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                  *       *       *       *       *




                 HERTZIAN WAVE WIRELESS TELEGRAPHY.

                    BY DR. J. A. FLEMING, F.R.S.

      [From the POPULAR SCIENCE MONTHLY, June-December, 1903.]

                  *       *       *       *       *

         [From the "Popular Science Monthly," June, 1903.]




               HERTZIAN WAVE WIRELESS TELEGRAPHY.[1]

                   BY DR. J. A. FLEMING, F.R.S.,

  PROFESSOR OF ELECTRICAL ENGINEERING, UNIVERSITY COLLEGE, LONDON.


The immense public interest which has been aroused of late years in
the subject of telegraphy without connecting wires has undoubtedly
been stimulated by the achievements of Mr. Marconi in effecting
communication over great distances by means of Hertzian waves. The
periodicals and daily journals, which are the chief avenues through
which information reaches the public, whilst eager to describe in a
sensational manner these wonderful applications of electrical
principles, have done little to convey an intelligible explanation of
them. Hence it appeared probable that a service would be rendered by
an endeavour to present an account of the present condition of
electric wave telegraphy in a manner acceptable to those unversed in
the advanced technicalities of the subject, but acquainted at least
with the elements of electrical science. It is the purpose of these
articles to attempt this task. We shall, however, limit the discussion
to an account of the scientific principles underlying the operation of
this particular form of wireless telegraphy, omitting, as far as
possible, references to mere questions of priority and development.

The practical problem of electric wave wireless telegraphy, which has
been variously called Hertzian wave telegraphy, Marconi telegraphy, or
spark telegraphy (_Funkentelegraphie_), is that of the production of
an effect called an electric wave or train of electric waves, which
can be sent out from one place, controlled, detected at another place,
and interpreted into an alphabetic code. Up to the present time, the
chief part of that intercommunication has been effected by means of
the Morse code, in which a group of long and short signs form the
letter or symbol. Some attempts have been made with more or less
success to work printing telegraphs and even writing or drawing
telegraphs by Hertzian waves, but have not passed beyond the
experimental stage, whilst wireless telephony by this means is still a
dream of the future.

We shall, in the first place, consider the transmitting arrangements
and, incidentally, the nature of the effect or wave transmitted; in
the second place, the receiving appliances; and, finally, discuss the
problem of the isolation or secrecy of the intelligence conveyed
between any two places.

The transmitter used in Hertzian wave telegraphy consists essentially
of a device for producing electric waves of a type which will travel
over the surface of the land or sea without speedy dissipation, and
the important element in this arrangement is the _radiator_, by which
these waves are sent out. It will be an advantage to begin by
explaining the electrical action of the radiator, and then proceed to
discuss the details of the transmitting appliances.

It will probably assist the reader to arrive most easily at a general
idea of the functions of the various portions of the transmitting
arrangements, and in particular of the radiator, if we take as our
starting point an analogy which exists between electric wave
generation for telegraphic purposes and air wave generation for sound
signal purposes. Most persons have visited some of the large
lighthouses which exist around our coasts and have there seen a steam
or air _siren_, as used for the production of sound signals during
fogs. If they have examined this appliance, they will know that it
consists, in the first place, of a long metal tube, generally with a
trumpet-shaped mouthpiece. At the bottom of this tube there is a fixed
plate with holes in it, against which revolves another similarly
perforated plate. These two plates separate a back chamber or wind
chest from the tube, and the wind chest communicates with a reservoir
of compressed air or a high-pressure steam boiler. In the
communication pipe there is a valve which can be suddenly opened for a
longer or shorter time. When the movable plate revolves, the
coincidence or non-coincidence of the holes in the two plates opens or
shuts the air passage way very rapidly. Hence when the blast of air or
steam is turned on, the flow is cut up by the revolving plates into a
series of puffs which inflict blows upon the stationary air in the
siren tube. If these blows come at the rate, say, of a hundred a
second, they give rise to aerial oscillations in the tube, which
impress the ear as a deep, musical note or roar; and this continuous
sound can be cut up by closing the valve intermittently into long and
short periods, and so caused to signal a letter according to the Morse
code, denoting the name of the lighthouse. In this case the object is
to produce: first, aerial vibrations in the tube, giving rise to a
train of powerful air waves; secondly, to intermit this wave-train so
as to produce an intelligible signal; and thirdly, to transmit this
wave as far as possible through space.

The production of a sound or air wave can only be achieved by
administering a very sudden blow to the general mass of the air in the
tube. This impulse must be sufficient to call into operation the
inertia and elastic qualities of the air. It is found, moreover, that
the amplitude of the resulting wave, or the loudness of the sound, is
increased by suitably proportioning the length of the siren pipe and
the frequency of the air puffs; whilst the distance at which it is
heard depends also in some degree upon the form of the mouthpiece.

Inside the siren tube, when it is in operation, the air molecules are
in rapid vibratory motion in the direction of the length of the tube.
If we could at any one instant examine the distribution and changes of
air pressure in the tube, we should find that at some places there are
large, and at others small, variations in air pressure. These latter
places are called the _nodes_ of pressure. At the pressure nodes,
however, we should find large variations in the velocity of the air
particles, and these points are called the _antinodes_ of velocity. In
those places at which the pressure variation is greatest, the velocity
changes are least, and _vice versa_. Outside the tube, as a result of
these air motions in it, we have a hemispherical air wave produced,
which travels out from the mouthpiece as a centre; and if we could
examine the distribution of air pressure and velocity through all
external space, we should find a distribution which is periodic in
space as well as time, constituting the familiar phenomenon of an air
wave.

Turning then to consider the production of an electric, instead of an
air wave, we notice in the first place that the medium with which we
are concerned is the _ether_ filling all space. This ether permits the
production of physical changes in it which are analogous to, but not
identical in nature with, the pressures and movements which constitute
a sound wave. The Hertzian radiator is an appliance for acting on the
ether as the siren acts on the air. It produces a wave in it, and it
can be shown that all the parts of the above described siren apparatus
have their electrical equivalents in the transmitter employed in
Hertzian wave wireless telegraphy.

To understand the nature of an electric wave we must consider, in the
first place, some properties of the ether. In this medium we can at
any place produce a state called _electric displacement_ or _ether
strain_ as we can produce compression or rarefaction in air; and, just
as the latter changes are said to be created by mechanical force, so
the former is said to be due to _electric force_. We can not define
more clearly the nature of this ether strain or displacement until we
know much more about the structure of the ether than we do at present.
We can picture to ourselves the operation of compressing air as an
approximation of the air molecules, but the difficulty of
comprehending the nature of an electric wave arises from the fact that
we cannot yet definitely resolve the notion of electric strain into
any simpler or more familiar ideas.

We have to be content, therefore, to disguise our present ignorance by
the use of some descriptive term, such as _electric strain_,
_electrostatic strain_ or _ether strain_, to describe the directed
condition of the space around a body in a state of electrification
which is produced by electric force. This electric strain is certainly
not of the nature of a compression in the ether, but much more akin to
a twist or rotational strain in a solid body.

For our present purpose it is not so necessary to postulate any
particular theory of the ether as it is to possess some consistent
hypothesis, in terms of which we can describe the phenomena which
will concern us. These effects are, as we shall see, partly states of
electrification on the surface or distributions of electric current in
wires or rods, and partly conditions in the space outside them, which
we are led to recognise as distributions of electric strain and of an
associated effect called _magnetic flux_.

We find such a theory at hand at the present time in the electronic
theory of electricity, which has now been sufficiently developed and
popularised to make it useful as a descriptive hypothesis.[2] This
theory has the great recommendation that it offers a means of
abolishing the perplexing dualism of ether and ponderable matter, and
gives a definite and, in a sense, objective meaning to the word
electricity. In this physical speculation, the chief subject of
contemplation is the electron, or ultimate particle of negative
electricity, which, when associated in greater or less number with a
matrix of some description, forms the atom of ponderable matter. To
avoid further hypothesis, this matrix may be called the _co-electron_;
and we shall adopt the view that a single chemical atom is a union of
a _co-electron_ with a surrounding envelope or group of electrons, one
or more of the latter being detachable. We need not stop to speculate
on the structure of the atomic core or co-electron, whether it is
composed of positive and negative electrons or of something entirely
different. The single electron is the indivisible unit or atomic
element of so-called negative electricity, and the neutral chemical
atom deprived of one electron is the unit of positive electricity. On
this hypothesis, the chemical atom is to be regarded as a microcosm, a
sort of a solar system in miniature, the component electrons being
capable of vibration relatively to the atomic centre of mass.
Furthermore, from this point of view it is the electron which is the
effective cause of radiation. It alone has a grip on the ether whereby
it is able to establish wave motion in the latter.

Dr. Larmor has developed in considerable detail an hypothesis of the
nature of an electron which makes it the centre or convergence-point
of lines of a self-locked ether strain of a torsional type. The notion
of an atom merely as a "centre of force" was one familiar to Faraday
and much supported by Boscovich and others. The fatal objection to the
validity of this notion as originally stated was that it offers no
possibility of explaining the inertia of matter. On the electronic
hypothesis, the source of all inertia is the inertia of the ether, and
until we are able to dissect this last quality into anything simpler
than the time-element involved in the production of an ether strain or
displacement, we must accept it as an ultimate fact, not more
elucidated because we speak of it as the inductance of the electron.

We postulate, therefore, the following ideas: We have to think of the
ether as a homogeneous medium in which a strain of some kind, most
probably of a rotational type, is possible. This strain appears only
under the influence of an appropriate stress called the electric
force, and disappears when the force is removed. Hence to create this
strain necessitates the expenditure of energy. An electron is a
centre or convergence-point of lines of permanent ether strain of such
nature that it cannot release itself. To obtain some idea of the
nature of such a structure, let us imagine a flat steel band formed
into a ring by welding the ends together. There is then no torsional
strain. If, however, we suppose the band cut in one place, one end
then given half a turn and the cut ends again welded, we shall have on
the band a self-locked twist, which can be displaced on the band, but
which can not release itself or be released except by cutting the
ring. Hence we see that to make an electron in an ether possessing
torsional elasticity would require creative energy, and when made, the
electron cannot destroy itself except by occupying simultaneously the
same place as an electron of opposite type. Every electron extends,
therefore, as Faraday said of the atom, throughout the universe, and
the properties that we find in the electron are only there because
they are first in the universal medium, the ether. Every line of ether
or electric strain must, therefore, be a self-closed line, or else it
must terminate on an electron and a co-electron.

So far we have only considered the electron at rest. If, however, it
moves, it can be mathematically demonstrated that it must give rise to
a second form of ether strain which is related to the electric strain
as a twist is related to a thrust or a vortex ring to a squirt in
liquid or a rotation to a linear progression. The ether strain which
results from the lateral movement of lines of electric strain is
called the _magnetic flux_, and it can be mathematically shown that
the movement of an electron, consisting when a rest of a radial
convergence of lines of electric strain, must be accompanied by the
production of self-closed lines of magnetic flux, distributed in
concentric circles or rings round it, the planes of these circles
being perpendicular to the direction of motion of the electron.

This electronic hypothesis, therefore, affords a basis on which we can
build up a theory affording an explanation of the nature of the
intimate connection known to exist between ether, matter and
electricity. The electron is the connecting link between them all, for
it is in itself a centre of convergent ether strain; isolated, it
presents itself as electricity of the negative or resinous kind; and,
in combination with co-electrons and other electrons, it forms the
atoms of ponderable matter. At rest the electron or the co-electron
constitutes an electric charge, and when in motion it is an electric
current. A steady flux or drift of electrons in one direction and
co-electrons in the opposite direction is a continuous electric
current, whilst their mere oscillation about a mean position is an
alternating current. Furthermore, the vibration of an electron, if
sufficiently rapid, enables it to establish what are called electric
waves in the ether, but which are really detached and self-closed
lines of ether strain distributed in a periodic manner through space.

We have, therefore, to start with, three conceptions concerning the
electron, viz.: Its condition when at rest; its state when in uniform
motion; and its operations when in vibration or rapid oscillation. In
the first case, by our fundamental supposition, it consists of lines
of ether strain of a type called the electric strain, radiating
uniformly in all directions. When in uniform motion, it can be shown
that these lines of electric strain tend to group themselves in a
plane perpendicular to the line of motion drawn through the electron,
and their lateral motion generates another class of strain called the
magnetic strain, disposed in concentric circles described round the
electron and lying in this equatorial plane.

The proof of the above propositions cannot be given verbally, but
requires the aid of mathematical analysis of an advanced kind. The
reader must be referred for the complete demonstration to the writings
of Professor J. J. Thomson[3] and Mr. Oliver Heaviside.[4]

In the third case, when the electron vibrates, we have a state in
which self-closed lines of electric strain and magnetic flux are
thrown off and move away through the ether constituting electric
radiation, The manner in which this happens was first described by
Hertz in a Paper on "Electric Oscillations treated according to the
Method of Maxwell."[5] As this phenomenon lies at the very root of
Hertzian wave wireless telegraphy, we must spend a moment or two in
its careful examination.

