstone or
iron bar when heated to redness loses its magnetism; and since that time
the influence of heat on the magnetic metals has been investigated
frequently, though not with any material or practical result.

For a man of Mr. Tesla's inventive ability, the problems in this field
have naturally had no small fascination, and though he has but glanced
at them, it is to be hoped he may find time to pursue the study deeper
and further. For such as he, the investigation must undoubtedly bear
fruit. Meanwhile he has worked out one or two operative devices worthy
of note.[12] He obtains mechanical power by a reciprocating action
resulting from the joint operations of heat, magnetism, and a spring or
weight or other force--that is to say he subjects a body magnetized by
induction or otherwise to the action of heat until the magnetism is
sufficiently neutralized to allow a weight or spring to give motion to
the body and lessen the action of the heat, so that the magnetism may be
sufficiently restored to move the body in the opposite direction, and
again subject the same to the demagnetizing power of the heat.

  [12] It will, of course, be inferred from the nature of these devices
       that the vibration obtained in this manner is very slow owing to
       the inability of the iron to follow rapid changes in temperature.
       In an interview with Mr. Tesla on this subject, the compiler
       learned of an experiment which will interest students. A simple
       horseshoe magnet is taken and a piece of sheet iron bent in the
       form of an L is brought in contact with one of the poles and
       placed in such a position that it is kept in the attraction of
       the opposite pole delicately suspended. A spirit lamp is placed
       under the sheet iron piece and when the iron is heated to a
       certain temperature it is easily set in vibration oscillating
       as rapidly as 400 to 500 times a minute. The experiment is very
       easily performed and is interesting principally on account of the
       very rapid rate of vibration.

Use is made of either an electro-magnet or a permanent magnet, and the
heat is directed against a body that is magnetized by induction, rather
than directly against a permanent magnet, thereby avoiding the loss of
magnetism that might result in the permanent magnet by the action of
heat. Mr. Tesla also provides for lessening the volume of the heat or
for intercepting the same during that portion of the reciprocation in
which the cooling action takes place.

In the diagrams are shown some of the numerous arrangements that may be
made use of in carrying out this idea. In all of these figures the
magnet-poles are marked N S, the armature A, the Bunsen burner or other
source of heat H, the axis of motion M, and the spring or the equivalent
thereof--namely, a weight--is marked W.

[Illustration: FIG. 232.]

[Illustration: FIG. 231.]

[Illustration: FIG. 233.]


In Fig. 231 the permanent magnet N is connected with a frame, F,
supporting the axis M, from which the arm P hangs, and at the lower end
of which the armature A is supported. The stops 2 and 3 limit the extent
of motion, and the spring W tends to draw the armature A away from the
magnet N. It will now be understood that the magnetism of N is
sufficient to overcome the spring W and draw the armature A toward the
magnet N. The heat acting upon the armature A neutralizes its induced
magnetism sufficiently for the spring W to draw the armature A away from
the magnet N and also from the heat at H. The armature now cools, and
the attraction of the magnet N overcomes the spring W and draws the
armature A back again above the burner H, so that the same is again
heated and the operations are repeated. The reciprocating movements thus
obtained are employed as a source of mechanical power in any desired
manner. Usually a connecting-rod to a crank upon a fly-wheel shaft would
be made use of, as indicated in Fig. 240.

[Illustration: FIG. 234.]

[Illustration: FIG. 236.]

[Illustration: FIG. 235.]

Fig. 232 represents the same parts as before described; but an
electro-magnet is illustrated in place of a permanent magnet. The
operations, however, are the same.

In Fig. 233 are shown the same parts as in Figs. 231 and 232, but they
are differently arranged. The armature A, instead of swinging, is
stationary and held by arm P', and the core N S of the electro-magnet is
made to swing within the helix Q, the core being suspended by the arm P
from the pivot M. A shield, R, is connected with the magnet-core and
swings with it, so that after the heat has demagnetized the armature A
to such an extent that the spring W draws the core N S away from the
armature A, the shield R comes between the flame H and armature A,
thereby intercepting the action of the heat and allowing the armature to
cool, so that the magnetism, again preponderating, causes the movement
of the core N S toward the armature A and the removal of the shield R
from above the flame, so that the heat again acts to lessen or
neutralize the magnetism. A rotary or other movement may be obtained
from this reciprocation.

Fig. 234 corresponds in every respect with Fig. 233, except that a
permanent horseshoe-magnet, N S is represented as taking the place of
the electro-magnet in Fig. 233.

In Fig. 235 is shown a helix, Q, with an armature adapted to swing
toward or from the helix. In this case there may be a soft-iron core in
the helix, or the armature may assume the form of a solenoid core, there
being no permanent core within the helix.

[Illustration: FIG. 237.]

[Illustration: FIG. 238.]

[Illustration: FIG. 239.]


Fig. 236 is an end view, and Fig. 237 a plan view, illustrating the
method as applied to a swinging armature, A, and a stationary permanent
magnet, N S. In this instance Mr. Tesla applies the heat to an auxiliary
armature or keeper, T, which is adjacent to and preferably in direct
contact with the magnet. This armature T, in the form of a plate of
sheet-iron, extends across from one pole to the other and is of
sufficient section to practically form a keeper for the magnet, so that
when the armature T is cool nearly all the lines of force pass over the
same and very little free magnetism is exhibited. Then the armature A,
which swings freely on the pivots M in front of the poles N S, is very
little attracted and the spring W pulls the same way from the poles into
the position indicated in the diagram. The heat is directed upon the
iron plate T at some distance from the magnet, so as to allow the magnet
to keep comparatively cool. This heat is applied beneath the plate by
means of the burners H, and there is a connection from the armature A or
its pivot to the gas-cock 6, or other device for regulating the heat.
The heat acting upon the middle portion of the plate T, the magnetic
conductivity of the heated portion is diminished or destroyed, and a
great number of the lines of force are deflected over the armature A,
which is now powerfully attracted and drawn into line, or nearly so,
with the poles N S. In so doing the cock 6 is nearly closed and the
plate T cools, the lines of force are again deflected over the same, the
attraction exerted upon the armature A is diminished, and the spring W
pulls the same away from the magnet into the position shown by full
lines, and the operations are repeated. The arrangement shown in Fig.
236 has the advantages that the magnet and armature are kept cool and
the strength of the permanent magnet is better preserved, as the
magnetic circuit is constantly closed.

In the plan view, Fig. 238, is shown a permanent magnet and keeper
plate, T, similar to those in Figs. 236 and 237, with the burners H for
the gas beneath the same; but the armature is pivoted at one end to one
pole of the magnet and the other end swings toward and from the other
pole of the magnet. The spring W acts against a lever arm that projects
from the armature, and the supply of heat has to be partly cut off by a
connection to the swinging armature, so as to lessen the heat acting
upon the keeper plate when the armature A has been attracted.

[Illustration: FIG. 240.]

[Illustration: FIG. 241.]

Fig. 239 is similar to Fig. 238, except that the keeper T is not made
use of and the armature itself swings into and out of the range of the
intense action of the heat from the burner H. Fig. 240 is a diagram
similar to Fig. 231, except that in place of using a spring and stops,
the armature is shown as connected by a link, to the crank of a
fly-wheel, so that the fly-wheel will be revolved as rapidly as the
armature can be heated and cooled to the necessary extent. A spring may
be used in addition, as in Fig. 231. In Fig. 241 the armatures A A are
connected by a link, so that one will be heating while the other is
cooling, and the attraction exerted to move the cooled armature is
availed of to draw away the heated armature instead of using a spring.

Mr. Tesla has also devoted his attention to the development of a
pyromagnetic generator of electricity[13] based upon the following laws:
First, that electricity or electrical energy is developed in any
conducting body by subjecting such body to a varying magnetic influence;
and second, that the magnetic properties of iron or other magnetic
substance may be partially or entirely destroyed or caused to disappear
by raising it to a certain temperature, but restored and caused to
reappear by again lowering its temperature to a certain degree. These
laws may be applied in the production of electrical currents in many
ways, the principle of which is in all cases the same, viz., to subject
a conductor to a varying magnetic influence, producing such variations
by the application of heat, or, more strictly speaking, by the
application or action of a varying temperature upon the source of the
magnetism. This principle of operation may be illustrated by a simple
experiment: Place end to end, and preferably in actual contact, a
permanently magnetized steel bar and a strip or bar of soft iron. Around
the end of the iron bar or plate wind a coil of insulated wire. Then
apply to the iron between the coil and the steel bar a flame or other
source of heat which will be capable of raising that portion of the iron
to an orange red, or a temperature of about 600° centigrade. When this
condition is reached, the iron somewhat suddenly loses its magnetic
properties, if it be very thin, and the same effect is produced as
though the iron had been moved away from the magnet or the heated
section had been removed. This change of position, however, is
accompanied by a shifting of the magnetic lines, or, in other words, by
a variation in the magnetic influence to which the coil is exposed, and
a current in the coil is the result. Then remove the flame or in any
other way reduce the temperature of the iron. The lowering of its
temperature is accompanied by a return of its magnetic properties, and
another change of magnetic conditions occurs, accompanied by a current
in an opposite direction in the coil. The same operation may be
repeated indefinitely, the effect upon the coil being similar to that
which would follow from moving the magnetized bar to and from the end of
the iron bar or plate.

  [13] The chief point to be noted is that Mr. Tesla attacked this
       problem in a way which was, from the standpoint of theory, and
       that of an engineer, far better than that from which some
       earlier trials in this direction started. The enlargement of
       these ideas will be found in Mr. Tesla's work on the pyromagnetic
       generator, treated in this chapter. The chief effort of the
       inventor was to economize the heat, which was accomplished by
       inclosing the iron in a source of heat well insulated, and by
       cooling the iron by means of steam, utilizing the steam over
       again. The construction also permits of more rapid magnetic
       changes per unit of time, meaning larger output.

The device illustrated below is a means of obtaining this result, the
features of novelty in the invention being, first, the employment of an
artificial cooling device, and, second, inclosing the source of heat and
that portion of the magnetic circuit exposed to the heat and
artificially cooling the heated part.

These improvements are applicable generally to the generators
constructed on the plan above described--that is to say, we may use an
artificial cooling device in conjunction with a variable or varied or
uniform source of heat.

[Illustration: FIG. 242.]

[Illustration: FIG. 243.]

Fig. 242 is a central vertical longitudinal section of the complete
apparatus and Fig. 243 is a cross-section of the magnetic armature-core
of the generator.

Let A represent a magnetized core or permanent magnet the poles of which
are bridged by an armature-core composed of a casing or shell B
inclosing a number of hollow iron tubes C. Around this core are wound
the conductors E E', to form the coils in which the currents are
developed. In the circuits of these coils are current-consuming devices,
as F F'.

D is a furnace or closed fire-box, through which the central portion of
the core B extends. Above the fire is a boiler K, containing water. The
flue L from the fire-box may extend up through the boiler.

G is a water-supply pipe, and H is the steam-exhaust pipe, which
communicates with all the tubes C in the armature B, so that steam
escaping from the boiler will pass through the tubes.

In the steam-exhaust pipe H is a valve V, to which is connected the
lever I, by the movement of which the valve is opened or closed. In such
a case as this the heat of the fire may be utilized for other purposes
after as much of it as may be needed has been applied to heating the
core B. There are special advantages in the employment of a cooling
device, in that the metal of the core B is not so quickly oxidized.
Moreover, the difference between the temperature of the applied heat and
of the steam, air, or whatever gas or fluid be applied as the cooling
medium, may be increased or decreased at will, whereby the rapidity of
the magnetic changes or fluctuations may be regulated.




CHAPTER XXXVII.

ANTI-SPARKING DYNAMO BRUSH AND COMMUTATOR.


In direct current dynamos of great electromotive force--such, for
instance, as those used for arc lighting--when one commutator bar or
plate comes out of contact with the collecting-brush a spark is apt to
appear on the commutator. This spark may be due to the break of the
complete circuit, or to a shunt of low resistance formed by the brush
between two or more commutator-bars. In the first case the spark is more
apparent, as there is at the moment when the circuit is broken a
discharge of the magnets through the field helices, producing a great
spark or flash which causes an unsteady current, rapid wear of the
commutator bars and brushes, and waste of power. The sparking may be
reduced by various devices, such as providing a path for the current at
the moment when the commutator segment or bar leaves the brush, by
short-circuiting the field-helices, by increasing the number of the
commutator-bars, or by other similar means; but all these devices are
expensive or not fully available, and seldom attain the object desired.

To prevent this sparking in a simple manner, Mr. Tesla some years ago
employed with the commutator-bars and intervening insulating material,
mica, asbestos paper or other insulating and incombustible material,
arranged to bear on the surface of the commutator, near to and behind
the brush.

In the drawings, Fig. 244 is a section of a commutator with an asbestos
insulating device; and Fig. 245 is a similar view, representing two
plates of mica upon the back of the brush.

In 244, C represents the commutator and intervening insulating material;
B B, the brushes. _d d_ are sheets of asbestos paper or other suitable
non-conducting material. _f f_ are springs, the pressure of which may be
adjusted by means of the screws _g g_.

