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[Illustration: _Frontispiece._ *Plate I. Electrical Conventions.*]




                           WIRELESS TELEGRAPH

                            CONSTRUCTION FOR

                                AMATEURS

                                   BY

                          ALFRED POWELL MORGAN


  EDITOR MECHANICAL AND ELECTRICAL DEPARTMENT OF THE "BOYS’ MAGAZINE"

             AUTHOR OF "WIRELESS TELEGRAPHY AND TELEPHONY"


                        _WITH 167 ILLUSTRATIONS_


                 *Third Edition, Revised and Enlarged*


                   WITH A COMPLETE DESCRIPTION OF THE

                            NEW WIRELESS LAW




                               NEW YORK:

                        D. VAN NOSTRAND COMPANY

                             25 PARK PLACE

                                  1914




                       Copyright, 1910, 1913, by

                        D. VAN NOSTRAND COMPANY




                            _Stanhope Press_

                          F. H. GILSON COMPANY

                             BOSTON. U.S.A.




PREFACE.


In this work, the author has endeavored to present a book embracing
practical information for those who may wish to build for private or
experimental use a set of wireless instruments which are more than toys
but yet not so expensive as the commercial apparatus.

Many books have been published on the subject of wireless telegraphy,
but in them the interests of the novice have been rather neglected and
in order to build an outfit he has been forced to rely upon a series of
disconnected articles published in the amateur periodicals.

It is the object of this book to show the construction of simple,
efficient instruments by means of clear drawings, and to give enough
elementary theory and practical hints to enable the experimenter to
build a size and type in keeping with his needs and resources.

The tiresome "how to make" style has been avoided as far as possible.
History and all unimportant details are omitted to give in their place a
concise explanation of the parts played by the different instruments and
the influence of developing their various factors.

A small lathe and a set of taps and dies are necessary to produce
apparatus having a good appearance, but a little ingenuity displayed in
adapting screws and parts of old electrical instruments oftentimes at
hand will make these tools unnecessary.

Ordinary precaution and plenty of time should be used in the work. It is
obvious that if a large coil is to be made, it is well to insure its
successful completion by painstaking care and the use of proper
materials. Neither is it wise to strain an instrument through becoming
impatient and using it before it is properly completed and adjusted.

Wherever possible instructions have been given regarding the adjustment
of the apparatus, but it is only by actual practice that the operator
will acquaint himself with the most efficient manipulation.

Extracts from articles contributed by the Author to _Popular
Electricity_ have been used in the chapters on Spark Gaps, Oscillation
Detectors and Telephone Receivers, through the courtesy of the editor,
Mr. H. W. Young.

In conclusion, the writer wishes to express his thanks to the United
Wireless Telegraph Company for views of their apparatus and to the other
firms who have loaned electrotypes and supplied information. To those
who have assisted in the preparation of the book, more especially to Mr.
Safford Adams, for numerous suggestions and criticisms, the Author
desires to express his full acknowledgments.


ALFRED POWELL MORGAN.

UPPER MONTCLAIR, N.J.

_June_, 1910.




PREFACE TO THE THIRD EDITION.


The success of the previous editions of this book has made a new and
third one necessary.

There have not been any startling changes or new discoveries made in the
field of wireless telegraphy since the first edition was published, but
the art has undergone a number of small changes and improvements which
have increased the efficiency and selectivity of the apparatus.

Since then a federal law restricting and controlling wireless telegraphy
has been passed. Its effect has been to place wireless telegraphy upon a
more certain basis, and to give a recognized standing to the amateur
experimenter.

This new law has been included in this edition in the form of an
appendix. The amateur will do well to read it carefully. Compliance with
its regulations will prove beneficial rather than a hindrance.

A fully illustrated chapter explaining exactly how to comply with the
law and how to build the apparatus required has been added. Complete
descriptions of several new types of detectors are also included.

All old matter has been thoroughly revised and several illustrations
replaced by ones more up-to-date and of direct interest.


ALFRED P. MORGAN.

UPPER MONTCLAIR, N.J.

_May_, 1913.




    PREFACE. ..........................................................
    PREFACE TO THE THIRD EDITION. .....................................
    CHAPTER I. INTRODUCTORY. ..........................................
      The Transmission and Reception of Electric Waves. ...............
    CHAPTER II. THE APPARATUS. ........................................
    CHAPTER III. AERIALS AND EARTH CONNECTIONS. .......................
    CHAPTER IV. INDUCTION COILS. ......................................
    CHAPTER V. INTERRUPTERS. ..........................................
    CHAPTER VI. TRANSFORMERS. .........................................
      Construction of a 2-K.W. Transformer. ...........................
    CHAPTER VII. OSCILLATION CONDENSERS AND LEYDEN JARS. ..............
    CHAPTER VIII. SPARK GAPS OR OSCILLATORS. ..........................
    CHAPTER IX. TRANSMITTING HELIXES. .................................
    CHAPTER X. KEYS. ..................................................
    CHAPTER XI. AERIAL SWITCHES AND ANCHOR GAPS. ......................
    CHAPTER XII. HOT WIRE AMMETER. ....................................
    CHAPTER XIII. OSCILLATION DETECTORS. ..............................
      "UNIVERSAL" DETECTOR. ...........................................
      ELECTROLYTIC DETECTORS. .........................................
      TANTALUM DETECTOR. ..............................................
      CRYSTAL DETECTORS. ..............................................
      LEAD PEROXIDE DETECTOR. .........................................
      THE MARCONI MAGNETIC DETECTOR. ..................................
    CHAPTER XIV. TUNING COILS AND TRANSFORMERS. .......................
    CHAPTER XV. RECEIVING CONDENSERS. .................................
    CHAPTER XVI. TELEPHONE RECEIVERS AND HEADBANDS. ...................
    CHAPTER XVII. OPERATION. ..........................................
      ELECTRICAL TERMS AND DEFINITIONS. ...............................
    CHAPTER XVIII. THE AMATEUR AND THE WIRELESS LAW. WHAT IT IS; HOW TO
    COMPLY; HOW TO SECURE A LICENSE. ..................................
      APPARATUS REQUIRED IN MANY INSTANCES IN ORDER TO COMPLY WITH THE
      WIRELESS LAW. ...................................................
      Receiving Apparatus. ............................................




    _Frontispiece._ *Plate I. Electrical Conventions.* ................
    Fig 1. Hertz Oscillator and Resonator. ............................
    Fig. 2. Hydraulic Oscillator. .....................................
    Fig. 3. "Hydraulic" Transmitter and Receptor. .....................
    Fig. 4. Simple Wireless Telegraph Transmitter and Receptor. .......
    Fig. 5. Electric Waves and Lines of Strain. .......................
    Fig. 6. Resonance Tube. ...........................................
    Fig. 7. Lag and Lead. .............................................
    Fig. 8. Tuned Hydraulic Transmitter and Receptor. .................
    Fig. 9. Tuned Wireless Telegraph Transmitter and Receptor .........
    Fig. 10. Long-distance Receiving Set. .............................
    Fig. 11. Murdock Receiving Set. ...................................
    Fig. 12. Clapp-Eastham Receiving Set. .............................
    Fig. 13. Prague Receiving Set. ....................................
    Fig. 14. Receiving Set. ...........................................
    Fig. 15. Receiving Set. ...........................................
    Fig. 16. Murdock Transmitting and Receiving Set. ..................
    Fig. 17. United Wireless Portable Outfit. .........................
    *Plate II. Aerial Systems.* .......................................
    Fig. 18. Electrose Insulators. ....................................
    Fig. 19. Guy Insulator. ...........................................
    Fig. 20. Insulating Tube. .........................................
    Fig. 21. High-tension Cable and Insulator. ........................
    Fig. 22. Flat-top T Aerial. .......................................
    Fig. 23. Ground Clamp. ............................................
    Fig. 24. Switch for Lightning Protection. .........................
    Fig. 25. Diagram of an Induction Coil. ............................
    Fig. 26. Induction Coil Core. .....................................
    Fig. 27. Theoretical and practical form of secondary. .............
    Fig. 28. Layer Winding for Small Coils. ...........................
    Fig. 29. Section Winder. ..........................................
    Fig. 30. Impregnator for Silk Covered Wire. .......................
    Fig. 31. Methods of Connecting the Secondary Sections. ............
    Fig. 32. Coil Case. ...............................................
    Fig. 33. Simple Interrupter. ......................................
    Fig. 34. Independent Interrupter. .................................
    Fig. 35. Details of Magnets. ......................................
    Fig. 36. Details of Moving Parts. .................................
    Fig. 37. Details of Standard and Screws. ..........................
    Fig. 38. Diagram of Connections for an Independent Interrupter. ...
    Fig. 39. Construction of a Paper Condenser. .......................
    Fig. 40. Wenhelt and Simon Electrolytic Interrupters. .............
    Fig. 41. Construction of Electrolytic Interrupters. ...............
    Fig. 42. Details of Electrolytic Interrupters. ....................
    Fig. 43. Electrolytic Interrupter. ................................
    Fig. 44. Assembly and Dimensions of Core. .........................
    Fig. 45. Fiber Head and Separator. ................................
    Fig. 46. Section Form. ............................................
    Fig. 47. Methods of Connecting Sections. ..........................
    Fig. 48. Assembly of Leg. .........................................
    Fig. 49. Transformer with One Secondary removed. ..................
    Fig. 50. Wiring Diagram. ..........................................
    Fig. 51. Clapp-Eastham 1/4-K.W. Transformer. ......................
    Fig. 52. United Wireless Motor-Generator set for supplying
    Alternating Current to the Transformer. ...........................
    Fig. 53. Simple Condenser. ........................................
    Fig. 54. Leyden Jar. ..............................................
    Fig. 55. "Aerial Switch." .........................................
    Fig. 56. Amco Oscillation Condenser. ..............................
    Fig. 57. Clapp-Eastham Oscillation Condenser. .....................
    Fig. 58. Methods of Varying Capacity. .............................
    Fig. 59. Spark Gaps. ..............................................
    Fig. 60. Spark Gap. ...............................................
    Fig. 61. Closely Coupled Helix. ...................................
    *Plate III. Transmitting Circuits.* ...............................
    Fig. 62. Prague Transmitting Helix. ...............................
    Fig. 63. Closely Coupled Tuning Circuit. ..........................
    Fig. 64. Loosely Coupled Transmitting Helix and Contact Clip. .....
    Fig. 65. Loosely Coupled Transmitting Circuit. ....................
    Fig. 66. United Wireless Helix, Spark Gap and Condenser. ..........
    Fig. 67. Morse Key fitted with Extension Lever. ...................
    Fig. 68. Wireless Key. ............................................
    Fig. 69. "United Wireless Type Key." ..............................
    Fig. 70. Connections for Aerial Switch. ...........................
    Fig. 71. Aerial Switches. .........................................
    Fig. 72. Detail of Contacts. ......................................
    Fig. 73. Details of Switch Parts. .................................
    Fig. 74. Method of Fastening Knife. ...............................
    Fig. 75. "T" Aerial Switch. .......................................
    Fig. 76. "United" Wireless Lightning Switch. ......................
    Fig. 77. Shoemaker Tuning Coil and Aerial Switch. .................
    Fig. 78. "United" Wireless Anchor Gaps. ...........................
    Fig. 79. Anchor Gap. ..............................................
    Fig. 80. Simple Hot Wire Meter. ...................................
    Fig. 81. Meter with Case Removed. .................................
    Fig. 82. Glass Compensating Strip. ................................
    Fig. 83. Details of "Hot Wire" Supports. ..........................
    Fig. 84. Details of Movement. .....................................
    Fig. 85. Complete Movement. .......................................
    Fig. 86. Side View of Hot Wire and Movement. ......................
    Fig. 87. United Wireless Hot Wire Ammeter. ........................
    Fig. 88. Universal Detector. ......................................
    Fig. 89. Details of Universal Detector. ...........................
    Fig. 90. Parts of Universal Detector. .............................
    Fig. 91. Bare Point Electrolytic Detector. ........................
    Fig. 92. Effect of Exposing too much Wire. ........................
    Fig. 93. Electrolytic Detector Circuits. ..........................
    Fig. 94. Electrolytic Detector. ...................................
    Fig. 95. Forming "Glass" Point. ...................................
    Fig. 96. Shoemaker Detector. ......................................
    Fig. 97. Shoemaker Detector Circuits. .............................
    Fig. 98. Lamp Detector. ...........................................
    Fig. 99. Simple Electrolytic Detector. ............................
    Fig. 100. Electrolytic Detector. ..................................
    Fig. 101. Details of Electrolytic Detector. .......................
    Fig. 102. Increasing the Sensitiveness of an Electrolytic Detector.
    Fig. 103. Tantalum Detector. ......................................
    Fig. 104. United Wireless Carborundum Detector (horizontal type). .
    Fig. 105. United Wireless Carborundum Detector (vertical type). ...
    Fig. 106. Clapp-Eastham Ferron Detector. ..........................
    Fig. 107. Silicon Crystal in Cup. .................................
    Fig. 108. Silicon Detector Circuits. ..............................
    Fig. 109. Perikon Detector Elements. ..............................
    Fig. 110. Perikon Detector. .......................................
    Fig. 111. Peroxide of Lead Detector. ..............................
    Fig. 112. Marconi Magnetic Detector. ..............................
    Fig. 113. Details of Transformer. .................................
    Fig. 114. Method of Joining Ends of Band. .........................
    Fig. 115. Pulley. .................................................
    Fig. 116. Pulley Bearings. ........................................
    Fig. 117. Circuit of Magnetic Detector. ...........................
    Fig. 118. Fleming Oscillation Valve. ..............................
    Fig. 119. Flame Audion. ...........................................
    Fig. 120. Circuit of Flame Audion. ................................
    Fig. 121. Double-slide Tuning Coil. ...............................
    Fig. 122. Sliders. ................................................
    Fig. 123. Double-slide Tuning Coil Circuits. ......................
    *Plate IV. Receiving Circuits. (Straightaway Aerial.)* ............
    Fig. 124. Murdock Double-slide Tuning Coil. .......................
    Fig. 125. United Wireless Receiving Set. ..........................
    Fig. 126. United Wireless Portable Receiving Set. .................
    Fig. 127. Oscillation Transformer. ................................
    Fig. 128. United Wireless Receiving Transformer. ..................
    Fig. 129. Details of Receiving Transformer. .......................
    Fig. 130. Slider for Loose Coupler. ...............................
    Fig. 131. Loosely Coupled Tuning Circuits. ........................
    Fig. 132. Combination Loosely and Closely Coupled Tuner. ..........
    Fig. 133. Clapp-Eastham Loose Coupler. ............................
    Fig. 134. A Highly Efficient Form of Loose Coupler. ...............
    Fig. 135. Potentiometer. ..........................................
    Fig. 136. Amco Potentiometer. .....................................
    *Plate V. Receiving Circuits.* ....................................
    Fig. 137. Tuning Circuit with and without an Adjustable Condenser. .
    Fig. 138. Tubular Condenser. ......................................
    Fig. 139. Variable Condenser. .....................................
    Fig. 140. Details of Variable Condenser. ..........................
    Fig. 141. Sliding Plate Variable Condenser. .......................
    Fig. 142. Types of Permanent Magnets. .............................
    Fig. 143. Grinding Tool. ..........................................
    Fig. 144. Parts of a Holtzer Cabot Receiver. ......................
    Fig. 145. Holtzer Cabot Head Set. .................................
    Fig. 146. Adjustable Head Band. ...................................
    Fig. 147. Marconi Station at Siasconset, Mass. ....................
    *Plate VI. DeForest and Marconi Systems.* .........................
    Fig. 148. Experimental Amateur Station of W. Haddon, Brooklyn, N. Y.
    Fig. 149. Complete Receiving Outfit Consisting of Receiving
    Transformer, Detector, Fixed Condenser, Loading Coil, Two Variable
    Condensers, Potentiometer, Battery, Switches, etc. ................
    Fig. 150. Receiving Outfit Consisting of Receiving Transformer,
    Fixed Condenser and Detector. .....................................
    Fig. 151. Amco Oscillation Helix. .................................
    Fig. 152. Details of Oscillation Helix Construction. ..............
    Fig. 153. Quenched Gap. ...........................................
    Fig. 154. Quenched Gap. ...........................................
    Fig. 155. Details of Disk and Ring. ...............................
    Fig. 156. Explanatory Drawing of Quenched Gap. ....................
    Fig. 157. Amco Rotary Gap. ........................................
    Fig. 158. Details of Revolving Parts of Rotary Gap. ...............
    Fig. 159. Details of Rotary Gap. ..................................
    Fig. 160. Methods of Preventing "Kick Back." ......................
    Fig. 161. Variometer. .............................................
    Fig. 162. Silicon Detector. .......................................
    Fig. 163. Pyron Detector. .........................................
    Fig. 164. Galena Detector. ........................................
    Fig. 165. Audion. .................................................
    Fig. 166. Audion Circuit. .........................................
    Fig. 167. Rotary Variable Condenser. ..............................




CHAPTER I. INTRODUCTORY.


Being desirous of keeping this book as far as possible within the limits
prescribed by the title, it is not possible to go deeply into the theory
of the propagation of electric waves, but at the same time it is not
deemed advisable to plunge suddenly into the construction of wireless
apparatus without giving some explanation of the underlying principles.

If the reader desires information upon this subject he is referred to
Fleming’s "Electric Wave Telegraphy" or the same author’s "Elementary
Manual of Radio-telegraphy and Radio-telephony."

The explanations given in this chapter do not involve any actual theory
of the transmission and reception of electric waves. They are merely
intended to show the train of actions which take place and may be
observed in a physical sense. With this purpose in view, several
references have been made to simple hydraulic apparatus and an analogy
drawn to render the explanation clearer.



The Transmission and Reception of Electric Waves.


Wireless telegraphy by means of electromagnetic waves may be divided
into four distinct operations, namely:

  1. The generation of electrical oscillations.
  2. The transformation of electrical oscillations into electrical
     waves.
  3. The transformation of electrical waves into electrical
     oscillations.
  4. The detection of the electrical oscillations.

The first two operations comprise those taking place at the transmitter,
while the last two, which are the converse of the first, are in evidence
only when receiving.

[Illustration: Fig 1. Hertz Oscillator and Resonator.]

Fig. 1 illustrates the original Hertz oscillator and resonator, which is
the simplest form a wireless installation may take. _T_ represents the
transmitting apparatus and _R_ the receptor. At the transmitting station
a telegraph key is placed in series with a battery and an induction
coil. Two large metal plates, _t_ and _t’_, are connected to the
opposite sides of the spark gap, which in turn is connected to the
secondary of the induction coil. When the key is pressed the electrical
circuit is completed and the voltage of the battery is raised
sufficiently by the induction coil to charge the metal plates _t_ and
_t’_.

The key serves to break the current into periods corresponding to the
dots and dashes of the telegraph code. When the high voltage of the
induction coil is impressed upon the plates they become charged, and
being of opposite polarity, when at a maximum the energy rushes across
the gap and produces a disruptive spark. Each discharge, although
appearing like a single spark passing in one direction, is in reality
made up of a large number of rapid oscillations or surgings. The first
passage of current serves to more than discharge the plates and they
become charged in the opposite direction. A reverse discharge then
occurs which also oversteps itself, and thus the oscillations go on, but
gradually become weaker and weaker until they die completely or are
damped out. The heated air of the spark gap becomes a conductor during
the passage of the spark, and the oscillations are enabled to surge back
and forth at the rate of 15,000 to 1,000,000 per second, although the
actual discharge may take only a fraction of a second.

[Illustration: Fig. 2. Hydraulic Oscillator.]

The generation of electrical oscillations may perhaps be made more clear
by reference to the hydraulic apparatus illustrated in Fig. 2. _T_ and
_T’_ are communicating tubes divided by an elastic membrane M. The tubes
may be likened to the metal plates _t_ and _t’_ or the arms of the
oscillator. The membrane may be likened to the layer of air between the
knobs which separates the opposite arms of the oscillator. _P_ is a pump
connected to the two tubes _T_ and _T’_, and the broken lines in the
apparatus represent water. The pump corresponds to the induction coil in
Fig. 1, and the water to the secondary currents of the induction coil.
When the pump is set in operation, the water is drawn from the tube _T_
and injected into _T’_. The pump valves prevent it from flowing back.
When the level becomes very high in _T’_, the great pressure distends
the membrane in the direction shown by the dotted line until finally it
bursts and the water is allowed to flow with a rush into the tube T. But
the inertia of the water causes it to rise higher in the tube than its
final position of equilibrium, while in returning and endeavoring to
seek its level its inertia carries it below this position. Thus the
water oscillates back and forth until finally it comes to rest.

Similarly the difference of potential of the oscillator arms is not
immediately equalized upon the breaking down of the air gap, and the
apparatus becomes the seat of extremely rapid electrical oscillations,
as explained above.

All space is supposed to be filled with a highly attenuated, invisible
and weightless medium called ether. When the electrical oscillations
surge back and forth through the arms of the oscillator, portions of the
energy are thrown off from the apparatus and travel in enlarging circles
like the ripples on a pond. These consist of lines of dielectric stress
or electrostatic flux which pass through the ether and constitute
electromagnetic waves.

The receptor or resonator _R_, Fig. 1, consists of a circle of wire
having in it a small spark gap capable of minute adjustment. Two metal
plates _r_ and _r’_ are sometimes attached to the opposite sides of the
spark gap. When the key is pressed at the transmitting station and waves
are sent out through the ether, they strike the resonator and set up
therein electrical oscillations which pass across the gap in the shape
of sparks.

[Illustration: Fig. 3. "Hydraulic" Transmitter and Receptor.]

To make the explanation clearer, let us consider Fig. 3 in which two
floats or blocks of wood are represented as resting on the surface of a
tank or pool of water. One float, _A_, is connected by a rope and pulley
so that by jerking the rope the float may be made to _oscillate_ and
cause little ripples or waves to pass outwards in a gradually enlarging
circle. When the waves reach the float, _B_, they cause it to rise and
fall with each wave or to oscillate and reproduce the movements of the
float, _A_. Likewise the oscillations set up by a wireless transmitter
are sent out into space to be caught and duplicated at the receiving
station. Of course this analogy to the propagation and reception of
electric waves is not the same as the true electrical actions, but is
merely a graphical, representation.

[Illustration: Fig. 4. Simple Wireless Telegraph Transmitter and
Receptor.]

The wireless telegraph outfit illustrated in Fig. 1 would not serve for
more than short distances of a few feet, and so a somewhat similar but
more efficient apparatus is employed in practice. Fig. 4 shows such a
system in its simplest form. In this case the secondary or high
potential leads of the induction coil are connected, one to an earth and
the other to an aerial or antenna composed of a number of bare copper
wires insulated and suspended from a mast.

All electrically charged bodies are surrounded by an electrostatic field
of force, the nature of which in theory is a state of strain.

The action of an induction coil connected as in Fig. 4 is to charge the
upper part of the aerial above the spark gap, say with negative
electricity and establish a field of force in its vicinity varying in
area from a few feet to several miles. When the charge reaches a certain
potential it is sufficient to puncture the layer of air in the gap and a
spark takes place, setting up electrical oscillations.

[Illustration: Fig. 5. Electric Waves and Lines of Strain.]

Previous to the rupture of the spark gap, _lines of electric strain or
force_ stretch from the aerial to the earth on all sides as in the
center of Fig. 5. A line of force may be defined as a curve drawn in the
electric field so that the direction of the curve is the same as that of
the electric intensity at that point.

The aerial and the earth act like the two metal plates in Fig. 1 or like
the opposite plates of a condenser. As soon as the air gap is punctured
it becomes conductive and the aerial charge rushes down into the earth.
With the discharge, the strain in the electrostatic field is released
and the aerial charge rushes down into the earth, but in so relaxing
produces a new current and builds up a strain around the antenna
opposite in direction to the first. This process repeats itself very
rapidly and electrical oscillations are thus set up in the antenna.
Every oscillation changes the direction of the magnetic flux or
dielectric strain and causes the imaginary lines which originally
stretched from the aerial to the earth to be displaced and the ends
terminating at the aerial to run down it and form semi-loops or inverted
"U’s" standing with their ends on the earth in a circular ripple around
the aerial and moving away from it with the speed of light. In Fig. 5
three oscillations are supposed to have taken place. The shortest
distance between two adjacent points at which the electric strain is at
a maximum in the same direction and period of time is the _wave length_
emitted by the aerial. The separate standing groups of dielectric strain
moving away from the antenna are electromagnetic waves. In the figure,
the adjacent groups are separated by half a wave length. These waves are
emitted at right angles to the transmitting aerial, whence they pass
through the ether to the other station. When they reach the receiving
aerial they set up electrical oscillations therein which are _too weak
to be perceptible in the shape of sparks_ as in the original Hertz
oscillator and resonator because of the great distance separating the
stations, so they are made to flow through a _detector_, which in Fig. 4
is represented as being a crystal of a mineral called silicon. When the
high frequency currents strike the silicon, they set up a weak pulsating
direct current. This action is due to a peculiar rectifying property of
the mineral. The direct current flows through the telephone receiver and
produces an audible sound. If the aerial and ground were connected
directly to the terminals of the telephone receiver, without the
silicon, the oscillations would not pass because of the impeding or
choking action of the electro-magnets in the telephone receivers.

*Tuning.*—It is sometimes desirable that messages should be made
selective or secretive. It is obvious that if there were several large
stations in the same neighborhood they could not all operate at the same
time unless some means of preventing the stations from receiving more
than one message at a time were possible. This is the object in view of
the so-called "tuning" of wireless telegraphy. It also accomplishes a
second purpose which is perhaps considered more important than the
first. The length of the aerial may be too great or too short for the
amount of energy and the length of the waves which it emits or receives.
When this is the case, the oscillations are quickly damped out and do
not generate very powerful waves or produce strong signals at the
receiving station and thus by properly adjusting the circuit all
undesirable messages may be cut out as well as the signaling range
greatly increased. Every electrical circuit has a definite period or
electrical length, determined by its inductance and capacity. A circuit
emits waves of only one length for given values of inductance and
capacity, and must also be of a certain length before it will respond to
waves sent out by another transmitter. The careful adjustment of a
circuit to emit or receive a given wave constitutes _tuning_.

[Illustration: Fig. 6. Resonance Tube.]

This may be made more clear by the comparison of an electrical circuit
with a column of air. Fig. 6 represents a cross section of a glass tube,
_T_, lying in a horizontal position and containing a cork, _C_, which
can be slid to various positions. By adjusting the cork we are able to
obtain various depths of air in the tube from its open end, _M_, to the
cork, _C_.

When a vibrating tuning fork, _F_, is held opposite the open mouth and
the cork slid back and forth it is found that the sound of the tuning
fork is greatly increased in volume at a certain position of the cork.
If the cork is then removed from this position the sound decreases in
intensity. When the cork is in such a position that the sound of the
fork is reenforced, we have secured resonance. When in this condition
and the prong of the vibrating fork is moving toward the open mouth of
the tube a "condensed" pulse of air travels down the tube and back
again, having been reflected at the cork and reaching _M_ just as the
prong of the fork begins its excursion away from the open mouth of the
tube. When the prong of the fork is moving away from _M_ a "rarefied"
pulse of air moves from _M_ to _C_ and back again by the time the prong
is ready to begin its next vibration. When the tube is not in resonance,
the successive condensations and rarefactions passing up and down the
air column interfere with one another and decrease instead of increase
the sound of the tuning fork.

If we substitute the sound waves emitted by the tuning fork for high
frequency oscillations and the air column for the electrical circuit we
may readily see that by adjusting its length, resonance can be produced.
If the length of the air column is measured it will be found that the
reenforcing of the sound of the fork reaches a maximum when the depth of
the air column is _one-fourth_ of the sound wave length given by the
fork. Likewise resonance is produced in wireless telegraphy when the
length of the circuit is _approximately one-fourth_ the length of the
waves. Vice versa, the wave emitted from an ordinary closed circuit
transmitter is _approximately four times_ the length of the aerial wire.
For example, an aerial 25 meters long will emit waves having a length in
the neighborhood of 100 meters.

As stated above, tuning is accomplished and resonance or syntony
established by varying the inductance and capacity of the circuit. The
capacity of a circuit may be defined as its relative ability to retain
an electrical charge, while inductance is the property of an electric
circuit by virtue of which lines of force are developed around it.

Capacity and inductance are opposite or reactive in their effects upon a
circuit. If the value of one is decreased the influence of the other in
increased. Fig. 7 and the following explanation will serve to illustrate
this.

[Illustration: Fig. 7. Lag and Lead.]

Alternating currents do not always keep step with the voltage impulses
of a circuit. If there is inductance in the circuit, the current will
lag behind the voltage, and if there is capacity, the impulses of the
current will lead. Fig. 7 _A_ illustrates the lag produced by inductance
and _B_ the lead produced by capacity. In _A_ the impulses of the
current, represented by the full line, occur a little _later_ than those
of the volts as represented by the dotted line. In _B_ the effect is
just the opposite and the current leads. These reactive effects of
inductance and capacity are very pronounced with the high frequency
currents of wireless telegraphy, and, as stated before, are the factors
which determine the period of the circuit.

[Illustration: Fig. 8. Tuned Hydraulic Transmitter and Receptor.]

Tuning is represented graphically in Fig. 8. The two floats _A_ and _B_
are not only resting on the surface of a pool of water as in Fig. 3 but
are also suspended from the springs _S_ and _S’_. The springs will have,
like a pendulum, a definite time of rising and falling, or period of
oscillation, depending upon their length. If we strike the float _A_ the
spring will cause the float to rise and fall at a definite rate and send
out a little wave or ripple with every oscillation. If the springs _S_
and _S’_ are of the same length, the float _B_ will be caused to
oscillate with every wave sent out by _A_, for, the periods of the
springs being equal, _B_ will be permitted to rise with a wave and fall
again just in time to be raised by the next oncoming ripple. On the
other hand, if the springs are of different lengths, _B_ may only rise
slightly and in falling meet an oncoming wave which will cause it to
rise before it has reached its lowest point and so dampen or weaken its
oscillations that they either do not become very strong or are entirely
obliterated. Thus several floats having different periods of oscillation
might be sending out ripples in the same pool, and the float _B_ could
be made to respond to any of them by adjusting the length of the spring.

We may also see in this illustration the part that tuning plays in
causing the apparatus to emit or receive more powerful impulses. When
the rope in the untuned apparatus illustrated in Fig. 3 is jerked, the
block _A_ oscillates only once or twice before a new jerk is required to
keep it in motion. In Fig. 8 it is quite the contrary, for when an
impulse has been given to the float _A_ it will oscillate much longer
than the untuned float before it requires to be set in motion again.
Likewise the float _B_ in Fig. 8 will oscillate longer and more
powerfully than the float _B_ in Fig. 3, when once it has been set in
motion.