Let us imagine two metal rods placed in line and constituting what is
called a linear oscillator. Let these rods have adjacent ends
separated by a very small air space, and let one rod be charged with
positive and the other with negative electricity. On the electronic
theory this is explained by stating that there is an accumulation of
electrons in one and of co-electrons in the other. These charges
create a distribution of electric strain throughout their
neighbourhood, which follows approximately the same law of
distribution as the lines of magnetic force of a bar magnet, and may
be roughly represented as in Fig. 1. Suppose then that the air gap is
destroyed, these charges move towards each other and disappear by
uniting, the lines of electric strain then collapse, and as they
shrink in give rise to circular lines of magnetic flux embracing the
rods. This external distribution of magnetism constitutes an electric
current in the rods produced by the movement of the two opposite
electric charges. At this stage it may be explained that the electrons
or atoms of electricity can in some cases make their way freely
between the atoms of ponderable matter. The former are incomparably
smaller than the latter, and in those cases in which this electronic
movement can take place easily, we call the material a good conductor.

[Illustration: FIG. 1.--LINES OF ELECTRIC STRAIN BETWEEN A POSITIVE
AND NEGATIVE ELECTRON AT REST.]

Suppose then the electric charges reappear in reversed positions and
go through an oscillatory motion. The result in the external space
would be the alternate production of lines of electric strain and
magnetic flux, the direction of these lines being reversed each half
cycle. Inside the rods we have a movement of electrons and
co-electrons to and fro, electric charges at the ends of the rods
alternating with electric currents in the rods, the charges being at a
maximum when the current is zero, and the current at a maximum when
the charges have for the moment disappeared. Outside the rods we have
a corresponding set of charges, lines of electric strain stretching
from end to end of the rod, alternating with rings of magnetic flux
embracing the rod. So far we have supposed the oscillation to be
relatively a slow one.

[Illustration: FIG. 2.--SUCCESSIVE STAGES IN THE DEFORMATION OF A LINE
OF STRAIN BETWEEN POSITIVE AND NEGATIVE ELECTRONS IN RAPID
OSCILLATION, SHOWING CLOSED LOOP OF ELECTRIC STRAIN THROWN OFF.]

Imagine next that the to and fro movement of the electrons or charges
is sufficiently rapid to bring into play the inertia quality of the
medium. We then have a different state of affairs. The lines of strain
in the external medium cannot contract or collapse quickly enough to
keep up with the course of events, or movements of the electrons in
the rods, and hence their regular contraction and absorption is
changed into a process of a different kind. As the electrons and
co-electrons, _i.e._, the electric charges, vibrate to and fro, the
lines of electric strain connecting them are nipped in and thrown off
as completely independent and closed lines of electric strain, and at
each successive alternation, groups or batches of these loops of
strain are detached from the rod, and, so to speak, take on an
independent existence. The whole process of the formation of these
self-closed lines of electric strain is best understood by examining a
series of diagrams which roughly represent the various stages of the
process. In Fig. 2 we have a diagram (_a_) the curved line in which
delineates approximately the form of one line of electric strain round
a linear oscillator, with spark gap in the centre, one half being
charged positively and the other negatively. Let us then suppose that
the insulation of the spark gap is destroyed, so that the opposite
electric charges rush together and oscillate to and fro. The strain
lines at each oscillation are then crossed or decussate, and the
result, as shown in Fig. 2, _d_, is that a portion of the energy of
the field is thrown off in the form of self-closed lines of strain
(see Fig. 2, _e_). At each oscillation of the charges the direction of
the lines of strain springing from end to end of the radiator is
reversed. It is a general property of lines of strain whether electric
or magnetic, that there is a tension along the line and a pressure at
right angles. In other words, these lines of electric strain are like
elastic threads, they tend to contract in the direction of their
length and press sideways on each other when in the same direction.
Hence it is not difficult to see that as each batch of self-closed
lines of strain is thrown off, the direction of the strain round each
loop is alternately in one direction and in the other. Hence these
loops of electric strain press each other out, and each one that is
formed squeezes the already formed loops further and further from the
radiator. The loops, therefore, march away into space (see Fig. 2,
_f_). If we imagine ourselves standing at a little distance at a point
on the equatorial line and able to see these loops of strain as they
pass, we should recognise a procession of loops, consisting of
alternately directed strain lines marching past. This movement through
the ether of self-closed lines of electric strain constitutes what is
called electric radiation.

Hence along a line drawn perpendicular to the radiator through its
centre, there is a distribution of electric strain normal to that
line, which is periodic in space and in time. Moreover, in addition to
these lines of electric strain, there are at right angles to them
another set of self-closed lines of magnetic flux. Alternated between
the instants when the electric charges at the ends of the radiator are
at their maximum, we have instants when the radiator rod is the seat
of an electric current, and hence the field round it is filled with
circular lines of magnetic flux coaxial with the radiator. As the
current alternates in direction each half period, these rings of
magnetic flux alternate in direction as regards the flux, and hence we
must complete our mental picture of the space round the radiator rods
when the charges are oscillating by supposing it filled with
concentric rings of magnetic flux which are periodically reversed in
direction, and have their maximum values at those instants and places
where the lines of electric strain have their zero values.
Accordingly, along the equatorial line we have two sets of strains in
the ether, distributed periodically in space and in time. First, the
lines of electric strain in the plane of the radiator, and, secondly,
the lines of magnetic flux at right angles to these. At any one point
in space these two changes, the strain and the flux, succeed each
other periodically, being, however, at right angles in direction. At
any one moment these two effects are distributed periodically or
cyclically through space, and these changes in time and space
constitute an _electric wave_ or electromagnetic wave.

We may then summarise the above statements by saying that the most
recent hypothesis as to the nature of electrical action and of
electricity itself is briefly comprised in the following statements:
The universally diffused medium called the ether has had created in
it certain centres of strain or radiating points from which proceed
lines of strain, and these centres of force are called electrons.
Electrons must, therefore, be of two kinds, positive and negative,
according to the direction of the strain radiating from the centre.
These electrons in their free condition constitute what we call
electricity, and the electrons themselves are the atoms of electricity
which, in one sense, is, therefore, as much material as that which we
call ordinary gross or ponderable matter.

Collocations of these electrons constitute the atoms of gross matter,
and we must consider that the individuality of any atom is not
determined merely by the identity of the electrons composing it, but
by the permanence of their arrangement or form. In any mass of
material substance there is probably a continual exchange of electrons
from one atom to another, and hence at any one given moment, whilst a
number of the electrons are an association forming material atoms,
there will be a further number of isolated but intermingled electrons,
which are called the free electrons. In substances which we call good
conductors, we must imagine that the free electrons have the power of
moving freely through or between the material atoms, and this movement
of the electrons constitutes a current of electricity; whilst a
superfluity of electrons of either type in any one mass of matter
constitutes what we call a charge of electricity. Hence an electrical
oscillation, which is merely a very rapid alternating current taking
place in a conductor, is on this hypothesis assumed to consist in a
rapid movement to and fro of the free electrons. We may picture to
ourselves, therefore, a rod of metal in which electrical oscillations
are taking place, as similar to an organ-pipe or siren tube in which
movements of the air particles are taking place to and fro, the free
electrons corresponding with the air particles.

Owing to the nature of the structure of an electron, it follows,
however, that every movement of an electron is accompanied by changes
in the distribution of the electric strain or ether strain taking
place throughout all surrounding space, and, as already explained,
certain very rapid movements of the electrons have the effect of
detaching closed lines of strain in the ether which move off through
space, forming, when cyclically distributed, an electric wave.

[Illustration: FIG. 3.--SIMPLE MARCONI RADIATOR. B, battery; I,
induction coil; K, signalling key; S, spark gap; A, aerial wire; E,
earth plate.]

We may next proceed to apply these principles to the explanation of
the action of the simplest form of Hertzian wave telegraphic radiator,
viz., the Marconi aerial wire. In its original form this consists of a
long vertical insulated wire, A, the lower end of which is attached to
one of the spark balls S of an induction coil, I, the other spark ball
being connected to earth E, and the two spark balls being placed a few
millimetres apart (see Fig. 3). When the coil is set in action
oscillatory or Hertzian sparks pass between the balls, electric
oscillations are set up in the wire and electric waves are radiated
from it. Deferring for the moment a more detailed examination of the
operations of the coil and at the spark gap, we may here say that the
action which takes place in the aerial wire is as follows: The wire is
first charged to a high potential, let us suppose, with negative
electricity. We may imagine this process to consist in forcing
additional electrons into it, the induction coil acting as an electron
pump. Up to a certain pressure the spark gap is a perfect insulator,
but at a critical pressure, which for spark gap lengths of four or
five millimetres and balls about one centimetre in diameter
approximates to three thousand volts per millmetre, the insulation of
the air gives way, and the charge in the wire rushes into the earth.
In consequence, however, of the inertia of the medium or of the
electrons, the charge, so to speak, overshoots the mark, and the wire
is then left with a charge of opposite sign. This again in turn
rebounds, and so the wire is discharged by a series of electrical
oscillations, consisting of alternations of static charge and electric
discharge. We may fasten our attention either on the events taking
place in the vertical wire or in the medium outside, but the two sets
of phenomena are inseparably connected and go on together. When the
aerial wire is statically charged, we may describe it by saying that
there is an accumulation of electrons or co-electrons in it. Outside
the wire there is, however, a distribution of electric strain the
strain lines proceeding from the wire to the earth (see Fig. 4).

[Illustration: FIG. 4.--LINES OF ELECTRIC STRAIN (DOTTED LINES)
EXTENDING BETWEEN A MARCONI AERIAL, A, AND THE EARTH _ee_ BEFORE
DISCHARGE.]

The wire has _capacity_ with respect to the earth, and it acts like
the inner coating of a Leyden jar, of which the dielectric is the air
and ether around it, and the outer coating is the earth's surface.
When the discharge takes place, we may consider that electrons rush
out of the wire and then rush back again into it. At the moment when
the electrons rush out of or into the aerial wire, we say there is an
electric current flowing into or out of the wire, and this electron
movement, therefore, creates the magnetic flux which is distributed in
concentric circles round the wire. This current, and, therefore,
motion of electrons, can be proved to exist by its heating effect upon
a fine wire inserted in series with the aerial, and in the case of
large aerials it may have a mean value of many amperes and a maximum
value of hundreds of amperes. Inside the aerial wire we have,
therefore, alternations of electric potential or charge and electric
current, or we may call it electron-pressure and electron-movement.

There is, therefore, an oscillation of electrons in the aerial wire,
just as in the case of an organ-pipe there is an oscillation of air
molecules in the pipe. Outside the aerial we have variations and
distributions of electric strain and magnetic flux. The resemblance
between the closed organ-pipe and the simple Marconi aerial is, in
fact, very complete. In the case of the closed organ-pipe, we have a
longitudinal oscillation of air molecules in the pipe. At the open end
or mouthpiece, where we have air moving in and out, the air movement
is alternating and considerable, but there is little or no variation
of air pressure. At the upper or closed end of the pipe we have great
variation of air pressure, but little or no air movement (see Fig. 5).

Compare this now with the electrical phenomena of the aerial. At the
spark ball or lower end we have little or no variation of potential or
electron pressure, but we have electrons rushing into and out of the
aerial at each half oscillation, forming the electric discharge or
current. At the upper or insulated end we have little or no current,
but great variations of potential or electron pressure. Supposing we
could examine the wire inch by inch, all the way up from the spark
balls at the bottom to the top, we should find at each stage of our
journey that the range of variation and maximum value of the current
in the wire became less and those of the potential became greater. At
the bottom we have nearly zero potential or no electric pressure, but
large current, and at the top end, no current, but great variation of
potential.

[Illustration: FIG. 5.--AMPLITUDE OF PRESSURE VARIATION IN A CLOSED
ORGAN PIPE, INDICATED BY THE ORDINATES OF THE DOTTED LINE _xy_.]

We can represent the amplitude of the current and potential values
along the aerial by the ordinates of a dotted line so drawn that its
distance from the aerial represents the potential oscillation or
current oscillation at that point (see Fig. 6).

This distribution of potential and current along the wire does not
necessarily imply that any one electron moves far from its normal
position. The actual movement of any particular air molecule in the
case of a sound wave is probably very small, and reckoned in
millionths of an inch. So also we must suppose that any one electron
may have a small individual amplitude of movement, but the
displacement is transferred from one to another. Conduction in a solid
may be effected by the movement of free electrons intermingled with
the chemical atoms, but any one electron may be continually passing
from a condition of freedom to one of combination.

[Illustration: FIG. 6.--(_a_) DISTRIBUTION OF ELECTRIC PRESSURE IN A
MARCONI AERIAL, A, (_b_) DISTRIBUTION OF ELECTRIC CURRENT IN A MARCONI
AERIAL, AS SHOWN BY THE ORDINATES OF THE DOTTED LINE _xy_.]