In Fig. 245 a simple arrangement is shown with two plates of mica or
other material. It will be seen that whenever one commutator segment
passes out of contact with the brush, the formation of the arc will be
prevented by the intervening insulating material coming in contact with
the insulating material on the brush.

[Illustration: FIG. 244.]

[Illustration: FIG. 245.]

Asbestos paper or cloth impregnated with zinc-oxide, magnesia, zirconia,
or other suitable material, may be used, as the paper and cloth are
soft, and serve at the same time to wipe and polish the commutator; but
mica or any other suitable material can be employed, provided the
material be an insulator or a bad conductor of electricity.

A few years later Mr. Tesla turned his attention again to the same
subject, as, perhaps, was very natural in view of the fact that the
commutator had always been prominent in his thoughts, and that so much
of his work was even aimed at dispensing with it entirely as an
objectionable and unnecessary part of dynamos and motors. In these later
efforts to remedy commutator troubles, Mr. Tesla constructs a commutator
and the collectors therefor in two parts mutually adapted to one
another, and, so far as the essential features are concerned, alike in
mechanical structure. Selecting as an illustration a commutator of two
segments adapted for use with an armature the coils or coil of which
have but two free ends, connected respectively to the segments, the
bearing-surface is the face of a disc, and is formed of two metallic
quadrant segments and two insulating segments of the same dimensions,
and the face of the disc is smoothed off, so that the metal and
insulating segments are flush. The part which takes the place of the
usual brushes, or the "collector," is a disc of the same character as
the commutator and has a surface similarly formed with two insulating
and two metallic segments. These two parts are mounted with their faces
in contact and in such manner that the rotation of the armature causes
the commutator to turn upon the collector, whereby the currents induced
in the coils are taken off by the collector segments and thence
conveyed off by suitable conductors leading from the collector segments.
This is the general plan of the construction adopted. Aside from certain
adjuncts, the nature and functions of which are set forth later, this
means of commutation will be seen to possess many important advantages.
In the first place the short-circuiting and the breaking of the armature
coil connected to the commutator-segments occur at the same instant, and
from the nature of the construction this will be done with the greatest
precision; secondly, the duration of both the break and of the short
circuit will be reduced to a minimum. The first results in a reduction
which amounts practically to a suppression of the spark, since the break
and the short circuit produce opposite effects in the armature-coil. The
second has the effect of diminishing the destructive effect of a spark,
since this would be in a measure proportional to the duration of the
spark; while lessening the duration of the short circuit obviously
increases the efficiency of the machine.

[Illustration: FIG. 246.]

[Illustration: FIG. 247.]

The mechanical advantages will be better understood by referring to the
accompanying diagrams, in which Fig. 246 is a central longitudinal
section of the end of a shaft with the improved commutator carried
thereon. Fig. 247 is a view of the inner or bearing face of the
collector. Fig. 248 is an end view from the armature side of a modified
form of commutator. Figs. 249 and 250 are views of details of Fig. 248.
Fig. 251 is a longitudinal central section of another modification, and
Fig. 252 is a sectional view of the same. A is the end of the
armature-shaft of a dynamo-electric machine or motor. A' is a sleeve of
insulating material around the shaft, secured in place by a screw, _a'_.

[Illustration: FIG. 248.]

[Illustration: FIG. 249.]

[Illustration: FIG. 250.]

The commutator proper is in the form of a disc which is made up of four
segments D D' G G', similar to those shown in Fig. 248. Two of these
segments, as D D', are of metal and are in electrical connection with
the ends of the coils on the armature. The other two segments are of
insulating material. The segments are held in place by a band, B, of
insulating material. The disc is held in place by friction or by screws,
_g' g'_, Fig. 248, which secure the disc firmly to the sleeve A'.

The collector is made in the same form as the commutator. It is composed
of the two metallic segments E E' and the two insulating segments F F',
bound together by a band, C. The metallic segments E E' are of the same
or practically the same width or extent as the insulating segments or
spaces of the commutator. The collector is secured to a sleeve, B', by
screws _g g_, and the sleeve is arranged to turn freely on the shaft A.
The end of the sleeve B' is closed by a plate, _f_, upon which presses a
pivot-pointed screw, _h_, adjustable in a spring, H, which acts to
maintain the collector in close contact with the commutator and to
compensate for the play of the shaft. The collector is so fixed that it
cannot turn with the shaft. For example, the diagram shows a slotted
plate, K, which is designed to be attached to a stationary support, and
an arm extending from the collector and carrying a clamping screw, L, by
which the collector may be adjusted and set to the desired position.

Mr. Tesla prefers the form shown in Figs. 246 and 247 to fit the
insulating segments of both commutator and collector loosely and to
provide some means--as, for example, light springs, _e e_, secured to
the bands A' B', respectively, and bearing against the segments--to
exert a light pressure upon them and keep them in close contact and to
compensate for wear. The metal segments of the commutator may be moved
forward by loosening the screw _a'_.

The line wires are fed from the metal segments of the collector, being
secured thereto in any convenient manner, the plan of connections being
shown as applied to a modified form of the commutator in Fig. 251. The
commutator and the collector in thus presenting two flat and smooth
bearing surfaces prevent most effectually by mechanical action the
occurrence of sparks.

The insulating segments are made of some hard material capable of being
polished and formed with sharp edges. Such materials as glass, marble,
or soapstone may be advantageously used. The metal segments are
preferably of copper or brass; but they may have a facing or edge of
durable material--such as platinum or the like--where the sparks are
liable to occur.

[Illustration: FIG. 251.]

[Illustration: FIG. 252.]

In Fig. 248 a somewhat modified form of the invention is shown, a form
designed to facilitate the construction and replacing of the parts. In
this modification the commutator and collector are made in substantially
the same manner as previously described, except that the bands B C are
omitted. The four segments of each part, however, are secured to their
respective sleeves by screws _g' g'_, and one edge of each segment is
cut away, so that small plates _a b_ may be slipped into the spaces thus
formed. Of these plates _a a_ are of metal, and are in contact with the
metal segments D D', respectively. The other two, _b b_, are of glass or
marble, and they are all better square, as shown in Figs. 249 and 250,
so that they may be turned to present new edges should any edge become
worn by use. Light springs _d_ bear upon these plates and press those in
the commutator toward those in the collector, and insulating strips _c
c_ are secured to the periphery of the discs to prevent the blocks from
being thrown out by centrifugal action. These plates are, of course,
useful at those edges of the segments only where sparks are liable to
occur, and, as they are easily replaced, they are of great advantage. It
is considered best to coat them with platinum or silver.

In Figs. 251 and 252 is shown a construction where, instead of solid
segments, a fluid is employed. In this case the commutator and collector
are made of two insulating discs, S T, and in lieu of the metal segments
a space is cut out of each part, as at R R', corresponding in shape and
size to a metal segment. The two parts are fitted smoothly and the
collector T held by the screw _h_ and spring H against the commutator S.
As in the other cases, the commutator revolves while the collector
remains stationary. The ends of the coils are connected to binding-posts
_s s_, which are in electrical connection with metal plates _t t_ within
the recesses in the two parts S T. These chambers or recesses are filled
with mercury, and in the collector part are tubes W W, with screws _w
w_, carrying springs X and pistons X', which compensate for the
expansion and contraction of the mercury under varying temperatures, but
which are sufficiently strong not to yield to the pressure of the fluid
due to centrifugal action, and which serve as binding-posts.

In all the above cases the commutators are adapted for a single coil,
and the device is particularly suited to such purposes. The number of
segments may be increased, however, or more than one commutator used
with a single armature. Although the bearing-surfaces are shown as
planes at right angles to the shaft or axis, it is evident that in this
particular the construction may be very greatly modified.




CHAPTER XXXVIII.

AUXILIARY BRUSH REGULATION OF DIRECT CURRENT DYNAMOS.


An interesting method devised by Mr. Tesla for the regulation of direct
current dynamos, is that which has come to be known as the "third brush"
method. In machines of this type, devised by him as far back as 1885, he
makes use of two main brushes to which the ends of the field magnet
coils are connected, an auxiliary brush, and a branch or shunt
connection from an intermediate point of the field wire to the auxiliary
brush.[14]

  [14] The compiler has learned partially from statements made on
       several occasions in journals and partially by personal inquiry
       of Mr. Tesla, that a great deal of work in this interesting line
       is unpublished. In these inventions as will be seen, the brushes
       are automatically shifted, but in the broad method barely
       suggested here the regulation is effected without any change in
       the position of the brushes. This auxiliary brush invention, it
       will be remembered, was very much discussed a few years ago, and
       it may be of interest that this work of Mr. Tesla, then unknown
       in this field, is now brought to light.

The relative positions of the respective brushes are varied, either
automatically or by hand, so that the shunt becomes inoperative when the
auxiliary brush has a certain position upon the commutator; but when the
auxiliary brush is moved in its relation to the main brushes, or the
latter are moved in their relation to the auxiliary brush, the electric
condition is disturbed and more or less of the current through the
field-helices is diverted through the shunt or a current is passed over
the shunt to the field-helices. By varying the relative position upon
the commutator of the respective brushes automatically in proportion to
the varying electrical conditions of the working-circuit, the current
developed can be regulated in proportion to the demands in the
working-circuit.

Fig. 253 is a diagram illustrating the invention, showing one core of
the field-magnets with one helix wound in the same direction throughout.
Figs. 254 and 255 are diagrams showing one core of the field-magnets
with a portion of the helices wound in opposite directions. Figs. 256
and 257 are diagrams illustrating the electric devices that may be
employed for automatically adjusting the brushes, and Fig. 258 is a
diagram illustrating the positions of the brushes when the machine is
being energized at the start.

_a_ and _b_ are the positive and negative brushes of the main or
working-circuit, and _c_ the auxiliary brush. The working-circuit D
extends from the brushes _a_ and _b_, as usual, and contains electric
lamps or other devices, D', either in series or in multiple arc.

M M' represent the field-helices, the ends of which are connected to the
main brushes _a_ and _b_. The branch or shunt wire _c'_ extends from the
auxiliary brush _c_ to the circuit of the field-helices, and is
connected to the same at an intermediate point, _x_.

[Illustration: FIG. 253.]

H represents the commutator, with the plates of ordinary construction.
When the auxiliary brush _c_ occupies such a position upon the
commutator that the electro-motive force between the brushes _a_ and _c_
is to the electro-motive force between the brushes _c_ and _b_ as the
resistance of the circuit _a_ M _c' c_ A is to the resistance of the
circuit _b_ M' _c' c_ B, the potentials of the points _x_ and Y will be
equal, and no current will flow over the auxiliary brush; but when the
brush _c_ occupies a different position the potentials of the points _x_
and Y will be different, and a current will flow over the auxiliary
brush to and from the commutator, according to the relative position of
the brushes. If, for instance, the commutator-space between the brushes
_a_ and _c_, when the latter is at the neutral point, is diminished, a
current will flow from the point Y over the shunt _c_ to the brush _b_,
thus strengthening the current in the part M', and partly neutralizing
the current in part M; but if the space between the brushes _a_ and _c_
is increased, the current will flow over the auxiliary brush in an
opposite direction, and the current in M will be strengthened, and in
M', partly neutralized.

By combining with the brushes _a_, _b_, and _c_ any usual automatic
regulating mechanism, the current developed can be regulated in
proportion to the demands in the working circuit. The parts M and M' of
the field wire may be wound in the same direction. In this case they are
arranged as shown in Fig. 253; or the part M may be wound in the
opposite direction, as shown in Figs. 254 and 255.

[Illustration: FIG. 254.]

It will be apparent that the respective cores of the field-magnets are
subjected to neutralizing or intensifying effects of the current in the
shunt through _c'_, and the magnetism of the cores will be partially
neutralized, or the points of greatest magnetism shifted, so that it
will be more or less remote from or approaching to the armature, and
hence the aggregate energizing actions of the field magnets on the
armature will be correspondingly varied.

In the form indicated in Fig. 253 the regulation is effected by shifting
the point of greatest magnetism, and in Figs. 254 and 255 the same
effect is produced by the action of the current in the shunt passing
through the neutralizing helix.

The relative positions of the respective brushes may be varied by moving
the auxiliary brush, or the brush _c_ may remain stationary and the core
P be connected to the main-brush holder A, so as to adjust the brushes
_a b_ in their relation to the brush _c_. If, however, an adjustment is
applied to all the brushes, as seen in Fig. 257, the solenoid should be
connected to both _a_ and _c_, so as to move them toward or away from
each other.

There are several known devices for giving motion in proportion to an
electric current. In Figs. 256 and 257 the moving cores are shown as
convenient devices for obtaining the required extent of motion with very
slight changes in the current passing through the helices. It is
understood that the adjustment of the main brushes causes variations in
the strength of the current independently of the relative position of
those brushes to the auxiliary brush. In all cases the adjustment should
be such that no current flows over the auxiliary brush when the dynamo
is running with its normal load.

In Figs. 256 and 257 A A indicate the main-brush holder, carrying the
main brushes, and C the auxiliary-brush holder, carrying the auxiliary
brush. These brush-holders are movable in arcs concentric with the
centre of the commutator-shaft. An iron piston, P, of the solenoid S,
Fig. 256, is attached to the auxiliary-brush holder C. The adjustment is
effected by means of a spring and screw or tightener.