[Illustration: Fig. 9. Tuned Wireless Telegraph Transmitter and
Receptor]

Fig. 9 shows a diagram of a simple wireless telegraph system employing
an inductance and capacity for tuning the circuits. When the induction
coil is in operation it charges a condenser. The condenser discharges
through the sending helix and across the spark gap. The sending helix is
merely a spiral coil of wire of large diameter, and constitutes the
greater part of the inductance in the circuit. Two movable contacts, _A_
and _B_, make connections with the helix. The spark gap, condenser and
lower portion of the helix up to the movable contact _A_ are known as
the closed circuit. By shifting _A_, more or less inductance may be
included in the closed circuit until resonance is secured. The aerial,
the inductance from the contact _B_ down, the condenser and the ground
compose the open circuit. By varying the contact _B_ more or less
inductance may be included in the open circuit and its period altered
until the oscillatory currents of both circuits flow in the same period
of time. The closed and open portions of the transmitting helix form an
auto transformer, and the voltages of the open circuit are raised above
those of the closed circuit.

The tuned receptor shown in Fig. 9 is the simplest form possible and is
known as the single slide system. The tuning coil or helix is much
longer in proportion to its diameter than the sending helix, and is made
of finer wire, since it does not carry such heavy currents. When the
contact is slid up or down on the tuning coil, the inductance of the
circuit is varied. Since the oscillating currents in the receiving
aerial have the same frequency as those in the radiating aerial, the
receptor must have the same relative values of inductance and capacity.
This condition is obtained by varying the slider until the signals in
the telephone receivers are the loudest.

In practice more than one sliding contact is used, and these together
with adjustable condensers make the circuit more complicated. These
devices are necessary because oscillations may be forced on a receptor
by a near-by transmitter unless other precautions than the "single
slider" are taken. Such circuits are illustrated in Plates IV and V.
With them it is possible to obtain a considerable degree of selectivity
and "tune out" an undesirable message.




CHAPTER II. THE APPARATUS.


It is generally the receiving apparatus which first attracts the
attention of the amateur operator, and so it will be considered first
here. An efficient receiving set consists of some form of _Detector,
Tuning Coil, Telephone Receivers_, and _Condenser_.

Other accessories such as adjustable condensers, potentiometer, battery
and testing buzzer improve the outfit and make it more complete.

The choice of the type of instruments must be left entirely to the
person who is constructing them. His resources will determine whether he
is to use 1,000 ohm telephone receivers built especially for wireless
work or ordinary ones having a resistance of only 75 ohms. It is
therefore best to read carefully the chapters devoted to the different
pieces of receiving apparatus and select the type of detector, tuning
coil, etc., which it is desirable to use before commencing the
construction of any.

For beginners, I would recommend an outfit consisting of a silicon
detector, a double slide tuning coil, a condenser of fixed capacity and
75-ohm telephone receivers. Such an outfit with a 50-foot aerial will
receive messages about 150 miles. If 1,000 ohm telephone receivers are
used, messages may be read up to 400 miles. Much depends upon the
location of the station and the ability of the operator.

A more elaborate and efficient set consists of an electrolytic or
"Perikon" detector, a transforming tuner, two adjustable condensers, a
potentiometer and a pair of 1,000 ohm telephone receivers. This outfit
and a 75-foot aerial could be made to receive 500 to 1,000 miles by a
careful operator.

Several cuts of wireless apparatus built for private installation are
shown both in this chapter and further through the book, to give an idea
of how the better instruments of this type are constructed and finished.

[Illustration: Fig. 10. Long-distance Receiving Set.]

Fig. 10 illustrates a selective receiving set built by the Long Distance
Wireless Company. The set is mounted on a mahogany base and the
instruments are finished in polished hard rubber and lacquered brass.
They comprise a detector stand so arranged that any of the sensitive
minerals used in wireless work may be used. The tuning coil is of the
double slide type. The condenser is sealed up in a square lacquered
brass tube fitted with hard rubber ends and binding posts. The
arrangement and construction of the outfit may be readily understood
from the cut.

[Illustration: Fig. 11. Murdock Receiving Set.]

Fig. 11 illustrates a receiving set manufactured by the Wm. J. Murdock
Company. The tuning coil is fitted with hard rubber composition ends and
is wound on a special core which is not affected by temperature changes.
The detector is of the crystal type and is of rather unique
construction, since the small fixed condenser is mounted in the base of
the detector itself.

The Clapp-Eastham set in Fig. 12 employs a receiving transformer which
makes great selectivity possible. The detector is mounted at the
left-hand side of the outfit. A very sensitive mineral called "Ferron"
is used in the detector. The fixed condenser is enclosed in a brass tube
fitted with hard rubber ends and is located on the front of the base,
directly in the center. A variable condenser of the rotary type is
placed at the right-hand corner.

[Illustration: Fig. 12. Clapp-Eastham Receiving Set.]

The Prague Electric Company manufacture the apparatus shown in Fig. 13.
The cabinet is mahogany and is fitted with a hard rubber cover. A fixed
condenser and a double slide tuning coil are mounted within the cabinet.
The sliders of the tuning coil project through two long slots in the
cabinet. A universal detector mounted on top of the cabinet is so
designed that any material may be experimented with or tested.

[Illustration: Fig. 13. Prague Receiving Set.]

Fig. 14 shows a receiving set built up from apparatus described in this
book. The cabinet is 12 x 16 inches and 4 inches deep. The wood should
be 1/2 inch thick, and in order to present a good appearance is
preferably of mahogany. As mahogany is sometimes very hard to procure
and expensive, some may find red birch an excellent substitute. When
stained with a mahogany stain it presents a fine appearance.

[Illustration: Fig. 14. Receiving Set.]

Varnishing and polishing are wasted time when applied to the average
amateur’s instruments in view of the rough handling and scratching which
they receive. The best plan is to stain the wood with an oil stain and
give it a wax finish. An oil stain contains no varnish but is merely
coloring matter and oil. A good coat should be applied with a wide brush
and the surplus stain immediately wiped off by rubbing the whole surface
with a piece of cheesecloth. As soon as the stain is thoroughly dry the
wood is waxed.

Cut up some beeswax into fine shreds and place it in a jar. Pour some
turpentine over the beeswax and let the mixture stand for five or six
hours, giving it an occasional stir. Allow it to stand further if
necessary until the wax melts and then add enough turpentine to give the
mixture a consistency similar to that of thick cream. Apply the
preparation to the wood with a rag, and then rub with a piece of clean
cheesecloth until the finish is hard and dry. Waxing produces a gloss
which is not so bright as a French polish but yet is more durable and
not so easily scratched or marred.

A double slide tuning coil made as described in Chapter XIV is mounted
on top of the cabinet in the rear. A "universal" detector is mounted in
the center, directly in front of the tuning coil. A tubular condenser of
fixed capacity is placed on the left-hand side of the detector, and a
potentiometer on the opposite side. A double point switch placed
directly in front of the detector enables the potentiometer to be
brought into play when a battery is used. Four binding posts are mounted
on the front of the cabinet. The ground and aerial are connected to the
left-hand pair and the telephone receivers to those on the right hand.
The wiring diagram is shown also in Fig. 14. By placing the switch _A_
on contact 1, the potentiometer is brought into use. When on contact 2,
the potentiometer is cut out. The switch _B_ must be opened when the
detector is not in use so as not to run down the battery.

[Illustration: Fig. 15. Receiving Set.]

Fig. 15 shows a receiving set somewhat similar to that shown in Fig. 14
but more elaborate and efficient. The cabinet in this case measures 14 x
22 inches and is 6 inches deep. A loosely coupled or transforming tuning
coil is used in place of the closely coupled double slide type. Two
variable condensers are mounted on either side of the tuning coil. The
detector and potentiometer occupy the space directly in front of the
tuning coil, while a fixed condenser is placed at their right. The two
switches for breaking the battery circuit and disconnecting the
potentiometer are in front of the detector. The aerial, ground and
telephone receiver leads are connected to binding posts mounted on the
front of the cabinet.

The batteries, in both cases, are placed inside the cabinet. The details
and construction of all the separate instruments will be found in the
respective chapters as denoted by the titles.

*Transmitting Range.*—A simple transmitting outfit capable of sending
about two miles consists of the necessary batteries, a one-inch spark
induction coil, a small zinc spark gap and a key. The connections of
such a transmitting outfit are shown in Fig. 4.

If the same coil is used with a transmitting helix and a condenser, the
range may be increased from 3 to 5 miles.

A 1 1/2-inch spark induction coil using a condenser and a transmitting
helix will send about 10 miles, and a 3-inch coil under the same
conditions about 20 miles. A 4 and a 6 inch coil will transmit about 30
and 40 miles respectively.

[Illustration: Fig. 16. Murdock Transmitting and Receiving Set.]

The one quarter kilowatt transformer with a helix and four two-quart
leyden jars or an equivalent condenser will transmit at least 50 miles
with a suitable aerial. Used as a one half kilowatt transformer with a
helix and eight two-quart leyden jars, it will send about 100 miles.

Of course, as in the case of the receiving outfits, these distances are
approximate and depend upon the location of the station, the nature of
the ground over which the messages are transmitted, the kind of receptor
used at the receiving station and the efficiency of the operator
himself.

Fig. 16 illustrates a complete Murdock transmitting and receiving set.
The transmitting outfit consists of a 15-watt induction coil giving
about a one-inch spark, a sending helix, oscillation condenser, a key
and a double pole double throw switch for changing the antenna and
ground from the transmitting to the receiving instruments or vice versa.
The spark gap is mounted on top of the coil. The receiving outfit is the
same as that illustrated in Fig. 11.

*A complete wireless station*—outside of the aerial consists primarily
of a source of electrical energy, a transformer or induction coil for
charging the oscillation condenser, an oscillation condenser, a
transmitting helix, a key for breaking the primary current, a spark gap,
an aerial switch, a hot wire ammeter for tuning the transmitting
circuits, a detector, a receiving tuner, auxiliary tuning apparatus such
as fixed and variable condensers, a potentiometer and battery, and a
pair of telephone receivers with a headband.

Other apparatus such as switches, insulators, anchor gaps, testing
buzzers, reactance coils, grounding switches, etc., have been described
in various places throughout the book and their use suggested whenever
it is of any advantage.

The choice of transmitting instruments, as with the receiving apparatus,
is left entirely with the experimenter so that he may suit his ideas and
means. Wherever possible the range and power of the instruments have
been given and suggestions made as to the other apparatus which should
be used in connection with them so that the completed outfit will bear
some sense of proportion.

[Illustration: Fig. 17. United Wireless Portable Outfit.]

Fig. 17 illustrates the portable wireless telegraph set manufactured by
the United Wireless Telegraph Company for army service and exploring
expeditions or isolated camps. The aerial and the mast can be unloaded,
erected, and all parts be ready for operation in fifteen minutes. The
mast is made of interchangeable wooden sections. The current for the
transmitter is furnished by a portable storage battery. The whole outfit
is capable of furnishing efficient service for distances of 25 to 30
miles.




CHAPTER III. AERIALS AND EARTH CONNECTIONS.


The aerial or antenna ordinarily consists of a number of wires elevated
in the air to emit or intercept the Hertzian waves. In fitting up a
wireless station the location and erection of an aerial are of prime
importance, and the successful reception and transmission of wireless
messages will depend largely upon its condition.

A few years ago the wireless antenna consisted of a metal plate high in
the air and having a wire suspended from it, but to-day usually exists
in one of the forms illustrated in Plate II.

The higher an aerial is placed above the surface of the earth, the wider
will be its electrostatic field, and consequently more powerful
electrical waves will be developed. But after a height of 180-200 feet
is attained, the engineering difficulties and the expenses increase so
rapidly that few stations exceed it. Other things being equal, the
increased range in transmitting varies as the square of the height of
the radiating wires. For example, a 25-foot aerial capable of
transmitting one mile theoretically will send waves 16 miles if made 100
feet high. The actual ratio is often greater, but much is dependent upon
the many meteorological conditions.

After the limit in a vertical direction has been reached, the only
remaining possibilities are to increase the surface and spread out
horizontally.

The flat top aerials are used on shipboard or wherever it is an
advantage to suspend the wires between two masts.

They are especially recommended for amateur use, since they need not be
so high as the other aerials, to be efficient. The flat top aerials are
_directive_, that is, they receive or radiate waves better in certain
directions. The bent or inverted L type is one of these and exhibits a
preference for waves coming from a direction opposite to that in which
its free end points. This directive action of an inverted L antenna may
be somewhat lessened if the leads are taken off at the center and it is
made a T aerial. This is the most common form of flat top aerial in use
on ships.

The inverted U type is not used extensively because the two opposite
leads or rat-tails make a centrally located operating room necessary.
The loop aerial is used by the United Wireless Company, in both their
ship and land stations. This type of aerial is well adapted to long
waves and close tuning.

The Lodge-Muirhead capacity aerial does not make use of a ground and is
rarely seen in this country. Lately the United States Signal Corps have
applied it to their balloons where an earth connection would be
impossible. The upper part of the balloon is covered with a network of
wires which serves as the upper aerial, and a second system of wires is
suspended below the balloon to take the place of the ground. By this
means they have had little difficulty in establishing successful
communication between the balloons and the earth.

The pyramid aerial is the type used by Marconi in long-distance
ultra-powerful stations, but is debarred from extensive installation on
account of the large cost of erection.

The cage and grid aerials are of the vertical type and are excellent
where a high support to elevate them can be secured. They are at present
used principally by the Massie and Stone Companies.

[Illustration: *Plate II. Aerial Systems.*]

The desirable feature of an aerial is a quantity known as its
electrostatic capacity and is measured by the charge required to raise
its potential one unit. An increase in capacity enables more energy to
be accumulated in the antenna, and consequently greater radiation
results. The capacity of an aerial may be increased by adding wires, but
must not be carried too far or the transmitting apparatus will not be
able to raise its potential sufficiently. Owing to an effect caused by
mutual induction between the wires, the lines of strain are not
distributed symmetrically, and the capacity will not vary directly but
rather approximately as the square root of the number of wires. In order
to decrease this action and use the surface most efficiently, the wires
should not be placed nearer than one-fiftieth of their length and
preferably farther apart.

The materials used for the insulation and suspension of an aerial must
be reliable, so that in event of bad weather the station will not lose
energy or be put out of working order because the aerial blew down.

Porcelain cleats or a string of porcelain insulating knobs make
inexpensive insulators. The standard insulator for wireless telegraph
work is the "Electrose" insulator. These are made of a molded
composition, and have iron rings set firmly in the ends so that they can
withstand a very heavy strain. Hard rubber is undesirable for an aerial
insulator because it becomes carbonized and covered with a conducting
layer.

[Illustration: Fig. 18. Electrose Insulators.]

High frequency currents permeate copper wire only about one
three-hundredth of an inch, and so, in order to increase the surface and
decrease the resistance, it is best to make the aerial of stranded wire.
A phosphor bronze wire for this purpose which is very flexible but still
does not sag or stretch, is composed of 7 strands of No. 20 B. S. gauge.
Such a wire 150 feet long suspended vertically and insulated from the
earth will have a capacity of from 0.0003 to 0.0004 of a microfarad.

The aerial must receive very particular attention if the station is one
kilowatt or over in power. In that case stranded wire is necessary. The
insulation of the aerial must be as thorough as possible, and proximity
to large conductors such as smokestacks, telephone lines, etc., avoided.
Rope stays and guys are advisable in order to prevent dissipation of
energy. If wire stays must be used they should be divided up at frequent
intervals by insulators.

Fig. 19 illustrates a guy insulator used by the United Wireless
Telegraph Company. It is made of two strips of well paraffined wood
separated by two porcelain knob insulators.

[Illustration: Fig. 19. Guy Insulator.]

Copper wire is the most desirable for an aerial. Iron wire must never be
used unless it is very heavily galvanized, and even then it is not to be
recommended. Aluminum wire is undesirable except for kite sustained
aerials. When used on an aerial and exposed to smoke and other fumes it
becomes quickly coated with a layer of oxide. All connections made in
aluminum wire must be soldered. This necessity may be better understood
when it is explained that electro-magnets on dynamos, etc., are
sometimes wound with bare aluminum wire and that the natural coating of
oxide on the wire is sufficient insulation to separate the turns.

[Illustration: Fig. 20. Insulating Tube.]

Where the aerial enters the building in which the instruments are
located it must be very carefully insulated. The simplest method is to
bore a hole through the wall and push a porcelain tube through it. The
rat-tail or leading-in wire is then passed through and the interstices
between it and the tube poured full of melted paraffin.

The best method is to bore a hole in the window pane and pass the wire
through a hard rubber insulating tube. Fig. 20 shows such a tube. It is
three-quarters of an inch in outside diameter and has an internal bore
of three-eighths of an inch. The tube is threaded throughout its entire
length. Two hard rubber flanges inch thick and 2 inches in diameter are
threaded to screw on the tube. The tube is inserted in the hole in the
window pane and the flanges screwed on either side. If a soft rubber
washer is placed between the hard rubber flanges and the pane there will
be less likelihood of cracking the glass. The leading-in wire is then
passed through the tube. The hole in the window pane may be bored by
using a copper tube having an external diameter equal to that of the
required hole. The tube is set in a brace and used like an ordinary bit,
but must be kept well smeared with emery and oil or else it will not
cut.

This method of leading in the rat-tail is the only one to be recommended
if the transmitter is one-quarter kilowatt or over in power.

[Illustration: Fig. 21. High-tension Cable and Insulator.]

The lead-in should be anchored just outside of the window so as to
relieve the glass pane and the tube from all strain. Pirelli cable or
the high-tension cable which is used for the secondary wiring of an
automobile is the best conductor to use for the aerial in the interior
of a building. The way to lead it over the ceiling is to support it on a
porcelain cleat similar to that shown in Fig. 21.

Many are under the erroneous impression that four times the length of
the aerial is the wave length which the station will emit. This is only
at the best a very rough approximation, for many undeterminable factors
such as the nature and location of surrounding objects, trees, etc., so
affect the capacity and inductance of the aerial that the wave length
must be determined empirically after the aerial is in operation.

The standard wave length of the United States Navy for ship
installations is 425 meters. An inverted L aerial, calculated before
erection to have a wave length as near as possible to this, has the
following dimensions: Four horizontal stranded phosphor bronze wires (7
strands No. 20 B. S.) each 160 feet long and spaced 5 feet apart, four
vertical wires 85 feet long and a 35-foot rat-tail.

It is always desirable that the wave length should be as long as
possible, for the waves will then travel farther and are not absorbed to
such an extent by trees, etc. The absorption due to trees is said to
vary as the fourth power of the frequency.

It is sometimes very convenient to calculate the strain on insulators or
masts caused by a horizontal antenna. This is easily found by the
following equation:

_P_ equals L² x W/8S

where _P_ is the required strain in lbs., _W_ the weight in lbs. per
foot of aerial, _L_ the length of the aerial and _S_ the sag of the wire
in feet.

When erecting an aerial, it is best to fasten a pulley at the top of the
supporting mast and hoist the aerial up after the pole is in position.
Then in case the wires become twisted or broken they may be lowered and
repaired without any difficulty.

*Erection of an Aerial.*—The average amateur aerial is generally from 40
to 60 feet high and supported at one end by a short pole placed on the
house and at the other end by a mast set in the ground or lashed to a
tree. Fig. 22 illustrates such an arrangement whereby a flat-top T
aerial is supported at one end by a short pole fastened to the house and
at the other end by a pole set in the ground.

[Illustration: Fig. 22. Flat-top T Aerial.]

The flat-top loop aerial is preferred by some amateurs and it is to be
recommended for receiving but is an inefficient radiator. When this type
of aerial is used the two leading-in wires should be connected to a
switch so that when the switch is closed they are connected. The aerial
may then be used as a straight-away aerial for transmitting, and by
opening the switch, as a loop aerial for receiving. This precaution is
advised when a loop aerial is to be used with a low-powered induction
coil as a transmitter, for otherwise there will be a loss of energy at
the anchor gap.

A large aerial is of no advantage when used with a small transformer or
induction coil because it cannot become properly charged.

To erect a flat-top T aerial, first select its location. If possible
take advantage of two trees and lash a short pole in the top of each, so
that the aerial may be raised up clear of the leaves. Another good plan
is to erect a pole at each end of the house. In any case, the distance
separating the poles must not be greater than three times the height
above ground or the directive action of the aerial will be very
pronounced. An aerial 50 to 60 feet high should have a length of from 80
to 100 feet. Stranded wire is no advantage for receiving, but must be
used if the transmitter is other than a small induction coil.

Secure two spruce sticks about 2 inches in diameter and 10 feet long.
Fasten an insulator 6 inches from each end of the spars and two more
each 3 J feet from the ends. This arrangement will separate each of the
four wires which compose the aerial by three feet.

The two spars are then laid on the ground at a distance apart equal to
the desired length of the aerial. Four wires, either stranded or No. 12
B. S. gauge copper, are cut to equal lengths and fastened to the
corresponding insulators. The middle of each wire is found and a long
copper wire soldered to it. These four wires constitute the rat-tail or
lead-in. They should be of the same length, and are not connected
together until they are about to enter the building. A short rope tied
to each end of the spars and fastened to the rope which passes over the
pulley on the top of the pole, serves as a bridle to prevent the aerial
from twisting. The aerial is then hoisted up but allowed to hang
slightly slack.

There is considerable difference of opinion among experts as to whether
or not the ends of the horizontal wires should be connected, and it is
impossible to say with good reason which method is better. However, when
erecting a flat top aerial, exercise every care to make all the wires of
exactly the same length.

An aerial of the size and type just described will send and receive the
following distances.

These distances are only approximate and will vary with the efficiency
of the operator and the location of the station.

When any of the transformers described later are used, the aerial should
be 80 to 100 feet high. In this case the last named receiving outfit
will have a range of from 500 to 1,000 miles.

*Ground Connections.*—The importance of a good earth or ground
connection can hardly be overestimated. Whenever possible commercial
stations are located on moist ground or near a body of water so that a
good ground may be secured by imbedding zinc or copper plates in the
earth or water. A ground on shipboard is easily secured by fastening a
conductor to one of the ship’s plates.

If the ground connection is poor, the natural period of the oscillation
circuit is made irregular and short, so that the currents are choked in
passing in and out of the earth. The result is an undesirable rise of
potential at the lower end of the aerial and often harmful sparking at
the ground connection. The transmitting and receiving ranges of a
station are very considerably reduced through a poor earth.

Ground connection can often be obtained in the country by immersing
metal plates in a well or a cistern. Where connection is made to a water
supply pipe some sort of a ground clamp should be used to insure a good
contact.

[Illustration: Fig. 23. Ground Clamp.]

An efficient earth for portable outfits may be quickly formed by
spreading a large area of wire netting over the ground.

Proper precautions for protection against lightning by grounding the
aerial outside of the building should be taken.

The wisest plan is to install a heavy single pole double throw switch
outside of the building where the rat-tail enters. The knife of the
switch should be connected to the aerial, one contact to the house lead
and the other to a heavy wire grounded on the outside of the building as
in Fig. 24. When the apparatus is not in use the aerial should be
grounded by throwing the switch on the grounded contact.

[Illustration: Fig. 24. Switch for Lightning Protection.]

The rulings of the National Board of Fire Underwriters governing this
class of work are appended below.

"1. Aerial conductors to be permanently and effectively grounded at all
times when the station is not in operation by a conductor not smaller
than No. 4 B. S. gauge copper wire, run in a direct line as possible to
water pipe on street side of said water pipe within the premises or to
some other equally satisfactory earth connection.

"2. Aerial conductors when grounded as above specified must be
effectually cut off from all apparatus within the building.

"3. Or the aerial to be permanently connected at all times to earth in
the manner specified above, through a short gap lightning arrester; said
arrester to have a gap of not over .015 of an inch between brass or
copper plates not less than 2 1/2 inches in length, parallel to the gap,
and 1 1/2 inches the other way, with a thickness of not less than
one-eighth of an inch, mounted on non-combustible, non-absorptive
insulating material of such dimensions as to give ample strength. Other
approved arresters of equally low resistance and equally substantial
construction may be used.

"4. In cases where the aerial is grounded as specified in paragraph 1,
the switch employed to join the aerial to the ground connection shall
not be smaller than a standard 100-ampere jack-knife switch.

"Notice of wiring done for these installations should be sent to the
Board, the same as for all other work."




CHAPTER IV. INDUCTION COILS.


Some means of charging the condenser which produces the oscillatory
discharge is necessary. An induction coil is the most practical for the
amateur.

The induction coil consists of a primary coil of wire wound around a
central iron core and surrounded by a secondary coil consisting of many
thousand turns of carefully insulated wire. The primary coil is
connected to a source of direct current which also includes an
interrupter to "make" and "break" the current in rapid succession. Every
"make" of the circuit and consequent magnetization of the core induces a
momentary inverse current in the secondary, and every "break" and
corresponding demagnetization a momentary direct current. Normally, the
induced currents would be equal, but by means of a condenser shunted
across the interrupter the circuit when "made" requires considerable
time for the current and magnetization of the core to reach a maximum
value, while when broken the demagnetization and current drop are nearly
instantaneous. The value of the induced electromotive force in a circuit
varies as the speed at which the magnetic lines of force cut the
circuit, and so the induced e.m.f. at "break" is thus rendered high
enough to leap across a gap in the shape of sparks.

The formulas connected with induction coils depend upon conditions which
are never met in actual practice and cannot be relied upon. To construct
a coil of a given size, it is necessary to use dimensions obtained
empirically. Therefore it is well for the amateur to stick closely to
lines and hints which are given here or which appear in some _up to
date_ book on induction coil building.

[Illustration: Fig. 25. Diagram of an Induction Coil.]

For a long time the induction coil was an expensive, inefficient
instrument, until wireless telegraphy demanded of it more rigid and
efficient design and construction. It was the aim of manufacturers to
produce the longest possible spark length with a minimum amount of
secondary wire. As a result of this demand, wireless coils are now made
with a core of large diameter and give heavier and thicker sparks. The
secondary in this case is short and uses wire of large cross section in
order to reduce the resistance and minimize the heating.

No one part of an induction coil may be developed to its maximum
efficiency without seriously influencing and lowering the efficiency of
the other parts. The following suggestions regarding the construction
are given that they may prove a useful guide to the amateur coil
builder. The parts will be considered in their natural order of
construction.

*Core.*—Some experimenters not quite familiar with the principles of
magnetism reason that if an induction coil were provided with a closed
core as the transformer, the efficiency of the coil would be materially
increased. But this is not so, for the magnetization and demagnetization
of the iron cannot take place rapidly enough in a closed core when an
interrupted direct current is employed in place of an alternating
current.

The core of an induction coil is therefore always straight. For the same
reason, it is never solid but is made up of a bundle of soft iron wires
in order that rapid changes in magnetism may take place. The wires are
always of as high a permeability¹ as possible so as to create a strong
magnetic field. Swedish or Russian iron of a good quality is the best,
as its hysteresis² losses are small. The smaller the diameter of the
wire the less will be the eddy current losses and heating, but the
permeability is also rendered less and the core will not be so
effective, as the amount of iron is thereby decreased and the oxidized
surface increased. No. 22 B. S. gauge wire is the best size for the
average core.

Wires of a good quality may be purchased already cut to various lengths.
To buy them in this form will save a great deal of the labor required in
building a core. If the wires are not quite straight they may be
straightened by rolling them, one at a time, between two boards. It is
best to reanneal the wires in the following manner. Place them in an
iron pipe and plug the ends of the pipe with clay. Then lay it in a coal
fire until the whole mass attains a red heat. The fire is then allowed
to die out gradually with the pipe and wires remaining in the ashes
until cool. When cool remove them from the pipe and rub each one with
emery paper until bright. After this cleaning, the wires are dipped in
hot water and dried. They are then dipped in a good quality of varnish
and allowed to dry again.

[Illustration: Fig. 26. Induction Coil Core.]

The varnish serves to interpose resistance to the eddy currents
generated in the core and renders the losses due to this cause much
less. A strong paper tube having an internal diameter equal to the
diameter of the finished core is made by rolling the paper on a form and
cementing with shellac. When perfectly dry. the tube is removed and the
wires tightly packed in it. The following table gives the core
dimensions for practical coils of different sizes.

*Primary Winding.*—The ratio of the number of primary turns of an
induction coil to the number of secondary turns bears no relation to the
ratio of the primary and the secondary currents. It has been found in
practice that two layers of wire wound tightly on the core constitute
the best primary. The primary should always be thoroughly shellacked or
covered with insulating varnish. Since there is almost no ventilation in
the primary the wire must be large enough to avoid all heating. A table
containing the various sizes of primary wires is given below.

In large coils, the inductance of the primary causes a "kick back" and
sparks are liable to pass between the adjacent turns. For this reason,
it is always well to use double cotton covered wire and to further
thoroughly insulate it by soaking the primary and core in a pan of
melted paraffin and allowing them to harden therein. Afterwards the pan
is slightly warmed to loosen the cake of paraffin and the excess of wax
removed by scraping with a blunt instrument so as not to injure the
wires. Paraffin contracts upon hardening, and the proper method to
impregnate a porous substance is to allow it to soak and become set in
it upon cooling.

A good method of reducing the "kick back" and also the size of the
condenser shunted across the interrupter is to form the primary of a
number of turns of smaller wire in parallel, the effect being to give a
conductivity equal to a single wire of large diameter and at the same
time to make a more compact winding of the primary on the core. This
method of winding is very desirable in large coils, as it reduces the
cross section of the primary and allows the secondary to be placed
nearer the core, where the magnetic field is the strongest.

The primary winding ought to occupy nearly the whole length of the core,
since there is no gain in carrying the end of the core very far beyond
the end of the primary, for most of the magnetic lines of force bend at
the end of the primary and return without passing through the extreme
ends of the core.

*Insulating Tube.*—The successful operation of an induction coil without
breaking down when under strain depends largely upon the insulating tube
which separates the primary and secondary. Hard rubber tubes are perhaps
the best. A tube may be easily built up of several layers of 1/2-inch
sheet hard rubber by steaming it so as to soften it and then wrapping it
around a form. The tube should fit the primary tightly and be about one
inch shorter than the core. After the tube is in place it is poured full
of beeswax and rosin in order to fill all interstices and prevent sparks
due to the condenser effect of the windings from jumping from the inside
of the tube to the primary.

*Secondary.*—A coil used as a wireless telegraph transmitter must have
wire of large cross section in its secondary so as to obtain a heavy
disruptive discharge. Numbers 34 and 32 B. S. are generally used for
small coils and numbers 30 and 28 B. S. for large coils. Silk covered
wire is the usual practice, but enameled wire is coming into use. Cotton
covered wire takes up too much space and has poorer insulating
qualities.

Enameled wire is insulated by a coating of cellulose acetate, which has
a dielectric strength of about twice that of cotton and takes up much
less room than silk. There is, then, with enameled wire a great saving
in space, and a greater number of turns may be placed on the secondary
without increasing its mean distance from the core. The following table
shows the comparative diameters of silk and enamel covered wires
suitable in size for use on the secondaries of induction coils.

In winding enameled wire it must be taken into consideration that the
insulation of enameled wire is rigid and has no give. Consequently, to
allow for expansion, enameled wire must be more loosely wound than fiber
or silk covered wire. The occasional insertion of a layer of paper in
winding will give room for expansion and at the same time not add
greatly to the diameter.

[Illustration: Fig. 27. Theoretical and practical form of secondary.]