So much for the events inside the wire, but now outside the wire its
electric charge is represented by lines of electric strain springing
from the aerial to the earth. It must be remembered that every line of
strain terminates on an electron or a co-electron. Hence, when the
discharge or spark takes place between the spark balls, the rapid
movement of the electrons in the wire is accompanied by a
redistribution and movement of the lines of strain outside. As the
negative charge flows out of the aerial the ends of the strain lines
abutting on to it run down the wire and are transferred to the earth,
and at the next instant this semi-loop of electric or ether strain,
with its ends on the earth, is pushed out sideways from the wire by
the growth of a new set of lines of ether strain in an opposite
direction. The process is best understood by consulting a series of
diagrams which represent the distribution and approximate form of a
few of the strain lines at successive instants (see Fig. 7). In
between the lines of formation of the successive strain lines between
the aerial and the earth, corresponding to the successive alternate
electric charges of the aerial with opposite sign, there are a set of
concentric rings of magnetic flux formed round it which are
alternately in opposite directions, and these expand out, keeping step
with the progress of the detached strain loops and having their planes
at right angles to the latter. As the semi-loops of electric strain
march outwards with their feet on the ground, these strain lines must
always be supposed to terminate on electrons, but not continually on
the same electrons. Since the earth is a conductor, we must suppose
that there is a continual migration of the electrons forming the atoms
of the earth, and that when one electron enters an atom, another
leaves it. Hence, corresponding to the electric wave in the space
above, there are electrical changes in the ground beneath. This view
is confirmed by the well-known fact that the achievement of Hertzian
wave telegraphy is much dependent on the nature of the surface over
which it is conducted, and can be carried on more easily over good
conducting material, like sea water, than over badly conducting dry
land.

[Illustration: FIG. 7.--SUCCESSIVE STAGES IN THE PRODUCTION OF A
SEMI-LOOP OF ELECTRIC STRAIN BY A MARCONI AERIAL RADIATOR.]

The matter may be viewed, however, from another standpoint. Good
conductors are opaque to Hertzian waves; in other words, are
non-absorptive. The energy of the electric wave is not so rapidly
absorbed when it glides over a sea surface as when it is passing over
a surface which is an indifferent conductor, like dry land. In fact,
it is possible by the improvement of the signals to detect a heavy
fall of rain in the space between two stations separated only by dry
land. It is, however, clear that on the electronic theory the
progression of the lines of electric strain can only take place if the
surface over which they move is a fairly good conductor, unless these
lines of strain form completely closed loops. Hence we may sum up by
saying that there are three set of phenomena to which we must pay
attention in formulating any complete theory of the aerial. The first
is the operation taking place in the vertical wire, which is described
by saying that electrical oscillations or vibratory movements of
electrons are taking place in it, and, on our adopted theory, it may
be said to consist in a longitudinal vibration of electrons of such a
nature that we may appropriately call the aerial an ether organ-pipe.
Then in the next place, we have the distribution and movement of the
lines of electric strain and magnetic flux in the space outside the
wire, constituting the electric wave; and lastly, there are the
electrical changes in the conductor over which the wave travels, which
is the earth or water surrounding the aerial. In subsequently dealing
with the details of transmitting arrangements, attention will be
directed to the necessity for what telegraphists call a "good earth"
in connection with Hertzian wave telegraphy. This only means that
there must be a perfectly free egress and ingress for the electrons
leaving or entering the aerial, so that nothing hinders their access
to the conducting surface over which the wave travels. There must be
nothing to stop or throttle the rush of electrons into or out of the
aerial wire, or else the lines of strain cannot be detached and and
travel away.

We may next consider more particularly the energy which is available
for radiation and which is radiated. In the original form of simple
Marconi aerial, the aerial itself when insulated forms one coating or
surface of a condenser, the dielectric being the air and ether around
it, and the other conductor being the earth. The electric energy
stored up in it just before discharge takes place is numerically equal
to the product of the capacity of the aerial and half the square of
the potential to which it is charged.

If we call C the capacity of the aerial in microfarads, and V the
potential in volts to which it is raised before discharge, then the
energy storage in joules E is given by the equation,

    E = (CV^{2}) / (2 · 10^{6}).

Since one joule is nearly equal to three-quarters of a foot-pound, the
energy storage in foot-pounds F is roughly given by the rule F =
(3/8)CV^{2}/10^{6}. For spark lengths of the order of five to fifteen
millimetres, the disruptive voltage in air of ordinary pressure is at
the rate of 3,000 volts per millimetre. Hence, if S stands for the
spark length in millimetres, and C for the aerial capacity in
microfarads, it is easy to see that the energy storage in foot-pound
is

    F = (27CS^{2}) / 8.

If the aerial consists of a stranded wire formed of 7/22 and has a
length of 150 feet, and is insulated and held vertically with its
lower end near the earth, it would have a capacity of about one three
ten-thousandths of a microfarad or 0·0003 mfd.[6] Hence, if it is used
as a Marconi aerial and operated with a spark gap of one centimetre in
length, the energy stored up in the wire before each discharge would
be only one-tenth (0·1) of a foot-pound.

By no means can all of this energy be radiated as Hertzian waves; part
of it is dissipated as heat and light in the spark, and yet such an
aerial can, with a sensitive receiver such as that devised by Mr.
Marconi, make itself felt for a hundred miles over sea in every
direction. This fact gives us an idea of the extremely small energy
which, when properly imparted to the ether, can effect wireless
telegraphy over immense distances. Of course, the minimum telegraphic
signal, say the Morse dot, may involve a good many, perhaps
half-a-dozen, discharges of the wire, but even then the amount of
energy concerned in affecting the receiver at the distant place is
exceedingly small.

The problem, therefore, of long-distance telegraphy by Hertzian waves
is largely, though not entirely, a matter of associating sufficient
energy with the aerial wire or radiator. There are obviously two
things which may be done; first, we may increase the capacity of the
aerial, and secondly, we may increase the charging voltage or, in
other words, lengthen the spark gap. There is, however, a well-defined
limit to this last achievement. If we lengthen the spark gap too much,
its resistance becomes too great and the spark ceases to be
oscillatory. We can make a discharge, but we obtain no radiation. When
using an induction coil, about a centimetre, or at most a centimetre
and a half, is the limiting length of oscillatory sparks; in other
words, our available potential difference is restricted to 30,000 or
40,000 volts. By other appliances we can, however, obtain oscillatory
sparks having a voltage of 100,000 or 200,000 volts, and so obtain
what Hertz called "active sparks" five or six centimetres in length.

Turning then to the question of capacity, we may enquire in the next
place how the capacity of an aerial wire can be increased. This has
generally been done by putting up two or more aerial wires in
contiguity and joining them together, and so making arrangements
called in the admitted slang of the subject "multiple aerials." The
measurement of the capacity of insulated wires can be easily carried
out by means of an appliance devised by the author and Mr. W. C.
Clinton, consisting of a rotating commutator which alternately charges
the insulated wire at a source of known electromotive force and then
discharges it through a galvanometer. If this galvanometer is
subsequently standardised, so that the ampere value of its deflection
is known, we can determine easily the capacity C of the aerial or
insulated conductor, reckoned in microfarads, when it is charged to a
potential of V volts, and discharged _n_ times a second through a
galvanometer. The series of discharges are equivalent to a current, of
which the value in amperes A is given by the equation

    A = (nVC) / (10^{6}),

and hence, if the value of the current resulting is known, we have the
capacity of the aerial or conductor expressed in microfarads, given by
the formula

    C = (A10^{6}) / (nV).

A series of experiments made on this plan have revealed the fact that
if a number of vertical insulated wires are hung up in the air and
rather near together, the electrical capacity of the whole of the
wires in parallel is not nearly equal to the sum of their individual
capacities. If a number of parallel insulated wires are separated by a
distance equal to about 3 per cent. of their length, the capacity of
the whole lot together varies roughly as the square root of their
number. Thus, if we call the capacity of one vertical wire in free
space unity, then the capacity of four wires placed rather near
together will only be about twice that of one wire, and that of
twenty-five wires will only be about five times one wire.

This approximate rule has been confirmed by experiments made with long
wires one hundred or two hundred feet in length in the open air. Hence
it points to the fact that the ordinary plan of endeavouring to obtain
a large capacity by putting several wires in parallel and not very far
apart is very uneconomical in material. The diagrams in Fig. 8 show
the various methods which have been employed by Mr. Marconi and others
in the construction of such multiple wire aerials. If, for instance,
we put four insulated stranded 7/22 wires each 100 feet long, about
six feet apart, all being held in a vertical position, the capacity of
the four together is not much more than twice that of a single wire.
In the same manner, if we arrange 150 similar wires, each 100 feet
long, in the form of a conical aerial, the wires being distributed at
the top round a circle 100 feet in diameter, the whole group will not
have much more than twelve times the capacity of one single wire,
although it weighs 150 times as much.

[Illustration: FIG. 8.--VARIOUS FORMS OF AERIAL RADIATOR. _a_, single
wire; _b_, multiple wire; _c_, fan shape; _d_, cylindrical; _g_,
Conical.]

The author has designed an aerial in which the wires, all of equal
length, are arranged sufficiently far apart not to reduce each other's
capacity.

As a rough guide in practice, it may be borne in mind that a wire
about one tenth of an inch in diameter and one hundred feet long, held
vertical and insulated, with its bottom end about six feet from the
ground, has a capacity of 0·0002 of a microfarad, if no other earthed
vertical conductors are very near it. The moral of all this is that
the amount of electric energy which can be stored up in a simple
Marconi aerial is very limited, and is not much more than one-tenth of
a joule or one-fourteenth of a foot-pound, per hundred feet of 7/22
wire. The astonishing thing is that with so little storage of energy
it should be possible to transmit intelligence to a distance of a
hundred miles without connecting wires.

One consequence, however, of the small amount of energy which can be
accumulated in a simple Marconi aerial is that this energy is almost
entirely radiated in one oscillation or wave. Hence, strictly
speaking, a simple aerial of this type does not create a train of
waves in the ether, but probably at most a single impulse or two.

[Illustration: FIG. 9.--MARCONI-BRAUN SYSTEM OF INDUCING ELECTROMOTIVE
FORCE IN AN AERIAL, A. B, battery; K, key; I, induction coil; S, spark
gap; C, Leyden jar; E, earth plate; _ps_, oscillation transformer.]

We shall later on consider some consequences which follow from this
fact. Meanwhile, it may be explained that there are methods by which
not only a much larger amount of energy can be accumulated in
connection with an aerial, but more sustained oscillations created
than by the original Marconi method. One of these methods originated
with Professor Braun, of Strasburg, and a modification was first
described by Mr. Marconi in a lecture before the Society of Arts of
London.[7] In this method the charge in the aerial is not created by
the direct application to it of the secondary electromotive force of
an induction coil, but by means of an induced electromotive force
created in the aerial by an oscillation transformer. The method due to
Professor Braun is as follows: A condenser or Leyden jar has one
terminal, say, its inside, connected to one spark ball of an induction
coil. The other spark ball is connected to the outside of the Leyden
jar or condenser through the primary coil of a transformer of a
particular kind, called an oscillation transformer (see Fig. 9). The
spark balls are brought within a few millimetres of each other. When
the coil is set in operation, the jar is charged and discharged
through the spark gap, and electrical oscillations are set up in the
circuit consisting of the dielectric of the jar, the primary coil of
the oscillation transformer and the spark gap. The secondary circuit
of this oscillation transformer is connected in between the earth and
the insulated aerial wire; hence, when the oscillations take place in
the primary circuit, they induce other oscillations in the aerial
circuit. But the arrangement is not very effective unless, as is
shown by Mr. Marconi, the two circuits of the oscillation transformer
are tuned together.

We shall return presently to the consideration of this form of
transmitter; meanwhile we may notice that by means of such an
arrangement it is possible to create in the aerial a far greater
charging electromotive force than would be the case if the aerial were
connected directly to one terminal of the secondary circuit of the
induction coil, the other terminal being to earth, and the two
terminals connected as usual by spark balls. By the inductive
arrangement it is possible to create in an aerial electromotive forces
which are equivalent to a spark of a foot in length, and when the
length of the aerial is also properly proportioned the potential along
it will increase all the way up, until at the top or insulated end of
the aerial it may reach an amount capable of giving sparks several
feet in length. From the remarks already made on the analogy between
the closed organ-pipe and the Marconi aerial wire, it will be seen
that the wave which is radiated from the aerial must have a wave
length four times that of the aerial if the aerial is vibrating in its
fundamental manner. It is also possible to create electrical
oscillations in a vertical wire which are the harmonics of the
fundamental.

All musicians are aware that in the case of an organ-pipe if the pipe
is blown gently it sounds a note which is called the fundamental of
the pipe. The celebrated mathematician, Daniel Bernouilli, discovered
that an organ-pipe can be made to yield a succession of musical notes
by properly varying the pressure of the current of air blown into it.
If the pipe is an open pipe, and if we call the frequency of the
primary note obtained when the pipe is gently blown, unity, then when
we blow more strongly the pipe yields notes which are the harmonics of
the fundamental one; that is to say, notes which have frequencies
represented by the numbers 2, 3, 4, 5, &c. If, however, the pipe is
closed at the top, then over-blowing the pipe makes it yield the odd
harmonics or the tones which are related to the primary tone in the
ratio of 3, 5, 7, &c., to unity. Accordingly, if a stopped pipe gives
as its fundamental the note C, its first overtone will be the fifth
above the octave or G'.

[Illustration: FIG. 10.--SEIBT'S APPARATUS FOR SHOWING STATIONARY
WAVES IN LONG SOLENOID A. I, induction coil; S, spark gap; L,
inductance coil; C_{1}C_{2}, Leyden jars; E, earth wire.]