In Fig. 257 instead of a solenoid, an iron tube inclosing a coil is
shown. The piston of the coil is attached to both brush-holders A A and
C. When the brushes are moved directly by electrical devices, as shown
in Figs. 256 and 257, these are so constructed that the force exerted
for adjusting is practically uniform through the whole length of motion.

[Illustration: FIG. 255.]

It is true that auxiliary brushes have been used in connection with the
helices of the field-wire; but in these instances the helices receive
the entire current through the auxiliary brush or brushes, and these
brushes could not be taken off without breaking the circuit through the
field. These brushes cause, moreover, heavy sparking at the commutator.
In the present case the auxiliary brush causes very little or no
sparking, and can be taken off without breaking the circuit through the
field-helices. The arrangement has, besides, the advantage of
facilitating the self-excitation of the machine in all cases where the
resistance of the field-wire is very great comparatively to the
resistance of the main circuit at the start--for instance, on arc-light
machines. In this case the auxiliary brush _c_ is placed near to, or
better still in contact with, the brush _b_, as shown in Fig. 258. In
this manner the part M' is completely cut out, and as the part M has a
considerably smaller resistance than the whole length of the field-wire
the machine excites itself, whereupon the auxiliary brush is shifted
automatically to its normal position.

[Illustration: FIG. 256.]

[Illustration: FIG. 257.]

In a further method devised by Mr. Tesla, one or more auxiliary brushes
are employed, by means of which a portion or the whole of the field
coils is shunted. According to the relative position upon the commutator
of the respective brushes more or less current is caused to pass through
the helices of the field, and the current developed by the machine can
be varied at will by varying the relative positions of the brushes.

[Illustration: FIG. 258.]

In Fig. 259, _a_ and _b_ are the positive and negative brushes of the
main circuit, and _c_ an auxiliary brush. The main circuit D extends
from the brushes _a_ and _b_, as usual, and contains the helices M of
the field wire and the electric lamps or other working devices. The
auxiliary brush _c_ is connected to the point _x_ of the main circuit by
means of the wire _c'_. H is a commutator of ordinary construction. It
will have been seen from what was said already that when the
electro-motive force between the brushes _a_ and _c_ is to the
electromotive force between the brushes _c_ and _b_ as the resistance of
the circuit _a_ M _c' c_ A is to the resistance of the circuit _b_ C B
_c c'_ D, the potentials of the points _x_ and _y_ will be equal, and no
current will pass over the auxiliary brush _c_; but if that brush
occupies a different position relatively to the main brushes the
electric condition is disturbed, and current will flow either from _y_
to _x_ or from _x_ to _y_, according to the relative position of the
brushes. In the first case the current through the field-helices will be
partly neutralized and the magnetism of the field magnets will be
diminished. In the second case the current will be increased and the
magnets gain strength. By combining with the brushes at _a b c_ any
automatic regulating mechanism, the current developed can be regulated
automatically in proportion to the demands of the working circuit.

In Figs. 264 and 265 some of the automatic means are represented that
maybe used for moving the brushes. The core P, Fig. 264, of the
solenoid-helix S is connected with the brush _a_ to move the same, and
in Fig. 265 the core P is shown as within the helix S, and connected
with brushes _a_ and _c_, so as to move the same toward or from each
other, according to the strength of the current in the helix, the helix
being within an iron tube, S', that becomes magnetized and increases the
action of the solenoid.

In practice it is sufficient to move only the auxiliary brush, as shown
in Fig. 264, as the regulation is very sensitive to the slightest
changes; but the relative position of the auxiliary brush to the main
brushes may be varied by moving the main brushes, or both main and
auxiliary brushes may be moved, as illustrated in Fig. 265. In the
latter two cases, it will be understood, the motion of the main brushes
relatively to the neutral line of the machine causes variations in the
strength of the current independently of their relative position to the
auxiliary brush. In all cases the adjustment may be such that when the
machine is running with the ordinary load, no current flows over the
auxiliary brush.

The field helices may be connected, as shown in Fig. 259, or a part of
the field helices may be in the outgoing and the other part in the
return circuit, and two auxiliary brushes may be employed as shown in
Figs. 261 and 262. Instead of shunting the whole of the field helices, a
portion only of such helices may be shunted, as shown in Figs. 260 and
262.

The arrangement shown in Fig. 262 is advantageous, as it diminishes the
sparking upon the commutator, the main circuit being closed through the
auxiliary brushes at the moment of the break of the circuit at the main
brushes.

[Illustration: FIG. 259.]

[Illustration: FIG. 260.]

[Illustration: FIG. 261.]

[Illustration: FIG. 262.]

[Illustration: FIG. 263.]

The field helices may be wound in the same direction, or a part may be
wound in opposite directions.

The connection between the helices and the auxiliary brush or brushes
may be made by a wire of small resistance, or a resistance may be
interposed (R, Fig. 263,) between the point _x_ and the auxiliary brush
or brushes to divide the sensitiveness when the brushes are adjusted.

[Illustration: FIG. 264.]

[Illustration: FIG. 265.]

The accompanying sketches also illustrate improvements made by Mr. Tesla
in the mechanical devices used to effect the shifting of the brushes, in
the use of an auxiliary brush. Fig. 266 is an elevation of the regulator
with the frame partly in section; and Fig. 267 is a section at the line
_x x_, Fig. 266. C is the commutator; B and B', the brush-holders, B
carrying the main brushes _a a'_, and B' the auxiliary or shunt brushes
_b b_. The axis of the brush-holder B is supported by two pivot-screws,
_p p_. The other brush-holder, B', has a sleeve, _d_, and is movable
around the axis of the brush-holder B. In this way both brush-holders
can turn very freely, the friction of the parts being reduced to a
minimum. Over the brush-holders is mounted the solenoid S, which rests
upon a forked column, _c_. This column also affords a support for the
pivots _p p_, and is fastened upon a solid bracket or projection, P,
which extends from the base of the machine, and is cast in one piece
with the same. The brush-holders B B' are connected by means of the
links _e e_ and the cross-piece F to the iron core I, which slides
freely in the tube T of the solenoid. The iron core I has a screw, _s_,
by means of which it can be raised and adjusted in its position
relatively to the solenoid, so that the pull exerted upon it by the
solenoid is practically uniform through the whole length of motion which
is required to effect the regulation. In order to effect the adjustment
with greater precision, the core I is provided with a small iron screw,
_s'_. The core being first brought very nearly in the required position
relatively to the solenoid by means of the screw _s_, the small screw
_s'_ is then adjusted until the magnetic attraction upon the core is the
same when the core is in any position. A convenient stop, _t_, serves to
limit the upward movement of the iron core.

To check somewhat the movement of the core I, a dash-pot, K, is used.
The piston L of the dash-pot is provided with a valve, V, which opens by
a downward pressure and allows an easy downward movement of the iron
core I, but closes and checks the movement of the core when it is pulled
up by the action of the solenoid.

To balance the opposing forces, the weight of the moving parts, and the
pull exerted by the solenoid upon the iron core, the weights W W may be
used. The adjustment is such that when the solenoid is traversed by the
normal current it is just strong enough to balance the downward pull of
the parts.

[Illustration: FIG. 266.]

[Illustration: FIG. 267.]

The electrical circuit-connections are substantially the same as
indicated in the previous diagrams, the solenoid being in series with
the circuit when the translating devices are in series, and in shunt
when the devices are in multiple arc. The operation of the device is as
follows: When upon a decrease of the resistance of the circuit or for
some other reason, the current is increased, the solenoid S gains in
strength and pulls up the iron core I, thus shifting the main brushes in
the direction of rotation and the auxiliary brushes in the opposite way.
This diminishes the strength of the current until the opposing forces
are balanced and the solenoid is traversed by the normal current; but if
from any cause the current in the circuit is diminished, then the weight
of the moving parts overcomes the pull of the solenoid, the iron core I
descends, thus shifting the brushes the opposite way and increasing the
current to the normal strength. The dash-pot connected to the iron core
I may be of ordinary construction; but it is better, especially in
machines for arc lights, to provide the piston of the dash-pot with a
valve, as indicated in the diagrams. This valve permits a comparatively
easy downward movement of the iron core, but checks its movement when it
is drawn up by the solenoid. Such an arrangement has the advantage that
a great number of lights may be put on without diminishing the
light-power of the lamps in the circuit, as the brushes assume at once
the proper position. When lights are cut out, the dash-pot acts to
retard the movement; but if the current is considerably increased the
solenoid gets abnormally strong and the brushes are shifted instantly.
The regulator being properly adjusted, lights or other devices may be
put on or out with scarcely any perceptible difference. It is obvious
that instead of the dash-pot any other retarding device may be used.




CHAPTER XXXIX.

IMPROVEMENT IN THE CONSTRUCTION OF DYNAMOS AND MOTORS.


This invention of Mr. Tesla is an improvement in the construction of
dynamo or magneto electric machines or motors, consisting in a novel
form of frame and field magnet which renders the machine more solid and
compact as a structure, which requires fewer parts, and which involves
less trouble and expense in its manufacture. It is applicable to
generators and motors generally, not only to those which have
independent circuits adapted for use in the Tesla alternating current
system, but to other continuous or alternating current machines of the
ordinary type generally used.

Fig. 268 shows the machine in side elevation. Fig. 269 is a vertical
sectional view of the field magnets and frame and an end view of the
armature; and Fig. 270 is a plan view of one of the parts of the frame
and the armature, a portion of the latter being cut away.

The field magnets and frame are cast in two parts. These parts are
identical in size and shape, and each consists of the solid plates or
ends A B, from which project inwardly the cores C D and the side bars or
bridge pieces, E F. The precise shape of these parts is largely a matter
of choice--that is to say, each casting, as shown, forms an
approximately rectangular frame; but it might obviously be more or less
oval, round, or square, without departure from the invention. It is also
desirable to reduce the width of the side bars, E F, at the center and
to so proportion the parts that when the frame is put together the
spaces between the pole pieces will be practically equal to the arcs
which the surfaces of the poles occupy.

The bearings G for the armature shaft are cast in the side bars E F. The
field coils are either wound on the pole pieces or on a form and then
slipped on over the ends of the pole pieces. The lower part or casting
is secured to the base after being finished off. The armature K on its
shaft is then mounted in the bearings of the lower casting and the
other part of the frame placed in position, dowel pins L or any other
means being used to secure the two parts in proper position.

[Illustration: FIG. 268.]

[Illustration: FIG. 269.]

[Illustration: FIG. 270.]

In order to secure an easier fit, the side bars E F, and end pieces, A
B, are so cast that slots M are formed when the two parts are put
together.

This machine possesses several advantages. For example, if we magnetize
the cores alternately, as indicated by the characters N S, it will be
seen that the magnetic circuit between the poles of each part of a
casting is completed through the solid iron side bars. The bearings for
the shaft are located at the neutral points of the field, so that the
armature core is not affected by the magnetic condition of the field.

The improvement is not restricted to the use of four pole pieces, as it
is evident that each pole piece could be divided or more than four
formed by the shape of the casting.




CHAPTER XL.

TESLA DIRECT CURRENT ARC LIGHTING SYSTEM.


At one time, soon after his arrival in America, Mr. Tesla was greatly
interested in the subject of arc lighting, which then occupied public
attention and readily enlisted the support of capital. He therefore
worked out a system which was confided to a company formed for its
exploitation, and then proceeded to devote his energies to the
perfection of the details of his more celebrated "rotary field" motor
system. The Tesla arc lighting apparatus appeared at a time when a great
many other lamps and machines were in the market, but it commanded
notice by its ingenuity. Its chief purpose was to lessen the
manufacturing cost and simplify the processes of operation.

We will take up the dynamo first. Fig. 271 is a longitudinal section,
and Fig. 272 a cross section of the machine. Fig. 273 is a top view, and
Fig. 274 a side view of the magnetic frame. Fig. 275 is an end view of
the commutator bars, and Fig. 276 is a section of the shaft and
commutator bars. Fig. 277 is a diagram illustrating the coils of the
armature and the connections to the commutator plates.

The cores _c c c c_ of the field-magnets are tapering in both
directions, as shown, for the purposes of concentrating the magnetism
upon the middle of the pole-pieces.

The connecting-frame F F of the field-magnets is in the form indicated
in the side view, Fig. 274, the lower part being provided with the
spreading curved cast legs _e e_, so that the machine will rest firmly
upon two base-bars, _r r_.

To the lower pole, S, of the field-magnet M is fastened, by means of
babbitt or other fusible diamagnetic material, the base B, which is
provided with bearings _b_ for the armature-shaft H. The base B has a
projection, P, which supports the brush-holders and the regulating
devices, which are of a special character devised by Mr. Tesla.

The armature is constructed with the view to reduce to a minimum the
loss of power due to Foucault currents and to the change of polarity,
and also to shorten as much as possible the length of the inactive wire
wound upon the armature core.

[Illustration: FIG. 271.]