The length of the secondary is generally not much more than one-half the
length of the core. Coils giving sparks up to 2 inches in length may be
wound in two sections or in layer windings, but the layer winding is not
recommended for coils giving sparks over one inch. It is best in a coil
of this kind to insert an occasional layer of paper. The paper should be
well shellacked or paraffined and be of a good grade of linen. It should
project about one-quarter of an inch from the ends of the secondary as
shown by the sectional drawing in Fig. 28.

This insertion of paper increases the insulation and renders the
liability of sparks jumping from layer to layer much less, as is the
case when the layers are very long.

[Illustration: Fig. 28. Layer Winding for Small Coils.]

The secondaries of large coils are made up of "pies" or "pancakes" from
one-eighth to three-eighths of an inch in thickness. The "pies" are
separated from each other by a triple thickness of blotting paper which
has been thoroughly dried and then soaked in melted paraffin. In cutting
the blotting paper, much labor may be saved if a metal template of the
required size is first cut from sheet brass and then laid on the
blotting paper, which is cut by scoring around the edge of the template
with a sharp knife.

[Illustration: Fig. 29. Section Winder.]

The "pies" are wound in a bobbin or form such as is shown in Fig. 29.

The disks or flanges are made of sheet brass and mounted on an arbor so
that the form may be placed in a lathe or some other contrivance for
revolving it. The core is beveled in order to facilitate the removal of
a completed "pie" from the winder. The flanges of the winder are clamped
against the core by two nuts placed on either side. The "pie" is removed
by unscrewing one of the nuts and removing one of the flanges.

In winding silk covered wire it is first passed through a mixture of
beeswax and rosin or a bath of melted paraffin. The excess of wax is
removed by passing the wire through a slit made in a pad of paper or by
rubbing against a piece of felt. Fig. 30 shows such a contrivance.

[Illustration: Fig. 30. Impregnator for Silk Covered Wire.]

The wire passes from the reel over an ordinary spool down into the pan
of paraffin, out of the paraffin, over another spool, and rubs against a
piece of felt to remove the surplus paraffin. The spools are mounted
with a screw and a washer so that they will turn without friction.

The wire is guided, when winding, by the fingers. If it is wrapped with
a piece of felt and held between the thumb and forefinger it will run
without friction and not cut the fingers. It is necessary that the wire
should be closely watched for kinks, etc. which would cause the wire to
break. Oftentimes the wire is broken but is held together by the
insulation. Therefore each "pie" should be tested for continuity when
completed. This is best accomplished by means of a galvanometer and
battery. All imperfect "pies" should be rejected, as one of them would
cause serious trouble if embodied in the coil. In soldering the
secondary wires, acid _must not be used_ as it soon corrodes the fine
wires. Rosin is the best flux for this purpose. When building a small
coil with a "layer" winding it is absolutely necessary that the wire
should be wound on in smooth even layers. In a built-up secondary having
"pies" not greater than 1/4 inch in thickness such great care is not
necessary.

[Illustration: Fig. 31. Methods of Connecting the Secondary Sections.]

Fig. 31 shows the methods of connecting up the pies or pancakes. In _A_,
the inside of one section is connected to the outside of the next, and
so on. The maximum voltage which can exist between the adjacent sections
in this case is equal to the e.m.f. generated by one "pie" and is equal
throughout. In _B_, the coils are connected alternately inside and out.
The voltage ranges from zero at the points where they are connected, to
a value equal to twice the e.m.f. developed by one section. It would
seem that there would be a saving in insulation space of one-half in the
first case, but it is not so since the connecting wire passes between
the "pies" and therefore the insulation must be twice as thick or
exactly equal to that in the second case. The latter method (Fig. 31 B)
is the best and most convenient. When the "pies" are connected in this
manner the current must flow through alternate sections in opposite
directions. To accomplish this it is not necessary to wind every
alternate coil in an opposite direction, but merely to turn them around
and connect them with the direction of their windings reversed as shown
by the arrows and the bevels in Fig. 31. The connections between the
sections must be very carefully soldered.

After the secondary is assembled the coil should be placed in a tight
receptacle or tank containing melted paraffin. The tank is then
connected to an air pump or aspirator and the air exhausted. The
diminution of pressure causes any air bubbles in the windings to expand
and be pumped out. After standing a while, the pressure of the
atmosphere is readmitted and the place of the bubbles will be occupied
by paraffin which has been forced in under pressure.

*Mounting.*—A coil for wireless work is best mounted as shown in Fig. 32
and used with an independent interrupter. The coil may then be placed
under the operating table or on the wall, out of harm’s way, and the
interrupter on the table, where it is handy to the adjustment of the
operator.

[Illustration: Fig. 32. Coil Case.]

The case is simply a rectangular hardwood box large enough to contain
the completed coil. Two binding posts mounted on the side of the box
connect with the primary winding and two on the top of the box lead to
the secondary terminals. The box is filled with boiled oil or melted
paraffin and sealed up by screwing on the lid. If desirable, the
secondary binding posts may be mounted on the top of a short piece of
hard rubber rod as illustrated in the drawing.

    ¹ Magnetic permeability is the conducting power for lines of
      magnetic force.

    ² See _hysteresis_ under Magnetic Detector.




CHAPTER V. INTERRUPTERS.


We now come to what is the greatest source of trouble and annoyance in
an induction coil, namely the interrupter. Too much importance cannot be
attached to this instrument, for upon it depends largely the
satisfactory working of the coil. The operation of an induction coil and
the part played by the interrupter were fully explained in the chapter
on induction coils.

An adjustable interrupter is necessary for large coils, that is, one not
only whose speed may be governed, but also the time and duration of the
break.

[Illustration: Fig. 33. Simple Interrupter.]

The rapidity of oscillation of a mechanical interrupter is a very
different thing from the speed of break. The ideal speed of break is
instantaneous. In wireless telegraphy, very faint signals are heard more
distinctly in telephone receivers if the rate of interruption at the
station sending them is high. The human ear is somewhat more sensitive
to sounds higher than those ordinarily produced in the telephone
receivers of a wireless receptor. This seems to argue the use of a
high-speed interrupter to make and break the current. But the effect on
the coil must also be considered.

In the first place, where a condenser is shunted across the terminals of
the secondary as is the case with a wireless transmitter, a high-speed
interrupter would be very likely to set up harmful oscillations in the
secondary of the coil itself.

Second, if too fast, the rise and fall of the secondary currents will be
caused to run into each other, since the break will occur before the
primary current has reached a maximum and the reverse secondary current
has died away.

Third, the diameter of the core of a wireless coil is generally much
larger than that of the ordinary coil, and if a very rapid interrupter
is employed there is not time enough to properly magnetize the core
before the current is broken.

Fourth, the strength of the losses in the core caused by the eddy
currents and hysteresis are proportional to the interruptions in the
primary circuit and therefore a low speed will be the most efficient. A
rapid interrupter requires a higher voltage and amperage than the same
interrupter run at a lower speed.

These are some of the reasons why it is very desirable to use an atomic
interrupter or one so adjustable that the rate of the time and duration
of the "make" and "break" may be closely regulated. An ideal interrupter
is designed to give the longest time possible after contact is
established and before the "break" occurs.

It does not pay to construct an interrupter for an induction coil giving
sparks up to 2 inches in length. The type of interrupter in use on
automobile coils is perfectly well adapted to small coils, and may be
purchased complete with the platinum points for as low a price as $1.50.

The mechanical break described below is designed so that various
adjustments are possible and it may be adapted to almost any coil. Since
it is independent, it need not be mounted directly on the coil, but may
be placed in the position most convenient to the operator for
adjustment. The interrupter will not operate coils well on an
electromotive force above 30 volts, for the excessive voltage causes a
spark at the contacts when the circuit is broken and prolongs the
decadence of the primary current.

[Illustration: Fig. 34. Independent Interrupter.]

*Independent Atomic Interrupter.*—Fig. 34 illustrates two views of the
interrupter. Current is furnished to the electromagnets by a six volt
battery independent of the source supplying the coil. The interrupter is
set in operation by closing the circuit breaker on the aerial switch.
When the primary circuit of the transmitter is then completed by
pressing the key, the coil will respond immediately because the
interrupter is already in vibration.

The electromagnets (Fig. 35) are a pair of four ohm telegraph sounder
magnets. A hole is bored in the center of the top of each magnet core
and threaded with an 8-32 tap so that the pole pieces may be fastened
thereto, The shape and dimensions of these projections, which must be
made of soft iron, are illustrated in Fig. 35.

[Illustration: Fig. 35. Details of Magnets.]

A soft iron yoke _Y_, 2 1/2 x 7/8 x 1/4 inches, connects the bottom of
the magnets and supports them in an upright position. An 8-32 machine
screw passing upward through the base and yoke holds them firmly. The
base is preferably of hard rubber 4 x 3 1/4 x 3/4 inches.

[Illustration: Fig. 36. Details of Moving Parts.]

The moving parts are illustrated in Fig. 36. The main spring, _D_, is a
strip of spring steel, 2 1/8 inches long, 1/2 inch wide, and 1/32 inch
thick. The soft iron armature, _A_, is fastened to the spring by means
of two small 4-36 machine screws. _M_ is a piece of brass rod, 1 1/2
inches long, bent in the form of a hook and threaded with a 4-36 die to
screw in a similarly threaded hole in the back of the armature _A_. The
hooked portion of _M_ is fitted with a small piece of hard rubber rod,
_R_, to insulate it where it comes into contact with the spring, _G_.
The spring, _D_, carries a second hook, _E_, riveted to the center of
the spring 1 5/8 inches from the lower end. The hook, which is about 3/8
inch long, passes through a hole in the top of the spring, _F_, and
engages it so that it is set in operation by the vibratory motion of the
spring, _D_. The spring, _F_, is 1 3/4 inches long, 5/16 inch wide, and
ir 1/64 inch thick. It carries a platinum rivet 3/4 inch from its lower
end. The spring, _G_, is 2 1/2 inches long, 5/16 inch wide and 1/64 inch
thick. A heavy platinum rivet is fastened 2 1/4 inches from the lower
end. An elongated hole, 1/4 inch long and 3/16 inch wide, permits the
hook, _M_, to pass through the opening. A 5/32 inch hole, 1 1/4 inches
from the bottom, allows the adjusting screw to pass through and make
contact with the platinum rivet on the spring, _F_.

Two rectangular pieces of brass, _O_, 1 1/4 x 1/2 x 5/16 inches are
fastened to the base to support the springs.

[Illustration: Fig. 37. Details of Standard and Screws.]

The standard, _U_, supporting the adjusting thumbscrews is a piece of
3/8-inch brass, 2 1/2 inches high. It tapers from 1 1/2 inches at the
bottom to 3/4 inch at the top. A hole 2 1/4 inches from the bottom is
threaded with a 10-32 tap to receive the thumbscrew, _B_. A second hole
1 1/4 inches from the base is threaded with an 8-32 tap to fit the
adjusting screw, _S_.

Both of the adjusting thumbscrews carry heavy platinum points. The
standard is held upright to the base by means of two machine screws
passing through the base.

A 3/32 inch brass rod 1 3/4 inches long is threaded to fit a hole in the
top of the armature. A sliding weight, _W_, may be clamped in any
position on the rod by means of a thumb-screw. Raising or lowering the
position of the weight decreases or increases the natural period of
vibration of the interrupter. Screwing the hook, _M_, in or out so as to
shorten or lengthen it, decreases or increases the ratio of the make to
the break.

[Illustration: Fig. 38. Diagram of Connections for an Independent
Interrupter.]

Fig. 38 shows a diagram of the connections of the interrupter. The
standard, the thumbscrew, _B_, and the spring, _G_, form part of the
primary circuit of the induction coil. The standard, the thumbscrew,
_S_, the spring, _F_, and the electromagnets are placed in series with a
six-volt battery and connected to the circuit breaker on the aerial
switch, so that when the switch is thrown in position for transmitting,
the interrupter will be set in operation.

A condenser must be shunted across the larger contacts of the
interrupter in order to hasten the demagnetization of the core of the
induction coil and create a higher e.m.f. in the secondary. The
condenser must be suited in size to the induction coil with which the
interrupter is to be used and so the following table is appended to
serve as a guide.

The condensers are built up of alternate sheets of tin foil and
paraffined paper. Connections are made to the sheets by means of tin
foil strips which project out alternately from opposite sides as in the
illustration.

The paper should be about two inches larger each way so as to leave a
one inch margin on all sides of the tin foil.

[Illustration: Fig. 39. Construction of a Paper Condenser.]

When the alternate sheets of tin foil and paper have all been assembled,
the condenser is warmed so as to soften the paraffin. It is then placed
between two flat boards and subjected to great pressure in a letter
press or a vice. The capacity of a pressed condenser is often several
times that of a condenser of the same dimensions but not pressed.

*Mercury Interrupters.*—The mercury turbine interrupter is one of the
most convenient and successful breaks in use. The construction is such
that a stream of mercury is made to play against a number of saw shaped
metal teeth. A spiral worm terminating in a nozzle at the top is rapidly
revolved by an electric motor. The lower end of the tubular worm dips in
a mercury reservoir, so that when the spiral is revolved the mercury is
caused by centrifugal action to rise in the tube and be thrown out in
the form of a jet at the upper end. When the revolving jet strikes one
of the metal teeth, the circuit is closed and the current flows from the
mercury jet into the teeth. When the mercury jet passes between the
openings between the teeth, the circuit is interrupted. By raising and
lowering the saw teeth so that the mercury strikes either the lower or
upper part of them, the ratio between the make and break may be made
smaller or larger. By regulating the speed of the motor driving the jet,
the number of interruptions may be varied from 10 to 10,000 per second.
The bottom and sides of the mercury reservoir are ribbed to prevent the
mercury from attaining a rotary motion.

A somewhat simpler and more easily constructed type of mercury
interrupter consists of a hard rubber disk having a brass rod running
through from the periphery to the center, where it connects with the
shaft. The lower edge of the disk dips at an angle in a mercury bath and
is rapidly revolved by an electric motor. When the rod is under the
surface of the mercury, the circuit is made through the mercury to the
rod. The circuit is broken when the rod is above the surface. The
mercury is covered with a layer of alcohol, which prevents excessive
sparking and makes a quicker break. An interrupter of this kind when run
by a motor of the magnetic attraction type is exceedingly simple.

The break of any of the mercury type interrupters when properly adjusted
is much quicker than the hammer spring break and gives thicker sparks.

After the mercury has been in use awhile it becomes churned up into
small globules of a black color, but may be easily cleaned and restored
for use by shaking up with some strong sulphuric acid. Care must be
taken that the mercury is perfectly dry and free from acid before
replacing in the interrupter.

*Electrolytic Interrupters.*—Fig. 40 shows a diagram of a Wehnelt
interrupter. The cathode or negative electrode is a lead plate immersed
in dilute sulphuric acid. The anode is a piece of platinum wire placed
in a porcelain tube and projecting through a small hole in the bottom,
so that only a very small surface of the wire is exposed to the liquid.
When a strong electrical current is passed through the acid electrolyte,
the current is very rapidly interrupted by the formation of gases on the
small platinum electrode. The speed of the interrupter is variable
through great ranges by moving the platinum electrode up or down and
changing the amount of surface exposed to the liquid. The only
disadvantage of this interrupter is that the electrolyte soon becomes
heated, and unless the interrupter is provided with a water jacket or
some device for cooling, the bubbles of gas do not form freely. A
potential of at least 40 volts is required to operate a Wehnelt or other
electrolytic break.

[Illustration: Fig. 40. Wenhelt and Simon Electrolytic Interrupters.]

A Wehnelt interrupter may easily be made by sealing a platinum wire in a
glass tube. It is well to make several such tubes with the platinum
projecting from one-sixteenth to one-quarter of an inch. The different
tubes will each have a different speed of interruption, and one should
be picked out which seems to be most suitable for the coil upon testing.
Connection to the platinum wire is established by filling the tube with
mercury and dipping a wire in it.

Fig. 40 also shows a diagram of a Simon electrolytic interrupter. It
consists of a vessel containing dilute sulphuric acid and divided into
two parts by a thin porcelain diaphragm having a small hole in the
center. A lead electrode dips into each of the divisions. The
interruption is caused by exceeding a certain current density in the
small hole at the diaphragm. Upon the passage of the current the liquid
is so heated that it becomes vaporized. The vapor is a poor conductor to
low voltages and so the current is broken. Immediately upon the
cessation of the current, the vapor condenses and the circuit is
established again. This cycle repeats itself with a speed depending upon
the size of the aperture and the amount of current flowing.

A crude form of this type of interrupter may be made by heating the end
of a test tube in a pin flame, and then blowing on the open end of the
tube so as to burst the soft glass and form a small hole. Several such
test tubes should be prepared having holes varying from 1/32 to 1/8 of
an inch in diameter. The one which gives the best results upon trial is
selected for use. A number of holes in a single tube, if not too many or
too large in diameter, increases the efficiency and the speed of
interruption. The tube should be immersed in a glass jar containing
dilute sulphuric acid. One lead electrode is placed inside of the test
tube and the other outside. It makes no difference which way the current
flows through this interrupter.

The Caldwell interrupter is a modification of the Simon type in which
the size of the aperture is made adjustable by means of a pointed glass
rod which may be raised or lowered in the hole and the speed of
interruption varied. An interrupter of the test tube type as described
above may be modified to this form by locating the hole directly in the
center of the bottom of the tube and inserting in it a hard glass rod
which has been drawn out to a point.

Electrolytic interrupters do not require any condenser connected across
the break.

[Illustration: Fig. 41. Construction of Electrolytic Interrupters.]

Fig. 41 shows in section more substantial forms of both the Wehnelt and
Simon-Caldwell interrupters. The containers are ordinary 5 x 7 inch
battery jars. They are fitted with covers made of two thicknesses, _C_
and _B_, of 3/4-inch wood. The upper piece, _C_, is 6 inches in
diameter, while the under one should fit snugly into the interior of the
jar. The wood must be boiled in paraffin to protect it from the action
of the acid. A slit is made in the left-hand side of both covers for the
passage of a lead electrode, _L_, 1 inch wide and 1/4 inch thick. The
upper end of the electrode is bent over and fitted with a binding post.

[Illustration: Fig. 42. Details of Electrolytic Interrupters.]

The mechanism for adjusting the interrupters is the same in both cases.
The dimensions are indicated in Fig. 42. A brass yoke, _Y_, is mounted
on the cover in the position shown. A 1/2-inch hole is bored through the
upper part of the yoke and a piece of brass tubing, _S_, 1 inch long
soldered in a vertical position in the hole. A 1/4-inch threaded brass
rod passes through the tube, 5. A groove is milled in A along its entire
length and engages a pin in the wall of _S_. The rod is thus enabled to
slide up and down in the tube but is prevented from revolving. A fiber
head, _H_, is fitted with a brass sleeve or bushing in its center. The
bushing should fit tightly into the fiber head and is threaded to fit
the rod, _A_. The electrode may then be carefully raised or lowered by
revolving the head. The tube, _V_, for the Simon-Caldwell interrupter is
a hard glass test tube. A 1/8-inch hole is blown in the bottom of the
tube. A hard glass rod, _G_, is drawn out to a point and fastened to the
lower end of the rod, _A_, by means of a short length of flexible rubber
tubing, _R_.

The tube, N, for the Wehnelt break is made from a piece of hard rubber
tubing 6 inches long, having a bore of J inch.

[Illustration: Fig. 43. Electrolytic Interrupter.]

The lower end is fitted with a spark plug porcelain. The porcelain must
fit the tube tightly and not leak. The electrode, _P_, is a piece of
brass wire which will just pass through the hole in the porcelain. The
upper end of the electrode, _P_, is soldered or fastened otherwise to
the lower end of the rod, _A_.

A small hole, _h_, should be made in the tubes, _N_ and _V_, above the
level of the electrolyte in the jar. When the interrupter is in
operation the electrolyte gradually rises in the tubes, and would
corrode the lower end of _A_ if it were not able to pass out through the
vents.

In the Simon-Caldwell interrupter, a strip of lead passes from the
binding post mounted on the foot of the yoke down inside of the test
tube. The size of the hole in the tube is regulated by revolving the
fiber head so that the glass pointed rod will be inserted in or
withdrawn from the hole.

The frequency of the interruption will also depend somewhat on the
concentration of the acid solution. It is therefore best to start with a
weak solution and add acid slowly until it is of the proper strength.




CHAPTER VI. TRANSFORMERS.


Where alternating current is available in commercial wireless telegraph
stations, the induction coil has been gradually superseded by the more
modern transformer for charging the oscillation condenser. Since the
transformer is less expensive to construct than an induction coil
capable of transmitting the same distance, it is favored by many amateur
experimenters. A one quarter kilowatt transformer has a sending range of
over 50 miles when used with a properly constructed aerial about 80 feet
high. This is probably the size best suited to the average private
installation.

The secondary of a wireless transformer seldom develops potentials
exceeding 15,000 to 20,000 volts, while those of an induction coil range
from 1 5,000 to 300,000 volts. However, the strength of the secondary
current of a transformer is so much greater than that of an induction
coil, that more powerful and penetrating waves are developed. For these
reasons a transformer is always rated by its output in watts or
kilowatts rather than by the spark length produced at the secondary
terminals. The spark of a one quarter kilowatt transformer is only 0.25
to 0.50 of an inch, while the spark of a one half kilowatt transformer
may be the same length but still represent more energy.

There are two distinct types of transformers in use, known as the "open"
and "closed," accordingly whether the core is straight like the core of
an induction coil or in the form of a hollow rectangle. The open core
type is used in the government stations and by the United Wireless
Telegraph Co. It is the simpler and more easily constructed of the two,
but is also less efficient and requires that more material be expended
to bring it up to a definite rating. In principle it is simply an
induction coil operated on alternating current minus the interrupter and
condenser. In view of the greater currents employed, the windings must
be larger and heavier than those of the induction coil to prevent
heating.

Before commencing the construction of a transformer one should read the
chapter on induction coils and use the same care emphasized there in
regard to building coils.

*Open Core Transformer.*—The transformer described below will transmit
from 20 to 75 miles and consume about 300 watts on the no volt 60 cycle
alternating current.

The core is 16 inches long and 2 inches in diameter. It is built up of
soft iron wires in the same manner as if it were the core of an
induction coil.

The primary is composed of two layers of No. 14 double cotton covered B.
S. gauge magnet wire. The primary is 14 inches long and is wrapped with
a layer of micanite cloth 3/8 inch thick to separate it from the
secondary winding.

It is never advisable to use shellacked cotton cloth as insulation. When
cotton is dried and shellacked, it is at first a good insulator, but
soon becomes capable of absorbing moisture. Shellac carbonizes at a low
temperature, and if a transformer or coil having any of this sort of
insulation entering into its construction is overheated, the insulation
is liable to become a conductor. Micanite cloth is the best insulation
for transformers. The dielectric strengths of the different forms of
micanite are shown by the following table.

The secondary is wound in ten sections over the micanite insulation.
Each section is 6 inches in diameter and 1 1/4 inches thick and is wound
with No. 30 B. S. single silk covered wire. The sections are separated
by disks of blotting paper 1/8 inch thick and 7 inches in diameter,
treated as described in Chapter IV. The completed transformer should be
placed in a box and covered with oil.

*A One Quarter Kilowatt Closed Core Transformer.* The simplest form of a
closed core transformer consists of two independent coils of insulated
wire wound upon an iron ring. When an alternating current is passed
through one of the coils, known as the primary, it generates a magnetic
flux in the iron core. The lines of force passing through the core
induce in the secondary coil an electromotive force the magnitude of
which is in nearly the same ratio to the primary inducing electromotive
force as the number of turns of wire in the secondary is to the number
of turns in the primary. For example, if it is desired to raise the
potential of the no volt alternating current to 22,000 volts, the number
of turns in the secondary of the transformer must be at least 200 times
the number in the primary.

A circular ring of iron wire presents several theoretical advantages as
a transformer core but it would be difficult to form and afterwards
place the windings in position. The core is therefore usually in the
form of a hollow rectangle, built up of very thin sheets or laminations
of soft iron carefully insulated from one another by a coat of varnish.
If the core were solid or the separate laminations not insulated from
one another, heavy currents, known as eddy currents, would be set up in
the iron and cause heating. A considerable loss in the efficiency of the
transformer would also result.

One half of both the secondary and the primary windings of a properly
designed transformer are placed on opposite sides or "legs" of the core
in order to reduce the magnetic leakage and increase the efficiency. The
only difficulty involved in such construction is the proper insulation
of the primary from the secondary, but if careful attention is given to
this point no difficulty will be experienced.

*Core.*—The dimensions and method of assembling the core laminations are
indicated by Fig. 44. Long strips are cut from soft Russian or Swedish
stovepipe iron. The strips, which are 1 3/4 inches wide are then cut up
into short lengths, one half of which are 7 3/4 inches long and the
other half 4 3/4 inches. Enough are cut to form two piles of each size 1
3/4 inches high when compressed. The completed core will then form a
hollow rectangle 9 1/2 x 6 1/2 x 1 3/4 inches.

[Illustration: Fig. 44. Assembly and Dimensions of Core.]

The strips must be dipped in some good insulating varnish such as P. &
B. compound and thoroughly dried before they are assembled. Both "legs"
(the longest sides) are laid on a table with the alternate ends
overlapping as shown by _A_ and _B_ in Fig. 44. After the short pieces
_C_ and _D_ have been slipped between the overlapping ends the whole
core is squared up. The strips _D_ are then carefully removed and one
end of the core thus left open until all the windings are in place.
Three or four layers of well varnished linen cloth are wound tightly
over the "legs" preparatory to winding on the primary.

*Primary.*—Four fiber heads, _H_, 4 3/4 inches square, 1/2 inch thick
and having a square hole 1 7/8 x 1 7/8 inches cut in the center are
required. One of the fiber heads is placed on each end of the assembled
"leg" as shown in Fig. 48.

[Illustration: Fig. 45. Fiber Head and Separator.]

The primary winding is wound in six layers, 4 1/2 inches long, three
layers on each "leg." About three pounds of No. 16 B. S. gauge double
cotton covered magnet wire are required for the winding. The terminals
of the two halves of the primary are led through the fiber heads at the
same end of the transformer. The windings are not to be carried close up
to the fiber heads but begin and end about 1/4 inch from them, so that
the remaining space may be filled by winding in a strip of micanite
cloth 1/4 inch wide. The primary and secondary windings are separated by
a strip of micanite cloth 5 inches wide, wound over both of the primary
windings close up to the heads until a layer 1/2 inch thick is formed.

*Secondary.*—The form on which the secondary sections are wound is
illustrated in Fig. 46. All the parts are cut out of wood except the
shaft and are made of the dimensions indicated. If the center of the
form is slightly tapered it will greatly facilitate the removal of the
completed sections. Sixteen sections are required. When removed from the
winder they will be in the form of hollow squares 4 1/2 x 4 1/2 x 7/16
inches.

[Illustration: Fig. 46. Section Form.]

About ten pounds of No. 34 B. S. gauge silk covered wire are required to
wind the sections. Cotton covered wire must not be used to avoid
expense, because with it a sufficient number of secondary turns cannot
be secured to bring the secondary current up to the proper voltage. By
observing explicitly the instructions and precautions given below no
trouble will be experienced in handling enameled wire and forming the
sections. The form should be placed in a lathe chuck or some other
machine which is convenient and whereby the form may be rapidly revolved
under the control of the operator.

Saw slots are cut in the wooden flanges and the center of the form as
shown in the illustration so that silk threads may be passed under and
around the completed section and tied so that a possible "cave in" of
the wire is prevented. After tying up the section should be removed from
the form by unscrewing the nut and taking off the flange.

[Illustration: Fig. 47. Methods of Connecting Sections.]

When winding the wire it must be very carefully watched for loops or
kinks and only be laid on in even layers. It must also be borne in mind
that enameled wire cannot be as tightly wound as fiber covered wire for
reasons heretofore explained. In case the wire becomes broken, it must
be smoothly spliced and soldered. Do not under any consideration use
acid as a flux or heat the wire with a flame. Acid will corrode the fine
wire, and the flame will badly oxidize or melt it. Use a short piece of
No. 8 B. S. gauge tinned copper wire set in a small file handle as a
soldering iron, and rosin as a flux. Paraffin some silk binding tape
such as dressmakers use and wrap the joint with a small piece. The
sections as they are removed from the winder must be taped and then
carefully marked with an arrow which points in the direction of the
winding.

Fig. 47 illustrates the two methods of connecting up the sections. It
will be noticed in the second method that the arrow denoting the
direction of winding points down on every alternate section. This does
not indicate necessarily that every alternate section is wound in an
opposite direction from the other, but that they have merely been turned
around so that the arrows come on a reverse side of the core and point
in an opposite direction. This precaution must be taken in order that
the current will flow through all the sections, and is made necessary
because the inside terminal of one section is connected to the inside
terminal of the adjacent section and the outside terminal of that
section is connected to the outside terminal of the next adjacent
section. The first method, _A_, illustrated in Fig. 44, is less
complicated and does not require this reversal, but for various reasons
is not to be recommended in place of _B_.

Eight of the completed and taped sections are placed on each "leg" of
the transformer, with one of the fiber separators between each pair as
in Figs. 48 and 49. When each "leg" has been completely assembled,
solder all the secondary terminals together so as to connect as in Fig.
47.

[Illustration: Fig. 48. Assembly of Leg.]

Then place the remaining fiber head, _S_, on each of the "legs" and
finish assembling the core by slipping in the end strips _D_.

Square the core up perfectly true and fasten by four fiber strips _M_,
Fig. 49, 9 3/4 inches long, 1 3/4 inches wide and 1/2-inch thick. The
strips are placed in the position shown in Fig. 46 and a hole _P_ bored
in the end of each. Four 1/4-inch bolts, two of which are 3 inches long
and two 3 1/2 inches, pass through the holes in the strips, so that when
the nuts are screwed on the core is clamped firmly. The two longer bolts
are placed at the same end of the transformer.

[Illustration: Fig. 49. Transformer with One Secondary removed.]

The terminals of the primary lead out to four binding posts mounted on
the fiber strips. The pillars which support the secondary binding posts
are fiber rods, 1 inch in diameter and 2 inches long. The lower end of
each is bored and tapped to fit the upper ends of the longer bolts which
clamp the fiber strips together. An insulating shield must be placed
between the two secondary windings to prevent sparks from jumping
across. A piece of fiber 5 x 5 x 1/8 inches will serve nicely for this
purpose. If the primary windings are placed in series the transformer
will consume about 300 watts. When the transformer is placed in a box
and the box filled with some boiled amber petroleum, the windings may be
connected in parallel and the transformer will consume about 500 watts.
It will then transmit over 100 miles providing the aerial is at least
100 feet high.

The wiring connections are diagramed in Fig. 50. A variable inductance
or reactance coil is connected in series with the primary circuit to
steady the current, as explained in the paragraph under reactance. A
reactance suitable for this transformer may be constructed by winding
two layers of No. 12 B. S. gauge cotton covered wire, six inches long,
around a hollow wooden tube made of cigar box wood. The core is built up
of soft sheet iron to form a rectangle 8 x 1 3/4 x 1 3/4 inches which
will just slide in and out of the tube. The windings should be about six
inches long.