As already remarked, the aerial wire or radiator as used in Marconi
telegraphy may be looked upon as a kind of ether organ-pipe or siren
tube, and its electrical phenomena are in every respect similar to the
acoustic phenomena of the ordinary closed organ-pipe. When the aerial
is sounding its fundamental ether note, the conditions which pertain
are that there is a current flowing into the aerial at the lower end,
but at that point the variation in potential is very small, whereas at
the upper end there is no current, but the variations of potential are
very large. Accordingly, we say that at the upper end of the aerial
there is an antinode of potential and a node of current, and at the
bottom an antinode of current and a node of potential. By altering the
frequency of the electrical impulses we can create in the aerial an
arrangement of nodes of current or potential corresponding to the
overtones of a closed organ-pipe. But whatever may be the arrangement
the conditions must always hold that there is a node of current at
the upper end and an antidote of current at the lower end. In other
words, there are large variations of current at the place where the
aerial terminates on the spark-gap and no current at the upper end.
The first harmonic is formed where there is a node of potential at
one-third of the length of the aerial from the top. In this case we
have a node of potential not only at the lower end of the wire, but at
two-thirds of the way up. In the same way we can create in the closed
organ-pipe, by properly overblowing the pipe, a region about
two-thirds of the way up the pipe, where the pressure changes in the
air are practically no greater than they are at the mouthpiece. We can
make evident visually in a beautiful manner the existence of similar
stationary electrical waves in an aerial by means of an ingenious
arrangement devised by Dr. Georg Seibt, of Berlin. It consists of a
very long silk-covered copper wire, A (see Fig. 10), wound in a close
spiral of single layer round a wooden rod six feet long and about two
inches in diameter. This rod is insulated, and at the lower end the
wire is connected to a Leyden jar circuit, consisting of a Leyden jar
or jars and an inductance coil, L, the inductance of which can be
varied. Oscillations are set up in this jar circuit by means of an
induction-coil discharge, and the lower end of the long spiral wire is
attached to one point on the jar circuit. In this manner we can
communicate to the bottom end of the long spiral wire a series of
electric impulses, the time period of which depends upon the capacity
of the jar and the inductance of the discharge circuit. We can,
moreover, vary this frequency over wide limits. Parallel to the long
spiral wire is suspended another copper wire, E (see Fig. 10), and
between this wire and the silk-covered copper wire discharges take
place due to the potential difference between each part of the wire
and this long aerial wire. If we arrange matters so that the impulses
communicated to the bottom end of the long spiral wire correspond to
its fundamental note or periodic time, then in a darkened room we
shall see a luminous glow or discharge between the vertical wire and
the spiral wire, which increases in intensity all the way up to the
top of the spiral wire. The luminosity of this brush discharge at any
point is evidence of the potential of the spiral wire at that point,
and its distribution clearly demonstrates that the difference of
potential between the spiral wire and the aerial increases all the way
up from the bottom to the top of the spiral wire. In the next place,
by making a little adjustment and by varying the inductance of the jar
circuit, we can increase the frequency of the impulses which are
falling upon the spiral wire; and then it will be noticed that the
distribution of the brush discharge or luminosity is altered, and that
there is a maximum now at about one-third of the height of the spiral
wire, and a dark place at about two-thirds of the height, and another
bright place at the top, thus showing that we have a node of potential
at about two-thirds the way up the wire (see Fig. 11), and we have
therefore set up in the spiral wire electrical oscillations
corresponding to the first overtone. It is possible to show in the
same way the existence of the second harmonic in the coil, but the
luminosity then becomes too faint to be seen at a distance.

[Illustration: FIG. 11.--HARMONIC OSCILLATIONS IN LONG SOLENOID SHOWN
WITH SEIBT'S APPARATUS.]

An interesting form of aerial devised by Professor Slaby, of Berlin,
depends for its action entirely on the fact that the electrical
oscillations set up in it which radiate are harmonics of the
fundamental tone.

[Illustration: FIG. 12.--NON-RADIATIVE CLOSED LOOP AERIAL.]

[Illustration: FIG. 13.--SLABY'S LOOP RADIATOR.]

A closed vertical loop, A_{1}A_{2} (see Fig. 12), is formed by
erecting two parallel insulated wires vertically a few feet apart and
joining them together at the top. At the bottom these wires are
connected, with the secondary terminals of an induction coil, a
condenser, C, or Leyden jar, being bridged across the terminals and a
pair of spark balls, S, inserted in one side of the loop. It will
readily be seen that on setting the coil in action, oscillations will
take place in these vertical wires, but that if the oscillations are
simply the fundamental note of the system, then at any moment
corresponding to a current going up one side of the loop of wire there
must be a current coming down the other. Accordingly, an arrangement
of this kind, forming what is called a closed circuit, will not
radiate or radiates but very feebly. Professor Slaby found, however,
that it might be converted into a powerful radiator if we give the two
sides of the loop unequal capacity or inductance and at the same time
earth one of the lower ends of the loop, as shown in Fig. 13. By this
means it is possible to set up in the loop electrical overtones or
harmonics of the fundamental oscillation, and if we cause the system
to vibrate so as to produce its first odd harmonic, there is a
potential node at the lower end of both vertical sides of the loop, a
potential node on both vertical sides at two-thirds of the way up, and
a potential antinode at the summit of the loop; then, under these
circumstances, the closed loop of wire is in the same electrical
condition as if two simple Marconi aerials, both emitting their first
odd harmonic oscillation, were placed side by side and joined together
at the top.

It is a little difficult without the employment of mathematical
analysis to explain precisely the manner in which earthing one side of
the loop or making the loop unsymmetrical as regards inductance has
the effect of creating overtones in it. The following rough
illustration may, however, be of some assistance. Imagine a long
spiral metallic spring supported horizontally by threads. Let this
represent a conductor, and let any movement to or fro of a part of the
spring represent a current in that conductor. Suppose we take hold of
the spring at one end, we can move it bodily to and fro as a whole. In
this case, every part of the spring is moving one way or the other in
the same manner at the same time. This corresponds with the case in
which the discharge of the condenser through the uniform loop
conductor is a flow of electricity, all in one direction one way or
the other. The current is in the same direction in all parts of the
loop at the same time, and, therefore, if the current is going up one
side of the loop it is at the same time coming down the other side.
Hence the two sides of the loop are always in exact opposition as
regards the effect of the current in them on the external space, and
the loop does not radiate. Returning again to the case of the spring.
Supposing that we add a weight to one end of the spring by attaching
to it a metal ball, and then move the other end to and fro with
certain periodic motion, it will be found quite easy to set up in the
spring a pulsatory motion resembling the movement of the air in an
open organ-pipe. Under these circumstances both ends of the spring
will be moving inwards or outwards at the same time, and the central
portions of the spring, although being pressed and expanded slightly,
are moving to and fro very little. This corresponds in the case of the
looped aerial with a current flowing up or down both sides at the same
time; in other words, when this mode of electrical oscillation is
established in the loop, its electrical condition is just that of two
simple Marconi aerials joined together at the top and vibrating in
their fundamental manner. Accordingly, if one side of the double loop
is earthed, we then have an arrangement which radiates waves.
Professor Slaby found that by giving one side of the loop less
inductance than the other, and at the same time earthing the side
having greater inductance at the bottom, he was able to make an
arrangement which radiated, not in virtue of the normal oscillations
of the condenser, but in virtue of the harmonic oscillations set up in
the conductor itself. The mathematical theory of this radiator has
been very fully developed by Dr. Georg Seibt.

It will be seen, therefore, that there are several ways in which we
may start into existence oscillations in an aerial. First, the aerial
may be insulated, and we may charge it to a high potential and allow
this charge suddenly to rush out. Although this process gives rise to
a disturbance in the ether, as already explained, it is analogous to a
pop or explosion in the air, rather than to a sustained musical note.
The exact acoustic analogue would be obtained if we imagine a long
pipe pumped full of air and then suddenly opened at one end. The air
would rush out, and, communicating a blow to the outer air, would
create an atmospheric disturbance appreciated as a noise or small
explosion. This is what happens when we cut the string and let the
cork fly out from a bottle of champagne. At the same time, the
inertia of the air rushing out of the tube would cause it to overshoot
the mark, and a short time after opening the valve the tube, so far
from containing compressed air, would contain air slightly rarefied
near its mouth, and this rarefication would travel back up the tube in
the form of wave motion, and, being reflected as condensation at the
closed end, travel down again; and so after being reflected once or
twice at the open or closed end, become damped out very rapidly in
virtue of both air friction and the radiation of the energy. In the
case, however, of the ordinary organ-pipe, we do not depend merely
upon a store of compressed air put into the pipe, but we have a store
of energy to draw upon in the form of the large amount of compressed
air contained in a wind chest, which is being continually supplied by
the bellows. This store of compressed air is fed into the organ-pipe,
with the result that we obtain a continuous radiation of sound waves.
The first case, in which the only store of energy is the compressed
air originally contained in the pipe, illustrates the operation of the
simple Marconi aerial. The second case, in which there is a larger
store of energy to draw upon, the organ-pipe being connected to a wind
chest, illustrates the Marconi-Braun method, in which an aerial is
employed to radiate a store of electric energy contained in a
condenser, gradually liberated by the aerial in the form of a series
of electrical oscillations and waves. In this arrangement the
condenser corresponds to the wind chest, and it is continually kept
full of electrical energy by means of the induction coil or
transformer, which answers to the bellows of the organ. From the
condenser, electrical energy is discharged each time the spark
discharge passes at a spark gap in the form of electrical oscillations
set up in the primary circuit of an oscillation transformer. The
secondary circuit of this transformer is connected in between the
earth and the aerial, and therefore may be considered as part of it,
and, accordingly, the energy which is radiated from the aerial is not
simply that which is stored up in it in virtue of its own small
capacity, but that which is stored up in the much larger capacity
represented by the primary condenser or, as it may be called, the
electrical wind chest. By the second arrangement we have therefore the
means of radiating more or less continuous trains of electric waves,
corresponding with each spark discharge. To create powerful
oscillations in the aerial, one condition of success is that there
shall be an identity in time-period between the circuit of the aerial
and that of the primary condenser. The aerial is an open circuit which
has capacity with respect to the earth, and it has also inductance,
partly due to the wire of the aerial and partly due to the secondary
circuit of the oscillation transformer in series with it. The primary
circuit or spark circuit has capacity--viz., the capacity of the
energy-storing condenser--and it has also inductance--viz., the
inductance of the primary circuit of the oscillation transformer. We
shall consider at a later stage more particularly the details of
syntonising arrangements, but meanwhile it may be said that one
condition for setting up powerful waves by means of the above
arrangement is that the electrical time-period of both the two
circuits mentioned shall be the same. This involves adjusting the
inductance and capacity so that the product of conductance and
capacity for each of these two circuits is numerically the same.
Instead of employing an oscillation transformer between the condenser
circuit and the aerial, the aerial may be connected directly to some
point on the condenser circuit at which the potential oscillations are
large, and we have then another arrangement devised by Professor Braun
(see Fig. 14). In this case, in order to accumulate large potential
oscillations at the top of the aerial, it is, as we have seen,
necessary that the length of the aerial shall be one quarter the
length of the wave. If, therefore, the electrical oscillations in the
condenser circuit are at the rate of N per second, in other words,
have a frequency N, the wave-length correponding to this frequency is
given by the expression,

    3×10^{10}/N cms.

The number 3×10^{10} is the value in centimetres per second of the
velocity of the electromagnetic wave, and is identical with that of
light. The corresponding resonant length of the aerial is therefore
one-fourth of this wave-length, or 3×10^{10}/4N. Generally speaking,
however, it will be found that with any length of aerial which is
practicable, say, 200 feet or 6,000 cms., this proportion necessitates
rather a high frequency in the primary oscillation circuit. In the
case considered--viz., for an aerial 200 feet in height--the
oscillations in the primary circuit must have a frequency of one and a
quarter million. This high frequency can only be obtained either by
greatly reducing the inductance of the primary discharge circuit, or
reducing the capacity. If we reduce the capacity, we thereby greatly
reduce the storage of energy, and it is not practicable to reduce the
inductance below a certain amount.

[Illustration: FIG. 14.--BRAUN'S RADIATOR. B, battery; I, induction
coil; K, key; S, spark-gap; L, inductance coil; C, condenser; A,
aerial.]

Summing up, it may be said that there are three, and, as far as the
writer is aware, at present only three, modes of exciting the
electrical oscillations in an aerial wire. First, the aerial may
itself be used as an electrical reservoir and charged to a high
potential and suddenly discharged to the earth. This is the original
Marconi method. The second method, due to Braun, consist of attaching
the aerial to some point on an oscillation circuit consisting of a
condenser, an inductance coil and a spark gap, in series with one
another, and charging and discharging the condenser across the spark
gap so as to create alterations of potential at some point on the
oscillation circuit. The length of the aerial must then be so
proportioned as above described that it is resonant to this frequency.
Thirdly, we may employ the arrangement involving an oscillation
transformer, in which the oscillations in the primary condenser
circuit are made to induce others in the aerial circuit, the
time-period of the two circuits being the same. This method may be
called the Braun-Marconi method. Professor Slaby has combined together
in a certain way the original Marconi simple aerial with the resonant
quarter-wave-length wire of Braun. He constructs what he calls a
_multiplicator_, which is really a wire wound into a loose spiral
connected at one point to an oscillation circuit consisting of a
condenser inductance, the length of this wire being proportioned so
that there is a great resonance or multiplication of tension or
potential at its free end. This free end is then attached to the lower
end of an ordinary Marconi aerial, and serves to charge it with a
higher potential than could be obtained by the use of the induction
coil directly attached to it.