It is well known that when the armature is revolved between the poles of
the field-magnets, currents are generated in the iron body of the
armature which develop heat, and consequently cause a waste of power.
Owing to the mutual action of the lines of force, the magnetic
properties of iron, and the speed of the different portions of the
armature core, these currents are generated principally on and near the
surface of the armature core, diminishing in strength gradually toward
the centre of the core. Their quantity is under some conditions
proportional to the length of the iron body in the direction in which
these currents are generated. By subdividing the iron core electrically
in this direction, the generation of these currents can be reduced to a
great extent. For instance, if the length of the armature-core is twelve
inches, and by a suitable construction it is subdivided electrically, so
that there are in the generating direction six inches of iron and six
inches of intervening air-spaces or insulating material, the waste
currents will be reduced to fifty per cent.

As shown in the diagrams, the armature is constructed of thin iron discs
D D D, of various diameters, fastened upon the armature-shaft in a
suitable manner and arranged according to their sizes, so that a series
of iron bodies, _i i i_, is formed, each of which diminishes in
thickness from the centre toward the periphery. At both ends of the
armature the inwardly curved discs _d d_, of cast iron, are fastened to
the armature shaft.

The armature core being constructed as shown, it will be easily seen
that on those portions of the armature that are the most remote from the
axis, and where the currents are principally developed, the length of
iron in the generating direction is only a small fraction of the total
length of the armature core, and besides this the iron body is
subdivided in the generating direction, and therefore the Foucault
currents are greatly reduced. Another cause of heating is the shifting
of the poles of the armature core. In consequence of the subdivision of
the iron in the armature and the increased surface for radiation, the
risk of heating is lessened.

The iron discs D D D are insulated or coated with some insulating-paint,
a very careful insulation being unnecessary, as an electrical contact
between several discs can only occur at places where the generated
currents are comparatively weak. An armature core constructed in the
manner described may be revolved between the poles of the field magnets
without showing the slightest increase of temperature.

[Illustration: FIG. 272.]

[Illustration: FIG. 273.]

The end discs, _d d_, which are of sufficient thickness and, for the
sake of cheapness, of cast-iron, are curved inwardly, as indicated in
the drawings. The extent of the curve is dependent on the amount of wire
to be wound upon the armatures. In this machine the wire is wound upon
the armature in two superimposed parts, and the curve of the end discs,
_d d_, is so calculated that the first part--that is, practically half
of the wire--just fills up the hollow space to the line _x x_; or, if
the wire is wound in any other manner, the curve is such that when the
whole of the wire is wound, the outside mass of wires, _w_, and the
inside mass of wires, _w'_, are equal at each side of the plane _x x_.
In this case the passive or electrically-inactive wires are of the
smallest length practicable. The arrangement has further the advantage
that the total lengths of the crossing wires at the two sides of the
plane _x x_ are practically equal.

[Illustration: FIG. 274.]

To equalize further the armature coils at both sides of the plates that
are in contact with the brushes, the winding and connecting up is
effected in the following manner: The whole wire is wound upon the
armature-core in two superimposed parts, which are thoroughly insulated
from each other. Each of these two parts is composed of three separated
groups of coils. The first group of coils of the first part of wire
being wound and connected to the commutator-bars in the usual manner,
this group is insulated and the second group wound; but the coils of
this second group, instead of being connected to the next following
commutator bars, are connected to the directly opposite bars of the
commutator. The second group is then insulated and the third group
wound, the coils of this group being connected to those bars to which
they would be connected in the usual way. The wires are then thoroughly
insulated and the second part of wire is wound and connected in the same
manner.

Suppose, for instance, that there are twenty-four coils--that is, twelve
in each part--and consequently twenty-four commutator plates. There will
be in each part three groups, each containing four coils, and the coils
will be connected as follows:

                        _Groups._   _Commutator Bars._
                      { First            1--5
  First part of wire  { Second          17--21
                      { Third            9--13

                      { First           13--17
  Second part of wire { Second           5--9
                      { Third           21--1

In constructing the armature core and winding and connecting the coils
in the manner indicated, the passive or electrically inactive wire is
reduced to a minimum, and the coils at each side of the plates that are
in contact with the brushes are practically equal. In this way the
electrical efficiency of the machine is increased.

[Illustration: FIG. 275.]

[Illustration: FIG. 276.]

The commutator plates _t_ are shown as outside the bearing _b_ of the
armature shaft. The shaft H is tubular and split at the end portion, and
the wires are carried through the same in the usual manner and connected
to the respective commutator plates. The commutator plates are upon a
cylinder, _u_, and insulated, and this cylinder is properly placed and
then secured by expanding the split end of the shaft by a tapering screw
plug, _v_.

[Illustration: FIG. 277.]

The arc lamps invented by Mr. Tesla for use on the circuits from the
above described dynamo are those in which the separation and feed of the
carbon electrodes or their equivalents is accomplished by means of
electro-magnets or solenoids in connection with suitable clutch
mechanism, and were designed for the purpose of remedying certain
faults common to arc lamps.

He proposed to prevent the frequent vibrations of the movable carbon
"point" and flickering of the light arising therefrom; to prevent the
falling into contact of the carbons; to dispense with the dash pot,
clock work, or gearing and similar devices; to render the lamp extremely
sensitive, and to feed the carbon almost imperceptibly, and thereby
obtain a very steady and uniform light.

In that class of lamps where the regulation of the arc is effected by
forces acting in opposition on a free, movable rod or lever directly
connected with the electrode, all or some of the forces being dependent
on the strength of the current, any change in the electrical condition
of the circuit causes a vibration and a corresponding flicker in the
light. This difficulty is most apparent when there are only a few lamps
in circuit. To lessen this difficulty lamps have been constructed in
which the lever or armature, after the establishing of the arc, is kept
in a fixed position and cannot vibrate during the feed operation, the
feed mechanism acting independently; but in these lamps, when a clamp is
employed, it frequently occurs that the carbons come into contact and
the light is momentarily extinguished, and frequently parts of the
circuit are injured. In both these classes of lamps it has been
customary to use dash pot, clock work, or equivalent retarding devices;
but these are often unreliable and objectionable, and increase the cost
of construction.

Mr. Tesla combines two electro-magnets--one of low resistance in the
main or lamp circuit, and the other of comparatively high resistance in
a shunt around the arc--a movable armature lever, and a special feed
mechanism, the parts being arranged so that in the normal working
position of the armature lever the same is kept almost rigidly in one
position, and is not affected even by considerable changes in the
electric circuit; but if the carbons fall into contact the armature will
be actuated by the magnets so as to move the lever and start the arc,
and hold the carbons until the arc lengthens and the armature lever
returns to the normal position. After this the carbon rod holder is
released by the action of the feed mechanism, so as to feed the carbon
and restore the arc to its normal length.

Fig. 278 is an elevation of the mechanism made use of in this arc lamp.
Fig. 279 is a plan view. Fig. 280 is an elevation of the balancing lever
and spring; Fig. 281 is a detached plan view of the pole pieces and
armatures upon the friction clamp, and Fig. 282 is a section of the
clamping tube.

M is a helix of coarse wire in a circuit from the lower carbon holder to
the negative binding screw -. N is a helix of fine wire in a shunt
between the positive binding screw + and the negative binding screw -.
The upper carbon holder S is a parallel rod sliding through the plates
S' S^{2} of the frame of the lamp, and hence the electric current passes
from the positive binding post + through the plate S^{2}, carbon holder
S, and upper carbon to the lower carbon, and thence by the holder and a
metallic connection to the helix M.

[Illustration: FIG. 278.]

[Illustration: FIG. 279.]

[Illustration: FIG. 280.]

[Illustration: FIG. 281.]

[Illustration: FIG. 282.]

The carbon holders are of the usual character, and to insure electric
connections the springs _l_ are made use of to grasp the upper carbon
holding rod S, but to allow the rod to slide freely through the same.
These springs _l_ may be adjusted in their pressure by the screw _m_,
and the spring _l_ maybe sustained upon any suitable support. They are
shown as connected with the upper end of the core of the magnet N.

Around the carbon-holding rod S, between the plates S' S^{2}, there is a
tube, R, which forms a clamp. This tube is counter-bored, as seen in the
section Fig. 282, so that it bears upon the rod S at its upper end and
near the middle, and at the lower end of this tubular clamp R there are
armature segments _r_ of soft iron. A frame or arm, _n_, extending,
preferably, from the core N^{2}, supports the lever A by a fulcrum-pin,
_o_. This lever A has a hole, through which the upper end of the tubular
clamp R passes freely, and from the lever A is a link, _q_, to the lever
_t_, which lever is pivoted at _y_ to a ring upon one of the columns
S^{3}. This lever _t_ has an opening or bow surrounding the tubular
clamp R, and there are pins or pivotal connections _w_ between the lever
_t_ and this clamp R, and a spring, _r^{2}_, serves to support or
suspend the weight of the parts and balance them, or nearly so. This
spring is adjustable.

At one end of the lever A is a soft-iron armature block, _a_, over the
core M' of the helix M, and there is a limiting screw, _c_, passing
through this armature block _a_, and at the other end of the lever A is
a soft iron armature block, _b_, with the end tapering or wedge shaped,
and the same comes close to and in line with the lateral projection _e_
on the core N^{2}. The lower ends of the cores M' N^{2} are made with
laterally projecting pole-pieces M^{3} N^{3}, respectively, and these
pole-pieces are concave at their outer ends, and are at opposite sides
of the armature segments _r_ at the lower end of the tubular clamp R.

The operation of these devices is as follows: In the condition of
inaction, the upper carbon rests upon the lower one, and when the
electric current is turned on it passes freely, by the frame and spring
_l_, through the rods and carbons to the coarse wire and helix M, and to
the negative binding post V and the core M' thereby is energized. The
pole piece M^{3} attracts the armature _r_, and by the lateral pressure
causes the clamp R to grasp the rod S', and the lever A is
simultaneously moved from the position shown by dotted lines, Fig. 278,
to the normal position shown in full lines, and in so doing the link _q_
and lever _t_ are raised, lifting the clamp R and S, separating the
carbons and forming the arc. The magnetism of the pole piece _e_ tends
to hold the lever A level, or nearly so, the core N^{2} being energized
by the current in the shunt which contains the helix N. In this position
the lever A is not moved by any ordinary variation in the current,
because the armature _b_ is strongly attracted by the magnetism of _e_,
and these parts are close to each other, and the magnetism of _e_ acts
at right angles to the magnetism of the core M'. If, now, the arc
becomes too long, the current through the helix M is lessened, and the
magnetism of the core N^{3} is increased by the greater current passing
through the shunt, and this core N^{3}, attracting the segmental
armature _r_, lessens the hold of the clamp R upon the rod S, allowing
the latter to slide and lessen the length of the arc, which instantly
restores the magnetic equilibrium and causes the clamp R to hold the rod
S. If it happens that the carbons fall into contact, then the magnetism
of N^{2} is lessened so much that the attraction of the magnet M will be
sufficient to move the armature _a_ and lever A so that the armature _b_
passes above the normal position, so as to separate the carbons
instantly; but when the carbons burn away, a greater amount of current
will pass through the shunt until the attraction of the core N^{2} will
overcome the attraction of the core M' and bring the armature lever A
again into the normal horizontal position, and this occurs before the
feed can take place. The segmental armature pieces _r_ are shown as
nearly semicircular. They are square or of any other desired shape, the
ends of the pole pieces M^{3}, N^{3} being made to correspond in shape.

In a modification of this lamp, Mr. Tesla provided means for
automatically withdrawing a lamp from the circuit, or cutting it out
when, from a failure of the feed, the arc reached an abnormal length;
and also means for automatically reinserting such lamp in the circuit
when the rod drops and the carbons come into contact.

Fig. 283 is an elevation of the lamp with the case in section. Fig. 284
is a sectional plan at the line _x x_. Fig. 285 is an elevation, partly
in section, of the lamp at right angles to Fig. 283. Fig. 286 is a
sectional plan at the line _y y_ of Fig. 283. Fig. 287 is a section of
the clamp in about full size. Fig. 288 is a detached section
illustrating the connection of the spring to the lever that carries the
pivots of the clamp, and Fig. 289 is a diagram showing the
circuit-connections of the lamp.

In Fig. 283, M represents the main and N the shunt magnet, both securely
fastened to the base A, which with its side columns, S S, are cast in
one piece of brass or other diamagnetic material. To the magnets are
soldered or otherwise fastened the brass washers or discs _a a a a_.
Similar washers, _b b_, of fibre or other insulating material, serve to
insulate the wires from the brass washers.

The magnets M and N are made very flat, so that their width exceeds
three times their thickness, or even more. In this way a comparatively
small number of convolutions is sufficient to produce the required
magnetism, while a greater surface is offered for cooling off the wires.

[Illustration: FIG. 286.]

[Illustration: FIG. 283.]

[Illustration: FIG. 285.]

[Illustration: FIG. 284.]

[Illustration: FIG. 287.]

[Illustration: FIG. 288.]

The upper pole pieces, _m n_, of the magnets are curved, as indicated in
the drawings, Fig. 283. The lower pole pieces _m' n'_, are brought near
together, tapering toward the armature _g_, as shown in Figs. 284 and
286. The object of this taper is to concentrate the greatest amount of
the developed magnetism upon the armature, and also to allow the pull to
be exerted always upon the middle of the armature _g_. This armature _g_
is a piece of iron in the shape of a hollow cylinder, having on each
side a segment cut away, the width of which is equal to the width of the
pole pieces _m' n'_.