[Illustration: Fig. 50. Wiring Diagram.]

Five half-gallon Leyden jars form about the right capacity for the
secondary of the transformer when the windings are in series. Twice that
number must be used when the windings are in parallel.

The secondary sections must always be kept in series, as otherwise the
voltage would not be high enough to properly charge the condenser.



Construction of a 2-K.W. Transformer.


*Core.*—Strips 2 1/2 inches wide are cut from soft sheet iron. One half
of them should be 11 1/2 inches long and the other half 6 1/2 inches.
Enough are cut to make two piles of each size 2 1/2 inches high. Both
sides (the longest strips) of the core are built up with the ends
overlapping as in Fig. 41.

The ends (the short pieces) are then slipped between the overlapping
ends of the long strips and the whole core squared up. The completed
core should have a cross section of 2 1/2 inches square and form a
hollow rectangle 8 x 14 inches. The strips of iron must be dipped in P.
& B. insulating varnish and dried before they are assembled.

*Primary.*—Four fiber heads 6 inches square, 1/2 inch thick and having a
hole 2 1/2 inches square are made. One of these is placed on the core
legs as shown by Fig. 48. Two or three layers of well varnished linen
are wound over the core preparatory to winding the primary. Room must be
left for the other head to be placed on the opposite end after the
windings are all in place. The primary consists of 100 double turns of
No. 12 B. S. gauge double cotton covered magnet wire. Fifty of the
double turns are wound on each leg. The four terminals of the primary
are led out through the fiber heads. Care should be taken to distinguish
them from each other so that they may be identified when the transformer
is completed. The primary is then wound with a strip of micanite or
empire cloth 8 inches wide until it measures 4 inches square over all.

*Secondary.*—The secondary coils, which are eight in number, are wound
on a form 4 inches square and 1 1/2 inches between the flanges. The
construction of the form is similar to that shown in Fig. 46 but is
larger. The slots are also necessary here so that the completed section
may be tied up.

About ten pounds of No. 30 B. S. gauge single silk covered wire are
required to wind the secondary. The sections are wound in smooth even
layers until they are six inches in outside diameter. They are then tied
up and removed from the winder. The sections are separated by sheets of
fiber 6 1/2 inches square on the outside, 1/2 inch thick, having a hole
4 inches square cut in the center. The sections are all connected in
series and the terminals soldered to strips of copper.

After the secondary coils are all in place and connected, the fiber head
is slipped on the end of the leg. Then the short lengths of the core,
which had been pulled out again after squaring the core up, are slipped
into place. The core is squared up again and fastened together by boring
a 3/8-inch hole completely through the core at each corner. Two strips
of fiber 12 x 2 1/2 x 1/2 inches are bored with corresponding holes in
their ends. These strips are placed at the end of the transformer, on
top of the core, and 1/4-inch bolts passed through the holes in the
fiber and the core. The bolts are wrapped with micanite cloth where they
pass through the core, and an insulating washer is placed under the
nuts, so that the iron core laminations are not electrically connected.
The nuts are tightened until the core is held firmly together.

The fiber strips also serve as insulated supports for the binding posts.
The copper terminals of the secondary lead to two binding posts mounted
on two fiber or hard rubber pillars 1 inch diameter and 4 inches high.
The rods are arranged as explained in the section under the heading of a
1/4-K.W. transformer.

The transformer is designed for use on a 60-cycle 110-volt current. It
may be used on 220 volts if the two primary coils are connected in
series. When the primaries are in series, and the transformer is used on
the 110-volt current, it will deliver a voltage of about 12,000 at the
secondary. With either primary alone the voltage will be about 12,000,
and with both in parallel about 25,000. It will then deliver a very
heavy current at the secondary and draw from the line about 20 amperes
in the primary. If used with a proper tuning helix, condenser and
aerial, the transformer is capable of sending about 300 miles under
favorable conditions.

If the transformer is to be used for long periods at a time, it is best
to place it in a tight wooden box 18 inches square and 12 inches deep.
The box is then filled with boiled linseed oil or amber petroleum.

A rheostat or impedance and reactance coil should be placed in series
with the transformer to regulate the current and also to prevent arcing
across the spark gap.

*Reactance.*—In Chapter I, the lag and lead of a circuit were explained
in connection with tuning. This is a property of every alternating
circuit and is brought to our notice again in the transformer which
charges the condenser. The current developed by a transformer is a
leading current, since the instantaneous values of the current do not
correspond to the proportionate values of the voltage supplying the
current. In order to force the current values of the charging current to
correspond with the voltage it is necessary to produce a "lag." This is
accomplished by means of an adjustable reactance in series with the
primary of the transformer.

A reactance or inductance suitable for the 2-K.W. transformer may be
made by building up a coil in the same manner as described under the
heading of the J-K.W. transformer. The reactance will have to be
somewhat larger on account of the heavier currents. The core is built up
of sheet iron to measure 2 1/2 x 2 1/2 x 10 inches when completed. The
coil is wound around a wooden form and is composed of about 100 turns of
No. 8 B. S. gauge double cotton covered magnet wire. By varying the
amount of core inserted in the hollow coil the energy may be adjusted as
desired.

[Illustration: Fig. 51. Clapp-Eastham 1/4-K.W. Transformer.]

Fig. 51 illustrates the 1/4-K.W. transformer manufactured by the
Clapp-Eastham Company. The core is so constructed that a small metal
tongue of soft iron projects from one side of the core towards the
opposite side between the windings, but is separated from the opposite
side by a small air gap. Several objects are accomplished by this
tongue, which gives rise to magnetic leakage; the inductance of the
primary is increased thereby to such an extent that the transformer is
self-controlling, so that it may be connected directly to the source of
alternating current supply of ordinary commercial frequencies and
potential, and the current flowing in this circuit be regulated by
varying the number of turns in the primary coil. As this magnetic
leakage gives rise to a loose coupling effect, the primary and secondary
circuits may be brought into resonance by placing a suitable capacity
across the secondary terminals. This condition of resonance brings the
power factor to a materially higher percentage. While the power factor
of the open or closed core transformer is seldom above 50%, this type of
transformer has a power factor of 80 to 90% when used with a suitable
condenser.

[Illustration: Fig. 52. United Wireless Motor-Generator set for
supplying Alternating Current to the Transformer.]

Another point of considerable advantage is the almost entire freedom
from arcing at the spark gap when this type of transformer is used. The
spark gap is connected directly across the secondary terminals of the
transformer and the condenser. The primary turns of the helix and the
spark gap are connected in series. When the transformer is in operation,
this condenser being across the secondary, the transformer is in
resonance and the condenser is charged to such a point that it will jump
the spark gap. At the instant that the spark passes, the secondary of
the transformer is practically short circuited through the spark gap. As
this circuit is now closed and the condenser out of circuit, the
secondary circuit of the transformer is no longer in resonance and the
energy immediately drops off, destroying at once the tendency for an arc
to form. As soon as the spark has passed, the condenser of course comes
in to play and the condition of resonance being reestablished the same
process is repeated. The Clapp-Eastham Company have made application for
a patent on any transformer employing this or any similar construction
for use in charging a condenser.




CHAPTER VII. OSCILLATION CONDENSERS AND LEYDEN JARS.


A condenser consists of two conducting surfaces separated by an
insulator or dielectric. Fig. 53 shows a diagram of a simple condenser
in which _A_ and _B_ are two tinfoil sheets separated by a sheet of
glass, _C_.

[Illustration: Fig. 53. Simple Condenser.]

If _A_ is connected by means of a wire to a static machine a positive
charge will collect on the glass at _A_ and induce a negative charge at
_B_, so that if _A_ and _B_ are connected to a small spark gap the
charge will leap the gap in the form of a spark.

When a condenser discharges through a coil of wire, the discharge
consists of a large number of exceedingly rapid oscillations or
surgings. The first passage of current more than empties the condenser
and it becomes charged in the opposite direction, that is, the
conducting coatings change their polarity. A reverse discharge then
occurs which also oversteps itself and the oscillations thus go on but
become rapidly weaker until they die completely. The time consumed in
the discharge may have been only a fraction of a second, but during that
short period the current perhaps oscillated several thousand times.

If a condenser is discharged through a conductor of high resistance the
discharge passes out slowly, and dies away gradually in one direction
without oscillating. One of the fundamental equations of wireless
telegraphy is therefore that there will be oscillations in a circuit if
the resistance in ohms is not greater than the square root of four times
the inductance in henries divided by the capacity of the condenser in
microfarads.

The capacity or the ability of a condenser to store electricity depends
upon the area and form of the conducting surfaces, the thickness of the
dielectric between them, and a factor known as the specific inductive
capacity of the dielectric. The unit of capacity is called the farad and
is defined as the condenser which would be raised to a potential of one
volt by a charge of one ampere flowing for one second. A condenser of
such a capacity is, because of its enormous size, impractical to
construct, and the unit ordinarily used is therefore the microfarad, or
one millionth of a farad.

Capacity may be calculated from the following formula:

Capacity equals K(A/D),

where _K_ equals a constant depending upon the specific inductive
capacity of the dielectric, _A_ the total area of tinfoil and _D_ the
thickness of the dielectric.

*Leyden Jars.*—Transmitting condensers in a wireless telegraph station
usually take the convenient form of a jar, coated inside and out with
tinfoil and known as a Leyden jar.

The jars should be of good Bohemian or Jena hard glass and coated with
tinfoil only for about three-quarters of their height, as otherwise the
discharge is liable to pass over the top. The tinfoil must be thick to
avoid blistering, and is stuck to the glass with shellac varnish. The
blistering of Leyden jars is a serious fault, for when this condition
exists, the capacity is thereby altered to such an extent that the
period of the closed circuit may be sufficiently altered to throw the
system out of tune and decrease the radiation of energy.

Considerable expense may be saved if the glass jars are purchased and
coated by the amateur. The best jars are those imported from Germany,
which have wide mouths so that they may be easily coated inside with
tinfoil.

[Illustration: Fig. 54. Leyden Jar.]

The jars must be _thoroughly_ cleaned and dried before they are coated.
Give the inside a thorough brushing over with shellac varnish, and
before it is dry, carefully insert the tinfoil and press it smoothly
against the glass. The outside of the jar is treated and coated in the
same manner. The inside and outside of the bottom are also coated by
cutting the tinfoil in circular pieces and shellacking them on.

[Illustration: Fig. 55. "Aerial Switch."]

The whole upper part of the jar is given one or two coats of shellac in
order to prevent the collection of moisture and brush discharging. A
wooden plug fitted in the top of the jar supports a brass rod,
terminating at the lower end in a chain or spiral spring which connects
with the inner coating. When trouble is experienced because of an
imperfect contact between the coating and the chain or rod, a layer of
brass filings an inch or two deep placed in the bottom of the jar will
remedy the difficulty. The upper end of the rod usually terminates in a
small brass ball or a binding post.

The wooden plug or cover is dried in an oven to expel all moisture and
then boiled in paraffin.

Small Leyden jars may be very conveniently made from six-inch test tubes
and mounted in a rack so that the capacity of the condenser will be
adjustable by removing one or more of the tubes. An ordinary test tube
rack such as is used in chemical laboratories serves very well for this
purpose. The tubes should be connected in parallel, that is, all the
outside coatings together and all the inside coatings together.

Figs. 55 and 56 illustrate condensers of this type which are on the
market. The tubes are all separately removable so that the capacity may
be adjusted.

[Illustration: Fig. 56. Amco Oscillation Condenser.]

*Glass Plate Condensers.*—Glass plate condensers offer several
advantages over Leyden jars and are coming into wide use. They are not
so bulky or expensive and, above all, do not blister.

Plate condensers are often placed in a rack and made adjustable by means
of movable contacts. Much the better plan is to place the plates in oil,
as this eliminates all corona or brush discharges and much sharper
tuning is rendered possible. The container is usually a tight wooden box
filled with oil or paraffin after the plates are in place.

It is impossible to state the size of condenser suitable for induction
coils of a given power or spark length, because many factors such as
inductance, length of aerial, etc., which differ in various stations,
influence the capacity. A condenser of convenient size suitable for
coils or small transformers consuming from 250 to 300 watts is that
described below. It is about the proper size for the small open core and
1/4-K.W. closed core transformers, described in the last chapter.

[Illustration: Fig. 57. Clapp-Eastham Oscillation Condenser.]

The glass plates may be secured by removing the emulsion from old 8 x 10
inch photograph plates. Hot water will soften the gelatin on the plates
so that it may be very easily scraped off. Twenty-four plates of this
size are required. The tinfoil is cut 8 x 8 inches, so that an inch
margin is left on all sides. The alternate sheets are connected together
by heavy tinfoil or thin copper foil strips. The condenser should be
placed in a convenient sized wooden box and poured full of paraffin.

The plate condenser shown in Fig. 57 is of .02 microfarad capacity. The
condenser is mounted in a plain wooden box with several binding posts
brought out, so that the capacity may be varied by connecting in various
sections. The condenser is manufactured and designed by the
Clapp-Eastham Company for use with the transformer illustrated in Fig.
51.

It is very necessary to have the transmitting condenser adjustable so
that its capacity may be varied, for the proper value depends upon the
wave length, spark frequency, power and persistency of the wave train.

When the condenser capacity is too small the spark will be somewhat
flaming like an arc, and the potential to which the aerial is charged
will be low. If too much capacity is used the spark will be very
irregular and intermittent.

[Illustration: Fig. 58. Methods of Varying Capacity.]

Fig. 58 shows condensers connected in series and in parallel and a
combination of the two. Two condensers of equal capacity connected in
parallel have twice the capacity of one, while in series they will have
only one-half the capacity of either. This may be otherwise stated as
the capacity in series is equal to the reciprocal of the sum of the
reciprocals of their capacities separately. By this means of connecting
either in series or in parallel almost any desired adjustment of
capacity may be brought about.

Oftentimes a high voltage may be divided between two condensers by
placing them in series and thus using them safely on a voltage which
would rupture either one alone. For example, two condensers built for
20,000 volts and to be used on 30,000 volts could be made to perform
this duty safely and only undergo a potential of 15,000 volts, which is
a large margin of safety.

It is obvious that if the capacity of the circuit were to remain
constant, four condensers connected up in series-parallel would be
necessary.

In case of several Ley den jars or condensers connected up in a
transmitting circuit, the leads or conducting wires connecting the
various units should all be of as nearly the same length as it is
possible to have them.

The resistance of metallic conductors to high frequency currents is
several times their normal resistance to constant currents. The larger
the diameter of the wire the greater is this ratio. This increase of
resistance is due to the fact that the high frequency currents permeate
wires only a very short distance. In the case of copper, the depth is
only about one three-hundredth of an inch and with other metals much
less. Therefore it is advisable to use as leads and conductors of large
condensers, stranded wires or flat ribbons of sheet copper in order to
present more surface and offer less resistance than solid conductors of
an equal cross sectional area.

Iron must never be used, as its resistance to these currents is over
fifty times that of copper.

After connections are once established between the jars or the condenser
units, they cannot be altered nor the capacity changed without re tuning
the circuits afterwards.




CHAPTER VIII. SPARK GAPS OR OSCILLATORS.


The oscillator or spark gap is one of the most important yet often the
most poorly adjusted part of a wireless station.

To obtain a good oscillatory discharge with little damping it is
necessary that the resistance of the circuit should be kept low, and
since the greatest part of the resistance is in the spark gap it would
appear as if this must be very short. While there are reasons for
keeping it short there are on the other hand very good reasons why it
should not be made too short, and the proper length should be a sort of
compromise to be determined by experiment.

If the gap is too short, the discharge will form an arc and the only
oscillations taking place will be those corresponding to the frequency
of the charging current. Power consumption is also in favor of a long
gap, since it causes the induction coil or transformer to draw less
current from the line. Another argument in favor of a long gap is the
fact that the condenser is charged to a higher voltage and more energy
stored up, which makes a greater distance of transmission possible. But
as stated above (and there always is a tendency for the amateur operator
to open out his gap as long as the sparks will continue to jump
steadily) the discharge will not oscillate but will merely set up a
unidirectional current.

There exists a proper gap length for a given circuit which will cause
that circuit to emit a maximum amount of energy and which may be
determined accurately only by means of a hot-wire ammeter placed in the
aerial circuit. The proper gap length is then indicated by the maximum
deflection of the meter.

The correct adjustment of the gap may be approximated by the experienced
operator. If too short, the spark will be hissing and flaming and is in
extreme cases red or yellow colored. It should be lengthened out until
it is thick and white and a slight increase in sound is noted. The spark
should not be, as commonly thought, stringy and crackling.

Arcing is often caused by insufficient condenser capacity in the
circuit. But if the capacity is increased to remedy the arcing, the gap
length should also be increased or otherwise the potential of the
condenser may not be sufficiently raised to permit a disruptive
discharge to take place.

[Illustration: Fig. 59. Spark Gaps.]

The best spark gap for use with a small untuned transmitter, making use
of neither tuning helix nor condenser, is shown at the left in Fig. 59.
It consists of two brass balls supported on suitable double binding
posts. The balls tend to thicken the spark and make it more disruptive
than if it passed between rods or points. The balls also increase the
potential required to leap the gap without increasing its length and
permit the aerial to become more highly charged before a discharge takes
place. For this reason the balls or knobs must always be kept free from
small rough spots or points by frequent polishing.

Silver has some peculiar property which makes it the most efficient
material for a spark gap, but its cost prohibits its use and so brass or
zinc, which are next in order, is generally used.

The type of gap illustrated at the right in Fig. 59 is very convenient
and efficient for small tuned transmitters employing coils of low power.

[Illustration: Fig. 60. Spark Gap.]

The zinc tips are pieces of 3/8-inch zinc battery rod 3/4 inch long.
They are bored and threaded to fit a pair of brass rods 1/8 inch
diameter and 2 1/2 inches long. The rods are supported by two double
binding posts and are fitted with two pieces of hard rubber rod 1/2 inch
diameter and 1 inch long to serve as handles. If desirable one of the
rods and binding posts may be threaded so that the adjustment of the gap
can be accomplished by revolving the handle and screwing the gap in or
out.

A gap of the size and type just described would not be suitable in
connection with a large transformer or an induction coil. The heavy
discharge of such powerful instruments would very rapidly heat the small
brass parts and corrode or even melt the zinc. The spark gap such as is
shown in Fig. 59 may be used with a very large coil or the two kilowatt
transformer if the same proportions are kept and it is made one-half
again as large. Using the dimensions shown in the figure, the spark gap
is suited to either the open core or the 1/4 and 1/2 kilowatt
transformers.

Two pieces of hard rubber rod, 3/4 inch diameter and 2 inches long serve
as the standards. Three grooves 1/8 inch wide, 1/8 inch deep and 1/8
inch apart better the appearance and reduce the liability of leakage but
are not necessary. Two holes 2 inches apart are bored in _S_, a strip of
brass 3 x 3/4 x 1/4 inches. Two screws pass through these holes into the
rubber standards _BB_ and hold _S_ firmly in position. The end of _S_ is
bent down at right angles and bored to receive a binding post. A hole is
bored in _S_ halfway between the standards and tapped to receive a
threaded brass rod 2 inches long and 1/4 inch diameter. The lower ends
of the hard rubber standards are each bored and tapped to receive a
screw which fastens them to the base. The electrodes are zinc or brass
cylinders 3/4 inch diameter and 5/8 inch long. The upper electrode is
adjustable by means of a knurled hard rubber head 2 inches diameter and
1/2 inch thick. The lower one is fastened to a brass plate 2 inches
long, 1 inch wide and 1/8 inch thick. One end of the plate is fitted
with a binding post. The base is a piece of hard rubber 5 1/2 x 3 inches
x 3/4 inch.




CHAPTER IX. TRANSMITTING HELIXES.


The transmitting helix or tuning coil supplies the greater part of the
inductance which is so necessary for the production of electrical
oscillations in the transmitting circuit. It consists merely of a few
turns of heavy copper or brass wire wound in a helix around a form. Two
or more movable contacts permit various amounts of the inductance to be
inserted in the open or closed circuits.

The tuning helixes described in this chapter are offered principally to
serve as guides. They have been designed as carefully as possible to
suit the transmitters for which they are recommended.

There are factors, such as the aerial, etc., which vary greatly in
stations of the same rated power and make it best to determine the
length of the helix by actual experiment.

A certain amount of inductance is necessary in the closed circuit for
the production of electrical oscillations and to transfer energy to the
open circuit. Inductance beyond that necessary to receive energy from
the closed circuit lessens the radiation and makes it necessary to
increase the period of the open circuit by adding wires to the aerial.

The open and closed circuits of a tuning helix constitute an oscillation
transformer, and the two circuits if they are very close together or
intertwined are said to be _closely coupled_. When separated or far
apart they are _loosely coupled_.

*Closely Coupled Helix.*—Fig. 61 illustrates a closely coupled tuning
helix, suitable for ordinary induction coils giving sparks up to 3 or 4
inches in length.

The heads of the helix are circular pieces of hard wood 1 inch thick and
12 inches in diameter. Six rectangular notches are made at equal
distances along the edges of the heads. Six uprights 3/4 x 1/2 inch and
8 3/4 inches long are fastened in the notches with small round headed
brass screws so that the heads are separated a distance of 6 3/4 inches.
Grooves are cut in the outside face of each of the uprights at a
distance of 7/8 inch apart. The wire forming the helix is 1/4 inch
brass. Brass wire is springy and retains its shape better than copper.
About twenty-two feet will be required. The wire is wound in the grooves
in the uprights and held in place by a few double pointed tacks placed
judiciously. The adjacent turns of wire will then have a separation of
about 5/8 inch.

[Illustration: Fig. 61. Closely Coupled Helix.]

The helix is raised above the level of the operating bench or table by
three small feet fastened to the under head, 120 degrees apart.

[Illustration: *Plate III. Transmitting Circuits.*]

If it is desired to use this helix with the one quarter K.W.
transformer, the diameter of the wire should be increased to 5/16 inch.
The two K.W. transformer will require seven turns of 1/2-inch hard drawn
brass wire two feet in diameter.

[Illustration: Fig. 62. Prague Transmitting Helix.]

When setting up a transmitting station, it is a good plan to coil up a
long piece of the wire to be used on the helix around a rough form. The
adjacent turns should be the same distance apart as they are to be on
the finished instrument. Tune up the station with this improvised helix
and ascertain the amount of inductance required. It is then easy to
design a helix containing the proper amount of wire. Four or five feet
extra should be included to allow plenty of range in case it is ever
necessary to make any changes in the aerial or condenser.

[Illustration: Fig. 63. Closely Coupled Tuning Circuit.]

Plate III illustrates the transmitting circuits of the various
commercial systems. The circuit used will determine the number of
binding posts and movable clips necessary. Fig. 63 gives the diagram of
a very good circuit.

*Loosely Coupled Helix*. The heads of the primary coil of the loosely
coupled helix illustrated in Fig. 64 are circular pieces of hard wood 1
inch thick and 18 inches in diameter. Six uprights 3/4 x 1/2 inch and 11
inches long are set into notches in the upper head and fastened to the
base so that the space between the heads for winding the wire is 10
inches.

[Illustration: Fig. 64. Loosely Coupled Transmitting Helix and Contact
Clip.]

A square wooden pillar 1 1/2 x 1 1/2 inches and 32 inches long passes
through a square hole 1 1/2 x 1 1/2 inches in the center of the heads
and projects 20 inches above the upper one. The heads of the secondary
coil are each 12 inches in diameter. The distance between them is also
12 inches. A square hole 1 1/2 inches x 1 1/2 inches in the center of
each of the heads permits the whole secondary coil to slide up and down
on the pillar. Several 1/4 inch holes bored 1 inch apart in the pillar
admit a small peg of the same diameter. The coupling between the two
coils is varied by changing the height of _S_ above _P_ and inserting
the peg to hold the upper coil in position.

The primary winding of the transformer should consist of about 20 turns
15 inches in diameter, and may range in size of wire from No. 2 to No. 8
B. S. gauge.

The secondary is 10 inches in diameter and will require about 40 turns
of No. 10 to No. 16 B. S. gauge.

The largest sizes of wire are for the one-quarter and one-half K.W.
transformers while the smaller sizes are best suited to small induction
coils.

Fig. 64 also illustrates a clip for making connections to the turns of
the helix. The handle is a piece of hard rubber rod 2 1/2 inches long
and 5/8 inch in diameter. A saw slot 1 inch deep is cut down the center
of one end. Two strips of spring brass 2 inches long, 5/8 inch wide and
3/64 inch thick are bent as shown in the illustration. The straight ends
are slipped in the slot in the handle and clamped together by boring a
hole and passing a machine screw through. The upper end may be fitted
with a nut or a binding post to facilitate connection. Flexible copper
ribbons or stranded wire should be the only conductor used for the
tuning leads.

[Illustration: Fig. 65. Loosely Coupled Transmitting Circuit.]

*Tuning a Transmitter.*—Fig. 62 shows a complete wiring diagram of a
transmitter with a hot wire ammeter inserted in the aerial circuit.

To tune such a transmitter place both contacts _A_ and _B_ together on a
turn of wire near the center of the helix. Set the transmitter in
operation by pressing the key and move both contacts together along the
various turns of wire until the meter shows a maximum reading. Then vary
_B_ alone until the reading is still higher. Shifting _A_ varies the
inductance in the closed circuit and _B_ that of the open circuit.

Fig. 65 illustrates the circuit of a loosely coupled transmitter. To
tune such a circuit it is necessary to connect a small needle spark gap
between the aerial and the ground.

[Illustration: Fig. 66. United Wireless Helix, Spark Gap and Condenser.]

A suitable spark gap for this purpose may be made of two darning needles
mounted in double binding posts.

Set the clips, _A_ and _B_, at the center turns of their respective
coils. Press the key to operate the transmitter and adjust the clip,
_A_, until the best spark is obtained in the small needle gap. Then
adjust _B_ until the spark is still better.

If several wires seem to give the same results, move the secondary a
little further away from the primary and try again. After securing the
best spark it is possible to obtain, remove the needle gap, which is
only used in testing. The hot wire ammeter should then be placed in
series with the aerial. By slowly and carefully adjusting the clips and
varying the coupling, the hot wire ammeter will indicate the proper
"tune."

The secondary of the loosely coupled tuning coil produces a "kick back"
effect on the primary, due to induction, and unless the two coils are
somewhat separated, the wave emitted from the transmitter will have more
than one "hump" and a person receiving the wave will be able to tune it
in on two places on his tuning coil.

A loosely coupled tuning coil does not radiate so much energy as a
closely coupled helix, but will, when properly tuned, emit a wave which
is not so highly damped. Therefore as far as distance is concerned the
loosely coupled type is perhaps the most efficient.




CHAPTER X. KEYS.


THE keys used in wireless work differ from those used in commercial wire
telegraphy in being much heavier and stronger, so as to conduct and
break the heavier currents without heating. Where very powerful currents
must be broken the contacts of the key usually operate under oil or in
the magnetic field of a pair of electromagnets. In other cases a small
key is used to operate a heavy relay. In the former type a metal arm
projects downward from the lever of an ordinary Morse telegraph key into
a compartment filled with oil. The arm is provided with a platinum point
which makes contact with another similar point on the bottom of the
compartment, so that the break is made under oil and the key remains
cool and does not arc. In the second form the same result is
accomplished by a magnetic "blowout." The "blowout" is merely an
electromagnet connected in series with the key and its poles at right
angles to the contacts of the key. When the circuit is broken the
magnetic field extinguishes the arc which tends to form between the
contacts of the key.

An ordinary Morse key may be used with coils operated on batteries if
they do not give more than a four-inch spark. Larger coils or
transformers, especially if they are operated on the 110-volt current,
require a heavier key. Where an ordinary Morse key is used with a 3-inch
or 4-inch coil it is a very good plan to provide an auxiliary conductor,
one end of which is fastened to the metal base and the other end to the
adjusting screw on the back of the lever. A piece of flexible lamp cord
is suitable for this purpose. This precaution saves the pivots of the
key from heating and possible burning by heavy currents.

[Illustration: Fig. 67. Morse Key fitted with Extension Lever.]

The most convenient method is to fit up an old Morse key in the manner
shown in Fig. 67. The lever is extended by a piece of aluminum or brass
6 inches long and 5/16 inch thick. The exact shape and the dimensions
are indicated in the illustration.

The extension lever is provided with a 1/2-inch round brass or nickel
steel contact 7/8 inch long. The lower end of the contact is bored and
tapped to receive a short machine screw. The other contact is a similar
piece projecting downwards from a small arm fastened to the top of a
standard 1/2 inch diameter and 2 inches long. The arm is a strip of
brass 1/2 inch wide, 1/4 inch thick, and 1 1/2 inches long, fastened to
the top of the standard with a machine screw. The brass standard is held
in an upright position by means of a small machine screw which passes
through the base. A wire connects with this screw to a heavy binding
post mounted on the base. Connection is made with contact on the
extension lever by means of a piece of heavy flexible lamp cord, which
also runs to a binding post mounted on the base. The contact is
insulated from the extension lever by means of two mica insulating
washers and a bushing. Avoid running the connection to the base of the
key so that the current must pass through the bearings. The contacts
should be filed until they are perfectly flat and square across and make
contact over their entire surface. When carefully constructed a key of
this type will carry without heating almost any currents in reach of the
amateur operator and which his induction coil or transformer will stand.

[Illustration: Fig. 68. Wireless Key.]

The plans and dimensions for a heavier key are shown in Fig. 68. A
wooden pattern of the base and bearings is made and taken to a brass
foundry where a casting in brass may be secured. The pattern should be
of the same shape and size as the finished article represented in the
figure. It is given a slight taper so that it may be easily withdrawn
from the sand mold. The brass casting is finished up square and smooth
by grinding on an emery wheel or by careful filing. The lever is a piece
of brass rod 1/2 inch square and 6 1/2 inches long. One end is fitted
with an ordinary hard rubber Morse key knob. The knurled nuts and
thumbscrews are of the same size as those used on an ordinary key, and
may be purchased from an electrical supply house.

[Illustration: Fig. 69. "United Wireless Type Key."]

The pivot is a piece of 3/16 inch round steel 1 inch long, and passes
through the lever 1 1/2 inches from the rear end. The ends are ground or
turned to sharp points and rest in bearings formed by boring a recess in
the ends of the thumbscrews. The thumbscrews pass through the center of
the bearing, standards 3/4 inch above the base. The lever is clamped to
the center of the pivot by means of a small machine screw. A heavy piece
of spring brass, _S_, 1/2 inch wide and 3 inches long, is fastened to
the base at one end with a short 8-32 screw. The other end is bent
upward so that it presses hard against the lower end of an adjusting
screw, which passes through the lever 1 1/4 inches forward of the pivot.
This spring serves as an auxiliary conductor and saves the key from
carrying an excessive current through its bearings. By adjusting the
thumbscrew, the tension of the key may be regulated to suit the
operator. The contacts of the key are 1/8-inch round nickel steel alloy
or pure silver, set in short 1/4-inch machine screws which fit into
correspondingly threaded holes in the base and the lever. This makes the
contacts easily removable for renewal. The lower contact is set in a
longer screw than the upper one so that connection may be made to it. It
is insulated from the base by a hard rubber brushing.