       *       *       *       *       *

We have next to consider the appliances for creating the necessary
charging electromotive force, and for storing and releasing this
charge at pleasure, so as to generate the required electrical
oscillations in the aerial.

It is essential that this generator should be able to create not only
large potential difference, but also a certain minimum electric
current. Accordingly, we are limited at the present moment to one of
two appliances--viz., the induction coil or the alternating current
transformer.

It will not be necessary to enter into an explanation of the action of
the induction coil. The coil generally employed for wireless
telegraphy is technically known as a ten-inch coil--_i.e._, a coil
which is capable of giving a ten-inch spark between pointed conductors
in air at ordinary pressure. The construction of a large coil of this
description is a matter requiring great technical skill, and is not to
be attempted without considerable previous experience in the
manufacture of smaller coils. The secondary circuit of a ten-inch coil
is formed of double silk-covered copper wire; generally speaking, the
gauge called No. 36, or else No. 34 S.W.G. is used, and a length of
ten to seventeen miles of wire is employed on the secondary circuit,
according to the gauge of wire selected. For the precautions necessary
in constructing the secondary coil, practical manuals must be
consulted.[8]

Very great care is required in the insulation of the secondary circuit
of an induction coil to be used in Hertzian wave telegraphy, because
the secondary circuit is then subjected to impulsive electromotive
forces lasting for a short time, having a much higher electromotive
force than that which the coil itself normally produces.

The primary circuit of a ten-inch coil generally consists of a length
of 300 or 400 feet of thick insulated copper wire. In such a coil the
secondary circuit would require about ten miles of No. 34 H.C. copper
wire, making 50,000 turns round the core. It would have a resistance
at ordinary temperatures of 6,600 ohms, and an inductance of 460
henrys. The primary circuit, if formed of 360 turns of No. 12 H.C.
copper wire, would have a resistance of 0·36 of an ohm, and an
inductance of 0·02 of a henry.

An important matter in connection with an induction coil to be used
for wireless telegraphy is the resistance of the secondary circuit.
The purpose for which we employ the coil is to charge a condenser of
some kind. If a constant electromotive force (V) is applied to the
terminals of a condenser having a capacity C, then the difference of
potential (_v_) of the terminals of the condenser at any time that the
contact is made is given by the expression:

    v = V(1 - e^{-t/RC}).

In the above equation, the letter e stands for the number 2·71828, the
base of the Napierian logarithms, and R is the resistance in series
with the condenser, of which the capacity is C, to which the
electromotive force is applied. This equation can easily be deduced
from first principles,[9] and it shows that the potential difference
_v_ of the terminals of the condenser does not instantly attain a
value equal to the impressed electromotive force V, but rises up
gradually. Thus, for instance, suppose that a condenser of one
microfarad is being charged through a resistance of one megohm by an
impressed voltage of 100 volts, the equation shows that at the end of
the first second after contact, the terminal potential difference of
the condenser will be only 63 volts, at the end of the second second,
86 volts, and so on.

Since _e_^{-10} is an exceedingly small number, it follows that in 10
seconds the condenser would be practically charged with a voltage
equal to 100 volts. The product CR in the above equation is called the
_time-constant_ of the condenser, and we may say that the condenser is
practically charged after an interval of time equal to ten times the
time-constant, counting from the moment of first contact between the
condenser and the source of constant voltage. The time-constant is to
be reckoned as the product of the capacity (C) in microfarads, by the
resistance of the charging circuit (R) in megohms. To take another
illustration. Supposing we are charging a condenser having a capacity
of one-hundreth of a microfarad, through a resistance of ten thousand
ohms. Since ten thousand ohms is equal to one-hundredth of a megohm,
the time-constant would be equal to one-ten-thousandth of a second,
and ten times this time-constant would be equal to a thousandth of a
second. Hence, in order to charge the above capacity through the above
resistance, it is necessary that the contact between the source of
voltage and the condenser should be maintained for at least
one-thousandth part of a second.

In discussing the methods of interrupting the circuit, we shall return
to this matter, but, meanwhile, it may be said that in order to secure
a small time-constant for the charging circuit, it is desirable that
the secondary circuit of the induction coil should have as low a
resistance as possible. This, of course, involves winding the
secondary circuit with a rather thick wire. If, however, we employ a
wire larger in size than No. 34, or at the most No. 32, the bulk and
the cost of the induction coil began to rise very rapidly. Hence, as
in all other departments of electrical construction, the details of
the design are more or less a matter of compromise. Generally
speaking, however, it may be said that the larger the capacity which
is to be charged, the lower should be the resistance of the secondary
circuit of the induction coil.

In the practical construction of induction coils for wireless
telegraphy, manufacturers have departed from the stock designs. We are
all familiar with the appearance of the instrument maker's induction
coil; its polished mahogany base, its lacquered brass fittings, and
its secondary bobbin constructed of and covered with ebonite. But such
a coil, although it may look very pretty on the lecture table, is yet
very unsuited to positions in which it may be used in connection with
Hertzian wave telegraphy.

Three important adjuncts of the induction coil are the primary
condenser, the interrupter and the primary key. The interrupter is the
arrangement for intermitting the primary current. We have in some way
or other to rapidly interrupt the primary current, and the torrent of
sparks that then appears between the secondary terminals of the coil
is due to the electromotive force set up in the secondary circuit at
each break or interruption of the primary circuit. We may divide
interrupters into five classes.

We have first the well-known hammer interrupter which Continental
writers generally attribute to Neef or Wagner.[10] In this
interrupter, the magnetisation of the iron core of the coil is caused
to attract a soft-iron block fixed at the top of a brass spring, and
by so doing to interrupt the primary circuit between two platinum
contacts. Mr. Apps, of London, added an arrangement for pressing back
the spring against the back contact, and the form of hammer that is
now generally employed is therefore called an Apps break.

As the ten-inch coil takes a primary current of ten amperes at sixteen
volts when in operation, it requires very substantial platinum
contacts to withstand the interruption of this current continuously
without damage. The small platinum contacts that are generally put on
these coils by instrument makers are very soon worn out in practical
wireless telegraph work. If a hammer break is used at all, it is
essential to make the contacts of very stout pieces of platinum, and
from time to time, as they get burnt away or roughened, they must be
smoothed up with a fine file. It does not require much skill to keep
the hammer contacts in good order and prevent them from sticking
together and becoming damaged by the break spark.

By regulating the pressure of the spring against the back contact, by
means of an adjusting screw, the rate at which the break vibrates can
be regulated, but as a rule it is not possible, with a hammer break,
to obtain more than about 800 interruptions per minute, or, say,
twelve a second. The hammer break is usually operated by the magnetism
of the iron core of the coil, but for some reasons it is better to
separate the break from the coil altogether, and to work it by an
independent electromagnet, which, however, may be excited by a current
from the same battery supplying the induction coil. For coils up to
the ten-inch size the hammer break can be used when very rapid
interruptions are not required. It is not in general practicable to
work coils larger than the ten-inch size with a platinum contact
hammer break, as such a butt contact becomes overheated and sticks if
more than ten amperes is passed through it. In the case of larger
coils, we have to employ some form of interrupter in which mercury or
a conducting liquid forms one of the contact surfaces.

The next class of interrupter is the vibrating or hand-worked mercury
break, in which a platinum or steel pin is made to vibrate in and out
of mercury. This movement may be effected by the attraction of an iron
armature by an electromagnet, or by the varying magnetism of the core
of the coil, or it may be effected more slowly by hand.

The mercury surface must be covered with water, alcohol, paraffin or
creosote oil to prevent oxidation and to extinguish the break spark.
The interruption of the primary current obtained by the mercury break
is more sudden than that obtained by the platinum contact in air, in
consequence of the more rapid extinction of the spark; hence the
sparks obtained from coils fitted with mercury interrupters are
generally from twenty to thirty per cent. longer than those obtained
from the same coil under the same conditions, with platinum contact
interrupters. The mercury breaks will not, however, work well unless
cleaned at regular intervals by emptying off the oil and rinsing well
with clean water, and hence they require rather more attention than
platinum interrupters. It is not generally possible to obtain so many
interruptions per minute with the simple vibrating mercury interrupter
as with the ordinary hammer interrupter. The mercury interrupter has,
however, the advantage that the contact time during which the circuit
is kept closed may be made longer than is the case with the hammer
break. Also, if fresh water is allowed to flow continuously over the
mercury surface, it can be kept clean, and the break will then operate
for considerable periods of time without attention. The mercury
interrupter may be worked by a separate electromagnet or by the
magnetism of the core of the induction coil.

The third class of interrupter may be called the motor interrupter, of
which a large number have been invented in recent years. In this
interrupter some form of a continuously-rotating electromotor is
employed to make and break a mercury or other liquid contact. In one
simple form the motor shaft carries an eccentric, which simply dips a
platinum point into mercury, or else a platinum horseshoe into two
mercury surfaces, making in this manner an interruption of the primary
circuit at one or two places. As a small motor can easily be run at
twelve hundred revolutions per minute, or twenty per second, it is
possible to secure easily in this manner a uniform rate of
interruption of the primary current at the rate of about twenty per
second. If, however, much higher speeds are employed, then the time of
contact becomes abbreviated, and the ability of the coil to charge a
capacity is diminished.

Professor J. Trowbridge has described an effective form of motor break
for large coils, in which the interruption is caused by withdrawing a
stout platinum wire from a dilute solution of sulphuric acid, and by
this means he increased the spark given by a coil provided with hammer
break and condenser from fifteen inches to thirty inches when using
the liquid break and no condenser.[11]

A good form of motor-interrupter, due to Dr. Mackenzie Davidson,
consists of a slate disc bearing pin contacts fixed on the prolonged
steel axle of a motor placed in an inclined position; the disc and the
lower part of the axle lie in a vessel filled one-third with mercury
and two-thirds with paraffin oil. The circuit is made and broken by
the revolution of the disc causing the pins to enter and leave the
mercury. The speed of the motor can be regulated by a small
resistance, and can be adapted to the electromotive force used in the
primary circuit. When the motor is running slowly the interrupter can
be used with a low electromotive force, that is to say, something
between twelve and twenty volts, but with a higher speed a large
electromotive force can be used without danger of overheating the
primary coil, and with an electromotive force of about fifty volts,
the interruptions may be so rapid that an unbroken arc of flame,
resembling an alternating-current arc, springs between the secondary
terminals of the coil.

Mr. Tesla has devised numerous forms of rotating mercury break. In
one, a star-shaped metal disc revolves in a box so that its points dip
into mercury covered with oil, and make and break contact. In another
form, a jet of mercury plays against a similar shaped rotating wheel.
For details, the reader must consult the fuller descriptions in _The
Electrical World_ of New York, Vol. XXXII., p. 111, 1898; also Vol.
XXXIII., p. 247; or _Science Abstracts_, Vol. II., pp. 46 and 47,
1898.

The fourth class of interrupter is called a turbine interrupter. In
this appliance, a jet of mercury is forced out of a small aperture by
means of a centrifugal pump, and is made to squirt against a metal
plate, and interrupted intermittently by a toothed wheel made of
insulating material rotated by the motor which drives the pump. The
current supplying the coil passes through or along this jet of
mercury, and is therefore rendered intermittent when the wheel
revolves. In the case of this interrupter, the duration of the
contacts, as well as the number of interruptions per second, is under
control, and for this reason better results are probably obtained with
it than with any other form of break.

A description of a turbine mercury break devised by M. Max Levy was
given in the _Elektrotechnische Zeitschrift_, Vol. XX., p. 717,
October 12, 1899 (see also _Science Abstracts_, Vol. III., p. 63,
abstract No. 165) as follows:--

A toothed wheel made of insulating material carries from 6 to 24
teeth, and can be made to rotate from 300 to 1,000 times per minute,
the interruptions being thus regulated between 5 and 400 per second.
By raising or lowering the position of the jet of mercury and that of
the plate against which it strikes, the duration of the contact can
be varied, so that it is possible to regulate this period without
disturbing the number of interruptions per second.

The sparks obtained from a coil worked with a turbine interrupter have
more quantity than the sparks obtained with any other interrupter
under similar conditions, and the coil can be worked with a far higher
voltage than is possible when using the hammer break. In this manner,
the appearance of the secondary sparks can be varied from the thin
snappy sparks given by the hammer break to the thick flame-like arc
sparks given by the electrolytic break. This break can be adapted for
any voltage from twelve to two hundred and fifty volts, and the
primary circuit cannot be closed before the interrupter is acting. The
mercury in the break is generally covered with alcohol or paraffin oil
to reduce oxidation, and the appliance is nearly noiseless when in
operation. The mercury has to be cleaned at intervals, if the
interrupter is much used. If alcohol is used to cover the mercury, the
cleaning is very simple; the break requires only to be rinsed under a
water tap. When paraffin oil is used, the cleaning is generally
effected with the help of a few ounces of sulphuric acid in a very few
minutes. It is best, however, to clean the mercury continuously by
allowing the water to flow over it.

The motor driving the centrifugal pump and the fan can be wound for
any voltage, and it is best to have it so arranged that this motor
works on the same battery which supplies the primary circuit of the
coil, the two circuits working parallel together. A rheostat can be
added to the motor circuit to regulate the speed.

The turbine break driven by an independent motor, which is kept always
running, has another advantage over the hammer break in practical
wireless telegraphy, viz., that a useful secondary spark can be
secured with a shorter time of closure of the primary circuit, since
there is no inertia to overcome as in the case of the hammer break.
This latter form has only continued in use because of its simplicity
and ease of management by ordinary operators.