The armature is soldered or otherwise fastened to the clamp _r_, which
is formed of a brass tube, provided with gripping-jaws _e e_, Fig. 287.
These jaws are arcs of a circle of the diameter of the rod R, and are
made of hardened German silver. The guides _f f_, through which the
carbon-holding rod R slides, are made of the same material. This has the
advantage of reducing greatly the wear and corrosion of the parts coming
in frictional contact with the rod, which frequently causes trouble. The
jaws _e e_ are fastened to the inside of the tube _r_, so that one is a
little lower than the other. The object of this is to provide a greater
opening for the passage of the rod when the same is released by the
clamp. The clamp _r_ is supported on bearings _w w_, Figs. 283, 285 and
287, which are just in the middle between the jaws _e e_. The bearings
_w w_ are carried by a lever, _t_, one end of which rests upon an
adjustable support, _q_, of the side columns, S, the other end being
connected by means of the link _e'_ to the armature-lever L. The
armature-lever L is a flat piece of iron in N shape, having its ends
curved so as to correspond to the form of the upper pole-pieces of the
magnets M and N. It is hung upon the pivots _v v_, Fig. 284, which are
in the jaw _x_ of the top plate B. This plate B, with the jaw, is cast
in one piece and screwed to the side columns, S S, that extend up from
the base A. To partly balance the overweight of the moving parts, a
spring, _s'_, Figs. 284 and 288, is fastened to the top plate, B, and
hooked to the lever _t_. The hook _o_ is toward one side of the lever or
bent a little sidewise, as seen in Fig. 288. By this means a slight
tendency is given to swing the armature toward the pole-piece _m'_ of
the main magnet.

The binding-posts K K' are screwed to the base A. A manual switch, for
short-circuiting the lamp when the carbons are renewed, is also fastened
to the base. This switch is of ordinary character, and is not shown in
the drawings.

The rod R is electrically connected to the lamp-frame by means of a
flexible conductor or otherwise. The lamp-case receives a removable
cover, _s^{2}_, to inclose the parts.

The electrical connections are as indicated diagrammatically in Fig.
289. The wire in the main magnet consists of two parts, _x'_ and _p'_.
These two parts may be in two separated coils or in one single helix,
as shown in the drawings. The part _x'_ being normally in circuit, is,
with the fine wire upon the shunt-magnet, wound and traversed by the
current in the same direction, so as to tend to produce similar poles, N
N or S S, on the corresponding pole-pieces of the magnets M and N. The
part _p'_ is only in circuit when the lamp is cut out, and then the
current being in the opposite direction produces in the main magnet,
magnetism of the opposite polarity.

The operation is as follows: At the start the carbons are to be in
contact, and the current passes from the positive binding-post K to the
lamp-frame, carbon-holder, upper and lower carbon, insulated return-wire
in one of the side rods, and from there through the part _x'_ of the
wire on the main magnet to the negative binding-post. Upon the passage
of the current the main magnet is energized and attracts the
clamping-armature _g_, swinging the clamp and gripping the rod by means
of the gripping jaws _e e_. At the same time the armature lever L is
pulled down and the carbons are separated. In pulling down the armature
lever L the main magnet is assisted by the shunt-magnet N, the latter
being magnetized by magnetic induction from the magnet M.

[Illustration: FIG. 289.]

It will be seen that the armatures L and _g_ are practically the keepers
for the magnets M and N, and owing to this fact both magnets with either
one of the armatures L and _g_ may be considered as one horseshoe
magnet, which we might term a "compound magnet." The whole of the
soft-iron parts M, _m'_, _g_, _n'_, N and L form a compound magnet.

The carbons being separated, the fine wire receives a portion of the
current. Now, the magnetic induction from the magnet M is such as to
produce opposite poles on the corresponding ends of the magnet N; but
the current traversing the helices tends to produce similar poles on the
corresponding ends of both magnets, and therefore as soon as the fine
wire is traversed by sufficient current the magnetism of the whole
compound magnet is diminished.

With regard to the armature _g_ and the operation of the lamp, the pole
_m'_ may be considered as the "clamping" and the pole _n'_ as the
"releasing" pole.

As the carbons burn away, the fine wire receives more current and the
magnetism diminishes in proportion. This causes the armature lever L to
swing and the armature _g_ to descend gradually under the weight of the
moving parts until the end _p_, Fig. 283, strikes a stop on the top
plate, B. The adjustment is such that when this takes place the rod R is
yet gripped securely by the jaws _e e_. The further downward movement of
the armature lever being prevented, the arc becomes longer as the
carbons are consumed, and the compound magnet is weakened more and more
until the clamping armature _g_ releases the hold of the gripping-jaws
_e e_ upon the rod R, and the rod is allowed to drop a little, thus
shortening the arc. The fine wire now receiving less current, the
magnetism increases, and the rod is clamped again and slightly raised,
if necessary. This clamping and releasing of the rod continues until the
carbons are consumed. In practice the feed is so sensitive that for the
greatest part of the time the movement of the rod cannot be detected
without some actual measurement. During the normal operation of the lamp
the armature lever L remains practically stationary, in the position
shown in Fig. 283.

Should it happen that, owing to an imperfection in it, the rod and the
carbons drop too far, so as to make the arc too short, or even bring the
carbons in contact, a very small amount of current passes through the
fine wire, and the compound magnet becomes sufficiently strong to act as
at the start in pulling the armature lever L down and separating the
carbons to a greater distance.

It occurs often in practical work that the rod sticks in the guides. In
this case the are reaches a great length, until it finally breaks. Then
the light goes out, and frequently the fine wire is injured. To prevent
such an accident Mr. Tesla provides this lamp with an automatic cut-out
which operates as follows: When, upon a failure of the feed, the arc
reaches a certain predetermined length, such an amount of current is
diverted through the fine wire that the polarity of the compound magnet
is reversed. The clamping armature _g_ is now moved against the shunt
magnet N until it strikes the releasing pole _n'_. As soon as the
contact is established, the current passes from the positive binding
post over the clamp _r_, armature _g_, insulated shunt magnet, and the
helix _p'_ upon the main magnet M to the negative binding post. In this
case the current passes in the opposite direction and changes the
polarity of the magnet M, at the same time maintaining by magnetic
induction in the core of the shunt magnet the required magnetism without
reversal of polarity, and the armature _g_ remains against the shunt
magnet pole _n'_. The lamp is thus cut out as long as the carbons are
separated. The cut out may be used in this form without any further
improvement; but Mr. Tesla arranges it so that if the rod drops and the
carbons come in contact the arc is started again. For this purpose he
proportions the resistance of part _p'_ and the number of the
convolutions of the wire upon the main magnet so that when the carbons
come in contact a sufficient amount of current is diverted through the
carbons and the part _x'_ to destroy or neutralize the magnetism of the
compound magnet. Then the armature _g_, having a slight tendency to
approach to the clamping pole _m'_, comes out of contact with the
releasing pole _n'_. As soon as this happens, the current through the
part _p'_ is interrupted, and the whole current passes through the part
_x_. The magnet M is now strongly magnetized, the armature _g_ is
attracted, and the rod clamped. At the same time the armature lever L is
pulled down out of its normal position and the arc started. In this way
the lamp cuts itself out automatically when the arc gets too long, and
reinserts itself automatically in the circuit if the carbons drop
together.




CHAPTER XLI.

IMPROVEMENT IN "UNIPOLAR" GENERATORS.


Another interesting class of apparatus to which Mr. Tesla has directed
his attention, is that of "unipolar" generators, in which a disc or a
cylindrical conductor is mounted between magnetic poles adapted to
produce an approximately uniform field. In the disc armature machines
the currents induced in the rotating conductor flow from the centre to
the periphery, or conversely, according to the direction of rotation or
the lines of force as determined by the signs of the magnetic poles, and
these currents are taken off usually by connections or brushes applied
to the disc at points on its periphery and near its centre. In the case
of the cylindrical armature machine, the currents developed in the
cylinder are taken off by brushes applied to the sides of the cylinder
at its ends.

In order to develop economically an electromotive force available for
practicable purposes, it is necessary either to rotate the conductor at
a very high rate of speed or to use a disc of large diameter or a
cylinder of great length; but in either case it becomes difficult to
secure and maintain a good electrical connection between the collecting
brushes and the conductor, owing to the high peripheral speed.

It has been proposed to couple two or more discs together in series,
with the object of obtaining a higher electro-motive force; but with the
connections heretofore used and using other conditions of speed and
dimension of disc necessary to securing good practicable results, this
difficulty is still felt to be a serious obstacle to the use of this
kind of generator. These objections Mr. Tesla has sought to avoid by
constructing a machine with two fields, each having a rotary conductor
mounted between its poles. The same principle is involved in the case of
both forms of machine above described, but the description now given is
confined to the disc type, which Mr. Tesla is inclined to favor for that
machine. The discs are formed with flanges, after the manner of
pulleys, and are connected together by flexible conducting bands or
belts.

The machine is built in such manner that the direction of magnetism or
order of the poles in one field of force is opposite to that in the
other, so that rotation of the discs in the same direction develops a
current in one from centre to circumference and in the other from
circumference to centre. Contacts applied therefore to the shafts upon
which the discs are mounted form the terminals of a circuit the
electro-motive force in which is the sum of the electro-motive forces of
the two discs.

It will be obvious that if the direction of magnetism in both fields be
the same, the same result as above will be obtained by driving the discs
in opposite directions and crossing the connecting belts. In this way
the difficulty of securing and maintaining good contact with the
peripheries of the discs is avoided and a cheap and durable machine made
which is useful for many purposes--such as for an exciter for
alternating current generators, for a motor, and for any other purpose
for which dynamo machines are used.

[Illustration: FIG. 290.]

[Illustration: FIG. 291.]

Fig. 290 is a side view, partly in section, of this machine. Fig. 291 is
a vertical section of the same at right angles to the shafts.

In order to form a frame with two fields of force, a support, A, is cast
with two pole pieces B B' integral with it. To this are joined by bolts
E a casting D, with two similar and corresponding pole pieces C C'. The
pole pieces B B' are wound and connected to produce a field of force of
given polarity, and the pole pieces C C' are wound so as to produce a
field of opposite polarity. The driving shafts F G pass through the
poles and are journaled in insulating bearings in the casting A D, as
shown.

H K are the discs or generating conductors. They are composed of copper,
brass, or iron and are keyed or secured to their respective shafts. They
are provided with broad peripheral flanges J. It is of course obvious
that the discs may be insulated from their shafts, if so desired. A
flexible metallic belt L is passed over the flanges of the two discs,
and, if desired, may be used to drive one of the discs. It is better,
however, to use this belt merely as a conductor, and for this purpose
sheet steel, copper, or other suitable metal is used. Each shaft is
provided with a driving pulley M, by which power is imparted from a
driving shaft.

N N are the terminals. For the sake of clearness they are shown as
provided with springs P, that bear upon the ends of the shafts. This
machine, if self-exciting, would have copper bands around its poles; or
conductors of any kind--such as wires shown in the drawings--may be
used.

       *       *       *       *       *

It is thought appropriate by the compiler to append here some notes on
unipolar dynamos, written by Mr. Tesla, on a recent occasion.


NOTES ON A UNIPOLAR DYNAMO.[15]

  [15] Article by Mr. Tesla, contributed to _The Electrical Engineer_,
       N. Y., Sept. 2, 1891.

It is characteristic of fundamental discoveries, of great achievements
of intellect, that they retain an undiminished power upon the
imagination of the thinker. The memorable experiment of Faraday with a
disc rotating between the two poles of a magnet, which has borne such
magnificent fruit, has long passed into every-day experience; yet there
are certain features about this embryo of the present dynamos and motors
which even to-day appear to us striking, and are worthy of the most
careful study.

Consider, for instance, the case of a disc of iron or other metal
revolving between the two opposite poles of a magnet, and the polar
surfaces completely covering both sides of the disc, and assume the
current to be taken off or conveyed to the same by contacts uniformly
from all points of the periphery of the disc. Take first the case of a
motor. In all ordinary motors the operation is dependent upon some
shifting or change of the resultant of the magnetic attraction exerted
upon the armature, this process being effected either by some mechanical
contrivance on the motor or by the action of currents of the proper
character. We may explain the operation of such a motor just as we can
that of a water-wheel. But in the above example of the disc surrounded
completely by the polar surfaces, there is no shifting of the magnetic
action, no change whatever, as far as we know, and yet rotation ensues.
Here, then, ordinary considerations do not apply; we cannot even give a
superficial explanation, as in ordinary motors, and the operation will
be clear to us only when we shall have recognized the very nature of the
forces concerned, and fathomed the mystery of the invisible connecting
mechanism.