The contacts must be directly above one another and in perfect alignment
or else they will pit and stick.




CHAPTER XI. AERIAL SWITCHES AND ANCHOR GAPS.


Some form of switch for quickly connecting the aerial and ground to
either the transmitting or receiving apparatus is necessary.

Low powered stations using an induction coil as a transmitter will find
a 250-volt double pole, double throw porcelain base switch to be
suitable. Such a switch is connected up according to the diagram shown
in Fig. 70. As clearly illustrated, the receiving apparatus is in use
when the switch is thrown to the right, and the transmitter is ready for
operation when the switch is in the left-hand position. The ease and
speed with which such a switch may be thrown from right to left or vice
versa may be much facilitated by fitting it with a larger and longer
handle than the one usually provided.

[Illustration: Fig. 70. Connections for Aerial Switch.]

An ordinary double pole, double throw switch should be mounted on the
wall within easy reach of the operator, while the two types of switch
about to be described are best situated directly on the operating table
alongside of the transmitting key.

All insulated parts of an aerial switch such as the base, insulating
bar, handle, etc., should be made of some good insulating material, as
porcelain, hard rubber or ebonite. All insulation which is used in power
switches such as slate, wood, marble, etc., is worthless and should not
be used.

If a loop aerial system is used, a switch is necessary, which will break
the primary transmitting current so that in case of an accidental touch
of the key while receiving, the high voltage discharge will not pass
across the anchor gap into the receptor and badly damage the same or
shock the operator. The connections of such a switch and the loop aerial
are shown in the wiring diagram of the De Forest system.

[Illustration: Fig. 71. Aerial Switches.]

A switch may be easily constructed from a 50 ampere, single throw,
triple pole, fuseless power switch. If it is mounted on a slate base
remove it and set it up on a base of the same size made of fiber or
porcelain. Flat unglazed tiles of various sizes are obtainable from tile
setters at a very low cost. They may be easily bored with an ordinary
steel twist drill. Such tiles are excellent insulators when not exposed
to moisture and are useful for bases, etc., in many cases. A heavy coat
of shellac varnish will make the tile impervious to moisture.

A hard rubber rod, _S_, 3 inches long and 5/8 inch diameter, is mounted
on both sides of the base in alignment with the contacts. A strip of
hard rubber, _Y_, 1 inch wide, 1/2 inch thick and 6 1/2 inches long is
fitted with two contacts similar to those mounted on the base of the
switch. The contacts are mounted at a distance apart equal to the
distance between the outside knife blades of the switch. A 1/8-inch hole
is bored through the strip at both ends, through the axis of the rods
along their entire length and through the base. A piece of brass rod 4
inches long and threaded at both ends with an 8-32 die is passed through
the holes and yoke, so that the rods may be held firmly to the base and
the strip to the tops of the rods by two nuts screwed on the ends of the
brass rod.

[Illustration: Fig. 72. Detail of Contacts.]

The upper contacts should be directly over the two outside lower ones.
They are made from a strip of brass 1/2 inch wide, 3 1/2 inches long and
1/16 inch thick (bent as in Fig. 72). A binding post is mounted on the
yoke directly above and connecting with each one of the contacts.

The middle pole of the switch is connected to the primary of the coil or
transformer, so that when the switch is down, the primary circuit is
completed, and when it is up it is broken.

[Illustration: Fig. 73. Details of Switch Parts.]

Some may prefer to make the complete switch, and in that case the
dimensions given in Fig. 73 may be of aid. The knife blades are 6 inches
long, 5/8 inch wide and 1/8 inch thick. Three are required. A 1/8-inch
hole is bored 3/8 inch from one end, and the other end is cut out in the
shape shown in the illustration. An insulating bar 5 x 3/4 x 5/8 inches
is made of hard rubber and three notches are cut in the back face, one
directly in the center and one 2 inches on either side.

[Illustration: Fig. 74. Method of Fastening Knife.]

Three 1/8-inch holes are bored one in the center of each groove and at
right angles to it. An 8-32 machine screw 3/4 inch long is placed in the
notch in the end of the knife blade and the screw passed through the
hole in the insulating bar. The blade may then be clamped tightly in
place by putting a nut on the end of the screw. The screw which holds
the middle knife blade in place is longer than the other two so that a
handle may be fitted on.

The bearings are made the same size and shape as the contacts
illustrated in Fig. 72, with the exception that a 1/8-inch hole is bored
through near the top so that a small bolt may be slipped in to hold the
knife blades in place.

The base of the switch measures 8 x 9 x 3/4 inches. The parts are
assembled as in Fig. 71.

Another type of aerial switch called a "T" switch is illustrated in the
same figure. A piece of 3/4-inch hard rubber is sawed in the shape of a
T, 3 1/2 inches high and 6 1/2 inches wide at the top. The upper
contacts are mounted on arms of the "T" and connected to binding posts
placed immediately above them, A slot, 1/2 inch wide and 3 inches long,
sawed in the center of the leg of the T, permits the middle knife blade
to move without obstruction.

[Illustration: Fig. 75. "T" Aerial Switch.]

A double pole, double throw switch may be easily adapted to form a "T"
switch. Two knives are cut from sheet brass, of the same shape as those
already on the switch but twice as long. The contacts at one end of the
porcelain base are removed and the bearings of the knife blade fastened
there in their stead. This forms a single throw, double pole switch
having knife blades twice the length of the original double pole, double
throw switch. A "T" shaped support for the upper contacts may be sawed
out of 1/2-inch hard rubber and smoothed up with a file. The support is
fastened upright by means of two screws which pass through holes in the
center of the base where the knife bearings originally were. The two
contacts which were removed from the base are fastened to the support
directly over the knife blades so that they make a good contact when the
switch is up. Two binding posts mounted above the contacts make
connection with them.

If desirable the switch may be fitted with a third knife blade and
contact so that when the switch is up in position for receiving, the
primary circuit of the induction coil or small transformer is open.

It is obvious that the dimensions, etc., need not be the same as those
given here but may be adapted to suit the size and design of the switch
which is to be altered. The knives should always be as long as
convenient and the contacts separated by a wide space.

[Illustration: Fig. 76. "United" Wireless Lightning Switch.]

[Illustration: Fig. 77. Shoemaker Tuning Coil and Aerial Switch.]

Figs. 76 and 77 illustrate the aerial switches used in the installations
of the United Wireless Telegraph Company. These illustrations give a
good idea of the long knife blades employed. The lightning switch is
fitted with a micrometer spark gap on top of the switch which is in
service when the handle is up. The small spark gap affords a path for
the lightning to jump into the ground, but cannot be bridged by the
receiving currents.

*Anchor Gaps.*—When a loop aerial system is employed for both
transmitting and receiving, a device known as an anchor gap becomes
necessary.

[Illustration: Fig. 78. "United" Wireless Anchor Gaps.]

The construction and connection of a simple anchor gap are shown in Fig.
79. A ring 1 3/4 inches outside and 1 1/4 inches inside is cut out of
5/8-inch hard rubber or fiber and smoothed up. Three 1/8-inch holes are
bored in the periphery of the ring at 120 degrees to each other. The
holes are threaded with a 10-24 tap. Three small binding posts are each
fitted with a 3/16-inch brass rod 7/8 inch long, having a 10-24 thread
so that they may be screwed into the holes in the ring.

[Illustration: Fig. 79. Anchor Gap.]

The gap between the ends of the two rods connected to the aerial and the
receiving apparatus is adjusted until it is about 1/8 inch long. The
third point is screwed in until it all but touches the other two.




CHAPTER XII. HOT WIRE AMMETER.


When a current passes through a conductor, it generates heat in
proportion to the resistance offered and the amount of current flowing.
Heat causes metals to expand sufficiently so that these two properties
may be applied to the construction of a hot wire ammeter for the
measurement of alternating currents of high frequency and potential.

The hot wire ammeter is placed in series with the aerial, so that by
noting the deflection of the pointer, the inductance, capacity and spark
gap may be adjusted until the meter gives a maximum reading.

[Illustration: Fig. 80. Simple Hot Wire Meter.]

A simple and crude form of meter which is sufficiently sensitive for
most experimental work is illustrated in Fig. 80. A piece of No. 36 B.
S. platinum wire is sealed in the bulb of an ordinary air thermometer.
When the wire becomes heated by a passing current of electricity, it
causes the air in the bulb to expand and change the height of the
colored liquid in the tube.

An air-thermometer is simply a glass tube of fine bore having a bulb
blown at the upper end and the free end immersed in a reservoir of ink
or some other colored liquid. The instrument is put in working order by
grasping the bulb in the palm of the hand, so that the warmth of the
hand will expand the air and cause some of it to escape from the lower
end of the tube. Upon removing the hand, the air will contract and suck
some of the liquid up into the tube. It should rise only about half way
to the bulb, and the tube should be about 18 inches long so as to leave
room for changes in the position of the column due to variations in the
outside atmosphere. A cardboard scale graduated in inches and reading
downward is fastened in back of the tube.

The tube should have a fine bore so as to make the instrument as
sensitive as possible. The best liquid to use is alcohol, colored with a
little aniline dye. Alcohol has a lower specific gravity than water, and
the column will be more sensitive to small changes of pressure. The same
figure shows a form of meter devised and used with success by the
author.

Two tubes are fitted to the bulb, a large one having a bore of about 0.1
of an inch and another about 0.04 inch. Connection is established by the
aid of two corks and a short length of glass tubing one inch in
diameter. The tubes are bent U shaped, and a little colored alcohol is
placed in each, so that the bottle reservoir is unnecessary. The tube of
large bore is fitted at the top with an ordinary glass stopcock such as
that used in chemical laboratories.

The stopcock is left open and the transmitter is set in operation by
holding down the key. The helix, etc., are adjusted until the larger
tube shows a maximum reading. The stop-cock is then closed and the
instruments further adjusted by noting the reading in the finer tube
which corresponds to much smaller changes in current. The finer bore
cannot at first be used alone because the large changes of current would
blow the liquid out of the tube. In lieu of a glass stopcock, a piece of
rubber tubing may be placed over the end of the tube and closed, when
necessary, with a pinch cock.

[Illustration: Fig. 81. Meter with Case Removed.]

Fig. 81 shows a more elaborate and sensitive form of meter which is not
only suitable for experimental outfits but may be used with good results
for more careful work. The advantage of the form of meter here described
is that it is "pivotless" so to speak, and contains no bearings which
require jewels to eliminate friction.

The "hot wire" is platinum, and in order to compensate for external
changes of the atmospheric temperature, is mounted on a strip of glass.
Glass and platinum expand at nearly the same rate, and the wire is thus
kept taut and prevented from changing the position of the pointer except
when the current passes.

[Illustration: Fig. 82. Glass Compensating Strip.]

Drill four 1/8-inch holes in a piece of window glass 6 inches long and 1
inch wide. The location of the holes is shown in Fig. 82. The two at the
ends serve to mount the standards, _A_ and _B_, and those at the center
to fasten down the strip to the base. The holes are drilled with a small
three-cornered file which has been broken off and set in a breast drill.
The broken end should be used to drill the glass and be kept thoroughly
lubricated with camphor and turpentine. With a little care and patience
the holes may be drilled without breaking the glass.

[Illustration: Fig. 83. Details of "Hot Wire" Supports.]

Two brass standards are fastened on each end of the glass. They are bent
out of sheet brass and are 3/4 inch high and 3/8 inch wide. A brass
spring of the same width and 1 1/4 inches long is clamped under one
standard. The standard which holds the spring in position is tapped for
a small thumbscrew which may be secured from a binding post. Solder a
small brass pin to the top of the spring and another one to the top of
the standard which is fastened at the opposite end of the glass strip.
Some paper or rubber washers must be placed between the feet of the
standards and the glass strip to prevent it from cracking when the
screws are tightened.

For a station up to one-half K.W. in power the hot wire must be No. 40
B. S. gauge platinum. For larger stations a single No. 36 wire may be
used or three No. 40 wires in parallel. The wire must be about 7 inches
long. Stretch it between the standard, _A_, and the spring, _C_. Wrap
the ends around the pins and solder them there, using as small amount of
solder as possible. The tension of the wire, which should be taut, is
adjusted with the thumbscrew.

Take a piece of the platinum wire about 1/2 inch long and make a little
eyelet at one end. Wrap the straight end around the center of the long
hot wire and tie a piece of silk in the eyelet.

[Illustration: Fig. 84. Details of Movement.]

The glass strip and its standards supporting the hot wire may then be
fastened to the baseboard of the instrument by means of two round headed
brass wood screws. Two rubber washers must be interposed between the
glass and the wood. A piece of 3/32 inch brass 1/2 inch wide and 3 1/4
inches long is bent in the shape shown by _F_ in Fig. 84. The upper end
is bored and tapped to receive a thumbscrew similar to the one in the
standard on the glass strip. Two brass springs 1/64 inch thick, 3/8 inch
wide and 1 1/4 inches long are soldered or riveted at opposite ends of
_F_ in the positions shown in Fig. 85. The springs should project one
inch from the upright. A small hook made from an ordinary pin is
soldered to the outside end of each.

[Illustration: Fig. 85. Complete Movement.]

The movement is shown in perspective by Fig. 85. _G_ is a rectangle of
very thin copper, 1/2 inch long and 1/4 inch wide, having a little
projection 1/4 inch long bent in a curve so that it forms a sextant of a
circle, of which the intersection of the diagonals of _G_ would be the
center.

The pointer is a piece of steel wire 5 inches long. It is slightly
flattened by hammering so that it will retain its shape and not curl.
About 1/2 inch is allowed to project through _G_ and is weighted with a
lead shot so as to partly counterbalance its weight.

Two loops of wire are fastened to the corners of _G_ by tying them in
holes which are bored there for that purpose. The wire is fine phosphor
bronze .003-.005 of an inch in diameter, which is used for suspending
the movements of delicate galvanometers. Pass the loops over the hooks
on the springs and adjust until the pointer moves horizontally. Then
fasten the wires permanently to the hooks by means of a small drop of
solder.

[Illustration: Fig. 86. Side View of Hot Wire and Movement.]

The movement is mounted in the position shown by Fig. 86. The silk
thread tied to the eyelet runs to the little sextant and is cemented at
the further end by means of a small drop of sealing wax. The scale is a
piece of sheet copper or brass, covered with white paper and calibrated
in degrees or made to read in amperes by connecting it in series with an
ammeter and a source of direct current. A rheostat should be included in
the circuit and the current varied so that various values may be marked
off. All the different points must be located by sending an actual
current of that value through the meter. An error is liable to result if
any of the points are marked by guesswork, for the divisions grow
smaller and smaller as they become farther away from zero. For example a
position of the pointer corresponding to 0.1 of an ampere will not be
half way between zero and 0.2 but will be nearer the 0.2 division.

The resistance of wires to high frequency currents is much higher than
their resistance to constant currents. This would seem at first to
indicate that our meter will give a higher reading for an equal current
value, when used with a high frequency current after being calibrated
with a direct current. But with wires of very small diameter such as No.
40 B. S. gauge there is almost no perceptible difference and
consequently no error unless the frequency of the oscillations exceeds
1,000,000 per second, which is very unlikely with the "spark" method of
wireless telegraphy.

Fig. 87 illustrates the form of hot wire ammeter used by the United
Wireless Telegraph Co. for tuning their installations.

[Illustration: Fig. 87. United Wireless Hot Wire Ammeter.]

The pivotless meter just described should be fitted with heavy binding
posts which are connected to the brass standards mounted on the glass
strip by means of stranded copper wire.

The meter should be fitted with a case and glass cover to exclude dust
and prevent injury to the working parts. It should be mounted in such a
position that the weight of the pointer is sufficient to keep the silk
thread taut so that when the wire expands the pointer which is normally
at zero will fall of its own weight. When the wire cools after the
current has ceased to flow, it will contract and draw the pointer up
again.

Platinum wire will give good results, but for more accurate work an
alloy known as platinoid is most suitable.

Detailed instructions for tuning the transmitting circuits by means of a
hot wire ammeter are given in the chapter on Transmitting Helixes.




CHAPTER XIII. OSCILLATION DETECTORS.



"UNIVERSAL" DETECTOR.


The purpose and position of the detector in a wireless telegraph system
has already received some notice in the first chapter, but its operation
and adjustment are so important that this chapter deserves the most
careful consideration. The receiving range of a station is not as much
dependent upon the aerial system as it is upon the adjustment of the
tuning circuits and the detector itself.

It is suggested that the amateur experimenter not confine his work to
receiving only with a single type of detector but rather accustom
himself to the different instruments.

During the past few years many wireless telegraph detectors have been
invented which lend themselves readily to amateur construction. It is
somewhat of a convenience to have a "universal" detector which with a
little manipulation may be used as more than one type and thus save
unnecessary expense and much labor.

The "universal" detector shown in Fig. 88 has been so designed as to
present a good appearance and at the same time be successfully operated
as an electrolytic, tantalum, peroxide of lead, silicon, carborundum or
any of the crystal type detectors.

[Illustration: Fig. 88. Universal Detector.]

[Illustration: Fig. 89. Details of Universal Detector.]

The standard, _R_, is a 3/4-inch hard rubber rod, 1 1/4 inches long,
with a 3/16 inch hole bored through its axis. A spring, _S_, is made
after the plan shown in Fig. 86. It is 2 inches long and 1/16 inch
thick. A brass collar 1/8 inch thick and 3/8 inch diameter is soldered
on the smaller end of the spring in order to so reenforce it that it may
be bored and threaded with an 8-32 tap to receive a thumbscrew. The
brass standard, _D_, is a small cylinder 3/8 inch high and 1/2 inch in
diameter. A 3/16 inch hole is bored through its axis. The arm, _A_, is
brass and measures 1 1/2 x 1/2 x 1/4 inches. The ends are rounded by
filing or grinding so that they coincide with the semi-circumference of
a circle having a diameter of 1/2 inch. Two holes are bored on the
center line 1/4 inch from each end.

One is a 1/8-inch hole and the other is threaded with a 10-32 tap to fit
the large adjusting screw. The adjusting screw, _H_, is 1 1/4 inches
long and has a 10-32 thread. A hard rubber head 1 inch thick and 1 1/4
inches in diameter is clamped to the upper end by means of two hexagonal
brass nuts. A small brass washer should be placed between the head and
each of the nuts to give it a more finished appearance and prevent the
nuts from marring the rubber.

[Illustration: Fig. 90. Parts of Universal Detector.]

The brass bed plate, _B_, is 1/8 inch thick, 3 inches long and 2 inches
wide. Two holes are drilled on the center line 9/16 inch and 7/8 inch
from either end. One is 3/8 inch in diameter and the other is threaded
with an 8-32 tap. An insulating bushing in the shape of a hard rubber
washer on the lower end of _R_, 1/8 inch thick and 3/8 inch in diameter,
has a 3/16-inch hole bored in its center and is fitted in the larger
hole in the bedplate. The whole detector is assembled and mounted on a
hard rubber base 4 x 5 x 4 inches. A brass binding rod, _M_, 3 1/4
inches long and having an 8-32 thread, is passed successively through
the arm, the brass standard, the spring, the hard rubber standard and
the bedplate. A hexagonal brass nut on the under side of the base and a
thumb nut on the brass arm serves to bind the whole tightly together.
Four binding posts are mounted on the four corners of the base. Two are
connected to the brass binding rod and two to the bedplate. This
completes the universal part of the detector. The remaining parts are
each described under the headings of the respective detectors to which
they belong.



ELECTROLYTIC DETECTORS.


*"Bare Point" Type.*—Although the electrolytic is the oldest of a long
line of very sensitive detectors,³ it still holds first rank when in the
hands of an experienced and skillful operator. It exists in two
different forms, but the more favored is that known as the Fessenden
"bare point" type, which consists of a very fine Woolaston platinum wire
(.001-.00002 of an inch in diameter) dipping in a small cup of dilute
acid. The acid is either 20 per cent chemically pure nitric or
sulphuric.

[Illustration: Fig. 91. Bare Point Electrolytic Detector.]

A large electrode of platinum wire or foil dips into or is sealed in the
bottom of the cup so as to make an electrical connection with the
liquid. The fine Woolaston wire is clamped over the cup in a holder
which permits of vertical adjustment, by means of a thumbscrew, so that
the depth of immersion in the acid may be regulated.

[Illustration: Fig. 92. Effect of Exposing too much Wire.]

Woolaston wire is covered with a comparatively thick coating of silver,
which before using must be removed from the end for about 1/32 inch by
dipping it in strong nitric acid, which will dissolve the silver and
expose the almost invisible platinum core. Too much of the fine platinum
core must not be exposed or else the surface tension of the acid will
cause the wire to curl over and present a large flat surface instead of
a fine point. This is a very necessary and important precaution, for the
detector is more sensitive as the area of contact between the fine wire
and the liquid is smaller.

Whenever this condition is reached the end of the wire should be cut off
with a pair of sharp scissors and a new point exposed.

[Illustration: Fig. 93. Electrolytic Detector Circuits.]

The detector circuit is shown in Fig. 93. The fine "bare point" is
always made the positive or anode of the battery circuit. Otherwise the
detector will not operate. A potentiometer must be shunted across the
terminals of the battery to reduce the voltage to a value just below
that which is required to break down the thin film of oxygen gas which
collects on the "bare point" and polarizes it or insulates it from the
liquid so that little or no battery current can flow. This film of gas
is caused by the electrolysis of the acid solution and the decomposition
of the water into hydrogen and oxygen gas.

When oscillations are set up in the receiving aerial and they surge
through the detector, a sufficient e.m.f. is generated to break down the
film of gas and permit the battery current to flow again. The passage of
current causes the signals in the telephone receivers.

The electrolytic cup for the universal detector is illustrated in Fig.
90. It is made of a piece of hard rubber rod 3/4 inch in diameter and
3/4 inch high. A recess 1/2 inch in diameter and 3/8 inch deep is cut in
the top to contain the acid. A small hole 1/4 inch deep is bored in the
under side and threaded with an 8-32 tap. A brass pin 1/2 inch long,
having a corresponding thread, is fitted in the hole. The pin may then
be screwed into the small hole in the bedplate. A piece of No. 30 B. S.
gauge platinum wire or a strip of platinum foil is clamped between the
bottom of the cup and the bedplate and then bent over the top of the cup
into the liquid.

[Illustration: Fig. 94. Electrolytic Detector.]

A 1/16-inch hole 1/4 inch deep is bored in the lower end of a thumbscrew
having an 8-32 thread. A piece of Woolaston wire 1/2 inch long is placed
in the center of the hole and tinfoil packed into the surrounding space
with the head of a sewing needle until the wire is held firmly in
position. The free end of the wire must then be dipped in some strong
nitric acid to remove the silver. The thumbscrew is placed in the collar
on the end of the spring of the universal detector and lowered until the
"bare point" almost touches the surface of the electrolyte in the cup
beneath. Pressure must then be applied to the spring by turning the
large adjusting screw until the "bare point" touches the liquid and a
click is heard in the telephone receivers and a faint bubbling sound is
also audible. The adjusting screw must then be slowly and carefully
turned in the opposite direction so as to raise the point until the
bubbling changes to a hissing sound. The point is then above the level
of the electrolyte in the cup but is still in contact with it because of
the capillary action of the fine wire and the liquid.

By using the large adjusting screw as much as possible, instead of the
small thumbscrew, the point is raised or lowered without giving it a
circular motion and much finer adjustment is made possible. The
potentiometer is adjusted until the hissing noise caused by excessive
battery voltage just disappears. The detector is then in its most
sensitive condition for receiving signals.

When the detector is in use for long periods, the potentiometer must be
frequently readjusted to compensate the gradual loss in voltage of the
battery. It is well to provide a small switch which will disconnect the
battery from the potentiometer when the detector is not in use. In the
same case the acid should be removed and placed in a tightly stoppered
bottle. A pipette or fountain pen filler furnishes the most convenient
means for filling or emptying the cup. The acid must be kept perfectly
pure and out of contact with all metals other than platinum. Great care
should be exercised in filling the cup, for the acid, if spilled, will
not only badly corrode the metal fittings, but will also provide a
current leak and seriously weaken the signals.

*Shoemaker and Stone Detectors.*—These two types of detectors make use
of "glass points," so called because the fine platinum wire is sealed in
a glass tube and the end of the tube is then ground down on a fine
oilstone until the platinum wire is exposed. This results in a very fine
contact area and insures constant immersion of the point without
readjustment.

[Illustration: Fig. 95. Forming "Glass" Point.]

The fine platinum wire for a glass point may be secured from one of the
flaming pocket cigar lighters making use of spongy platinum. The center
of a thick walled glass tube is softened by heat and contracted as shown
at _A-B_ in Fig. 95. After cooling, the tube is cut in half at the point
indicated by the dotted line. The platinum wire is placed in the
contracted end of the tube and carefully fused in so that about one-half
of the wire, which is about 1/2 inch long, is embedded in the glass. The
contracted end of the tube containing the wire should be closed.
Connection is established to the upper end of the fine platinum wire by
filling the tube with mercury and dipping a piece of flexible conductor
in the mercury. The upper end of the tube is closed and the mercury
prevented from escaping by a small dab of sealing wax.

The "point" is slowly and carefully rubbed on a fine oilstone kept well
wet with water. The tube must be held in a vertical position so that the
glass will be ground away at right angles. When it is thought that the
platinum wire has been exposed by the grinding, connect the flexible
conductor to one pole of a battery. The other pole of the battery is
connected to a pair of sensitive telephone receivers and the telephone
receivers to a vessel containing dilute acid. If the platinum wire is
exposed, a sharp click will be heard in the telephone receivers when the
"point" is dipped in the acid. Do not confuse the sharp click with the
sound which may be occasioned because the outside of the glass tube is
damp or wet.

After the point has been sufficiently ground, disconnect the testing
apparatus and connect the free end of the flexible conductor to a
binding post placed on the end of the detector spring, _S_. The detector
circuit is similar to that of the "bare point" type.

[Illustration: Fig. 96. Shoemaker Detector.]

The illustration shows what is sometimes called a "primary cell"
detector because it furnishes its own current and does not require a
battery. A Stone detector may be very easily changed to one of the
Shoemaker type by substituting an amalgamated zinc rod for the platinum
wire anode which makes connection with the liquid in the cup. This
combination of platinum and zinc results in an electromotive force of
about 0.7 volt, and the telephone receivers are connected directly to
the terminals of the detector without any local battery or
potentiometer. The electrolyte in the cup must be a 20 per cent solution
of pure sulphuric acid, as nitric acid would dissolve the zinc in a very
few minutes. The zinc must be kept well amalgamated with mercury.

[Illustration: Fig. 97. Shoemaker Detector Circuits.]

The Shoemaker system makes use of a loop aerial, and the circuits with a
single and double coil tuner are illustrated in Fig. 97. It is not
necessary to use these, and the detector will operate just exactly as
well on a "straightaway" aerial.

*Lamp Detector.*—All electrolytic detectors, more especially those of
the "glass point" type, are subject to the annoyance of "burn-outs."
That is, the fine platinum wire melts when receiving strong signals from
a near-by station.

In such case, the "bare point" must be lowered until it again makes
contact with the liquid, and the "glass point" reground until the wire
is again exposed.

When this trouble comes often it is very convenient to have at hand a
simple detector which will not burn out and which may be substituted for
the usual one when great sensitiveness is not required.

[Illustration: Fig. 98. Lamp Detector.]

Such an instrument is made by snipping off the tip of a small
incandescent electric lamp and removing the filament with a wire. One of
the leading-in wires is broken off as close as possible to the glass
stub and the globe half filled with a 20 per cent acid solution. The
broken wire must be made the negative or cathode and connected like a
Fessenden or Stone detector. This lamp detector though crude will give
good service without burning out when used to receive from near-by
stations.

In place of a lamp detector, a glass point having a larger wire than
that of the cigar lighter may be used instead of the usual point, but it
will not be so sensitive.

Fig. 99 illustrates a simple form of electrolytic detector which is not
so sensitive as that shown in Fig. 100 but is still very serviceable.

The cup is made from the carbon of an old dry cell, the brass connecting
cap serving very well to make the connections to. It has a recess about
1/2 inch in diameter and 1/4 inch deep cut in the top to contain the
electrolyte. The cup should be about one inch high. A file will smooth
up any rough edges and give it a good appearance.

[Illustration: Fig. 99. Simple Electrolytic Detector.]

The yoke is made of a piece of 1/8-inch sheet brass about 3/4 inch wide,
bent in the shape shown in the illustration. Two small holes are drilled
in the feet, to serve to fasten the yoke firmly to the base and also to
make connection to.

The adjusting screw may be made from the screw taken from the carbon of
an old dry cell. To permit of accurate adjustment, it should be fitted
with a large head made from a piece of 1/4-inch hard rubber or fiber cut
in a circle about 1 1/4 inches in diameter. Bore a small hole about 1/8
inch in diameter through the center of the head and force it on the
screw. A nut screwed on the under side will then clamp it tightly
against the brass head. A hole is bored in the center of the yoke and a
battery nut which will fit the adjusting screw soldered directly under
it.

The platinum wire may be either soldered to the adjusting screw or
fastened with tinfoil in the method which has been described.

The cup and yoke are best mounted on a piece of hard rubber 1/2 inch
thick, 3 inches wide and 4 inches long. A binding post is placed near
each of the four corners.

It is possible to do extremely fine and long distance work with the
detector illustrated in Fig. 100. It is so arranged that the "bare
point" need not necessarily be revolved when making an adjustment, and
so it is possible to place it in a very sensitive condition.

A brass standard, _U_, 1 1/4 inches long is cut from a piece of 1/2-inch
rod. A hole bored in the top and bottom of the standard is threaded with
an 8-32 tap. A brass rod, _R_, 2 inches long is threaded with an 8-32
die throughout its entire length. One end is screwed in the top of _U_.

A piece of brass tubing, _P_, 1 1/4 inches long and having an internal
bore of 1/2 inch is slipped over _U_. A slot cut in _P_ fits over a
small pin set in _U_ and permits _P_ to be slid up and down but not to
turn around.

[Illustration: Fig. 100. Electrolytic Detector.]

A head in the form of a circular brass washer, _E_, 1/8 inch thick, 1/2
inch in diameter and having a 5/32-inch hole bored in the center is
soldered in the top of the tube, _P_.

A circular piece of hard rubber, _H_, 2 inches in diameter and 1/2 inch
thick is fitted with a brass bushing having a hole in the center with an
8-32 thread to screw on the rod, _R_.

A spiral spring is placed around _R_ between _U_ and the head _E_. A
small brass washer should be placed between _H_ and _E_ in order to
eliminate friction. When _H_ is turned in one direction, the spring will
cause _P_ to rise, and when turned in the other direction it will be
lowered.

A brass arm, _A_, 1/4 x 1/4 x 1 1/2 inches carries a small thumbscrew,
_r_, at one end, while the other end is soldered to _P_ as shown in Fig.
96. The Woolaston wire is soldered to _T_.