The mercury turbine interrupter has been extensively adopted both in
the German and British navies in connection with induction coils used
for wireless telegraphy.

Lastly we have the electrolytic interrupters, the first of which was
introduced by Dr. Wehnelt, of Charlottenburg, in the year 1899, and
modified by subsequent inventors. In its original form, a glass vessel
filled with dilute sulphuric acid (one of acid to five or else ten
parts of water) contains two electrodes of very different sizes; one
is a large lead electrode formed of a piece of sheet lead laid round
the interior of the vessel, and the other is a short piece of platinum
wire projecting from the end of a glass or porcelain tube. The smaller
of these electrodes is made the positive, and the large one the
negative. If this electrolytic cell is connected in series with the
primary circuit of the induction coil (the condenser being cut out)
and supplied with an electromotive force from forty to eighty volts,
an electrolytic action takes place which interrupts the current
periodically.[12] An enormous number of interruptions can, by suitable
adjustment, be produced per second, and the appearance of a discharge
from the secondary terminals of the coil, while using the Wehnelt
break, more resembles an alternate-current arc than the usual
disruptive spark.

At the time when the Wehnelt break was first introduced, great
interest was excited in it, and the technical journals in 1899 were
full of discussions as to the theory of its operation.[13] The general
facts concerning the Wehnelt break are that the electrolyte must be
dilute sulphuric acid in the proportion of one of acid to five or ten
of water. The large lead plate must be the cathode or negative pole,
and the anode or positive pole must be a platinum wire, about a
millimetre in diameter, and projecting one or two millimetres from the
pointed end of a porcelain, glass or other acid-proof insulating tube.
The aperture through which the platinum wire works must be so tight
that acid cannot enter, yet it is desirable that the platinum wire
should be capable of being projected more or less from the aperture by
means of an adjusting screw. The glass vessel which contains these two
electrodes should be of considerable size, holding, say, a quart of
fluid, and it is better to include this vessel in a larger one in
which water can be placed to cool the electrolyte, as the latter gets
very warm when the break is used continuously. If such an electrolytic
cell has a continuous electromotive force applied to it tending to
force a current through the electrolyte from the platinum wire to the
lead plate, we can distinguish three stages in its operation, which
are determined by the electromotive force and the inductance in the
circuit. First, if the electromotive force is below sixteen or twenty
volts, then ordinary and silent electrolysis of the liquid proceeds,
bubbles of oxygen being liberated from the platinum wire and hydrogen
set free against the lead plate. If the electromotive force is raised
above twenty-five volts, then if there is no inductance in the
circuit, the continuous flow of current proceeds, but if the circuit
of the electrolyte possesses a certain minimum inductance, the
character of the current flow changes, and it becomes intermittent,
and the cell acts as an interrupter, the current being interrupted
from 100 to 2,000 times per second, according to the electromotive
force and the inductance of the circuit. Under these conditions, the
cell produces a rattling noise and a luminous glow appears round the
tip of the platinum wire. Thus, in a particular case, with an
inductance of 0·004 millihenry in the circuit of a Wehnelt break, no
interruption of the circuit took place, but with one millihenry of
inductance in the circuit, and with an electromotive force of 48
volts, the current became intermittent at the rate of 930 per second,
and by increasing the voltage to 120 volts, the intermittency rose to
1,850 a second.

The Wehnelt break acts best as an interrupter with an electromotive
force from 40 to 80 volts. At higher voltages a third stage sets in:
the luminous glow round the platinum wire disappears, and it becomes
surrounded with a layer of vapour, as observed by MM. Violle and
Chassagny; the interruptions of current cease, and the platinum wire
becomes red hot. If there is no inductance in the circuit, the
interrupter stage never sets in at all, but the first stage passes
directly into the third stage. In the first stage bubbles of oxygen
rise steadily from the platinum wire, and in the interrupted stage
they rise at longer intervals, but regularly. The cell will not,
however, act as a break at all unless some inductance exists in the
circuit.

In applying the Wehnelt break to an induction coil, the condenser is
discarded and also the ordinary hammer break, and the Wehnelt break is
placed in circuit with the primary coil. In some cases, the inductance
of the primary coil alone is sufficient to start the break in
operation, but with voltages above 50 or 60, it is generally necessary
to supplement the inductance of the primary coil by another inductive
coil. The best form of Wehnelt break for operating induction coils is
the one with multiple anodes (see Dr. Marchant, _The Electrician_,
Vol. XLII., p. 841, 1899), and when it has to be used for long
periods, the cathode may advantageously be formed of a spiral of lead
pipe, through which cold water is made to circulate.

Another form of electrolytic break was introduced by Mr. Caldwell. In
this, a vessel containing dilute sulphuric acid is divided into two
parts. In the partition is a small hole, and in the two compartments
are electrodes of sheet lead. The small hole causes an intermittency
in the current which converts the arrangement into a break. Mr.
Campbell Swinton modified the above arrangement by making the
partition to consist of a sort of porcelain test-tube with a hole in
the bottom. This hole can be more or less plugged up by a glass rod
drawn out to a point, and this is used to more or less close the hole.
This porcelain vessel contains dilute acid and stands in a larger
vessel of acid, and lead electrodes are placed in both compartments.
The current and intermittency can be regulated by more or less closing
the aperture between the two regions.

When the Wehnelt break is applied to an ordinary ten-inch induction
coil, and the inductance of the primary circuit and the electromotive
force varied until the break interrupts the current regularly and with
the frequency of some hundred a second, the character of the secondary
discharge is entirely different from its appearance with the ordinary
hammer break. The thin blue lightning-like sparks are then replaced by a
thicker mobile flaming discharge, which resembles an alternating-current
arc, and, when carefully examined or photographed, is found to consist
of a number of separate discharges superimposed upon one another in
slightly different positions.

Many theories have been adopted as to the action of the break, but
time will not permit us to examine these. Professor S. P. Thompson and
Dr. Marchant have suggested a theory of resonance.[14] One difficulty
in explaining the action of the break is created by the fact that it
will not work if the platinum wire is made a cathode.

Although the Wehnelt break has some advantages in connection with the
use of the induction coil for Röntgen ray work, its utility as far as
regards Hertzian wave telegraphy is not by any means so marked. It has
already been explained that, in order to charge a condenser of a given
capacity at a constant voltage, the electromotive force must be
applied for a certain minimum time, which is determined by the value
of the capacity and the resistance of the secondary circuit of the
induction coil. If the coil is a ten-inch coil and has a secondary
resistance of, say, 6,000 ohms, and if the capacity to be charged has
a value, say, of one-thirtieth of a microfarad, then the time-constant
of the circuit is 1/5,000 of a second. Therefore, the contact with the
condenser must be maintained for at least 1/500 of a second, during
the time that the secondary electromotive force of the coil is at its
maximum, so that the condenser may become charged to a voltage which
the coil is then capable of producing.

In the induction coil, the electromotive force generated in the
secondary coil at the "break" of the primary current is higher than
that at the "make," and this electromotive force, other things being
equal, depends upon the rate at which the magnetism of the iron core
dies away, and its duration is shorter in proportion as the whole time
occupied in the disappearance of the magnetism is less. The Wehnelt
break does not increase the actual secondary electromotive force, nor
apparently its duration, but it greatly increases the number of times
per second this electromotive force makes it appearance. Hence this
break increases the current, but not the electromotive force in the
secondary coil. It, therefore, does not assist us in the direction
required--viz., in prolonging the duration of the secondary
electromotive force to enable larger capacities to be charged.

The important point in connection with the working of a coil used for
charging a condenser is not the length of spark which the coil can
give alone, but the length of spark which can be obtained between
small balls attached to the secondary terminals, when these terminals
are also connected to the two surfaces of the condenser. Thus, a coil
may give a ten-inch spark if worked alone, but on a capacity of
one-thirtieth of a microfarad it may not be able to give more than a
five-millimetre spark. Hence, in describing the value of a coil for
wireless telegraph purposes, it is not the least use to state the
length of spark which the coil will give between the pointed
conductors in air, but we must know the spark length which it will
give between brass balls, say, 1 centimetre in diameter, connected to
the secondary terminals, when these terminals are also short-circuited
by a stated capacity, the spark not exceeding that length at which it
becomes non-oscillatory.

A good way of describing the value of an induction coil for wireless
telegraph purposes is to state the length of oscillatory spark which
can be produced between balls one centimetre in diameter connected to
the secondary terminals, when these balls are short-circuited by a
condenser having a capacity, say, of one-hundredth of a microfarad,
and also one-tenth of a microfarad.

If a hammer or motor interrupter is employed with the coil, then a
primary condenser must be connected across the points between which
the primary circuit is broken. This condenser generally consists of
sheets of tinfoil alternated with sheets of paraffin paper, and for a
ten-inch coil may have a capacity of about 0·4 or 0·5 of a
microfarad.[15]

Lord Rayleigh discovered that if the interruption of the primary
circuit is sufficiently sudden and complete, as when the primary
circuit is severed by a bullet from a gun, the primary condenser can
be removed and yet the sparks obtained from the secondary circuit are
actually longer than those obtained with the condenser and the
ordinary break.[16]

In the use, however, of the coil for Hertzian wave telegraphy, with
all interrupters except the Wehnelt break a condenser of suitable
capacity must be joined across the break points.

Turning in the next place to the primary key, or signalling
interrupter, it is necessary to be able to control the torrent of
sparks between the secondary terminals of the coil, and to cut them up
into long and short periods in accordance with the letters of the
Morse alphabet. This is done by means of the primary key. The primary
key generally consists of an ordinary massive single contact key with
heavy platinum contacts. As the current to be interrupted amounts to
about ten amperes and is flowing in a highly inductive circuit, the
spark at break is considerable. If the attempt is made to extinguish
this spark by making the contacts move rapidly away from one another
through a long distance, in other words, by using a key with a wide
movement, then the speed at which the signals can be set is greatly
diminished. The speed of sending greatly depends upon the time taken
to move the key up and down between sending two dots, and hence a
short range key sends quicker than a long range key. If it is desired
to use a short range key, then some method must be employed to
extinguish the spark at the contacts. This is done in one of three
ways: Either by using a high resistance coil to short-circuit these
contacts, or by a condenser, or by a magnetic blow-out, as in the case
of an electric tramcar circuit controller. Of these, the magnetic
blow-out is probably the best.

Mr. Marconi has designed a signalling key which performs the function
not only of interrupting the primary circuit, but at the same time
breaks connection between the receiving appliance and the aerial.

The author has designed for signalling purposes a multiple contact key
which interrupts the circuit simultaneously in ten or twelve different
places. The particular point about this break is the means which are
taken to make the twelve interruptions absolutely simultaneous. If
these interruptions are not simultaneous, the spark always takes place
at the contact which is broken first, but if the circuit is
interrupted in a dozen places quite simultaneously, then the spark is
cut up into a dozen different portions, and the spark at each contact
is very much diminished. By this break, voltages up to two thousand
volts may be quite easily dealt with.

Various forms of break have been devised in which the circuit is
broken under oil or insulating fluids, but, generally speaking, these
devices are not very portable, and a dry contact between platinum
surfaces with appropriate means for cutting up the spark and blowing
it out so that the mechanical movement of the switch may be small is
the best thing to use.

The signalling key is really a very important part of the transmitting
arrangement, because whatever may be the improvements in receiving
instruments, it is not possible to receive faster than we can send. A
great many statements have appeared in the daily papers as to the
possibility of receiving hundreds of words a minute by Hertzian wave
telegraphy, but the fact remains that whatever may be the sensibility
of the receiving appliance, the rate at which telegraphy of any kind
can be conducted is essentially dependent upon the rate at which the
signals can be sent, and this in turn is largely dependent upon the
mechanical movement which the key has to make to interrupt the primary
circuit, and so interrupt the secondary discharge.

In order to make the separation of the contact points of the switch as
small as possible, and yet prevent an arc being established, various
blow-out devices have been employed. The simplest arrangement for this
purpose is a powerful permanent magnet so placed that its inter-polar
field embraces the contact points and is at right angles to them.

As already explained, the applicability of the induction coil in
wireless telegraphy is limited by the fact of the high resistance of
the secondary circuit and the small current that can be supplied from
it. Data are yet wanting to show what is the precise efficiency of the
induction coil, as used in Hertzian wave telegraphy, but there are
reasons for believing that it does not exceed 50 or 60 per cent.

Where large condensers have to be charged--in other words, where we
have to deal with larger powers--we are obliged to discard the
induction coil and to employ the alternating-current transformer. But
this introduces us to a new class of difficulties. If an
alternating-current transformer wound for a secondary voltage, say, of
20,000 or 30,000 volts, has its primary circuit connected to an
alternator, then if the secondary terminals, to which are connected
two spark balls, are gradually brought within striking distance of one
another, the moment we do this an alternating-current arc starts
between these balls. If the transformer is a small one, there is no
difficulty in extinguishing this arc by withdrawing the secondary
terminals, but if the transformer is a large one, say, of ten or
twenty kilowatts, dangerous effects are apt to ensue when such an
experiment is tried. The short circuiting of the secondary circuit
almost entirely annuls the inductance of the primary circuit. There
is, therefore, a rush of current into the transformer, and if it is
connected to an alternator of low armature resistance the fuses are
generally blown and other damage done.