Considered as a dynamo machine, the disc is an equally interesting
object of study. In addition to its peculiarity of giving currents of
one direction without the employment of commutating devices, such a
machine differs from ordinary dynamos in that there is no reaction
between armature and field. The armature current tends to set up a
magnetization at right angles to that of the field current, but since
the current is taken off uniformly from all points of the periphery, and
since, to be exact, the external circuit may also be arranged perfectly
symmetrical to the field magnet, no reaction can occur. This, however,
is true only as long as the magnets are weakly energized, for when the
magnets are more or less saturated, both magnetizations at right angles
seemingly interfere with each other.

For the above reason alone it would appear that the output of such a
machine should, for the same weight, be much greater than that of any
other machine in which the armature current tends to demagnetize the
field. The extraordinary output of the Forbes unipolar dynamo and the
experience of the writer confirm this view.

Again, the facility with which such a machine may be made to excite
itself is striking, but this may be due--besides to the absence of
armature reaction--to the perfect smoothness of the current and
non-existence of self-induction.

If the poles do not cover the disc completely on both sides, then, of
course, unless the disc be properly subdivided, the machine will be very
inefficient. Again, in this case there are points worthy of notice. If
the disc be rotated and the field current interrupted, the current
through the armature will continue to flow and the field magnets will
lose their strength comparatively slowly. The reason for this will at
once appear when we consider the direction of the currents set up in the
disc.

[Illustration: FIG. 292.]

Referring to the diagram Fig. 292, _d_ represents the disc with the
sliding contacts B B' on the shaft and periphery. N and S represent the
two poles of a magnet. If the pole N be above, as indicated in the
diagram, the disc being supposed to be in the plane of the paper, and
rotating in the direction of the arrow D, the current set up in the disc
will flow from the centre to the periphery, as indicated by the arrow A.
Since the magnetic action is more or less confined to the space between
the poles N S, the other portions of the disc may be considered
inactive. The current set up will therefore not wholly pass through the
external circuit F, but will close through the disc itself, and
generally, if the disposition be in any way similar to the one
illustrated, by far the greater portion of the current generated will
not appear externally, as the circuit F is practically short-circuited
by the inactive portions of the disc. The direction of the resulting
currents in the latter may be assumed to be as indicated by the dotted
lines and arrows _m_ and _n_; and the direction of the energizing field
current being indicated by the arrows _a b c d_, an inspection of the
figure shows that one of the two branches of the eddy current, that is,
A B' _m_ B, will tend to demagnetize the field, while the other branch,
that is, A B' _n_ B, will have the opposite effect. Therefore, the
branch A B' _m_ B, that is, the one which is _approaching_ the field,
will repel the lines of the same, while branch A B' _n_ B, that is, the
one _leaving_ the field, will gather the lines of force upon itself.

In consequence of this there will be a constant tendency to reduce the
current flow in the path A B' _m_ B, while on the other hand no such
opposition will exist in path A B' _n_ B, and the effect of the latter
branch or path will be more or less preponderating over that of the
former. The joint effect of both the assumed branch currents might be
represented by that of one single current of the same direction as that
energizing the field. In other words, the eddy currents circulating in
the disc will energize the field magnet. This is a result quite contrary
to what we might be led to suppose at first, for we would naturally
expect that the resulting effect of the armature currents would be such
as to oppose the field current, as generally occurs when a primary and
secondary conductor are placed in inductive relations to each other. But
it must be remembered that this results from the peculiar disposition in
this case, namely, two paths being afforded to the current, and the
latter selecting that path which offers the least opposition to its
flow. From this we see that the eddy currents flowing in the disc partly
energize the field, and for this reason when the field current is
interrupted the currents in the disc will continue to flow, and the
field magnet will lose its strength with comparative slowness and may
even retain a certain strength as long as the rotation of the disc is
continued.

The result will, of course, largely depend on the resistance and
geometrical dimensions of the path of the resulting eddy current and on
the speed of rotation; these elements, namely, determine the retardation
of this current and its position relative to the field. For a certain
speed there would be a maximum energizing action; then at higher speeds,
it would gradually fall off to zero and finally reverse, that is, the
resultant eddy current effect would be to weaken the field. The reaction
would be best demonstrated experimentally by arranging the fields N S,
N' S', freely movable on an axis concentric with the shaft of the disc.
If the latter were rotated as before in the direction of the arrow D,
the field would be dragged in the same direction with a torque, which,
up to a certain point, would go on increasing with the speed of
rotation, then fall off, and, passing through zero, finally become
negative; that is, the field would begin to rotate in opposite direction
to the disc. In experiments with alternate current motors in which the
field was shifted by currents of differing phase, this interesting
result was observed. For very low speeds of rotation of the field the
motor would show a torque of 900 lbs. or more, measured on a pulley 12
inches in diameter. When the speed of rotation of the poles was
increased, the torque would diminish, would finally go down to zero,
become negative, and then the armature would begin to rotate in opposite
direction to the field.

To return to the principal subject; assume the conditions to be such
that the eddy currents generated by the rotation of the disc strengthen
the field, and suppose the latter gradually removed while the disc is
kept rotating at an increased rate. The current, once started, may then
be sufficient to maintain itself and even increase in strength, and then
we have the case of Sir William Thomson's "current accumulator." But
from the above considerations it would seem that for the success of the
experiment the employment of a disc _not subdivided_[16] would be
essential, for if there should be a radial subdivision, the eddy
currents could not form and the self-exciting action would cease. If
such a radially subdivided disc were used it would be necessary to
connect the spokes by a conducting rim or in any proper manner so as to
form a symmetrical system of closed circuits.

  [16] Mr. Tesla here refers to an interesting article which appeared
       in July, 1865, in the _Phil. Magazine_, by Sir W. Thomson, in
       which Sir William, speaking of his "uniform electric current
       accumulator," assumes that for self-excitation it is desirable
       to subdivide the disc into an infinite number of infinitely thin
       spokes, in order to prevent diffusion of the current. Mr. Tesla
       shows that diffusion is absolutely necessary for the excitation
       and that when the disc is subdivided no excitation can occur.

The action of the eddy currents may be utilized to excite a machine of
any construction. For instance, in Figs. 293 and 294 an arrangement is
shown by which a machine with a disc armature might be excited. Here a
number of magnets, N S, N S, are placed radially on each side of a metal
disc D carrying on its rim a set of insulated coils, C C. The magnets
form two separate fields, an internal and an external one, the solid
disc rotating in the field nearest the axis, and the coils in the field
further from it. Assume the magnets slightly energized at the start;
they could be strengthened by the action of the eddy currents in the
solid disc so as to afford a stronger field for the peripheral coils.
Although there is no doubt that under proper conditions a machine might
be excited in this or a similar manner, there being sufficient
experimental evidence to warrant such an assertion, such a mode of
excitation would be wasteful.

But a unipolar dynamo or motor, such as shown in Fig. 292, may be
excited in an efficient manner by simply properly subdividing the disc
or cylinder in which the currents are set up, and it is practicable to
do away with the field coils which are usually employed. Such a plan is
illustrated in Fig. 295. The disc or cylinder D is supposed to be
arranged to rotate between the two poles N and S of a magnet, which
completely cover it on both sides, the contours of the disc and poles
being represented by the circles _d_ and _d^{1}_ respectively, the upper
pole being omitted for the sake of clearness. The cores of the magnet
are supposed to be hollow, the shaft C of the disc passing through them.
If the unmarked pole be below, and the disc be rotated screw fashion,
the current will be, as before, from the centre to the periphery, and
may be taken off by suitable sliding contacts, B B', on the shaft and
periphery respectively. In this arrangement the current flowing through
the disc and external circuit will have no appreciable effect on the
field magnet.

[Illustration: FIG. 293.]

[Illustration: FIG. 294.]

But let us now suppose the disc to be subdivided spirally, as indicated
by the full or dotted lines, Fig. 295. The difference of potential
between a point on the shaft and a point on the periphery will remain
unchanged, in sign as well as in amount. The only difference will be
that the resistance of the disc will be augmented and that there will be
a greater fall of potential from a point on the shaft to a point on the
periphery when the same current is traversing the external circuit. But
since the current is forced to follow the lines of subdivision, we see
that it will tend either to energize or de-energize the field, and this
will depend, other things being equal, upon the direction of the lines
of subdivision. If the subdivision be as indicated by the full lines in
Fig. 295, it is evident that if the current is of the same direction as
before, that is, from centre to periphery, its effect will be to
strengthen the field magnet; Whereas, if the subdivision be as indicated
by the dotted lines, the current generated will tend to weaken the
magnet. In the former case the machine will be capable of exciting
itself when the disc is rotated in the direction of arrow D; in the
latter case the direction of rotation must be reversed. Two such discs
may be combined, however, as indicated, the two discs rotating in
opposite fields, and in the same or opposite direction.

[Illustration: FIG. 295.]

[Illustration: FIG. 296.]

Similar disposition may, of course, be made in a type of machine in
which, instead of a disc, a cylinder is rotated. In such unipolar
machines, in the manner indicated, the usual field coils and poles may
be omitted and the machine may be made to consist only of a cylinder or
of two discs enveloped by a metal casting.

Instead of subdividing the disc or cylinder spirally, as indicated in
Fig. 295, it is more convenient to interpose one or more turns between
the disc and the contact ring on the periphery, as illustrated in Fig.
296.

A Forbes dynamo may, for instance, be excited in such a manner. In the
experience of the writer it has been found that instead of taking the
current from two such discs by sliding contacts, as usual, a flexible
conducting belt may be employed to advantage. The discs are in such case
provided with large flanges, affording a very great contact surface. The
belt should be made to bear on the flanges with spring pressure to take
up the expansion. Several machines with belt contact were constructed by
the writer two years ago, and worked satisfactorily; but for want of
time the work in that direction has been temporarily suspended. A number
of features pointed out above have also been used by the writer in
connection with some types of alternating current motors.




PART IV.

APPENDIX.--EARLY PHASE MOTORS AND THE TESLA MECHANICAL AND ELECTRICAL
OSCILLATOR.




CHAPTER XLII.

MR. TESLA'S PERSONAL EXHIBIT AT THE WORLD'S FAIR.

While the exhibits of firms engaged in the manufacture of electrical
apparatus of every description at the Chicago World's Fair, afforded the
visitor ample opportunity for gaining an excellent knowledge of the
state of the art, there were also numbers of exhibits which brought out
in strong relief the work of the individual inventor, which lies at the
foundation of much, if not all, industrial or mechanical achievement.
Prominent among such personal exhibits was that of Mr. Tesla, whose
apparatus occupied part of the space of the Westinghouse Company, in
Electricity Building.

This apparatus represented the results of work and thought covering a
period of ten years. It embraced a large number of different alternating
motors and Mr. Tesla's earlier high frequency apparatus. The motor
exhibit consisted of a variety of fields and armatures for two, three
and multiphase circuits, and gave a fair idea of the gradual evolution
of the fundamental idea of the rotating magnetic field. The high
frequency exhibit included Mr. Tesla's earlier machines and disruptive
discharge coils and high frequency transformers, which he used in his
investigations and some of which are referred to in his papers printed
in this volume.

Fig. 297 shows a view of part of the exhibits containing the motor
apparatus. Among these is shown at A a large ring intended to exhibit
the phenomena of the rotating magnetic field. The field produced was
very powerful and exhibited striking effects, revolving copper balls and
eggs and bodies of various shapes at considerable distances and at great
speeds. This ring was wound for two-phase circuits, and the winding was
so distributed that a practically uniform field was obtained. This ring
was prepared for Mr. Tesla's exhibit by Mr. C. F. Scott, electrician of
the Westinghouse Electric and Manufacturing Company.

[Illustration: FIG. 297.]

A smaller ring, shown at B, was arranged like the one exhibited at A but
designed especially to exhibit the rotation of an armature in a rotating
field. In connection with these two rings there was an interesting
exhibit shown by Mr. Tesla which consisted of a magnet with a coil, the
magnet being arranged to rotate in bearings. With this magnet he first
demonstrated the identity between a rotating field and a rotating
magnet; the latter, when rotating, exhibited the same phenomena as the
rings when they were energized by currents of differing phase. Another
prominent exhibit was a model illustrated at C which is a two-phase
motor, as well as an induction motor and transformer. It consists of a
large outer ring of laminated iron wound with two superimposed,
separated windings which can be connected in a variety of ways. This is
one of the first models used by Mr. Tesla as an induction motor and
rotating transformer. The armature was either a steel or wrought iron
disc with a closed coil. When the motor was operated from a two phase
generator the windings were connected in two groups, as usual. When used
as an induction motor, the current induced in one of the windings of the
ring was passed through the other winding on the ring and so the motor
operated with only two wires. When used as a transformer the outer
winding served, for instance, as a secondary and the inner as a primary.
The model shown at D is one of the earliest rotating field motors,
consisting of a thin iron ring wound with two sets of coils and an
armature consisting of a series of steel discs partly cut away and
arranged on a small arbor.