[Illustration: Fig. 101. Details of Electrolytic Detector.]

A small carbon cup 3/4 x 3/4 inch serves to hold the electrolyte. A
3/8-inch hole is bored 1/4 inch deep in the bottom of the cup and poured
full of melted lead. The lead is then bored and tapped to fit a machine
screw which fastens the cup to the base. Connection is made from a
binding post to the machine screw. A second binding post is connected to
the screw which fits into the bottom of _U_ and holds it to the base.

If desirable a circular piece of hard wood, _F_, may be turned out and
glued to the base around the cup in order to give it a more finished
appearance.

The thumbscrew, _T_, is used to lower the "bare point" until it almost
touches the liquid, and then the large head, _H_, is brought into play
to make the finer adjustment.

*Increasing the Sensitiveness of an Electrolytic Detector.* The
sensitiveness of an electrolytic detector may be increased in three
ways, viz., by connecting two detectors in series, by warming the
electrolyte and by agitating it.

The first method is clearly apparent.

The second is accomplished by placing the detector over a sand bath and
gently warming it. It will then show a marked increase in the strength
of the signals at a temperature of about 30 C. This increase will
continue to rise with the temperature until it reaches a maximum at
about 60 C.

[Illustration: Fig. 102. Increasing the Sensitiveness of an Electrolytic
Detector.]

Branly discovered that a fine stream of gas passed through the
electrolyte in order to agitate it increases the strength of the signals
in the phones. He devised a detector provided with two extra platinum
terminals sealed in the cup. When connected in series with a battery and
an adjustable resistance, these terminals cause electrolysis of the
water, and a fine stream of oxygen and hydrogen gas flows through the
acid electrolyte. The stream of gas agitates the liquid just
sufficiently so that when oscillations strike the detector they augment
the breaking down of the film of gas which collects on the fine platinum
point. This results in an increase in the battery current flowing
through the telephone receivers of from two to four times and a
corresponding increase in the volume of sound. The adjustable resistance
is used to regulate the decomposition of the electrolyte and formation
of gas, for if this proceeds too rapidly an undesirable rumbling noise
will be produced in the telephone receivers.



TANTALUM DETECTOR.


The tantalum detector is especially suitable for the amateur
experimenter because its change in resistance when struck by
oscillations is so great that high resistance telephone receivers are
not necessary. Its normal resistance is about 1000-2000 ohms, and this
sometimes drops as low as 125-100 ohms when struck by oscillations of
ordinary strength. It is not nearly so sensitive as the electrolytic or
crystal detectors, but gives very much louder tones in the telephone
receivers when used for short distance work.

[Illustration: Fig. 103. Tantalum Detector.]

The detector is easily made by substituting a piece of tantalum wire for
the Woolaston wire of an ordinary electrolytic detector. The dilute acid
solution is removed from the cup and replaced by some pure mercury. The
connections remain the same as for the "bare point." The potentiometer
is adjusted until the potential of the battery is in the neighborhood of
0.2-0.4 volt.

The tantalum wire may be easily secured by breaking the globe of a
tantalum lamp and using a piece of the filament. It is best to snip off
the lamp tip before breaking the globe. This precaution admits the air
and prevents an explosion which would shatter the glass and scatter the
filament in fragments.

If the universal detector is used with a tantalum point, turn the small
thumbscrew until the wire almost touches the surface of the mercury.
Then lower it with the large adjusting screw until the tantalum touches
the surface and a sharp click is heard in the telephone receivers.
Adjust the potentiometer until the signals are the loudest.



CRYSTAL DETECTORS.


Certain minerals and crystals, principally members of the carbon and
sulphur groups, possess the peculiar property of rectifying electrical
oscillations and converting them into a pulsating direct current. These
crystals conduct the current better in one direction than in the other.
In the case of a current having a potential of ten volts and applied to
the ends of a carborundum crystal, the current may be one hundred times
greater when flowing in one direction than when flowing in the other.
This ratio decreases as the voltage is raised, for with 25 volts it may
be only about forty times greater. The crystals when properly inserted
in the aerial circuit are enabled to rectify the oscillations and
produce sounds in the telephone receivers without the aid of a battery.

The following is a partial list of the minerals and crystals exhibiting
these properties to a sufficient extent that they are of value as
oscillation detectors in wireless telegraphy.

In the case of iron pyrites the writer has found that a specimen of this
mineral containing very little or no copper as an impurity does not
exhibit these properties to an appreciable extent.

[Illustration: Fig. 104. United Wireless Carborundum Detector
(horizontal type).]

In order to use the universal detector for minerals, a special contact
similar to that shown in Fig. 90 must be made. The contact is bored and
threaded on its under side to fit a brass pin 3/4 inch long and having
an 8-32 thread. The other end of the pin screws into the hole in the
bedplate. The large knurled portion of the contact permits it to be
raised or lowered without the fingers coming in contact with the
crystal. The crystal is clamped between the contact and the spring, _S_.
The position is varied until a sensitive spot is found and then the
pressure is carefully regulated by means of the large adjusting screw
until the signals in the telephone receivers are the loudest. If
possible avoid touching the crystals with the fingers, as the oil and
dirt, even though it cannot always be seen, spoils their value for long
distance work. Use instead a pair of steel forceps.

[Illustration: Fig. 105. United Wireless Carborundum Detector (vertical
type).]

The United Wireless Telegraph Co. makes use of carborundum in the
detectors shown in Figs. 104 and 105. The principal advantage of
carborundum over such substances as silicon, etc., is that it is not
affected by the heavy discharge of the transmitting apparatus and does
not require a new adjustment after each period of sending. All the
crystals will not work, and so a large cake should be purchased and the
desired crystals selected. The dark blue portions of the mass, which are
the hardest, will give the clearest tones in the telephone receivers,
and are preferable to the lighter colored crystals. Since the crystals
conduct better in one direction than in the other, as explained above,
the adjustment must be made with the view of determining in which
position the particular crystal will work the best.

Carborundum will produce sounds in the telephone receivers without the
aid of any battery, but for careful work a battery and a potentiometer
are necessary.

The other crystals given in the column merely require that the telephone
receivers be connected to the detector terminals as in the wiring
diagram in Fig. 108.

The Clapp-Eastham detector makes use of a crystal of iron pyrites held
in a brass retaining cup beneath the metal contact point. It is not
affected by strong signals and requires no battery or potentiometer.
When adjusted it will remain in a sensitive condition for a long time
without further attention.

[Illustration: Fig. 106. Clapp-Eastham Ferron Detector.]

*Silicon Detector.*—While the silicon and "perikon" detectors are
classed as mineral or crystal detectors they deserve special attention.

[Illustration: Fig. 107. Silicon Crystal in Cup.]

Silicon gives fair results if a crystal is placed between two metal
electrodes as, for instance, between the contact and spring of the
"universal" detector, but is much more sensitive when properly mounted.
A brass cup such as that shown in Fig. 90 is made and the interior
brightened by scraping with a file. The cup is then poured full of a
molten fusible alloy and the silicon pressed in it until it cools and
becomes set. It should then present an appearance similar to that shown
by _A_ in Fig. 107. The silicon is ground down by rubbing on the surface
of a clean oilstone kept well wet with water, until the surface is flat
and shows a polish.

[Illustration: Fig. 108. Silicon Detector Circuits.]

The cup containing the silicon is placed over the hole in the bedplate
of the universal detector. A knurled brass thumbscrew having a point on
its lower end is screwed into the collar on the spring, _S_, and brought
to bear on the polished surface of the silicon. The pressure may be
easily regulated by means of the large adjusting screw until the signals
in the telephone receivers are the loudest. It is not advisable to
fasten the cup to the bedplate but merely to brighten the bottom so as
to insure a good contact. The cup may then be moved around so that
different portions of the polished surface of the silicon may be brought
into play when desirable.

If the knurled brass thumbscrew is fitted with a platinum point which
can be brought to bear on the surface of the silicon, the efficiency of
the detector will be materially increased.

When mounting silicon or other crystals some careless experimenters use
lead or a metal having a high melting point instead of a fusible alloy.
This is poor policy because the high temperature coats the surface of
the crystals and the interior of the cup with a non-conducting layer
which destroys the sensitiveness and makes it unfit for long distance
work. A fusible alloy melting at about the boiling point of water or
even lower should be used. Such alloys are usually composed of tin, lead
and bismuth. The addition of a little cadmium serves to make the fusing
point considerably lower in each case. The alloys may be prepared by the
experimenter from the following formulae, or are obtainable from a firm
manufacturing fire plugs for automatic fire extinguishers.

The lead should be melted first and then the bismuth, tin and cadmium
added in the order named.

*Perikon Detector.*—The Perikon detector is one of the latest types to
come into extensive use. It consists of two crystals, zincite and
chalcopyrites,⁴ set in cups in the manner just described and placed in
contact with each other. The minerals are mounted similar to those in
Fig. 105. The zincite should present a rather flat surface with the
grain of the crystal parallel to the sides of the cup so that the top
surface corresponds to the end of a stick of wood sawed at right angles
to the grain. More than one crystal of zincite is usually set in the
same cup. The chalcopyrites should present a rather blunt point. The cup
containing the chalcopyrites is the smaller and is bored and threaded to
fit a thumbscrew which passes through the collar in the spring, S, of
the "universal" detector. The bottom of the cup containing the zincite
is brightened so as to insure a good contact and then placed on the bed
plated under the cup containing the chalcopyrites which is fastened to
the thumbscrew. The zincite may then be moved around until the most
sensitive portion is found. The chalcopyrites is lowered until it comes
into contact with the zincite and then the pressure regulated by means
of the large adjusting screw.

[Illustration: Fig. 109. Perikon Detector Elements.]

The Perikon detector gives excellent results without a battery and is
preferably used in that manner. If a battery is used, a potentiometer to
lower the voltage is necessary.

When adjusting this or the carborundum detector where a battery is used,
the pressure must be very carefully regulated until it is found to be
the best. When the pressure is light the signals in the phones are due
to an imperfect contact, and when it is slightly increased the
rectifying properties of the crystal are brought into play.

The Perikon detector illustrated in Fig. 110 is somewhat similar to that
used for commercial work.

The standards or posts supporting the cups which contain the elements
are brass rods 1/2 inch square and 1 1/2 inches high. A hole is bored in
the bottom of each and threaded with an 8-32 tap to receive a machine
screw which passes through the base and holds them in an upright
position. A hole is bored 1 1/8 inches from the bottom, in the face of
one standard and threaded with an 8-32 tap. A brass rod 1 1/4 inches
long, carrying at one end a cup 1 inch in diameter and 3/8 inch deep, is
threaded to fit in the hole in the standard. The zincite is mounted in
this cup.

[Illustration: Fig. 110. Perikon Detector.]

The other standard is cut in half with a hack saw and a 1/8-inch hole
bored 1/4 inch deep in the axis of each piece. A pin, 1/2 inch long, is
set in the lower half by soldering it in the hole. The upper half of the
standard is placed over the pin and left free to move when twisted. A
1/8-inch brass tube, 1 inch long, passes through the upper part of the
standard. A 1/8-inch brass rod, 1 3/4 inches long, passes through the
tube.

The small cup containing the zincite is mounted on one end of the rod
and a hard rubber handle on the other.

A brass spring is placed between the cup and the standard in order to
press the chalcopyrites against the zincite. The cup is mounted out of
center so that by revolving it and twisting the standard at the same
time the chalcopyrites may be brought into contact with any portion of
the zincite. By screwing the rod supporting the zincite cup in or out of
the standard the pressure with which the two elements are pressed
together may be regulated.

The base of the detector is hard rubber of the dimensions indicated in
the illustration. Four binding posts on each corner of the base are
necessary. The detector is connected in a similar manner to the silicon
detector shown in Fig. 108. If a battery is used the circuit should be
like that of the "bare point" electrolytic, and the current must flow
from the zincite to the chalcopyrites.



LEAD PEROXIDE DETECTOR.


The peroxide of lead detector makes use of no liquids, but still may be
classed as an electrolytic since its action is of that nature.

[Illustration: Fig. 111. Peroxide of Lead Detector.]

It consists of a pellet of lead peroxide held between an electrode of
lead and one of platinum. Contrary to most other detectors, the
resistance is increased upon the passage of electrical oscillations. The
oscillations stimulate chemical action and increase a
counter-electromotive force sufficiently so that a decrease in the
current sent through the phones by the local battery takes place. The
action may be outlined more in detail as follows. The current of the
local battery decomposes part of the lead peroxide into its components,
lead and oxygen. The lead ions are positively electrified and so they
tend to pass upward toward the lead electrode which is negative. The
negative ions of oxygen gas tend to pass downwards towards the platinum.
But the lead and platinum electrodes with the intervening lead peroxide
constitute a small cell acting independently of the local battery and
sending a current in the opposite direction. This counter-electromotive
force tends to send the ions in an opposite direction to that in which
they are sent by the battery current. Upon the passage of electrical
oscillations this counter electromotive force is increased and
sufficient ions sent out in opposition to those of the battery current
so that an appreciable drop in the current flowing through the telephone
receivers takes place. The sudden current drop produces a sound in the
receivers.

The lead pellets may be secured from a druggist who can mold them in his
tablet press. They should be subjected to as great a pressure as
possible in order to reduce resistance and prevent crumbling.

A piece of platinum foil about 1/2 inch square is placed beneath the
pellet on the crystal electrode. A piece of clean, bright sheet lead 3/8
inch in diameter and 1/8 inch thick is laid on the pellet and the whole
clamped together by tightening the thumbscrew passing through the collar
on the spring, _S_. The detector is connected up similar to the "bare
point," The platinum is made the positive of the local battery.
Adjustment is secured by regulating the pressure.

It is very necessary that the pellets be kept dry, as otherwise a loud
singing and hissing noise, due to the decomposition of the water, will
render the reception of signals very difficult.



THE MARCONI MAGNETIC DETECTOR.


When an oscillatory discharge takes place through a coil of wire
surrounding a needle, it magnetizes the needle in a totally different
manner from a voltaic current. The needle will have several poles
throughout its length, many of them reversed. Rutherford applied this
phenomenon to the detection of electrical oscillations, but it remained
for Marconi to improve it and give the magnetic detector its existing
form. This type of detector is very sensitive, free from all adjustment
and is not made inoperative by the heavier discharge of the transmitter
during each period of sending.

[Illustration: Fig. 112. Marconi Magnetic Detector.]

A small transformer is provided with a core composed of a band or cord
of iron wires in the form of an endless belt which passes around two
pulleys kept in motion by a clockwork motor. The band revolves in the
field of a strong horseshoe magnet and passes directly over the poles
after issuing from the transformer bobbin, so that the portion
approaching the bobbin are constantly in a state of increasing
magnetism. The actual operation is based upon the property of iron
called hysteresis, for the magnetism of the core lags behind that of the
permanent magnet and is of a different degree from what it ought to be,
in view of its position in the vicinity of the permanent magnet. The
moment the oscillations pass through the primary coil of the
transformer, this lag is set free and the magnetism assumes its full
value. The change in magnetism induces a current in the secondary, which
registers as a sound in the telephone receivers.

[Illustration: Fig. 113. Details of Transformer.]

The primary coil is made up of a single layer of No. 36 B. S. gauge silk
covered wire wound on a thin walled glass or hard rubber tube. The ends
of the tube, which is 2 inches long and 1/4 inch external diameter, are
slightly flared so as not to chafe the band. The primary is thoroughly
shellacked and covered with a single layer of paper.

The secondary is also of No. 36 B. S. silk covered wire and is wound
between two disks of hard rubber, 1/4 inch thick and 1 1/2 inches in
diameter, placed 3/8 inch apart in the center of the secondary and the
intervening space wound full of wire.

The terminals of both the primary and secondary are extended to binding
posts mounted on the case of the instrument.

[Illustration: Fig. 114. Method of Joining Ends of Band.]

The core or revolving band is made by winding 100 strands of No. 36 silk
covered soft iron wire between two small pegs, placed a distance apart,
equal to twice the circumference of the oval formed by the two pulleys.
The wire is all wound in the same direction. It should be carefully
removed from the pegs and kept taut while it is slightly twisted,
doubled, and then further twisted into a rope or cord. The ends are
threaded together with a separate piece of insulated wire, into a link
which will pass easily through the primary tube.

[Illustration: Fig. 115. Pulley.]

The pulleys around which the band revolves are made of hard wood. They
are 4 inches in diameter and 3/8 inch thick and have a V-shaped groove
cut in the edge. In order to minimize friction and wear, it is advisable
to fit them with a bearing which may be made out of brass tubing and a
couple of washers. One of the pulleys is geared to a clockwork motor so
that the band makes a complete revolution about once every two minutes.
An old eight day clock may be adapted for this purpose, or, what is much
better, the motor from an old phonograph.

[Illustration: Fig. 116. Pulley Bearings.]

The horseshoe magnet is mounted with its north pole pointing towards and
nearly touching the middle of the outside of the secondary. The south
pole is placed opposite the end of the primary tube which is on the side
towards which the band is revolving, that is, the band in revolving
passes first over the north pole and then over the south. Two magnets
are sometimes used with their north poles together in the center of the
secondary, and a south pole opposite each end of the primary tube.

[Illustration: Fig. 117. Circuit of Magnetic Detector.]

The commercial instrument is placed in a glass covered case which
protects it from dust and injury. The clockwork motor is concealed in
the lower part of the case. This is a good plan but an experimental
detector may be merely mounted on a flat wooden base as illustrated in
Fig. 112.

The secondary terminals are connected directly to the telephone
receivers while the primary leads to the aerial and the ground. When the
detector is started up it should make a very slight hissing sound in the
telephone receivers as the band passes slowly through the coils. This
shows the instrument to be in good working order and ready for the
reception of signals.

*The Audion.*—Dr. Lee DeForest was led by the flickering of a sensitive
gas flame to investigate whether or not it would respond to Hertzian
vibrations as well as to those of heat and sound. His experiments led to
the invention of the audion, a peculiar instrument making use of ionized
gas for its operation.

The audion consists of an incandescent lamp having a metallic filament,
on either side of which are a grid and a plate made of nickel. When the
filament is lighted it throws off ions which act as a relay to high
frequency oscillations passing between the plate and the grid. A
properly constructed audion is exceedingly sensitive and produces very
loud tones in the telephone receivers. It has the further advantages of
entire absence of adjustment except the governing of the battery
voltage, and is capable of extremely fine tuning.

[Illustration: Fig. 118. Fleming Oscillation Valve.]

Fleming originated the oscillation valve illustrated in Fig. 118. It
consists of an ordinary incandescent lamp with a carbon filament, having
a metal cylinder, _C_, placed around the filament, but attached to an
independently insulated platinum wire sealed in the glass. When the lamp
is lighted by passing a current through the filament, the incandescent
carbon liberates negative ions. If oscillations are then set up in a
circuit which includes a pair of sensitive telephone receivers and is
formed by connecting the negative terminal of the filament with the
platinum cylinder, negative electricity will be enabled to pass from the
filament to the cylinder but not in the opposite direction, and so
sounds will be produced in the telephone receivers. High frequency
oscillations themselves could not be made to pass through the telephone
receivers because of the choking action of the iron cores of the
electromagnets.

[Illustration: Fig. 119. Flame Audion.]

The simple but sensitive form of detector illustrated in Fig. 119 is not
of practical value for commercial work, but is very interesting as the
progenitor of the audion, and provides a good field for amateur
investigation. Its only drawback is that the gas flame is very difficult
to keep steady and every flicker registers as a sound in the telephone
receivers.

A Bunsen burner using coal gas furnishes the flame, and a salt of an
alkaline metal heated in the flame, the ions. The hydroxides of caesium,
potassium and sodium give the best results in the order named.

The salt is contained in a piece of trough-shaped platinum foil, about
3/8 inch long and 1/16 inch wide. This trough is made the cathode or
negative of the telephone circuit and placed in the outer oxidizing
flame just above its juncture with the interior reducing flame and must
be kept incandescent. The upper electrode or anode is a piece of
platinum wire about 1/16 inch above the trough.

[Illustration: Fig. 120. Circuit of Flame Audion.]

The arrangement and construction of the detector is clearly indicated by
the drawing so that it is unnecessary to go into details. The block,
_E_, which fits on the tube of the Bunsen burner, is made of fiber. Two
double binding posts, _D_, are fastened to _E_ to support the rods, _R_,
which are fitted at the tops with binding posts, _B_, into which the
electrodes may be clamped.

Twelve dry cells are connected with a multiple point switch so that an
electromotive force of 6-18 volts, varying in steps of one cell at a
time, may be secured. The flame is best provided with a mica chimney to
protect it from drafts. By keeping plenty of salt in the trough and
carefully adjusting the voltage, this detector may be made marvelously
sensitive.

    ³ The different detectors in order of their sensitiveness are
      electrolytic, perikon, magnetic, silicon, carborundum.

    ⁴ Peacock ore or bornite, which consists of about 60 parts of
      copper, 14 parts of iron and 26 parts of sulphur, may be
      substituted for the chalcopyrites with excellent results.




CHAPTER XIV. TUNING COILS AND TRANSFORMERS.


A tuning coil is merely a variable inductance wound in single layer on a
suitable form.

Fig. 121 illustrates a double slide tuner. The base is a piece of hard
wood, 12 inches long, 1 inch thick and 5 1/2 inches wide. Two wooden
heads 4 x 4 x 3/4 inches support the form upon which the coil is wound.

[Illustration: Fig. 121. Double-slide Tuning Coil.]

The form is a piece of wooden curtain pole, 9 inches long and 3 inches
in diameter. Some may prefer to use a cardboard tube in place of the
curtain pole. A tube can be made by winding a long strip of cardboard 9
inches wide around a suitable form and cementing the layers together
with shellac. The liberal use of shellac will stiffen the tube and cause
it to better retain its shape. The tube is held tightly between the two
heads by means of a brass rod which passes through the center and is
clamped by two nuts.

A square brass rod 10 1/2 inches long is fastened to the center of the
top of the heads and a similar rod to the center of the front face.

[Illustration: Fig. 122. Sliders.]

Fig. 122 shows two forms of sliders. The first one is the better and to
be preferred. A short square brass tube, _S_, fits snugly upon the
square brass rod, _R_. It cannot turn around but is free to slide back
and forth. A strip of spring brass, _C_, is soldered to the lower face
of the square tube. It is bent in a double turn and a punch mark made
near the lower end as shown in the illustration. The indentation is made
with a center punch, but should not be deep enough to break through the
metal.

[Illustration: Fig. 123. Double-slide Tuning Coil Circuits.]

The little projection on the under side of _C_ caused by the punch mark
is the only part of the slider which should make contact with the wire
on the tuning coil. It should slide easily but firmly along the wires
and touch only one at a time. Long distance signals will be considerably
weakened if the slider touches more than one wire at a time and
short-circuits a turn.

[Illustration: *Plate IV. Receiving Circuits. (Straightaway Aerial.)*]

The slider, _B_, is similar to _A_ except that it has a short length of
brass tubing, _T_, soldered to the under side of _S_ in place of the
brass strip, _C_. A small ball bearing which just fits the bore of the
tube is pushed down into contact with the wire by means of a small
spiral spring inside of the tube. Both sliders are fitted with a hard
rubber handle so that they may be adjusted without the ringers coming
into contact with the metal.

[Illustration: Fig. 124. Murdock Double-slide Tuning Coil.]

[Illustration: Fig. 125. United Wireless Receiving Set.]

Two good circuits employing the double slide tuner are given in Fig.
123. Plates IV and V illustrate the oscillation or tuning circuits of
the most prominent receptor systems.

If a loop aerial is used, more than one tuning coil is necessary as
shown by the loop aerial oscillation circuits in Plate V.

Figs. 125 and 126 illustrate the instruments employed for receiving by
the United Wireless Company.

[Illustration: Fig. 126. United Wireless Portable Receiving Set.]

In Fig. 125 the handles which are attached to the sliders of the tuning
coils project through long slots cut in the top and one side of the
cabinet.

The tuning coils in the portable outfit are mounted in a vertical
position in back of the aerial switch.

*Loosely Coupled Tuning Coil.*—By the use of a loosely coupled receiving
tuner or transformer, the range of a station is considerably increased,
as is also the strength of the signals, and much finer tuning and
selectivity made possible.

Fig. 127 illustrates the construction of such an oscillation
transformer.

The base is wood and measures 14 x 5 1/2 x 1 inches. The primary winding
is wound on a cardboard or fiber tube 4 1/4 inches long, having an
internal diameter of 2 3/4 inches and an external diameter of 3 inches.
The heads, _L_ and _M_, are the same size as those of the double slide
tuning coil. The head, _L_, has a circular hole 2 3/4 inches in diameter
cut in the center in order to permit the secondary coil to slide in and
out of the primary.

[Illustration: Fig. 127. Oscillation Transformer.]

The secondary coil is a piece of round curtain pole 2 1/2 inches in
diameter and 3 inches long. A 5/16-inch hole is bored through its axis.
The head, _K_, of the secondary coil is 3 3/4 x 3 3/4 x 3/4 inches. A
ten-point switch on _K_ is so connected that it divides the secondary
into ten equal parts and permits any number of the divisions to be used
as desired.

[Illustration: Fig. 128. United Wireless Receiving Transformer.]

A wooden post, _J_, 2 1/2 inches high and 1 1/4 inches wide, supports
one end of a 1/4-inch brass rod upon which the secondary slides back and
forth.

[Illustration: Fig. 129. Details of Receiving Transformer.]

No. 24 B. S. gauge copper wire may be used for winding both the primary
and secondary. It is also the proper size to use on the double slide
tuner. The best method is to use bare wire, wound with a thread so that
a thread is interposed between adjacent turns of the winding. Give the
whole winding one or two coats of thick shellac and allow it to harden.
Then use a strip of sandpaper to remove the shellac in a long narrow
path immediately below the sliders so that they may make contact with
the wire.

[Illustration: Fig. 130. Slider for Loose Coupler.]

Some may prefer to make a loosely coupled tuner in which the inductance
of both coils is adjustable by means of a sliding contact. In such a
case the slider on the secondary coil must be constructed as illustrated
in Fig. 130. The contact is long and narrow so that it can touch the
innermost turns, when placed within the primary. By slipping the slider
off the end of the rod and reversing it, the contact can be made to
touch the turns next to the head. The square brass rod is set in a notch
cut in the coil head so that the rod is flush with the top.

When tuning a receiving transformer, place both variable condensers in a
halfway position and adjust the sliding contacts, first on the primary
and then on the secondary, until the signals are the loudest. Then
adjust the condensers.

[Illustration: Fig. 131. Loosely Coupled Tuning Circuits.]

To cut out an undesirable station, vary the coupling between the two
coils by sliding the secondary away from the primary. When several turns
on the secondary seem to give the same results also vary the coupling.

[Illustration: Fig. 132. Combination Loosely and Closely Coupled Tuner.]

Fig. 132 illustrates the wiring diagram of a combination loosely and
closely coupled tuner. Two sliding contacts are placed on the primary
coil of the receiving transformer and connected with a double pole
double throw switch as in the diagram. When the switch is thrown on
contacts 1 and 2, the primary is connected to the detector as a double
slide tuner, and when on 3 and 4 both the primary and secondary are
brought into use as a transformer.

This arrangement may seem cumbersome and is recommended only as a
convenience in experimenting. A loosely coupled tuning coil is capable
of exact tuning, and unless one understands how to use it, he may not
hear a station because the tuner is not properly adjusted. By using the
double slide tuner first and then throwing the switch so as to tune in
on the transformer, this difficulty may be eliminated.

[Illustration: Fig. 133. Clapp-Eastham Loose Coupler.]

*Potentiometer.*—A potentiometer is not properly classed under the
heading of tuning coils, but the construction may be made so similar
that it well appears here.

The potentiometer is merely a variable resistance shunted across the
terminals of the detector battery in the manner illustrated in the
numerous detector circuits. It is used to reduce the voltage of the
battery to a value slightly below the critical voltage of the detector.
The critical voltage of a detector is the voltage at which its action
commences. In the case of an electrolytic detector, it is the voltage
required to break down the thin film of gas which collects on the "bare
point."

[Illustration: Fig. 134. A Highly Efficient Form of Loose Coupler.]

In construction, the potentiometer illustrated in Fig. 135 is in reality
a small edition of a double slide tuning coil. It is wound with No. 28
B. S. gauge German silver wire. Three binding posts are mounted on the
base, two of them connecting with the ends of the coil and one with the
sliding contact.

[Illustration: Fig. 135. Potentiometer.]

In a finely balanced circuit where long distance work and close tuning
are desired, the potentiometer must be non-inductive.

[Illustration: Fig. 136. Amco Potentiometer.]

This may be accomplished by using two potentiometers wound in opposite
directions from one another and connected in series. The two terminals
of the windings are then connected across the battery and the sliding
contacts led to the detector.

[Illustration: *Plate V. Receiving Circuits.*]




CHAPTER XV. RECEIVING CONDENSERS.


Condensers play an important part in tuning and adjusting the receiving
circuits of a wireless telegraph station. They are inserted in the
circuits for various purposes. In some places a small condenser shunts
the detector to somewhat equalize any small changes in capacity which
might occur in the detector and throw the circuit out of tune. In other
cases where selectivity is desired they provide a path for undesirable
oscillations and allow them to flow into the ground without passing
through the detector. Wherever the double slide tuner or the Fessenden
single slider circuit is used, a condenser must be inserted in the
circuit to prevent the wire of the tuning coil from short-circuiting the
telephone receivers and battery.

[Illustration: Fig. 137. Tuning Circuit with and without an Adjustable
Condenser.]

The value of a condenser may be readily appreciated by the following
experiment. Connect up a detector according to the diagram shown by _A_
of Fig. 137. This circuit will give good results and the signals will be
clear. But change it to that shown in _B_, by connecting one side of a
variable condenser to the upper contact of the tuning coil and leading
the other side to the ground. Considerable selectivity may now be
attained and the signals will be 50 per cent louder.

To be of any value for a receiving circuit the capacity of the condenser
must be adjustable, but there are many places where a fixed condenser is
of service.

The average capacity of such condensers generally ranges around .003 of
a microfarad, but cannot be predetermined, as it depends upon many
factors which vary greatly in different stations. Even if specific
dimensions were given for the construction of a paper condenser of
stated capacity, the experimenter would very seldom succeed in
constructing his condenser and have it of this value when finished. The
paper used as the dielectric and the pressure applied to the condenser
would make the difference.

The best plan is to build one up in the form of a roll as follows. Three
thin tin foil strips 3 1/2 inches wide and four feet long are separated
by strips of thin paraffined paper 4 inches wide and four and one-half
feet long. The two outside strips of tin foil are connected together and
form one terminal of the condenser. The middle strip of tin foil is the
other. The outside strips of tin foil are covered with paraffined paper
and the whole rolled up. If thin paper and tin foil are used the
condenser will form a roll 4 inches long and less than an inch in
diameter.

Two or three such condensers should be constructed, and one of them
connected up in the circuit where it is desirable to use it. By
unrolling a little of the condenser and cutting one or two inches off at
a time the proper size may be determined.