Let us supppse, then, that the secondary terminals of the transformer
are also connected to a condenser. On bringing together the spark
balls connected with the secondary terminals we may have one or more
oscillatory discharges, but the process will not be continuous,
because the moment that the alternating-current arc starts between the
spark balls it reduces their difference of potential to a
comparatively low value, and hence the charge taken by the condenser
is very small, and, moreover, the circuit is not interrupted
periodically so as to re-start a train of oscillations.

When, therefore, we desire to employ an alternating-current
transformer as a source of electromotive force, although it may have
the advantage that the resistance of the secondary circuit of the
transformer is generally small compared with that of the secondary
circuit of an induction coil, yet, nevertheless, we are confronted
with two practical difficulties: (1) How to control the primary
current flowing into the transformer, and (2) how to destroy the
alternating-current arc between the spark balls and reduce the
discharge entirely to the disruptive or oscillatory discharge of the
condenser.

The control over the current can be obtained, in accordance with a
plan suggested by the author, by inserting in the primary circuit of
the transformer two variable choking coils. The form in which it is
preferred to construct these is that of a cylindrical bobbin standing
upon a laminated cross-piece of iron. These bobbins can have let down
into them an =E=-shaped piece of laminated iron, so as to complete the
magnetic circuit, and thus raise the inductance of the bobbin. By
placing two of these variable choking coils in series with the primary
circuit, the current is under perfect control. We can fix a minimum
value below which the current shall not fall, by adjusting the
position of the cores of these two choking coils, and we can then
cause that current to be increased up to a certain limit which it
cannot exceed, by short-circuiting one of these choking coils by an
appropriate switch. Several ways have been suggested for extinguishing
the alternating current arc which forms between the spark balls
connected to the secondary terminals when these are brought within a
certain distance of one another. One of these is due to Mr. Tesla. He
places a strong electromagnet so that its lines of magnetic flux pass
transversely between the spark balls. When the discharge takes place
the electric arc is blown out, but if the balls are short-circuited by
a condenser the oscillatory discharge of the condenser still takes
place across the spark gap. Professor Elihu Thomson achieves the same
result by employing a blast of air thrown on the spark gap. This has
the effect of destroying the alternating-current arc, but still leaves
the oscillating discharge of the condenser. The action is somewhat
tedious to explain in words, but it can easily be understood that the
blast of air, by continually breaking down the alternating-current arc
which tends to form, allows the condenser connected to the spark balls
to become charged with the potential of the secondary circuit of the
transformer, and that this condenser then discharges across the spark
gap, producing an oscillatory discharge in the usual manner. The
author has found that, without the use of any air blast or
electromagnet, simple adjustment of the double choking coil in the
primary circuit of the transformer, as above described, is sufficient
to bring about the desired result, when the capacity of the condenser
is adjusted to be in resonance.

Another method, which has been adopted by M. d'Arsonval, is to cause
the spark to pass between two balls placed at the extremities of
metal rods, which are in rapid rotation like the spokes of a wheel. In
this case, the draught of air produced by the passage of the spark
balls blows out the arc and performs the same function as the blast of
air in Professor Elihu Thomson's method. When these adjustments are
properly made, it is possible, by means of a condenser and an
alternating-current transformer supplied with current from an
alternator, to create a rapidly intermittent oscillatory discharge,
the sparks of which succeed one another so quickly that it appears
almost continuous. When using a large transformer and condenser, the
noise and brilliancy of these sparks are almost unbearable, and the
eyes may be injured by looking at this spark for more than a moment.
In the construction of transformers intended to be used in this
manner, very special precautions have to be taken in the insulation of
the primary and secondary circuits, and the insulation of these from
the core.

It may be remarked in passing that experimenting with large
high-tension transformers coupled to condensers of large capacity is
exceedingly dangerous work, and the greatest precautions are necessary
to avoid accident. In the light, however, of sufficient experience
there is no difficulty in employing high-tension transformers in the
above-described manner, and in obtaining electromotive forces of
upwards of a hundred thousand volts supplied through transformers
capable of yielding any required amount of current.

On occasions where continuous current alone is available, a motor
generator has to be employed converting the continuous current into an
alternating current. This is best achieved by the employment of a
small alternator directly coupled to a continuous-current motor; or by
providing the shaft of a continuous-current motor with two rings
connected to two opposite portions of its armature, so that when
continuous current is supplied to the brushes pressing against the
commutator, an alternating current can be drawn off from two other
brushes touching the above-mentioned insulated rings.

The next element of importance in the transmitting arrangement is the
spark gap. In the case of those transmitters employing an ordinary
induction coil, the secondary spark, or the discharge of any condenser
connected to the secondary terminals can be taken between the brass
balls about half an inch or one inch in diameter, with which the
terminals of the secondary coil are usually furnished; and it is
generally the custom to allow this spark discharge to take place in
air at ordinary pressure. In the very early days of his work Mr.
Marconi adopted the discharger devised by Professor Rhigi, in which
the spark takes place between two brass balls placed in vaseline or
other highly insulating oil.[17] But whatever advantage may accrue
from using oil as the dielectric in which the spark discharge takes
place, when carrying out simple laboratory experiments on Hertzian
waves, there is no advantage in the case of wireless telegraphy. The
Rhigi discharger was, therefore, soon discarded. If discharges having
large quantity are passed through oil, it is rapidly decomposed or
charred, and ceases to retain the special insulating and
self-restoring character which is necessary in the medium in which an
oscillating spark is formed. The conditions when the discharges of
large condensers are passed between spark balls are entirely different
from those when the quantity of the spark, or to put it in more exact
language, the current passing, is very small. In the case of Hertzian
experiments it is necessary, as shown by Hertz, to maintain a high
state of polish on the spark balls when they are employed for the
production of short waves of small energy, but when we are dealing
with large quantities of energy at each discharge, those methods which
succeed for laboratory experiments are perfectly impracticable. The
conditions necessary to be fulfilled by a discharger for use in
Hertzian wave telegraphy are that the surfaces shall maintain a
constant condition and not be fused or eaten away by the spark, and,
next, that the medium in which the discharge takes place shall not be
decomposed by the passage of the spark, but shall maintain the
property of giving way suddenly when a certain critical pressure is
reached, and passing instantly from a condition in which it is a very
perfect insulator to one in which it is a very good conductor; and,
thirdly, that on the cessation of the discharge, the medium shall
immediately restore itself to its original condition.

When using the ordinary ten-inch induction coil, and when the capacity
charged by it does not exceed a small fraction of a microfarad, it is
quite sufficient to employ brass or steel balls separated by a certain
distance in air, at the ordinary pressure, as the arrangement of the
discharger. When, however, we come to deal with the discharges of very
large condensers, at high electromotive forces, then it is necessary
to have special arrangements to prevent the destruction of the
surfaces between which the spark passes, or their continual
alteration, and many devices have been invented for this purpose. The
author has devised an arrangement which fulfils the above conditions
very perfectly for use in large power stations, but the details of
this cannot be made public at the present time.

       *       *       *       *       *

We have to consider in connection with this part of the subject the
dielectric strength of air under different pressures and for different
thicknesses. It was shown by Lord Kelvin, in 1860, that the dielectric
strength of very thin layers of air is greater than that of thick
layers.[18] The electric force, reckoned in volts per centimetre,
required to pierce a thickness of air from two to ten millimetres in
thickness, at atmospheric pressure, may be taken at 30,000 volts per
centimetre. The same force in electrostatic units is represented by
the number 100, since a gradient of 300 volts per centimetre
corresponds to a force of one electrostatic unit. It appears also that
for air and other gases there is a certain minimum voltage
(approximately 400 volts) below which no discharge takes place,
however near the conducting surfaces may be approximated. In this
particular practical application, however, we are only concerned with
spark lengths which are measured in millimetres or centimetres, lying,
say, between one or two millimetres and five or six centimetres. Over
this range of spark length we shall not generally be wrong in
reckoning the voltage required to produce a spark between metal balls
in air at the ordinary pressure to be given by the rule:

    _Disruptive voltage_ = 3,000 × _spark-gap length in millimetres_.

If, however, the air pressure is increased above the normal by
including the spark balls in a vessel in which air can be compressed,
then the spark length, corresponding to a given potential difference,
very rapidly decreases. Mr. F. J. Jervis-Smith[19] found that by
increasing the air pressure from one atmosphere to two atmospheres
round a pair of spark balls he reduced the spark length given by a
certain voltage from 2·5 to 0·75 centimetre.

Professor R. A. Fessenden has also made some interesting observations
on the effect of using compressed air round spark gaps. He found that
if a certain voltage between metal surfaces would yield a spark four
inches in length, at the ordinary pressure of the air, if the spark
balls were enclosed in a cylinder, the air round them compressed at
50lb. per square inch, the spark length for the same potential
difference of the balls was only one quarter of an inch, or
one-sixteenth of its former value.

The writer has also made experiments with an apparatus designed to
study the effect of compressed air round the spark gap. The
experimental arrangements are as follows: A ten-inch induction coil
has one of its terminals connected to the internal coating of a
battery of Leyden jars. The external coating is connected through the
primary coil of an oscillation transformer with the other secondary
terminal of the coil, and these secondary terminals are also connected
to a spark gap consisting of two brass balls enclosed in a glass
vessel into which air can be forced by a pump, the air pressure being
measured by a gauge. The balls in the glass vessel are set at a
distance of about three millimetres apart. The secondary circuit of
the oscillation transformer is connected to another pair of spark
balls, the distance of which can be varied.

Suppose we begin with the air in the glass vessel containing the balls
connected to the secondary terminals of the induction coil, which may
be called the secondary balls, at atmospheric pressure, and create
oscillatory discharges in the primary coil of the oscillation
transformer, we have a spark between the balls, which may be called
the tertiary balls, connected to the secondary terminals of the
oscillation transformer. If the secondary balls are placed, say, three
millimetres apart, the air in the glass vessel enclosing them being at
the ordinary atmospheric pressure, then with one particular
arrangement of jars used, a spark twenty-five or twenty-six
millimetres long between the tertiary balls will take place. Suppose,
then, we increase the pressure of the air round the secondary balls,
pumping it by degrees to 10, 20, 30, 40 and 50lb. per square inch
above the atmospheric pressure. We find that the spark between the
tertiary balls will gradually leap a greater and greater distance, and
when the pressure of the air is 50lb. per square inch, we can obtain a
fifty-millimetre spark between the tertiary balls, whereas when the
air in the glass vessel is at atmospheric pressure, we can only obtain
a spark between the tertiary balls of half that length.

This experiment demonstrates that the effect of compressing the air
round the secondary terminals of the induction coil is to greatly
increase the difference of potential between these balls before the
spark passes. In fact, it requires about double the voltage to force a
spark of the same length through air compressed at 50lb. on the square
inch that it does to make a spark of identical length between the same
balls in air at normal pressure. This shows that there is a very great
advantage in taking the discharge spark in compressed air. A better
effect can be produced by substituting dry gaseous hydrochloric acid
for air at ordinary pressures.

One other incidental advantage is that the noise of the spark is very
much reduced. The continual crackle, of the discharge spark of the
induction coil in connection with wireless telegraphy is very annoying
to sensitive ears, but in this manner we can render it perfectly
silent.

Professor Fessenden also states that when the spark balls are
surrounded by compressed air, and if one of the balls is connected
with a radiator, the compression of the air, although it shortens the
spark-gap corresponding to a given voltage, does not in any way
increase the radiation. When, however, the air in the spark-ball
vessel is compressed to 60lb. in the square inch, there is a marked
increase in the effective radiation, and at 80lb. per square inch the
energy emitted in the form of waves is nearly three and a-half times
greater than at 50lb., the potential difference between the balls
remaining the same.

This effect is no doubt connected with the fact that the production of
a wave, whether in ether or in any other material, is not so much
dependent upon the absolute force applied as upon the suddenness of
its application. To translate it into the language of the electronic
theory, we may say that the electron radiates only whilst it is being
accelerated, and that its radiating power, therefore, depends not so
much upon its motion as upon the rate at which its motion is changing.

The advantage in using compressed air round the spark gap is that we
can increase the effective potential difference between the balls
without rendering the spark non-oscillatory. In air of the ordinary
pressure there is a certain well-defined limit of spark length for
each voltage, beyond which the discharge becomes non-oscillatory, but
by the employment of spark balls in compressed air, we can increase
the potential difference between the balls corresponding to a given
distance apart before a discharge takes place, or employ higher
potentials with the same length of spark gap. In addition to this, we
have, perhaps, the production of a more effective radiation, as
asserted by Fessenden, when the air pressure exceeds a certain
critical value.

The next element which we have to consider in the transmitting
arrangements is a condenser of some kind for storing the energy which
is radiated at intervals. Where a condenser other than the aerial is
employed for storing the electric energy which is to be radiated by
the aerial, some form of it must be constructed which will withstand
high potentials. As the dielectric for such a condenser, only two
materials seem to be of any practical use, viz., glass and micanite.
Glass condensers in the form of Leyden jars have been extensively
employed, but they have the disadvantage that they are very bulky in
proportion to their electrical capacity. The instrument maker's quart
Leyden jar has a capacity of about one-five hundredth of a microfarad,
but it occupies about 150 cubic inches or more. Professor Braun has
employed in his transmitting arrangements condensers consisting of
small glass tubes like test tubes, lined on the inside and outside
with tinfoil, which are more economical in space. The author has found
that condensers for this purpose are best made of sheet glass about
one-eighth or one-tenth of an inch in thickness, coated to within one
inch of their edge on both sides with tinfoil, and arranged in a
vessel containing resin or linseed oil, like the plates of a storage
battery. M. d'Arsonval has employed micanite, but although this
material has a considerably higher dielectric strength than glass, it
is much more expensive to obtain a given capacity by means of micanite
than by glass, although the bulk of the condenser for a given capacity
is less.