At E is shown one of the first rotating field or induction motors used
for the regulation of an arc lamp and for other purposes. It comprises a
ring of discs with two sets of coils having different self-inductions,
one set being of German silver and the other of copper wire. The
armature is wound with two closed-circuited coils at right angles to
each other. To the armature shaft are fastened levers and other devices
to effect the regulation. At F is shown a model of a magnetic lag motor;
this embodies a casting with pole projections protruding from two coils
between which is arranged to rotate a smooth iron body. When an
alternating current is sent through the two coils the pole projections
of the field and armature within it are similarly magnetized, and upon
the cessation or reversal of the current the armature and field repel
each other and rotation is produced in this way. Another interesting
exhibit, shown at G, is an early model of a two field motor energized by
currents of different phase. There are two independent fields of
laminated iron joined by brass bolts; in each field is mounted an
armature, both armatures being on the same shaft. The armatures were
originally so arranged as to be placed in any position relatively to
each other, and the fields also were arranged to be connected in a
number of ways. The motor has served for the exhibition of a number of
features; among other things, it has been used as a dynamo for the
production of currents of any frequency between wide limits. In this
case the field, instead of being energized by direct current, was
energized by currents differing in phase, which produced a rotation of
the field; the armature was then rotated in the same or in opposite
direction to the movement of the field; and so any number of
alternations of the currents induced in the armature, from a small to a
high number, determined by the frequency of the energizing field coils
and the speed of the armature, was obtained.

[Illustration: FIG. 298.]

The models H, I, J, represent a variety of rotating field, synchronous
motors which are of special value in long distance transmission work.
The principle embodied in these motors was enunciated by Mr. Tesla in
his lecture before the American Institute of Electrical Engineers, in
May, 1888[17]. It involves the production of the rotating field in one
of the elements of the motor by currents differing in phase and
energizing the other element by direct currents. The armatures are of
the two and three phase type. K is a model of a motor shown in an
enlarged view in Fig. 298. This machine, together with that shown in
Fig. 299, was exhibited at the same lecture, in May, 1888. They were the
first rotating field motors which were independently tested, having for
that purpose been placed in the hands of Prof. Anthony in the winter of
1887-88. From these tests it was shown that the efficiency and output of
these motors was quite satisfactory in every respect.

  [17] See Part I, Chap. III, page 9.

[Illustration: FIG. 299.]

It was intended to exhibit the model shown in Fig. 299, but it was
unavailable for that purpose owing to the fact that it was some time ago
handed over to the care of Prof. Ayrton in England. This model was
originally provided with twelve independent coils; this number, as Mr.
Tesla pointed out in his first lecture, being divisible by two and
three, was selected in order to make various connections for two and
three-phase operations, and during Mr. Tesla's experiments was used in
many ways with from two to six phases. The model, Fig. 298, consists of
a magnetic frame of laminated iron with four polar projections between
which an armature is supported on brass bolts passing through the frame.
A great variety of armatures was used in connection with these two and
other fields. Some of the armatures are shown in front on the table,
Fig. 297, and several are also shown enlarged in Figs. 300 to 310. An
interesting exhibit is that shown at L, Fig. 297. This is an armature of
hardened steel which was used in a demonstration before the Society of
Arts in Boston, by Prof. Anthony. Another curious exhibit is shown
enlarged in Fig. 301. This consists of thick discs of wrought iron
placed lengthwise, with a mass of copper cast around them. The discs
were arranged longitudinally to afford an easier starting by reason of
the induced current formed in the iron discs, which differed in phase
from those in the copper. This armature would start with a single
circuit and run in synchronism, and represents one of the earliest types
of such an armature. Fig. 305 is another striking exhibit. This is one
of the earliest types of an armature with holes beneath the periphery,
in which copper conductors are imbedded. The armature has eight closed
circuits and was used in many different ways. Fig. 304 is a type of
synchronous armature consisting of a block of soft steel wound with a
coil closed upon itself. This armature was used in connection with the
field shown in Fig. 298 and gave excellent results.

[Illustration: FIG. 300.]

[Illustration: FIG. 301.]

[Illustration: FIG. 302.]

[Illustration: FIG. 303.]

[Illustration: FIG. 304.]

[Illustration: FIG. 305.]

[Illustration: FIG. 306.]

[Illustration: FIG. 307.]

[Illustration: FIG. 308.]

[Illustration: FIG. 309.]

[Illustration: FIG. 310.]

Fig. 302 represents a synchronous armature with a large coil around a
body of iron. There is another very small coil at right angles to the
first. This small coil was used for the purpose of increasing the
starting torque and was found very effective in this connection. Figs.
306 and 308 show a favorite construction of armature; the iron body is
made up of two sets of discs cut away and placed at right angles to each
other, the interstices being wound with coils. The one shown in Fig. 308
is provided with an additional groove on each of the projections formed
by the discs, for the purpose of increasing the starting torque by a
wire wound in these projections. Fig. 307 is a form of armature
similarly constructed, but with four independent coils wound upon the
four projections. This armature was used to reduce the speed of the
motor with reference to that of the generator. Fig. 300 is still another
armature with a great number of independent circuits closed upon
themselves, so that all the dead points on the armature are done away
with, and the armature has a large starting torque. Fig. 303 is another
type of armature for a four-pole motor but with coils wound upon a
smooth surface. A number of these armatures have hollow shafts, as they
have been used in many ways. Figs. 309 and 310 represent armatures to
which either alternating or direct current was conveyed by means of
sliding rings. Fig. 309 consists of a soft iron body with a single coil
wound around it, the ends of the coil being connected to two sliding
rings to which, usually, direct current was conveyed. The armature shown
in Fig. 310 has three insulated rings on a shaft and was used in
connection with two or three phase circuits.

All these models shown represent early work, and the enlarged engravings
are made from photographs taken early in 1888. There is a great number
of other models which were exhibited, but which are not brought out
sharply in the engraving, Fig. 297. For example at M is a model of a
motor comprising an armature with a hollow shaft wound with two or three
coils for two or three-phase circuits; the armature was arranged to be
stationary and the generating circuits were connected directly to the
generator. Around the armature is arranged to rotate on its shaft a
casting forming six closed circuits. On the outside this casting was
turned smooth and the belt was placed on it for driving with any desired
appliance. This also is a very early model.

On the left side of the table there are seen a large variety of models,
N, O, P, etc., with fields of various shapes. Each of these models
involves some distinct idea and they all represent gradual development
chiefly interesting as showing Mr. Tesla's efforts to adapt his system
to the existing high frequencies.

On the right side of the table, at S, T, are shown, on separate
supports, larger and more perfected armatures of commercial motors, and
in the space around the table a variety of motors and generators
supplying currents to them was exhibited.

The high frequency exhibit embraced Mr. Tesla's first original apparatus
used in his investigations. There was exhibited a glass tube with one
layer of silk-covered wire wound at the top and a copper ribbon on the
inside. This was the first disruptive discharge coil constructed by him.
At U is shown the disruptive discharge coil exhibited by him in his
lecture before the American Institute of Electrical Engineers, in May,
1891.[18] At V and W are shown some of the first high frequency
transformers. A number of various fields and armatures of small models
of high frequency apparatus as shown at X and Y, and others not visible
in the picture, were exhibited. In the annexed space the dynamo then
used by Mr. Tesla at Columbia College was exhibited; also another form
of high frequency dynamo used.

  [18] See Part II, Chap. XXVI., page 145.

[Illustration: FIG. 311.]

In this space also was arranged a battery of Leyden jars and his large
disruptive discharge coil which was used for exhibiting the light
phenomena in the adjoining dark room. The coil was operated at only a
small fraction of its capacity, as the necessary condensers and
transformers could not be had and as Mr. Tesla's stay was limited to one
week; notwithstanding, the phenomena were of a striking character. In
the room were arranged two large plates placed at a distance of about
eighteen feet from each other. Between them were placed two long tables
with all sorts of phosphorescent bulbs and tubes; many of these were
prepared with great care and marked legibly with the names which would
shine with phosphorescent glow. Among them were some with the names of
Helmholtz, Faraday, Maxwell, Henry, Franklin, etc. Mr. Tesla had also
not forgotten the greatest living poet of his own country, Zmaj Jovan;
two or three were prepared with inscriptions, like "Welcome,
Electricians," and produced a beautiful effect. Each represented some
phase of this work and stood for some individual experiment of
importance. Outside the room was the small battery seen in Fig. 311, for
the exhibition of some of the impedance and other phenomena of interest.
Thus, for instance, a thick copper bar bent in arched form was provided
with clamps for the attachment of lamps, and a number of lamps were kept
at incandescence on the bar; there was also a little motor shown on the
table operated by the disruptive discharge.

As will be remembered by those who visited the Exposition, the
Westinghouse Company made a line exhibit of the various commercial
motors of the Tesla system, while the twelve generators in Machinery
Hall were of the two-phase type constructed for distributing light and
power. Mr. Tesla, also exhibited some models of his oscillators.




CHAPTER XLIII.

THE TESLA MECHANICAL AND ELECTRICAL OSCILLATORS.


On the evening of Friday, August 25, 1893, Mr. Tesla delivered a lecture
on his mechanical and electrical oscillators, before the members of the
Electrical Congress, in the hall adjoining the Agricultural Building, at
the World's Fair, Chicago. Besides the apparatus in the room, he
employed an air compressor, which was driven by an electric motor.

Mr. Tesla was introduced by Dr. Elisha Gray, and began by stating that
the problem he had set out to solve was to construct, first, a mechanism
which would produce oscillations of a perfectly constant period
independent of the pressure of steam or air applied, within the widest
limits, and also independent of frictional losses and load. Secondly, to
produce electric currents of a perfectly constant period independently
of the working conditions, and to produce these currents with mechanism
which should be reliable and positive in its action without resorting to
spark gaps and breaks. This he successfully accomplished in his
apparatus, and with this apparatus, now, scientific men will be provided
with the necessaries for carrying on investigations with alternating
currents with great precision. These two inventions Mr. Tesla called,
quite appropriately, a mechanical and an electrical oscillator,
respectively.

The former is substantially constructed in the following way. There is a
piston in a cylinder made to reciprocate automatically by proper
dispositions of parts, similar to a reciprocating tool. Mr. Tesla
pointed out that he had done a great deal of work in perfecting his
apparatus so that it would work efficiently at such high frequency of
reciprocation as he contemplated, but he did not dwell on the many
difficulties encountered. He exhibited, however, the pieces of a steel
arbor which had been actually torn apart while vibrating against a
minute air cushion.

With the piston above referred to there is associated in one of his
models in an independent chamber an air spring, or dash pot, or else he
obtains the spring within the chambers of the oscillator itself. To
appreciate the beauty of this it is only necessary to say that in that
disposition, as he showed it, no matter what the rigidity of the spring
and no matter what the weight of the moving parts, in other words, no
matter what the period of vibrations, the vibrations of the spring are
always isochronous with the applied pressure. Owing to this, the results
obtained with these vibrations are truly wonderful. Mr. Tesla provides
for an air spring of tremendous rigidity, and he is enabled to vibrate
big weights at an enormous rate, considering the inertia, owing to the
recoil of the spring. Thus, for instance, in one of these experiments,
he vibrates a weight of approximately 20 pounds at the rate of about 80
per second and with a stroke of about 7/8 inch, but by shortening the
stroke the weight could be vibrated many hundred times, and has been, in
other experiments.

To start the vibrations, a powerful blow is struck, but the adjustment
can be so made that only a minute effort is required to start, and, even
without any special provision it will start by merely turning on the
pressure suddenly. The vibration being, of course, isochronous, any
change of pressure merely produces a shortening or lengthening of the
stroke. Mr. Tesla showed a number of very clear drawings, illustrating
the construction of the apparatus from which its working was plainly
discernible. Special provisions are made so as to equalize the pressure
within the dash pot and the outer atmosphere. For this purpose the
inside chambers of the dash pot are arranged to communicate with the
outer atmosphere so that no matter how the temperature of the enclosed
air might vary, it still retains the same mean density as the outer
atmosphere, and by this means a spring of constant rigidity is obtained.
Now, of course, the pressure of the atmosphere may vary, and this would
vary the rigidity of the spring, and consequently the period of
vibration, and this feature constitutes one of the great beauties of the
apparatus; for, as Mr. Tesla pointed out, this mechanical system acts
exactly like a string tightly stretched between two points, and with
fixed nodes, so that slight changes of the tension do not in the least
alter the period of oscillation.

The applications of such an apparatus are, of course, numerous and
obvious. The first is, of course, to produce electric currents, and by a
number of models and apparatus on the lecture platform, Mr. Tesla showed
how this could be carried out in practice by combining an electric
generator with his oscillator. He pointed out what conditions must be
observed in order that the period of vibration of the electrical system
might not disturb the mechanical oscillation in such a way as to alter
the periodicity, but merely to shorten the stroke. He combines a
condenser with a self-induction, and gives to the electrical system the
same period as that at which the machine itself oscillates, so that both
together then fall in step and electrical and mechanical resonance is
obtained, and maintained absolutely unvaried.

Next he showed a model of a motor with delicate wheelwork, which was
driven by these currents at a constant speed, no matter what the air
pressure applied was, so that this motor could be employed as a clock.
He also showed a clock so constructed that it could be attached to one
of the oscillators, and would keep absolutely correct time. Another
curious and interesting feature which Mr. Tesla pointed out was that,
instead of controlling the motion of the reciprocating piston by means
of a spring, so as to obtain isochronous vibration, he was actually able
to control the mechanical motion by the natural vibration of the
electro-magnetic system, and he said that the case was a very simple
one, and was quite analogous to that of a pendulum. Thus, supposing we
had a pendulum of great weight, preferably, which would be maintained in
vibration by force, periodically applied; now that force, no matter how
it might vary, although it would oscillate the pendulum, would have no
control over its period.