As explained in Chapter VII, two equal condensers connected in series
have one-half the capacity of either. By connecting them in parallel the
capacity is the sum of the two. In this manner it is not hard to first
find the approximate capacity, which gives the best results before
cutting the condenser.

[Illustration: Fig. 138. Tubular Condenser.]

When the proper value is found, place the condenser in a brass tube
about 5 inches long and 1 inch in diameter. The tube is fitted with hard
rubber flanges to close the ends. A binding post is mounted on each
flange, and connects with the terminal of the condenser to which it is
nearest.

Mounting a condenser in this manner gives it a much better appearance
and it occupies less space than otherwise. Fixed condensers are used in
two cases only, to shunt the condenser and to prevent the tuning coil
from short-circuiting the telephone receivers. In any other position
they are worthless.

*Variable Condensers.*—A simple form of adjustable condenser which may
be quickly made for a special experiment is constructed in the following
manner. A wooden curtain pole 2 inches in diameter and 18 inches long is
covered with a layer of tin foil. The tin foil must be laid on smoothly
and cemented with shellac. A layer of paraffined paper is then placed
over the tin foil. A piece of sheet copper or tin 6 3/4 inches wide and
18 inches long is rolled up in a cylinder to fit over the rod with its
tin foil and paper. The rod is fastened at one end to a base board about
20 inches long and 4 inches wide, by means of a bracket, or it may be
mounted on a smaller base in a vertical position.

Connections are made to the tin foil and to the outside metal tube. By
sliding the tube back and forth on the rod the condenser may be given a
variable capacity.

[Illustration: Fig. 139. Variable Condenser.]

The best variable condensers are constructed so that the dielectric
between the two plates is formed by air. There are consequently no
losses of energy in the condenser, for this mode of construction
eliminates all dielectric hysteresis.

Fig. 139 shows such a variable condenser and Fig. 140 the details. It is
possible to do very close, selective tuning with this instrument. The
efficiency of the receiving circuit may be considerably improved if one
is used wherever a condenser is required.

A rectangular box is built up in the manner shown in Fig. 135. The
sides, _D_, are 6 inches long, 4 11/16 inches wide and 1/2 inch thick.
The top, _A_, is 6 inches long, 1 13/16 inches wide and 1/2 inch thick.
The bottom, _B_, has the same width and thickness but is 11 inches long.
Eleven grooves are cut in _A_ and _B_ as shown in the cross section. The
grooves are 1/16 inch wide, 3/16 inch deep and 1/16 inch apart. They are
formed by setting the blade of a circular saw so that it projects 3/16
inch above the table or bed. After cutting one groove, the guide is
moved 1/8 inch and another cut made.

[Illustration: Fig. 140. Details of Variable Condenser.]

Eleven metal plates, _P_, are required, six fixed and five movable. They
are made of No. 22 gauge sheet brass and measure 4 x 6 inches. Six of
them are placed in the grooves in the box in such a manner that an empty
groove is left between each one. The six plates are then electrically
connected together by soldering a strip of brass across their rear ends.

[Illustration: Fig. 141. Sliding Plate Variable Condenser.]

Five slots each 1/4 inch deep and 1/4 inch apart are made with a hack
saw in a piece of brass, _C_, 1 1/2 inches long, 3/4 inch wide and 1/2
inch thick. A small pin 1/2 inch long is set in the center of _C_ on the
opposite side from the saw cuts. A fiber handle 1/2 inch in diameter and
1 1/2 inches long is fastened to the pin.

The five movable brass plates are set in the grooves between the six
fixed plates. They are allowed to project three or four inches out of
the box. The yoke, _C_, is then soldered across the ends so that each
one of the movable plates fits into its corresponding saw cut.

The capacity of the condenser is varied by sliding the movable plates
back and forth between the fixed plates.

A binding post is soldered to the yoke, _C_, and another one to the
strip which holds the fixed plates together. Connections are made to the
binding posts with lamp cord or some other flexible conductor which will
not interfere with the movement of the plates.

The instrument is finished by staining the woodwork and giving it a coat
of varnish or shellac.




CHAPTER XVI. TELEPHONE RECEIVERS AND HEADBANDS.


A pair of high resistance telephone receivers in nice adjustment
constitute one of the most sensitive electrical instruments in existence
and will detect an exceedingly weak current.

The only type of receiver of much service in wireless telegraphy is that
known as a watch case or pony receiver. It is small and compact so that
it may be attached to a headband and clamped against the ear.

[Illustration: Fig. 142. Types of Permanent Magnets.]

The permanent magnets of a watch-case receiver are usually in the form
of either a ring or a horseshoe as shown by Fig. 142. The first form has
"consequent" poles and is considered somewhat superior to the horseshoe,
since the lines of force are not so liable to pass across the pole
pieces before they pass through the electromagnets and the diaphragm.

The ordinary low resistance telephone receiver is perfectly well suited
to the telephone work for which it was designed and adjusted, and will
give good service on a wireless receptor for short distances, but can be
considerably improved by following the suggestions and instructions
given below.

The principal objection to the ordinary receiver is that it does not
contain enough turns of wire on its bobbins. This is easily remedied by
carefully rewinding them with a very fine silk covered, pure copper
magnet wire no larger than No. 40 B. S. gauge. This will increase the
number of turns and also the resistance, but it must not be inferred
that resistance is to be desired. This is a common impression of
amateurs who do not understand the underlying principle, that the
strength of an electromagnet varies directly as the number of turns of
wire, multiplied by the amperes flowing through the magnet. When a
telephone receiver is wound with a finer wire the resistance is
increased, cutting down both the current and the strength of the magnet.
But if pure copper wire is used, and the winding not carried beyond the
point where the circumference of the outside layer becomes twice as
great as the circumference of the first layer, the number of turns
increases faster than the resistance and the magnet strength is
considerably greater than before the receiver was rewound.

No. 40 B. S. gauge silk covered wire is often used for this purpose, but
the best results are obtained with enameled covered wire of the same
size. It is possible to wind almost three times as much of the enameled
wire on a telephone bobbin as silk wire of the same size. The difference
is due to the thickness of the insulation. An ordinary double pole
watch-case receiver will have a resistance of 800-1,000 ohms when wound
with silk covered wire and 1,500-1,800 ohms when wound with enameled
wire.

To rewind a telephone receiver, first unscrew the cap and remove the
diaphragm, then remove the bobbins by unloosening the screws with the
aid of a screw driver. Unwind the old wire and examine the empty bobbins
to see that wherever the wire is liable to come into contact with the
metal that it is well insulated with paraffined paper or some other
equally good material. Then wind the new wire on in smooth even layers
and when it is completed fasten the bobbins back on the permanent
magnets and connect them up. The current should flow through in opposite
directions so that the north pole of one and the south pole of the other
is on top. Do not trust splice connections but solder them using acid as
a flux.

The Navy Department specifies that its wireless receivers shall be wound
with copper wire of not less than 0.0015 inch in diameter and the
diaphragm to have a diameter of 1 3/4 inches and a thickness of 0.004
inch. The resistance of the coils is specified at 1,000-1,100 ohms.
There is not much advantage in greatly exceeding the number of turns
possible with this winding, for to obtain them a much finer wire than
No. 40 B. S. gauge is necessary and the ratio between resistance and
turns becomes greater.

The second objection to the ordinary receiver is that the diaphragms are
very often too thick. A receiver having a thin diaphragm is preferable
because when a weak current is sent through the coils, the change in
magnet strength is greater. But this may be carried to excess and the
diaphragm made so thin that it cannot absorb sufficient lines of force
to properly play its part. The best thickness then for a diaphragm can
only be determined experimentally and depends much upon the diameter.
The distance from the poles and the strength of the magnets will also
have considerable bearing on the thickness. The ordinary phone will be
very sensitive and give clear tones with diaphragms ranging from
.01-.004 inch.

The relation between the thickness and the diameter is shown by the
following: If the diaphragm of a receiver is increased in diameter, the
tones will become more distinct, but if the increase is carried too far
they will become indistinct and the only remedy is to thicken the
diaphragm. Likewise if after clearness is secured the diaphragm is
thickened so that the tones again become indistinct, the diameter must
be increased.

[Illustration: Fig. 143. Grinding Tool.]

The third objection is that such receivers are not carefully and
properly adjusted. The adjustment is also a matter of experiment and is
accomplished by comparison of the receiver in question with one which is
known to be in a sensitive condition. The adjusting may be done by means
of the tool shown in Fig. 143.

This tool is made from an ordinary file by grinding off the teeth on one
side save for a distance of about 3/4 inch in the middle. The grinding
may be done on an emery wheel. The part (_a_) is used for filing the
pole pieces and thus making the distance between them and the diaphragm
greater. The tool has the advantage over an ordinary file of permitting
the poles to be filed without removal from the receiver and without
grooving the diaphragm bed or the receiver case.

To lessen the distance between the poles and the diaphragm, lay the
receiver bed downward on a piece of fine emery paper and rub with a
circular scouring motion. If the emery paper is placed on a perfectly
flat surface no trouble will be experienced in grinding the bed down
evenly.

When filing the pole pieces rub with the same circular motion so as to
grind off all sides evenly. Test from time to time by passing a straight
edge over the bed in all directions while holding to the light and
looking between the straight edge and the poles. In this manner the
distance separating the diaphragm and the poles may be gauged and
whether or not it is the same on all sides. Bear in mind that if the
diaphragm is thin, the attraction of the permanent magnets will cause it
to bend in towards the poles.

In case you have a pair of receivers built for wireless work, which
appear to be in good condition but do not give their former results, the
last thing to do is to tamper with the adjustment. The most common
cause, when the tones are impaired, is dirt or dust accumulated on the
poles or diaphragm and damping its vibration. The cap should be
carefully unscrewed and the diaphragm examined to see if it is bent. If
so, replace with a new one of the same size. Remove any dirt or filings,
and if the diaphragm is rusty clean it by laying it on a flat surface
and rubbing it with a piece of fine emery paper. Then give it a thin
coat of colorless lacquer. Examine the magnets and pole pieces to see if
they have become loosened and if so tighten them.

Or the trouble may be that the permanent magnets have lost part of their
magnetism, and almost any receiver which has been in use for some length
of time will bear having its magnets strengthened. If they are found to
be weak they should be removed and remagnetized. This is accomplished by
winding a coil of No. 18 B. S. gauge wire around them and sending a
heavy direct current through for a few minutes.

[Illustration: Fig. 144. Parts of a Holtzer Cabot Receiver.]

In carrying out any of these suggestions remember to work with one
receiver at a time, keeping the other for comparison, so that by
repeated tests you may tell whether or not an improvement is being made
and when well enough is reached, let it alone.

Fig. 144 shows the construction of a telephone receiver manufactured by
the Holtzer Cabot Co. of Brookline, Mass. The permanent magnets and
bobbins are mounted in a metal cup, _B_, which supports the diaphragm,
_A_. The metal cup is enclosed in a hard rubber shell, _C_, and fitted
with a cap, _D_.

The complete receivers are mounted on an adjustable headband and fitted
with pneumatic ear cushions which make them set more comfortably and
shut out extraneous noise. These receivers are wound to all resistances
used in the wireless field but, for the experimenter, those having a
resistance of 1000 ohms apiece will give the best all around results.

[Illustration: Fig. 145. Holtzer Cabot Head Set.]

It is very desirable that the receivers should be fitted with a
comfortable headband which will hold the telephones in close adjustment
to the ears. Fig. 146 shows the construction of such a band.

[Illustration: Fig. 146. Adjustable Head Band.]

Two brass straps, 1 inch wide, 12 inches long and 1/16 inch thick are
necessary. Two 1/8-inch holes, _mm_, are bored in them 3/16 inch from
one end and the end bent up at an angle along a line 3/8 inch from the
end as shown by the drawing. A cover is made for the straps, by sewing
two strips of leather 1 1/4 inches wide and 12 inches long, together
along their edges. The covers may then be slipped over the straps. Two
1/16-inch brass strips, 3/8 inch wide and a little longer than one-half
the circumference of the watch-case receiver to be fitted to the head
band, are bent into stirrups as shown in the illustration by _Y_. The
ends of the stirrups are fitted with two pivots, _PP_, which are riveted
in a hole. Two holes or recesses are bored in the shell of each
receiver, on the sides 180 degrees apart. The pivots, _PP_, fit into
these holes and form bearings in which the receivers can turn and adjust
themselves to the ears. The stirrup has two small holes, _hh_, bored 3/8
inch apart at the top and tapped to receive a small screw. The screws
pass through the holes, _mm_, and hold the stirrups at the end of their
respective strap.

The leather-covered straps are bound together by means of two clamps
which permit the head band to be adjusted to suit the wearer. The
clamps, _C_, are made from 1/16-inch brass 3/8 inch wide. Their form and
construction may be best understood from the drawing. A small screw eye
is cut in half and soldered to the center of the upper side of the clamp
so that the receiver cord may be passed through the eyelet, _E_.

The telephone receivers should be connected in series by means of a
flexible telephone cord.




CHAPTER XVII. OPERATION.


The first essential in order to manage a wireless station, after
learning the construction and handling of the instruments, is to acquire
a thorough knowledge of the telegraph codes.

The two codes in use in wireless service are the Morse and the
Continental. They are given below.

In some cases, the intending wireless operator has had some experience
with a Morse sounder and then it is merely a matter of transition and of
accustoming the ear to a new sound. However it is always best to learn
the Morse code first as Continental is merely an adaptation in which no
space characters appear.

A beginner may learn to receive most easily by communicating with
another person to whom it is also new. They should first memorize all
the letters of the code and practice transmitting before commencing any
communication.

It is a great mistake for a beginner to start by writing down the dots
and dashes as he receives them on paper. He should make an effort to
translate them and set the characters and words down directly. This at
first will sacrifice speed but will make a better operator and enable
one to become proficient sooner than if he begins otherwise.

In case two complete sets of wireless apparatus are not convenient, one
may learn to read from a buzzer connected to a key and a battery. The
signals may be read directly from the sound of the buzzer itself, but if
a pair of telephone receivers are connected across the terminals, a
sound will be produced in the phones similar to that of a wireless
receptor.

In sending, avoid a short, choppy or jerky style and handle the key with
a light but firm touch. Keep the dots and dashes firm and of the proper
relative length, as also the spaces between letters and words.

The following suggestions and instructions adapted from the Rules and
Regulations governing Naval Wireless Telegraph Stations may be of value
and service to many in operating their instruments.

"The operating room should be well lighted and free from vibration. The
room should have a well insulated entrance for the aerial and should be
fitted with an operating table about two and one-half feet wide, not
less than seven feet long and of a convenient height for working the
sending key.

The table should be of dry, well-seasoned wood and the instruments
should be mounted on the table at safe sparking distances from each
other.

"The connections should all be as direct as possible and well insulated.
High potential leads should be kept away from low potential leads and
where they cross it should be at nearly right angles.

"Sending key contacts must be kept clean and flat with surfaces parallel
to each other.

"All sliding contacts, especially in the receiving tuning coils, should
be kept clean and bright and free from foreign matter. A sending set
working at low power with all connections good, closed and open circuits
in resonance, no sparking from edge of condenser, jar or plates, no glow
from aerial and no sparking to rigging, is utilizing its power much more
efficiently than the same set pushed to the limit with high resistance
connections, sparking at all points and out of resonance."

[Illustration: Fig. 147. Marconi Station at Siasconset, Mass.]

It is a good plan for any amateur to keep a note book in which he can
record the various distances he has been able to receive and transmit.
He should try more than one circuit and jot down the results. It is then
possible to bring a station up to an efficiency which cannot be reached
in any other manner.



ELECTRICAL TERMS AND DEFINITIONS.


_Accumulator:_
       A cell whose positive and negative electrodes are formed or
       deposited by a current flowing from a separate source.

_Aerial:_
       A network or a number of wires insulated from surrounding objects
       and suspended in the air to emit or intercept electromagnetic
       waves.

_Alternating Current:_
       An electric current, of which the direction of flow reverses a
       number of times per second.

_Ampere:_
       The quantity of current which will flow through a resistance of
       one ohm under a potential of one volt. The unit of current.

_Ampere Hour:_
       The quantity of electricity passed when flowing at the rate of
       one ampere for one hour.

_Anode:_
       The positive terminal of a broken circuit or a source of
       electricity. It is the carbon of a cell or the pole at which a
       current enters a solution.

_Capacity:_
       The relative ability of a conductor to receive and retain an
       electric charge.

_Circuit:_
       An unbroken conducting path for an electric current.

_Condenser:_
       A device for receiving and storing up electrostatic energy.

_Cycle:_
       The full period of reversal of an alternating current. A 60-cycle
       current is one making 60 complete reversals per second.

_Detector, Wireless:_
       An instrument for detecting oscillations in the aerial of a
       receiving station. It either varies its own internal resistance
       or generates a weak intermittent direct current similar in time
       and duration with the signals emitted by the transmitting
       station.

_Dielectric:_
       A non-conductor or insulator. It usually refers to the material
       interposed between the oppositely charged coatings of a
       condenser.

_Direct Current:_
       A current flowing in one direction only.

_Electrode:_
       The terminal of an open electric circuit or a conductor carrying
       a current and immersed in an electrolyte.

_Electrolysis:_
       The separation of a chemical compound into its elements by the
       action of an electric current.

_Electrolyte:_
       A solution which will conduct a current of electricity.

_Electromagnet:_
       A mass of iron which is magnetized by the passage of a current of
       electricity through a coil of wire wound around the mass and
       insulated therefrom.

_Farad:_
       The unit of electrical capacity. A condenser having a capacity of
       one farad would be raised to a potential of one volt by one
       ampere flowing for one second.

_Field of Force:_
       The space which is under magnetic stress in the neighborhood of a
       magnet or a wire carrying a current.

_Helix, Transmitting:_
       A coil of heavy wire which furnishes the inductance for the
       closed oscillation circuit of a wireless transmitter and acts as
       an auto-transformer to raise the voltage of the high frequency
       currents.

_Henry:_
       The unit of induction. It is the induction in a circuit when the
       electromotive force induced in this circuit is one volt, while
       the inducing current varies at the rate of one ampere per second.

_Inductance:_
       The property of an electric circuit whereby lines of force are
       developed around it.

_Induction Coil:_
       An instrument or device consisting of two independent coils of
       wire wound around an iron core, and which by magnetic induction
       steps up an intermittent direct current from a low to a high
       voltage.

_Insulator:_
       A non-conductor or a substance impervious to the passage of
       electricity.

_Key:_ A device for making and breaking a current into periods
       corresponding to the dots and dashes of the telegraph code.

_Kilowatt:_
       1,000 watts. See watt.

_Leyden Jar:_
       A static condenser which will store up static electricity and is
       cylindrical in form. So-called because it was originated in
       Leyden, Holland.

_Multiple:_
       The term expressing the connection of several pieces of
       electrical apparatus in parallel with each other so that the
       current is divided between them.

_Ohm:_ The unit of resistance. It is arbitrarily taken as the resistance
       of a column of mercury, one square millimeter in cross sectional
       area and 106 centimeters in height.

_Parallel:_
       The same meaning as multiple. Parallel circuits are those which
       start at a common point and end at a common point.

_Polarization:_
       The collection of hydrogen upon the positive electrode of a
       primary cell with a consequent loss of voltage.

_Potential:_
       Voltage or electrical force.

_Resistance:_
       The quality of an electrical conductor whereby it opposes the
       passage of an electric current. The unit of resistance is the
       ohm.

_Rheostat:_
       A variable resistance to regulate the strength of an electric
       current.

_Series:_
       Opposed to parallel or multiple. Instruments in series are so
       connected that the current passes from one to the other and does
       not divide.

_Spark Gap:_
       An air gap or open space between two electrodes for the passage
       of a high voltage discharge.

_Storage Battery:_
       See accumulator.

_Transformer:_
       A device for stepping up or stepping down the voltage of an
       alternating current by means of magnetic induction.

_Tuning Coil:_
       A variable inductance for changing the period of the receptor
       circuit.

_Volt:_
       The unit of electrical force or potential. The electromotive
       force which, if steadily applied to a conductor whose resistance
       is one ohm, will produce a current of one ampere.

_Voltmeter:_
       An instrument for measuring voltage.

_Watt:_
       Unit of work. It is the rate of work of one ampere flowing under
       a potential of one volt. Seven hundred and forty-six watts
       represent one electrical horsepower.

[Illustration: *Plate VI. DeForest and Marconi Systems.*]




CHAPTER XVIII. THE AMATEUR AND THE WIRELESS LAW. WHAT IT IS; HOW TO
COMPLY; HOW TO SECURE A LICENSE.


On August 13, 1912, Congress enacted a "Wireless Law" to regulate radio
communication. The whole law may be found in the appendix of this book,
but briefly as far as the amateur is concerned it is as follows:

An amateur may not use transmitting apparatus which is sufficiently
powerful to send radio signals across any of the boundaries of the state
in which he is located and which can be detected by a sensitive
receiving set located just beyond the state boundary, nor can he use
apparatus which is powerful enough to interfere with the reception of
signals by others from beyond the state boundaries unless he has a
_license_.

An amateur may receive messages from anywhere at any time without a
license provided that his station is not also fitted with transmitting
apparatus.

In other words if the amateur possesses a receiving outfit only which is
in working order or if he has both transmitter and receptor and the
former is not powerful enough to send signals out of the state in which
he lives, or to interfere with the reception of messages by another when
the messages come from beyond the state boundary a _license_ is
unnecessary.

This is of course somewhat unfair for those living near the center of
large states for they may operate almost as they please with ordinary
instruments with no fear of the signals going beyond the border, while
those living within a few miles of another state must secure a license.

If an amateur has a license he may transmit messages beyond the state
border, but he must not employ a wave length greater than 200 meters or
a power input into the transmitter of more than 1 K.W. without special
permission.

[Illustration: Fig. 148. Experimental Amateur Station of W. Haddon,
Brooklyn, N. Y.]

If the amateur is within five nautical miles of an army or navy station
equipped with radio apparatus his power input must not be more than 1
K.W.

*Complying with Law.*—After a license is secured, or rather as a matter
of fact in order to secure it, the wave length must not be greater than
200 meters. In order to secure such a wave length the effective portion
of the aerial cannot usually be made greater than 115 feet in
consideration of the amount which the lead-in, helix and ground wire
add.

[Illustration: Fig. 149. Complete Receiving Outfit Consisting of
Receiving Transformer, Detector, Fixed Condenser, Loading Coil, Two
Variable Condensers, Potentiometer, Battery, Switches, etc.]

Such an aerial will serve well for transmitting purposes, but is too
short for receiving very long distances. If the amateur desires to pick
up long distance messages he must employ two aerials, a short one for
transmitting and a long one for receiving. If it is desirable to use the
long aerial for both transmitting and receiving its wave length may be
brought down to 200 meters or under, while transmitting by placing a
large glass plate condenser of the proper capacity in series with the
aerial. It may be short-circuited with a suitable switch when receiving.

The law also says that the transmitting wave must be pure, and must be
sharply tuned, which means that the wave must be of one length and not,
as is very often the case, composed of two or more waves of different
lengths.

In order to comply with this restriction, the use of an oscillation
helix or loosely coupled helix is necessary. A rotary or quenched gap is
also necessary in place of the ordinary spark gap.

The construction of all three of these instruments is described farther
on.

*How to Obtain a License.*—In order to obtain a license send to the
nearest Radio Inspector; Radio Inspectors are stationed at the Custom
Houses at the following cities: Boston, Mass.; New York, N. Y.;
Savannah, Ga.; Baltimore, Md.; Cleveland, Ohio; Chicago, Ill.; Seattle,
Wash.; New Orleans, La.; San Francisco, Cal.; and obtain a copy of The
Wireless Law, 8-6412, The Regulations Governing Radio Communication, The
Berlin International Radio-telegraphic Convention, and the necessary
Application Forms.

Look over the Wireless Law, the Regulations and the Berlin Convention
pamphlet, then fill out Form 756. This is an application for an
operator’s license.

[Illustration: Fig. 150. Receiving Outfit Consisting of Receiving
Transformer, Fixed Condenser and Detector.]

If the amateur is not already in possession of a certificate of skill as
an operator it will be necessary to find out, from the commandant at the
nearest one of the stations, listed on page 4 of the Regulations as
being the places where examinations are given, on what days the
examinations are held. If the amateur is able to pass the examination,
an _operator’s license_ will be issued. Then fill out Form 757 and
forward it to the Radio Inspector. If he thinks your station conforms to
the regulations without inspection the license will be issued; if not,
the license will be issued after inspection.

There are no fees to pay in connection with securing any of the
licenses.



APPARATUS REQUIRED IN MANY INSTANCES IN ORDER TO COMPLY WITH THE
WIRELESS LAW.


*OSCILLATION HELIX*

The oscillation helix has almost become a necessity in order to comply
with the regulations of the Wireless Law regarding wave form, except in
those stations where a quenched gap is used.

[Illustration: Fig. 151. Amco Oscillation Helix.]

The wave emitted by many stations is not pure. It is composed usually of
two or three separate waves of different lengths instead of all the
energy being confined to oscillations of one period. It is possible to
tune such a wave in two or more places or "humps," as they are called,
on the tuning coil. It is obvious that a wave possessing such humps
cannot be closely tuned and is liable to interfere with the signals of
another station. This is one of the principal causes of interference.

The reason for this phenomenon is simple. The action of a transmitter is
to first charge a condenser. When the potential of the condenser rises
to sufficient value it discharges across the spark gap and sets up
oscillations in the _closed_ circuit. These oscillations immediately
induce oscillations in the _open_ circuit or aerial system and part of
the energy passes off into the ether as electro-magnetic waves. However,
the oscillations in the _aerial_ system do not immediately die away
after the oscillations in the closed circuit cease during the interim
until the next condenser discharge, but continue to surge and react upon
the dosed circuit to sufficient extent to induce therein currents which
surge back and forth long after the current from the condenser discharge
has died away.

We might call the oscillations due to the condenser discharge _primary
oscillations_ and those induced in the aerial thereby _secondary
oscillations_. Those which are then set up in the closed circuit by the
reaction of the _secondary_ currents are _tertiary_. This third train of
oscillations persist after the secondary currents have died away, and
induce another set of oscillations in the aerial which send out a second
set of electromagnetic waves differing in length from the first.

The oscillations which take place after the initial surge in the closed
and open circuits are naturally somewhat weak. By using an oscillation
helix in which the primary and secondary are separated from each other
it is possible to eliminate the third and fourth trains of oscillations
and all others having a tendency to follow, by placing the circuits
apart so that the weak oscillations are not strong enough* to react
across the intervening space. The immediate oscillations set up by the
condenser discharge are strong enough to act across the space and set up
powerful oscillations in the aerial.

[Illustration: Fig. 152. Details of Oscillation Helix Construction.]

A hot wire ammeter placed in the aerial circuit of a transmitter
employing an oscillation transformer will not indicate as much current
as if placed in the same position in a circuit using an ordinary helix;
but in spite of the fact, a transmitter using an oscillation helix will
send farther because the energy is concentrated in waves of one length.

The construction of one type of oscillation helix has already been
outlined on page 92. The form shown in Fig. 152 has no special
advantages over the other but is preferred by many experimenters.

It is of the "pancake" type, so-called because of the flat form of the
windings which are made in the shape of a spiral of brass ribbon set in
a slotted frame.

The dimensions of the helix are clearly apparent from the drawing. The
primary is composed of seven turns of brass ribbon 1/2 inch wide and
1/16 inch thick. The secondary should have from 10 to 15 turns of ribbon
3/8 x 1/16. The coils may be slid back and forth on the brass rod so
that the distance between them is variable. Connection is made to the
coils by means of suitable clips. A clip similar to that shown on page
92, but made to snap on a flat ribbon instead of a round wire, will
serve the purpose.

*QUENCHED SPARK GAP.*

A "quenched" gap is made up of a number of brass or copper disks
accurately turned to a true surface and separated by mica or rubber
rings about .01 inch thick. The spark discharge takes place in the
air-tight space at the center of the disks, inside of the mica rings.

The quenched gap has several advantages over other forms. It is
practically noiseless and the nuisance of a crashing discharge may be
avoided by its use.

The large surface offered to the spark by the disks cools the spark and
quickly stops the oscillations in the closed circuit, and thereby leaves
the open circuit and aerial system free to vibrate in its own period and
therefore radiates _pure_ waves. By pure wave a wave of one length is
meant.

A quenched gap cannot be used on a set of over 1 K.W. power without
artificial cooling by an air blast.

[Illustration: Fig. 153. Quenched Gap.]

[Illustration: Fig. 154. Quenched Gap.]

Fig. 154 shows an efficient form of quenched gap for use in stations up
to 1 K.W. in power.

The disks are shown in detail in Fig. 155. They are cast out of copper
and then turned perfectly true and smooth in a lathe. After surfacing,
the discharge surface should be heavily silver plated and buffed smooth.

[Illustration: Fig. 155. Details of Disk and Ring.]

The disks are piled on a marble base with a mica ring between each. They
are clamped down by a strong set screw mounted on a heavy brass yoke.
Enough pressure should be brought to bear to force the plates tightly
together and make them air tight.

[Illustration: Fig. 156. Explanatory Drawing of Quenched Gap.]

The number of disks required is governed by the voltage of the charging
condenser. Generally speaking it is one section of .01-inch gap for each
thousand volts delivered by the secondary of the transformer. It is very
important to secure just the proper number of disks. If properly
adjusted, the quenched gap will give one discharge for each alternation
of the current and produce a musical tone.

The quenched gap is placed in the same position in the transmitting
circuit as any other form of gap.

*ROTARY GAPS.*

Rotary gaps are divided into two general classes, the synchronous gap
and the non-synchronous gap.

The former usually consists of one or more stationary electrodes and a
rotating member made like a star wheel with projecting spokes. This
rotary member is attached directly to the shaft of the alternator or
motor generator and arranged so that a spoke always comes opposite a
stationary member at the exact moment that the maximum of potential is
obtained in the condenser. Such an arrangement permits one discharge for
each alternation of the current and produces a pure musical note easily
distinguished in the telephone receivers at a distant station.

In the non-synchronous rotary gap the wheel is driven at a high rate of
speed without any regard to synchronism with the alternations of the
current.

The rotary gap shown in Fig. 157 is of the non-synchronous type.

[Illustration: Fig. 157. Amco Rotary Gap.]

The rotating member is cast from an alloy of equal parts of zinc and
aluminum. It is necessary to first make a wooden pattern from which the
casting may be made. The details of the wheel are shown in Fig. 158. The
casting must be placed in a lathe chuck and turned true. It is mounted
on a hard rubber disk 2 7/8 inches in diameter and 1/4 of an inch thick.
The disk serves to insulate the revolving electrodes from the motor
shaft. The "rotor" is mounted upon the shaft by means of a small brass
bushing which passes through the center of the disk.

[Illustration: Fig. 158. Details of Revolving Parts of Rotary Gap.]

The motor must be well built and capable of running at high speed. A
"Juno" motor will be found very satisfactory. When running free its
speed is about 4500 r.p.m. With the rotor in place the speed is about
3600 r.p.m.

The motor should be mounted on a heavy marble base capable of absorbing
any little vibration that the gap may be subject to when running at high
speed.

The stationary electrodes are made in the same manner as those for an
ordinary gap and consists of two flanged zinc electrodes mounted upon
threaded brass rods supported by two hexagonal standards. The axis of
the electrodes should be the same height above the base as that of the
motor shaft.

The rotor should be carefully balanced so that it is practically free
from vibration by boring small holes in the back face so as to make the
weight on opposite sides equal.

[Illustration: Fig. 159. Details of Rotary Gap.]

The motor may be driven by a battery or from the same source that
supplies the transformer, in series with two or three suitable lamps. A
motor wound to run directly from the 110-volt line or a higher potential
must have its fields wound with very fine wire and is apt to give
trouble through "burn-outs," due to "kick back." When the motor is
operated on batteries or is wound for running in series with a lamp the
danger is lessened.

A rotary gap is placed in the transmitting circuit in the same position
as any other gap. Its use will result in a wonderful increase in the
transmitting range of almost any station, for not only will the amount
of energy passing through the aerial be raised, but the clear musical
tone given off is more plainly distinguishable at a greater distance in
the receiving station than a spark of the ordinary sort.

*"KICK BACK."*

The oscillations taking place in the closed circuit and aerial system of
a wireless transmitter continue to surge after the current in the
condenser has dropped below a certain value, and react upon the primary
winding of the coil or transformer by induction and produce high
voltage, high frequency currents termed "kick back," in wireless
telegraph parlance.

"Kick back," wherever it exists to an appreciable extent, is liable to
damage insulation and cause possible "burn-outs." The "kick-back"
preventers illustrated in the accompanying diagrams will be found an
efficient method of avoiding this danger.

[Illustration: Fig. 160. Methods of Preventing "Kick Back."]

The first method shows an ordinary pressed telephone condenser of about
two microfarads capacity connected directly across the A. C. mains near
the transformer terminals, in series with two 6-ampere fuses. The
condenser is shunted by a small spark gap made of needle points with a
very small space, about .005 of an inch, between them.

The second method is an elaboration of the first and shows two sets of
condensers in series with fuses and bridged by spark gaps with a
"ground" through a third condenser connected between. This second method
is the best and is often used to prevent delicate instruments, such as a
voltmeter, from the effects of "kick back."

A proper "kick-back" preventer is part of the Fire Insurance
Underwriters requirements for a wireless telegraph station.



Receiving Apparatus.


*THE VARIOMETER.*

A variometer is a tuning device in which two coils of wire are placed in
series and connected so that the turns can be made to oppose one
another. One coil is movable and by turning it the currents flowing
through the adjacent coils oppose each other and decrease the
self-induction of the whole and consequently the period of the circuit.

[Illustration: Fig. 161. Variometer.]

The accompanying illustration shows the constructive details of an
efficient form of variometer.

The coils are wound around two cardboard cylinders. One cylinder is 6
inches in diameter and the other 5 inches. Both are 2 inches long. The
large cylinder is wound with twenty-five turns of No. 22 B. S. gauge
single silk covered wire wound in two sections so as to leave a space
3/8 inch wide in the center. The small cylinder is wound in the same
manner with the same size of wire but contains thirty turns so as to
make the length of wire in each of the coils practically equal.

The space in the center of each of the coils allows a supporting rod to
pass through without interfering with the wire. The shaft is a piece of
5/16 _brass_ rod about 7 1/2 inches long. The upper end is provided with
a knob and a pointer. The large coil is mounted on the under side of a
wooden cover made to fit a containing case of the proper size to receive
the variometer and of a style similar to that ordinarily used to enclose
a variable condenser.

The shaft passes through the larger coil, at two points diametrically
opposite, but should not fit tightly. It also passes through the smaller
coil but is fastened to the latter by means of a cross bar so that when
the knob is turned the coil will revolve also. The coils are connected
by means of a piece of braided wire or flexible conductor, long enough
so as not to interfere with the movement of the inner coil.

The outside terminals of the coils terminate in binding posts mounted on
the top of the case. When making a variometer be careful not to employ
any iron or steel in its construction, not even iron screws.

The variometer is placed in the receiving circuit by connecting it in
series with the aerial before it reaches the tuning coil or loose
coupler.

*NEW CRYSTAL DETECTORS.*

*Silicon Detector.*—The silicon detector is always interesting because
it was one of the first mineral rectifiers to come into extended use.
The photograph shown below illustrates one of the latest forms of the
silicon detector.

[Illustration: Fig. 162. Silicon Detector.]

The large cup supported by the left-hand standard contacts several
pieces of fused silicon embedded in fusible alloy. The right-hand
standard supports a movable "offset" cup the same as that used on the
Perikon type of Detector. A small piece of _arsenic_ is mounted in this
cup and may be brought to bear against any portion of any of the silicon
crystals.

*Pyron Detector.*—The Pyron detector is not new, but the photograph
shows a very simple and efficient form in which a fine wire is brought
to bear against a crystal of iron pyrites mounted in a small cup.

[Illustration: Fig. 163. Pyron Detector.]

*Galena Detector.*—Although _galena_ is named in the list of sensitive
minerals on page 132 it has come into such extensive use as a detector
since the first edition of this book to be worthy of special attention.
Galena detectors are often spoken of as "cat-whisker" detectors because
of the long fine wire used to secure a delicate contact with the
mineral.

The mineral is usually placed in a cup and held in position by imbedding
in fusible alloy or clamped with a set screw. The best surface of the
mineral should be selected by testing previous to imbedding it in the
cup. Contact is made with the surface of the mineral by means of a piece
of No. 30 phosphor bronze wire mounted on the end of a short brass rod
fitted with an adjusting screw so that by turning or twisting the
surface of the mineral may be "searched" and the tension varied.

[Illustration: Fig. 164. Galena Detector.]

Such an arrangement is illustrated in Fig. 164. The base is a hard
rubber block 3 3/4 x 1 3/4 x 1/2. The binding posts are of the type
commonly used on electrical instruments. One of the posts is pivoted by
placing a spring washer under the head of the screw which holds the post
to the base. A short piece of brass rod fitted with a hard rubber knob
passes through the wire hole in the post. A piece of No. 30 phosphor
bronze wire is soldered to the end of the rod. By twisting the post and
sliding the rod any portion of the mineral surface may be selected.
Twisting the rod varies the tension of the contact.

The galena detector is connected in the receiving circuit in the same
manner as other detectors of the mineral type.

*THE AUDION.*

*The Audion*—is finding favor in many amateur wireless stations, because
of its almost entire lack of adjustment and of the loud clear signals
which it gives even when used for long distance work.

The illustration shows the latest form of audion, the plate and the grid
both being on the same side of the filament.

The filament of an audion is usually lighted by means of a six-volt
storage battery. A small battery rheostat placed in series with the
battery serves to regulate the amount of current flowing through the
filament. Only one filament is used at a time, the other being saved as
a reserve in case the first burns out.

[Illustration: Fig. 165. Audion.]

The higher voltage necessary to operate an audion is supplied by a
battery consisting of fifteen flashlight batteries, each flashlight
battery being composed of three separate cells. The batteries are
connected to an eight-point switch so that throwing the switch on the
first point will connect five sets of batteries. The second point places
five more in circuit and each additional point one set only. It is also
a good plan to connect a four-point switch and three separate cells of
battery in series with the eight-point switch so that they may be added
to the circuit one at a time and the potential varied more closely than
the steps on the eight-point switch permit. The maximum voltage of such
a battery is approximately 56 volts.

The diagram in Fig. 166 shows exactly how an audion is connected. The
wires _A_ and _B_ are the terminals leading to the tuning coil or loose
coupler. It is not necessary that the telephone receivers used with an
audion be wound to a high resistance in order to secure good results.

[Illustration: Fig. 166. Audion Circuit.]

The audion is placed in operation by turning the rheostat until the
filament lights brightly. Then adjust the voltage of the high potential
battery until the signals are clearest. If the voltage is too high the
audion will become filled with a bluish light and the voltage should be
immediately reduced. The signals will be loudest when the filament is
brilliantly lighted. No more current should be passed through the
filament than is necessary to render the signals plain, for if "forced"
its life will be limited to only a few hours.

[Illustration: Fig. 167. Rotary Variable Condenser.]




                              *APPENDIX.*

                           [PUBLIC No. 264.]

                               [S. 6412.]

                An Act to Regulate Radio Communication.

_Be it enacted by the Senate and House of Representatives of the United
States of America in Congress assembled._ That a person, company, or
corporation within the jurisdiction of the United States shall not use
or operate any apparatus for radio communication as a means of
commercial intercourse among the several States, or with foreign
nations, or upon any vessel of the United States engaged in interstate
or foreign commerce, or for the transmission of radiograms or signals
the effect of which extends beyond the jurisdiction of the State or
Territory in which the same are made, or where interference would be
caused thereby with the receipt of messages or signals from beyond the
jurisdiction of the said State or Territory, except under and in
accordance with a license, revocable for cause, in that behalf granted
by the Secretary of Commerce and Labor upon application therefor; but
nothing in this Act shall be construed to apply to the transmission and
exchange of radiograms or signals between points situated in the same
State: _Provided_, That the effect thereof shall not extend beyond the
jurisdiction of the said State or interfere with the reception of
radiograms or signals from beyond said jurisdiction; and a license shall
not be required for the transmission or exchange of radiograms or
signals by or on behalf of the Government of the United States, but
every Government station on land or sea shall have special call letters
designated and published in the list of radio stations of the United
States by the Department of Commerce and Labor. Any person, company, or
corporation that shall use or operate any apparatus for radio
communication in violation of this section, or knowingly aid or abet
another person, company, or corporation in so doing, shall be deemed
guilty of a misdemeanor, and on conviction thereof shall be punished by
a fine not exceeding five hundred dollars, and the apparatus or device
so unlawfully used and operated may be adjudged forfeited to the United
States.

SEC. 2. That every such license shall be in such form as the Secretary
of Commerce and Labor shall determine and shall contain the
restrictions, pursuant to this Act, on and subject to which the license
is granted; that every such license shall be issued only to citizens of
the United States or Porto Rico or to a company incorporated under the
laws of some State or Territory or of the United States or Porto Rico,
and shall specify the ownership and location of the station in which
said apparatus shall be used and other particulars for its
identification and to enable its range to be estimated; shall state the
purpose of the station, and, in case of a station in actual operation at
the date of passage of this Act, shall contain the statement that
satisfactory proof has been furnished that it was actually operating on
the above-mentioned date; shall state the wave length or the wave
lengths authorized for use by the station for the prevention of
interference and the hours for which the station is licensed for work;
and shall not be construed to authorize the use of any apparatus for
radio communication in any other station than that specified. Every such
license shall be subject to the regulations contained herein, and such
regulations as may be established from time to time by authority of this
Act or subsequent Acts and treaties of the United States. Every such
license shall provide that the President of the United States in time of
war or public peril or disaster may cause the closing of any station for
radio communication and the removal therefrom of all radio apparatus, or
may authorize the use or control of any such station or apparatus by any
department of the Government, upon just compensation to the owners.

SEC. 3. That every such apparatus shall at all times while in use and
operation as aforesaid be in charge or under the supervision of a person
or persons licensed for that purpose by the Secretary of Commerce and
Labor. Every person so licensed who in the operation of any radio
apparatus shall fail to observe and obey regulations contained in or
made pursuant to this Act or subsequent Acts or treaties of the United
States, or any one of them, or who shall fail to enforce obedience
thereto by an unlicensed person while serving under his supervision, in
addition to the punishments and penalties herein prescribed, may suffer
the suspension of the said license for a period to be fixed by the
Secretary of Commerce and Labor not exceeding one year. It shall be
unlawful to employ any unlicensed person or for any unlicensed person to
serve in charge or in supervision of the use and operation of such
apparatus, and any person violating this provision shall be guilty of a
misdemeanor, and on conviction thereof shall be punished by a fine of
not more than one hundred dollars or imprisonment for not more than two
months, or both, in the discretion of the court, for each and every such
offense: _Provided_, That in case of emergency the Secretary of Commerce
and Labor may authorize a collector of customs to issue a temporary
permit, in lieu of a license, to the operator on a vessel subject to the
radio ship Act of June twenty-fourth, nineteen hundred and ten.

SEC. 4. That for the purpose of preventing or minimizing interference
with communication between stations in which such apparatus is operated,
to facilitate radio communication, and to further the prompt receipt of
distress signals, said private and commercial stations shall be subject
to the regulations of this section. These regulations shall be enforced
by the Secretary of Commerce and Labor through the collectors of customs
and other officers of the Government as other regulations herein
provided for.

The Secretary of Commerce and Labor may, in his discretion, waive the
provisions of any or all of these regulations when no interference of
the character above mentioned can ensue.

The Secretary of Commerce and Labor may grant special temporary licenses
to stations actually engaged in conducting experiments for the
development of the science of radio communication, or the apparatus
pertaining thereto, to carry on special tests, using any amount of power
or any wave lengths, at such hours and under such conditions as will
insure the least interference with the sending or receipt of commercial
or Government radiograms, of distress signals and radiograms, or with
the work of other stations.

In these regulations the naval and military stations shall be understood
to be stations on land.

                              REGULATIONS.

                          NORMAL WAVE LENGTH.

First. Every station shall be required to designate certain definite
wave length as the normal sending and receiving wave length of the
station. This wave length shall not exceed six hundred meters or it
shall exceed one thousand six hundred meters. Every coastal station open
to general public service shall at all times be ready to receive
messages of such wave lengths as are required by the Berlin convention.
Every ship station, except as hereinafter provided, and every coast
station open to general public service shall be prepared to use two
sending wave lengths, one of three hundred meters and one of six hundred
meters, as required by the international convention in force:
_Provided_, That the Secretary of Commerce and Labor may, in his
discretion, change the limit of wave length reservation made by
regulations first and second to accord with any international agreement
to which the United States is a party.

                          OTHER WAVE LENGTHS.

Second. In addition to the normal sending wave length all stations,
except as provided hereinafter in these regulations, may use other
sending wave lengths: _Provided_, That they do not exceed six hundred
meters or that they do exceed one thousand six hundred meters: _Provided
further_, That the character of the waves emitted conforms to the
requirements of regulations third and fourth following.

                          USE OF A "PURE WAVE"

Third. At all stations if the sending apparatus, to be referred to
hereinafter as the "transmitter," is of such a character that the energy
is radiated in two or more wave lengths, more or less sharply defined,
as indicated by a sensitive wave meter, the energy in no one of the
lesser waves shall exceed ten per centum of that in the greatest.

                         USE OF A "SHARP WAVE."

Fourth. At all stations the logarithmic decrement per complete
oscillation in the wave trains emitted by the transmitter shall not
exceed two-tenths, except when sending distress signals or signals and
messages relating thereto.

                    USE OF "STANDARD DISTRESS WAVE."

Fifth. Every station on shipboard shall be prepared to send distress
calls on the normal wave length designated by the international
convention in force, except on vessels of small tonnage unable to have
plants insuring that wave length.

                          SIGNAL OF DISTRESS.

Sixth. The distress call used shall be the international signal of
distress: dot dot dot, dash dash dash, dot dot dot.

         USE OF "BROAD INTERFERING WAVE" FOR DISTRESS SIGNALS.

Seventh. When sending distress signals, the transmitter of a station on
shipboard may be tuned in such a manner as to create a maximum of
interference with a maximum of radiation.

               DISTANCE REQUIREMENT FOR DISTRESS SIGNALS.

Eighth. Every station on shipboard, wherever practicable, shall be
prepared to send distress signals of the character specified in
regulations fifth and sixth with sufficient power to enable them to be
received by day over sea a distance of one hundred nautical miles by a
shipboard station equipped with apparatus for both sending and receiving
equal in all essential particulars to that of the station first
mentioned.

                         FOR DISTRESS SIGNALS.

Ninth. All stations are required to give absolute priority to signals
and radiograms relating to ships in distress; to cease all sending on
hearing a distress signal; and, except when engaged in answering or
aiding the ship in distress, to refrain from sending until all signals
and radiograms relating thereto are completed.

           REDUCED POWER FOR SHIPS NEAR A GOVERNMENT STATION.

Tenth. No station on shipboard, when within fifteen nautical miles of a
naval or military station, shall use a transformer input exceeding one
kilowatt, nor, when within five nautical miles of such a station, a
transformer input exceeding one-half kilowatt, except for sending
signals of distress, or signals or radiograms relating thereto.

                          INTERCOMMUNICATION.

Eleventh. Each shore station open to general public service between the
cost and vessels at sea shall be bound to exchange radiograms with any
similar shore station and with any ship station without distinction of
the radio systems adopted by such stations, respectively, and each
station on shipboard shall be bound to exchange radiograms with any
other station on shipboard without distinction of the radio systems
adopted by each station respectively.

It shall be the duty of each such shore station, during the hours it is
in operation, to listen in at intervals of not less than fifteen minutes
and for a period not less than two minutes, with the receiver tuned to
receive messages of three hundred meter wave lengths.

                           DIVISION OF TIME.

Twelfth. At important seaports and at all other places where naval or
military and private or commercial shore stations operate in such close
proximity that interference with the work of naval and military stations
can not be avoided by the enforcement of the regulations contained in
the foregoing regulations concerning wave lengths and character of
signals emitted, such private or commercial shore stations as do
interfere with the reception of signals by the naval and military
stations concerned shall not use their transmitters during the first
fifteen minutes of each hour, local standard time. The Secretary of
Commerce and Labor may, on the recommendation of the department
concerned, designate the station or stations which may be required to
observe this division of time.

            GOVERNMENT STATIONS TO OBSERVE DIVISION OF TIME.

Thirteenth. The naval or military stations for which the above mentioned
division of time may be established shall transmit signals or radiograms
only during the first fifteen minutes of each hour, local standard time,
except in case of signals or radiograms relating to vessels in distress,
as hereinbefore provided.

                       USE OF UNNECESSARY POWER.

Fourteenth. In all circumstances, except in case of signals or
radiograms relating to vessels in distress, all stations shall use the
minimum amount of energy necessary to carry out any communication
desired.

               GENERAL RESTRICTIONS ON PRIVATE STATIONS.

Fifteenth. No private or commercial station not engaged in the
transaction of bona fide commercial business by radio communication or
in experimentation in connection with the development and manufacture of
radio apparatus for commercial purposes shall use a transmitting wave
length exceeding two hundred meters, or a transformer input exceeding
one kilowatt, except by special authority of the Secretary of Commerce
and Labor contained in the license of the station: _Provided_, That the
owner or operator of a station of the character mentioned in this
regulation shall not be liable for a violation of the requirements of
the third or fourth regulations to the penalties of one hundred dollars
or twenty-five dollars, respectively, provided in this section unless
the person maintaining or operating such station shall have been
notified in writing that the said transmitter has been found, upon tests
conducted by the Government, to be so adjusted as to violate the said
third and fourth regulations, and opportunity has been given to said
owner or operator to adjust said transmitter in conformity with said
regulations.

     SPECIAL RESTRICTIONS IN THE VICINITIES OF GOVERNMENT STATIONS.

Sixteenth. No station of the character mentioned in regulation fifteenth
situated within five nautical miles of a naval or military station shall
use a transmitting wave length exceeding two hundred meters or a
transformer input exceeding one-half kilowatt.

       SHIP STATIONS TO COMMUNICATE WITH NEAREST SHORE STATIONS.

Seventeenth. In general, the shipboard stations shall transmit their
radiograms to the nearest shore station. A sender on board a vessel
shall, however, have the right to designate the shore station through
which he desires to have his radiograms transmitted. If this can not be
done, the wishes of the sender are to be complied with only if the
transmission can be effected without interfering with the service of
other stations.

    LIMITATIONS FOR FUTURE INSTALLATIONS IN VICINITIES OF GOVERNMENT
                               STATIONS.

Eighteenth. No station on shore not in actual operation at the date of
the passage of this Act shall be licensed for the transaction of
commercial business by radio communication within fifteen nautical miles
of the following naval or military stations, to wit: Arlington,
Virginia; Key West, Florida; San Juan, Porto Rico; North Head and
Tatoosh Island, Washington; San Diego, California; and those established
or which may be established in Alaska and in the Canal Zone; and the
head of the department having control of such Government stations shall,
so far as is consistent with the transaction of governmental business,
arrange for the transmission and receipt of commercial radiograms under
the provisions of the Berlin convention of nineteen hundred and six and
future international conventions or treaties to which the United States
may be a party, at each of the stations above referred to, and shall fix
the rates therefor, subject to control of such rates by Congress. At
such stations and wherever and whenever shore stations open for general
public business between the coast and vessels at sea under the
provisions of the Berlin convention of nineteen hundred and six and
future international conventions and treaties to which the United States
may be a party shall not be so established as to insure a constant
service day and night without interruption, and in all localities
wherever or whenever such service shall not be maintained by a
commercial shore station within one hundred nautical miles of a naval
radio station, the Secretary of the Navy shall, so far as is consistent
with the transaction of governmental business, open naval radio stations
to the general public business described above, and shall fix rates for
such service, subject to control of such rates by Congress. The receipts
from such radiograms shall be covered into the Treasury as miscellaneous
receipts.

                          SECRECY OF MESSAGES.

Nineteenth. No person or persons engaged in or having knowledge of the
operation of any station or stations shall divulge or publish the
contents of any messages transmitted or received by such station, except
to the person or persons to whom the same may be directed, or their
authorized agent, or to another station employed to forward such message
to its destination, unless legally required so to do by the court of
competent jurisdiction or other competent authority. Any person guilty
of divulging or publishing any message, except as herein provided,
shall, on conviction thereof, be punishable by a fine of not more than
two hundred and fifty dollars or imprisonment for a period of not
exceeding three months, or both fine and imprisonment, in the discretion
of the court.

                               PENALTIES.

For violation of any of these regulations, subject to which a license
under sections one and two of this Act may be issued, the owner of the
apparatus shall be liable to a penalty of one hundred dollars, which may
be reduced or remitted by the Secretary of Commerce and Labor, and for
repeated violations of any of such regulations, the license may be
revoked.

For violation of any of these regulations, except as provided in
regulation nineteenth, subject to which a license under section three of
this Act may be issued, the operator shall be subject to a penalty of
twenty-five dollars, which may be reduced or remitted by the Secretary
of Commerce and Labor, and for repeated violations of any such
regulations, the license shall be suspended or revoked.

SEC. 5. That every license granted under the provisions of this Act for
the operation or use of apparatus for radio communication shall
prescribe that the operator thereof shall not wilfully or maliciously
interfere with any other radio communication. Such interference shall be
deemed a misdemeanor, and upon conviction thereof the owner or operator,
or both, shall be punishable by a fine of not to exceed five hundred
dollars or imprisonment for not to exceed one year, or both.

SEC. 6. That the expression "radio communication" as used in this Act
means any system of electrical communication by telegraphy or telephony
without the aid of any wire connecting the points from and at which the
radiograms, signals, or other communications are sent or received.

SEC. 7. That a person, company, or corporation within the jurisdiction
of the United States shall not knowingly utter or transmit, or cause to
be uttered or transmitted, any false or fraudulent distress signal or
call or false or fraudulent signal, call, or other radiogram of any
kind. The penalty for so uttering or transmitting a false or fraudulent
distress signal or call shall be a fine of not more than two thousand
five hundred dollars or imprisonment for not more than five years, or
both, in the discretion of the court, for each and every such offense,
and the penalty for so uttering or transmitting, or causing to be
uttered or transmitted, any other false or fraudulent signal, call, or
other radiogram shall be a fine of not more than one thousand dollars or
imprisonment for not more than two years, or both, in the discretion of
the court, for each and every such offense.

SEC. 8. That a person, company, or corporation shall not use or operate
any apparatus for radio communication on a foreign ship in territorial
waters of the United States otherwise than in accordance with the
provisions of sections four and seven of this Act and so much of section
five as imposes a penalty for interference. Save as aforesaid, nothing
in this Act shall apply to apparatus for radio communication on any
foreign ship.

SEC. 9. That the trial of any offense under this Act shall be in the
district in which it is committed, or if the offense is committed upon
the high seas or out of the jurisdiction of any particular State or
district the trial shall be in the district where the offender may be
found or into which he shall be first brought.

SEC. 10. That this Act shall not apply to the Phillippine Islands.

SEC. 11. That this Act shall take effect and be in force on and after
four months from its passage.


Approved, August 13, 1912.




                             LIST OF WORKS

                                   ON

                           ELECTRICAL SCIENCE

                       PUBLISHED AND FOR SALE BY

                        D. VAN NOSTRAND COMPANY,

                       *25 Park Place, New York.*

*ABBOTT, A. V. The Electrical Transmission of Energy.*
       A Manual for the Design of Electrical Circuits. _Fifth Edition,
       enlarged and rewritten._ With many Diagrams, Engravings and
       Folding Plates. 8vo., cloth, 675 pp Net, $5.00

*ARNOLD, E. Armature Windings of Direct-Current Dynamos.*
       Extension and Application of a general Winding Rule. Translated
       from the original German by Francis B. DeCress. Illustrated. 8vo.
       cloth, 124 pp $2.00

*ASHE, S. W. Electricity Experimentally and Practically Applied.*
       _Second Edition._ 422 illustrations. 12mo., cloth, 375 pp. Net,
       $2.00

*ASHE, S. W., and KEILEY, J. D. Electric Railways Theoretically and
Practically Treated.*
       Illustrated. 12mo., cloth.

*Vol. I. Rolling Stock.*
       _Second Edition._ 285 pp Net, $2.50

*Vol. II. Substations and Distributing Systems.*
       296 pp Net, $2. 50

*ATKINSON, A. A. Electrical and Magnetic Calculations.*
       For the use of Electrical Engineers and others interested in the
       Theory and Application of Electricity and Magnetism. _Fourth
       Edition_, revised Illustrated. 12mo, cloth, 310 pp Net, $1.50

*ATKINSON, PHILIP. The Elements of Dynamic Electricity and Magnetism.*
       _Fourth Edition._ Illustrated. 12mo., cloth, 405 pp. $2.00

  - *Elements of Electric Lighting,* including Electric Generation,
    Measurement, Storage, and Distribution. _Tenth Edition_, fully
    revised and new matter added. Illustrated. 12mo., cloth, 280 pp
    $1.50
  - *Power Transmitted by Electricity and Applied by the Electric
    Motor,* including Electric Railway Construction. Illustrated.
    _Fourth Edition_, fully revised and new matter added. 12mo., cloth,
    241 pp. $2.00

*AUSTIN, E. Single-Phase Electric Railways.*
       346 illustrations 8 x 11, cloth, 308 pp Net, $5.00

*AYRTON, HERTHA. The Electric Arc.*
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*SWINBURNE, JAS., and WORDINGHAM, C. H. The Measurement of Electric
Currents. Electrical Measuring Instruments. Meters for Electrical
Energy.*
       Edited, with Preface, by T. Commerford Martin. Folding Plate and
       Numerous Illustrations. 16mo., cloth, 241 pp. (No. 109 Van
       Nostrand’s Science Series.) 50 cents

*SWOOPE, C. WALTON. Lessons in Practical Electricity: Principle
Experiments, and Arithmetical Problems. An Elementary Textbook.*
       With numerous Tables, Formulae, and two large Instruction Plates.
       _Fifteenth Edition, revised_. Illustrated. 5vo., cloth, 462 pp
       Net, $2.00

*THIESS, J. B. and JOY, G. A. Toll Telephone Practice.*
       273 illustrations. 8vo., cloth, 433 pp Net, $3.50

*THOM, C., and JONES, W. H. Telegraphic Connections, embracing recent
methods in Quadruplex Telegraphy.*
       20 Colored Plates. 8vo., cloth, 59 pp $1.50

*THOMPSON, S. P., Prof. Dynamo-Electric Machinery.*
       With an Introduction and Notes by Frank L. Pope and H. R. Butler.
       Illustrated 16vo., cloth, 214 pp. (No. 66 Van Nostrand’s Science
       Series.) 50 cents

  - *Recent Progress in Dynamo-Electric Machines.*
           Being a Supplement to "Dynamo-Electric Machinery."
           Illustrated. 16mo., cloth, 113 pp. (No. 75 Van Nostrand’s
           Science Series.) 50 cents


*TOWNSEND, FITZHUGH. Alternating Current Engineering.*
       Illustrated. 8vo, paper, 32 pp Net, 75 cents

*UNDERBILL, C. R. Solenoids, Electromagnets and Electromagnetic
Windings.*
       _Second Edition_. 218 Illustrations. 12mo., cloth, 345 pp Net,
       $2.00

*URQUHART, J. W. Electroplating.*
       _Fifth Edition_. Illustrated. 12mo., cloth, 230 pp $2.00

  - *Electrotyping.* Illustrated. 12mo., cloth, 228 pp $2.00

*VOSMAER, A. Ozone.* Its Manufacture and Uses.
       76 illustrations. 6 x 9, cloth, 210 pp Net, $2.50

*WADE, E. J. Secondary Batteries: Their Theory, Construction, and Use.*
       _Second Edition, corrected_ 265 illustrations. 8vo., cloth, 501
       pp. Net, $4.00

*WADSWORTH, C. Primary Battery Ignition.*
       A simple practical pocket guide on the construction, operation,
       maintenance, and testing of primary batteries for automobile,
       motorboat, and stationary engine ignition service. 26
       illustrations. 5 x 7, cloth, 79 pp Net, 0.50

*WALKER, FREDERICK. Practical Dynamo Building for Amateurs. How to Wind
for any Output.*
       _Third Edition_. Illustrated. 16mo., cloth, 104 pp. (No. 68 Van
       Nostrand’s Science Series.) 50 cents.

  - *Electricity in Mining.* Illustrated. 8vo., cloth, 385 pp $.50

*WATT, ALEXANDER. Electroplating and Refining of Metals*
       _New Edition_, rewritten by Arnold Philip. Illustrated 8vo.,
       cloth, 704 pp Net, $4.50

  - *Electro-metallurgy*
           _Fifteenth Edition_. Illustrated 12mo., cloth, 225 pp $1.00


*WEBB, H. L. A Practical Guide to the Testing of Insulated Wires and
Cables*
       _Sixth Edition_. Illustrated. 12mo., cloth., 118pp $1.00

*WILKINSON, H. D. Submarine Cable-Laying, Repairing, and Testing.*
       _Second Edition, completely revised_. 313 illustrations. 8vo.,
       cloth, 580 pp Net, $6.00

*WILSON, J. F. Essentials of Electrical Engineering.*
       300 illustrations. 6 x 9, cloth, 355 pp Net, $2.50

*WRIGHT, J. Testing, Fault Localization and General Hints for Linemen.*
       19 Illustrations. 16mo., cloth, 88 pp. (Installation Serious
       Manuals.) Net, 50 cents.

*YOUNG, J. ELTON. Electrical Testing for Telegraph Engineers.*
       Illustrated. 8vo., cloth, 264 pp Net, $4.00

*ZEIDLER, J., and LUSTGARTEN, J. Electric Arc Lamps: Their Principles,
Construction and Working.*
       160 Illustrations. 8vo., cloth, 188 pp Net, $2.00




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