Mr. Tesla also described a very interesting phenomenon which he
illustrated by an experiment. By means of this new apparatus, he is able
to produce an alternating current in which the E. M. F. of the impulses
in one direction preponderates over that of those in the other, so that
there is produced the effect of a direct current. In fact he expressed
the hope that these currents would be capable of application in many
instances, serving as direct currents. The principle involved in this
preponderating E. M. F. he explains in this way: Suppose a conductor is
moved into the magnetic field and then suddenly withdrawn. If the
current is not retarded, then the work performed will be a mere
fractional one; but if the current is retarded, then the magnetic field
acts as a spring. Imagine that the motion of the conductor is arrested
by the current generated, and that at the instant when it stops to move
into the field, there is still the maximum current flowing in the
conductor; then this current will, according to Lenz's law, drive the
conductor out of the field again, and if the conductor has no
resistance, then it would leave the field with the velocity it entered
it. Now it is clear that if, instead of simply depending on the current
to drive the conductor out of the field, the mechanically applied force
is so timed that it helps the conductor to get out of the field, then it
might leave the field with higher velocity than it entered it, and thus
one impulse is made to preponderate in E. M. F. over the other.

With a current of this nature, Mr. Tesla energized magnets strongly, and
performed many interesting experiments bearing out the fact that one of
the current impulses preponderates. Among them was one in which he
attached to his oscillator a ring magnet with a small air gap between
the poles. This magnet was oscillated up and down 80 times a second. A
copper disc, when inserted within the air gap of the ring magnet, was
brought into rapid rotation. Mr. Tesla remarked that this experiment
also seemed to demonstrate that the lines of flow of current through a
metallic mass are disturbed by the presence of a magnet in a manner
quite independently of the so-called Hall effect. He showed also a very
interesting method of making a connection with the oscillating magnet.
This was accomplished by attaching to the magnet small insulated steel
rods, and connecting to these rods the ends of the energizing coil. As
the magnet was vibrated, stationary nodes were produced in the steel
rods, and at these points the terminals of a direct current source were
attached. Mr. Tesla also pointed out that one of the uses of currents,
such as those produced in his apparatus, would be to select any given
one of a number of devices connected to the same circuit by picking out
the vibration by resonance. There is indeed little doubt that with Mr.
Tesla's devices, harmonic and synchronous telegraphy will receive a
fresh impetus, and vast possibilities are again opened up.

Mr. Tesla was very much elated over his latest achievements, and said
that he hoped that in the hands of practical, as well as scientific men,
the devices described by him would yield important results. He laid
special stress on the facility now afforded for investigating the effect
of mechanical vibration in all directions, and also showed that he had
observed a number of facts in connection with iron cores.

[Illustration: FIG. 312.]

The engraving, Fig. 312, shows, in perspective, one of the forms of
apparatus used by Mr. Tesla in his earlier investigations in this field
of work, and its interior construction is made plain by the sectional
view shown in Fig. 313. It will be noted that the piston P is fitted
into the hollow of a cylinder C which is provided with channel ports
O O, and _I_, extending all around the inside surface. In this
particular apparatus there are two channels O O for the outlet of the
working fluid and one, _I_, for the inlet. The piston P is provided with
two slots S S' at a carefully determined distance, one from the other.
The tubes T T which are screwed into the holes drilled into the piston,
establish communication between the slots S S' and chambers on each side
of the piston, each of these chambers connecting with the slot which is
remote from it. The piston P is screwed tightly on a shaft A which
passes through fitting boxes at the end of the cylinder C. The boxes
project to a carefully determined distance into the hollow of the
cylinder C, thus determining the length of the stroke.

Surrounding the whole is a jacket J. This jacket acts chiefly to
diminish the sound produced by the oscillator and as a jacket when the
oscillator is driven by steam, in which case a somewhat different
arrangement of the magnets is employed. The apparatus here illustrated
was intended for demonstration purposes, air being used as most
convenient for this purpose.

A magnetic frame M M is fastened so as to closely surround the
oscillator and is provided with energizing coils which establish two
strong magnetic fields on opposite sides. The magnetic frame is made up
of thin sheet iron. In the intensely concentrated field thus produced,
there are arranged two pairs of coils H H supported in metallic frames
which are screwed on the shaft A of the piston and have additional
bearings in the boxes B B on each side. The whole is mounted on a
metallic base resting on two wooden blocks.

[Illustration: FIG. 313.]

The operation of the device is as follows: The working fluid being
admitted through an inlet pipe to the slot I and the piston being
supposed to be in the position indicated, it is sufficient, though not
necessary, to give a gentle tap on one of the shaft ends protruding
from the boxes B. Assume that the motion imparted be such as to move the
piston to the left (when looking at the diagram) then the air rushes
through the slot S' and tube T into the chamber to the left. The
pressure now drives the piston towards the right and, owing to its
inertia, it overshoots the position of equilibrium and allows the air to
rush through the slot S and tube T into the chamber to the right, while
the communication to the left hand chamber is cut off, the air of the
latter chamber escaping through the outlet O on the left. On the return
stroke a similar operation takes place on the right hand side. This
oscillation is maintained continuously and the apparatus performs
vibrations from a scarcely perceptible quiver amounting to no more than
1 of an inch, up to vibrations of a little over 3/8 of an inch,
according to the air pressure and load. It is indeed interesting to see
how an incandescent lamp is kept burning with the apparatus showing a
scarcely perceptible quiver.

To perfect the mechanical part of the apparatus so that oscillations are
maintained economically was one thing, and Mr. Tesla hinted in his
lecture at the great difficulties he had first encountered to accomplish
this. But to produce oscillations which would be of constant period was
another task of no mean proportions. As already pointed out, Mr. Tesla
obtains the constancy of period in three distinct ways. Thus, he
provides properly calculated chambers, as in the case illustrated, in
the oscillator itself; or he associates with the oscillator an air
spring of constant resilience. But the most interesting of all, perhaps,
is the maintenance of the constancy of oscillation by the reaction of
the electromagnetic part of the combination. Mr. Tesla winds his coils,
by preference, for high tension and associates with them a condenser,
making the natural period of the combination fairly approximating to the
average period at which the piston would oscillate without any
particular provision being made for the constancy of period under
varying pressure and load. As the piston with the coils is perfectly
free to move, it is extremely susceptible to the influence of the
natural vibration set up in the circuits of the coils H H. The
mechanical efficiency of the apparatus is very high owing to the fact
that friction is reduced to a minimum and the weights which are moved
are small; the output of the oscillator is therefore a very large one.

Theoretically considered, when the various advantages which Mr. Tesla
holds out are examined, it is surprising, considering the simplicity of
the arrangement, that nothing was done in this direction before. No
doubt many inventors, at one time or other, have entertained the idea of
generating currents by attaching a coil or a magnetic core to the piston
of a steam engine, or generating currents by the vibrations of a tuning
fork, or similar devices, but the disadvantages of such arrangements
from an engineering standpoint must be obvious. Mr. Tesla, however, in
the introductory remarks of his lecture, pointed out how by a series of
conclusions he was driven to take up this new line of work by the
necessity of producing currents of constant period and as a result of
his endeavors to maintain electrical oscillation in the most simple and
economical manner.




INDEX.


Alternate Current Electrostatic Apparatus 392

Alternating Current Generators for High Frequency 152, 374, 224

Alternating Motors and Transformers 7

American Institute Electrical Engineers Lecture 145

Anthony, W. A., Tests of Tesla Motors 8

Apparatus for Producing High Vacua 276

Arc Lighting, Tesla Direct, System 451

Auxiliary Brush Regulation 438


Biography, Tesla 4

Brush, Anti-Sparking 432

Brush, Third, Regulation 438

Brush, Phenomena in High Vacuum 226


Carborundum Button for Tesla Lamps 140, 253

Commutator, Anti-Sparking 432

Combination of Synchronizing and Torque Motor 95

Condensers with Plates in Oil 418

Conversion with Disruptive Discharge 193, 204, 303

Current or Dynamic Electricity Phenomena 327


Direct Current Arc Lighting 451

Dischargers, Forms of 305

Disruptive Discharge Coil 207, 221

Disruptive Discharge Phenomena 212

Dynamos, Improved Direct Current 448


Early Phase Motors 477

Effects with High Frequency and High Potential Currents 119

Electrical Congress Lecture, Chicago. 486

Electric Resonance 340

Electric Discharges in Vacuum Tubes 396

Electrolytic Registering Meter 420

Eye, Observations on the 294


Flames, Electrostatic, Non-Consuming 166, 272

Forbes Unipolar Generator 468, 474

Franklin Institute Lecture 294


Generators, Pyromagnetic 429


High Potential, High Frequency:

  Brush Phenomena in High Vacuum 226
  Carborundum Buttons 140, 253
  Disruptive Discharge Phenomena 212
  Flames, Electrostatic, Non-Consuming 166, 272
  Impedance, Novel Phenomena 194, 338
  Lighting Lamps Through Body 359
  Luminous Effects with Gases 368
  "Massage" with Currents 394
  Motor with Single Wire 234, 330
  "No Wire" Motors 235
  Oil Insulation of Induction Coils 173, 221
  Ozone, Production of 171
  Phosphorescence 367
  Physiological Effects 162, 394
  Resonance 340
  Spinning Filament 168
  Streaming Discharges of High Tension Coil 155, 163
  Telegraphy without Wires 346


Impedance, Novel Phenomena 194, 338

Improvements in Unipolar Generators 465

Improved Direct Current Dynamos and Motors 448

Induction Motors 92

Institution Electrical Engineers Lecture 198


Lamps and Motor operated on a Single Wire 330

Lamps with Single Straight Fiber 183

Lamps containing only a Gas 188

Lamps with Refractory Button 177, 239, 360

Lamps for Simple Phosphorescence 187, 282, 364

Lecture, Tesla before:

 American Institute Electrical Engineers 145
 Royal Institution 124
 Institution Electrical Engineers 198
 Franklin Institute and National Electric Light Association 294
 Electrical Congress, Chicago 486

Lighting Lamps Through the Body 359

Light Phenomena with High Frequencies 349

Luminous Effects with Gases at Low-Pressure 368


"Magnetic Lag" Motor 67

"Massage" with Currents of High Frequency 394

Mechanical and Electrical Oscillators 486

Method of obtaining Direct from Alternating currents 409

Method of obtaining Difference of Phase by Magnetic Shielding 71

Motors:

  With Circuits of Different Resistance 79
  With Closed Conductors 9
  Combination of Synchronizing and Torque 95
  With Condenser in Armature Circuit 101
  With Condenser in one of the Field Circuits 106
  With Coinciding Maxima of Magnetic Effect in Armature and Field 83
  With "Current Lag" Artificially Secured 58
  Early Phase 477
  With Equal Magnetic Energies in Field and Armature 81
  Or Generator, obtaining Desired Speed of 36
  Improved Direct Current 448
  Induction 92
  "Magnetic Lag" 67
  "No Wire" 235
  With Phase Difference in Magnetization of Inner and Outer Parts
          of Core 88
  Regulator for Rotary Current 45
  Single Circuit, Self-starting Synchronizing 50
  Single Phase 76
  With Single Wire to Generator 234, 330
  Synchronizing 9
  Thermo-Magnetic 424
  Utilizing Continuous Current Generators 31


National Electric Light Association Lecture 294

"No Wire" Motor 235


Observations on the Eye 294

Oil, Condensers with Plates in 418

Oil Insulation of Induction Coils 173, 221

Oscillators, Mechanical and Electrical 486

Ozone, Production of 171

Phenomena Produced by Electrostatic Force 318

Phosphorescence and Sulphide of Zinc 367

Physiological Effects of High Frequency 162, 394

Polyphase Systems 26

Polyphase Transformer 109

Pyromagnetic Generators 429

Regulator for Rotary Current Motors 45

Resonance, Electric, Phenomena of 340

"Resultant Attraction" 7

Rotating Field Transformers 9

Rotating Magnetic Field 9

Royal Institution Lecture 124

Scope of Lectures 119

Single Phase Motor 76

Single Circuit, Self-Starting Synchronizing Motors 50

Spinning Filament Effects 168

Streaming Discharges of High Tension Coil 155, 163

Synchronizing Motors 9

Telegraphy without Wires 246

Transformer with Shield between Primary and Secondary 113

Thermo-Magnetic Motors 424

Thomson, J. J., on Vacuum Tubes 397, 402, 406

Thomson, Sir W., Current Accumulator 471

Transformers:

  Alternating 7
  Magnetic Shield 113
  Polyphase 109
  Rotating Field 9

Tubes:

  Coated with Yttria, etc. 187
  Coated with Sulphide of Zinc, etc. 290, 367

Unipolar Generators 465

Unipolar Generator, Forbes 468, 474

Yttria, Coated Tubes 187

Zinc, Tubes Coated with Sulphide of 367