To store up a certain amount of electric energy in a condenser, we
require a certain definite volume of dielectric, no matter how we may
arrange it, and the volume required per unit of energy is determined
by the dielectric strength of the material. Thus, for instance,
ordinary sheet glass cannot be safely employed with a greater electric
force than is represented by 20,000 volts for one-tenth of an inch in
thickness, or, say, a potential gradient of 160,000 volts per
centimetre. This is equivalent to an electric force of about 500
electrostatic units. This may be called the safe-working force. The
electrostatic capacity of a condenser formed of two metal surfaces a
foot square separated by glass three millimetres in thickness is
between 1/360 and 1/400 of a microfarad. If this condenser is charged
to 20,000 volts, we have stored up in it half a joule of electric
energy, and the volume of the dielectric is 270 cubic centimetres.
Hence, to store up in a glass condenser electric energy represented by
one joule at a pressure of 20,000 volts, we require 500 cubic
centimetres of glass, and it will be found that if we double the
pressure and double the thickness of the glass, we still require the
same volume.[20] Hence, in the construction of high-tension condensers
to store up a given amount of energy, the economical problem is how to
obtain the greatest energy-storing capacity for the least money.
Glass fulfils this condition better than any other material. Although
some materials may have very high dielectric strength, such as paper
saturated with various oils, or resins, yet they cannot be used for
the purpose of making condensers to yield oscillatory discharges,
because the oscillations are damped out of existence too soon by the
dielectric.

In arranging condensers to attain a given capacity, regard has to be
taken of the fact that for a given potential difference there must be
a certain total thickness of dielectric, and that if condensers of
equal size are being arranged in parallel it adds to their capacity,
whilst joining them in series divides their capacity. If N equal
condensers or Leyden jars have each a capacity represented by C, and
if they are joined _n_ in series and _m_ in parallel, the joint
capacity of the whole number is _m_C/_n_, where the product _mn_ = N.

Passing on next to the consideration of oscillation transformers of
various kinds--these are appliances of the nature of induction coils
for transforming the current or electromotive force of electrical
oscillations in a required ratio. These coils are, however, destitute
of any iron core, and they generally consist of coils of wire wound on
a fibre, wooden or ebonite frame, and must be immersed in a vat of oil
to preserve the necessary insulation. No dry insulation of the nature
of indiarubber or gutta-percha will withstand the high pressures that
are brought to bear upon the circuits of an oscillation transformer.
In constructing these transformers we have to set aside all previous
notions gathered from the design of low-frequency iron-core
transformers. The chief difficulty we have to contend against in the
construction of an effective oscillation transformer is the inductance
of the primary circuit and the magnetic leakage that takes place. In
other words, the failure of the whole of the flux generated by the
primary circuit to pass through or be linked with the secondary
circuit. Mr. Marconi has employed an excellent form of oscillation
transformer, in the design of which he was guided by a large amount of
experience. In this transformer the two circuits are wound round a
square wooden frame. The primary circuit consists of a number of
strands of thick insulated cable laid on in parallel, so that it
consists of only one turn of a stranded conductor. The secondary
circuit consists of a number of turns, say, ten to twenty, of thinner
insulated wire laid over the primary circuit and close to it, so that
the transformer has the transformation ratio of one to ten or one to
twenty. In the arrangements devised and patented by Mr. Marconi, these
two circuits, with their respective capacities in series with them,
are tuned to one another, so that the time-period of each circuit is
exactly the same, and without this tuning the device becomes
ineffective as a transformer.[21] There is no advantage in putting a
number of turns on the primary circuit, because such multiplication
simply increases the inductance, and, therefore, diminishes the
primary current in the same ratio which it multiplies the turns, and
hence the magnetic field due to the primary circuit remains the same.
Where it is desired to put a number of turns upon a coil, and yet at
the same time keep the inductance down, the writer has adopted the
device of winding a silk or hemp rope well paraffined between the
turns of the circuit, so as to keep them further apart from one
another, and as the inductance depends on the turns per centimetre,
this has the effect of reducing the inductance.

The next and most important element in any transmitting station is the
aerial or radiator, and it was the introduction of this element by Mr.
Marconi which laid the foundation for Hertzian wave telegraphy as
opposed to mere experiments with the Hertzian waves. We may consider
the different varieties of aerial which have been evolved from the
fundamental idea. The simple single Marconi aerial consists of a bare
or insulated wire, generally about 100ft. or 150ft. in length,
suspended from a sprit attached to a tall mast. As these masts have
generally to be erected in exposed positions, considerable care has to
be taken in erecting them with a large margin of strength. To the end
of a sprit is attached an insulator of some kind, which may be a
simple ebonite rod, or sometimes a more elaborate arrangement of oil
insulators, and to the lower end of this insulator is attached the
aerial wire. As at the top of the aerial we have to deal with
potentials capable sometimes of giving sparks several feet in length,
the insulation of the upper end of the aerial is an important matter.

In the original Marconi system, the lower end of the aerial was simply
attached to one spark ball connected to one terminal of the induction
coil, and the other terminal and spark ball were connected to the
earth. In this arrangement, the aerial acted not only as radiator, but
as energy-storing capacity, and as already explained, its radiating
power was on that account limited. The earth connection is an
important matter. For long distance work, a good earth is essential.
This earth must be made by embedding a metal plate in the soil, and
many persons are under the impression that the efficiency of the earth
plate depends upon its area, but this is not the fact. It depends much
more upon its shape, and principally upon the amount of its "edge." It
has been shown by Professor A. Tanakadate, of Japan, that if a metal
plate of negligible resistance is embedded in an infinite medium
having a resistivity _r_, the electrical conductance of this plate is
equal to 4pi/_r_ times the electrostatic capacity of the same plate
placed in a dielectric of infinite extent. Hence in designing an
earth plate, we have to consider not how to give it the utmost amount
of surface, but how to give it the greatest electrostatic capacity,
and for this purpose it is far better to divide a given amount of
metal into long strips radiating out in different directions, rather
than to employ it in the form of one big square or circular plate. The
importance of the "good earth" will have been seen from our discussion
on the mode of formation of electric waves. There must be a perfectly
free access for the electrons to pass into and out of the aerial.
Hence, if the soil is dry, or badly conductive in the neighbourhood,
we have to go down to a level at which we get a good moist earth. In
fact, the precautions which have to be taken in making a good earth
for Hertzian wave telegraphy are exactly those which should be taken
in making a good earth for a lightning conductor.

Whilst on the subject of aerials, a word may be said on the
localisation of wireless telegraph stations on the Marconi system. For
reasons which were explained previously, the transmission of signals
is effected more easily over water than over dry land, and it is
hindered if the soil in the neighbourhood of the sending station is a
poor conductor. Hence, all active Hertzian wave telegraph stations,
like all active volcanoes, are generally found near the sea. In those
cases in which a multiple aerial has to be put up consisting of many
wires, one mast may be insufficient to support the structure, and
several masts arranged in the form of a square or a circle have to be
employed. The illustrated papers have reproduced numerous pictures of
the Marconi power stations at Poldhu in Cornwall, Glace Bay in Nova
Scotia, and Cape Cod in the United States. In these stations, after
preliminary failures to obtain the necessary structural strength with
ordinary masts, tall lattice girder wooden towers have been built,
about 215 feet in height, well stayed against wind pressure, and which
so far have proved themselves capable of withstanding any storm of
wind which has come against them.

An important question in connection with the sending power of an
aerial is that of the relation of its height to the distance covered.
Some time ago Mr. Marconi enunciated a law, as the result of his
experiments, connecting these two quantities, which may be called
Marconi's Law. He stated that the height of the aerial to cover a
given distance, other things remaining the same, varies as the square
root of the distance. Let D be the distance and let L be the length of
the aerial, then if both the transmitting and receiving aerial are the
same height, we may say that D varies as L^{2}. This relation may be
theoretically deduced as follows:--Any given receiving apparatus for
Hertzian wave telegraphy requires a certain minimum energy to be
imparted to it to make it yield a signal. If the resistance and the
capacity of the receiver is taken as constant, this minimum working
energy is proportional to the square of the electromotive force set up
in the receiving aerial by the impact on it of the electric waves.
This electromotive force varies as the length of the receiving aerial
and as the magnetic force due to the wave cutting across it, and the
magnetic force varies as the current in the transmitting aerial, and
therefore, for any given voltage varies as the capacity, and therefore
as the length of the transmitting aerial. If, therefore, the
transmitting and receiving aerial have the same length, the minimum
energy varies as the square of the electromotive force in the
receiving aerial, and therefore as the fourth power of the length of
either aerial, since the electromotive force varies as the product of
the lengths of the aerials. Hence, when the distance between the
aerials is constant, the minimum working energy varies as the fourth
power of the height of either aerial, but when the lengths of the
aerials are constant, the energy caught up by the receiving aerial
must vary inversely as the square of the distance D between the
aerials. Hence, if we call _e_ this minimum working energy, _e_ must
vary as 1/D^{2} when L is constant, or as L^4 when D is constant, and
since _e_ is a constant quantity for any given arrangements of
receiver and transmitter, it follows that when the height of aerial
and distance vary, the ratio L^4/D^2 is constant, or, in other words,
D^2 varies as L^4 or D varies as L^2--_i.e._, distance varies as the
square of the height of the aerial, which is Marconi's Law. The curve,
therefore, connecting height of aerial with sending distance for given
arrangements is a portion of a parabola.

Otherwise, the law may be stated in the form L = _a_[\sq]{D}, where _a_
is a numerical coefficient. If L and D are both measured in metres,
then, for recent Marconi apparatus as used on ships, _a_ = 0·15 roughly.
(See a report on experiments made for the Italian Navy, 1900-1901, by
Captain Quintino Bonomo--"Telegrafia senza fili," Rome, 1902.)

This law, however, must not be used without discretion. After Mr.
Marconi had transmitted signals across the British Channel, some
people, forgetting that a little knowledge is a dangerous thing,
predicted that aerials a thousand feet in height would be required to
signal across the Atlantic, but Mr. Marconi has made such improvements
of late years in the receiving arrangements that he has been able to
receive signals over three thousand miles in 1903 with aerials only
thirty-three per cent. longer than those which, in 1899, he employed
to cover twenty miles across the English Channel.

[Illustration: FIG. 15.--ALTERNATING-CURRENT DOUBLE-TRANSFORMATION
POWER PLANT FOR GENERATING ELECTRIC WAVES (Fleming). _a_, alternator;
H_{1}H_{2}, choking coil; K, signalling key; T, step-up transformer;
S_{1}S_{2} spark-gap; C_{1}C_{2} condensers; T_{1}T_{2}, oscillation
transformers; A, aerial; E, earthplate.]

We turn, in the next place, to the consideration of those devices for
putting more power into the aerial than can be achieved when the
aerial itself is simply employed as the reservoir of energy. Professor
Braun, of Strassburg, in 1899, described a method for doing this by
inducing oscillations in the aerial by means of an oscillation
transformer, these oscillations being set up by the discharges from a
Leyden jar or battery of Leyden jars, which formed the reservoir of
energy. The induction coil is employed to produce a rapidly
intermittent series of electrical oscillations in the primary coil of
an oscillation transformer by the discharge through it of a Leyden
jar. Mr. Marconi immensely improved this arrangement, as described by
him in a lecture given before the Society of Arts on May 17, 1901, by
syntonising the two circuits and making the circuit, consisting of the
capacity of the aerial and the inductance of the secondary circuit of
the oscillation transformer, have the same time-period as the circuit
consisting of the Leyden jars, or energy-storing condenser, and the
primary circuit of the oscillation transformer, and by so doing
immensely added to the power and range of the apparatus.

Starting from these inventions of Braun and Marconi, the author
devised a double transmission system in which the oscillations are
twice transformed before being generated in the aerial, each time with
a multiplication of electromotive force and a multiplication of the
number of groups of oscillations per second. This arrangement can best
be understood from the diagram (see Fig. 15).

In this case a transformer, T, or transformers receive alternating
low-frequency current from an alternator, _a_, being regulated by
passing through two variable choking coils, H_{1} and H_{2}, so as to
control it. This alternating current is transformed up from a
potential of two thousand to twenty, forty or a hundred thousand, and
is employed to charge a large condenser, C_{1}, which discharges
across a primary spark-gap, S_{1}, through the primary coil of an
oscillation transformer, T_{1}. The secondary circuit of the
oscillation transformer is connected to a second pair of spark balls,
S_{2}, which in turn are connected by a secondary condenser, C_{2},
and the primary circuit of a third transformer, T_{2} and the
secondary circuit of this last transformer are inserted between a
Marconi aerial, A, and the earth E. When all these circuits are tuned
to resonance by Mr. Marconi's methods, we have an enormously powerful
arrangement for creating electric waves, or rather trains of electric
waves, sent out from the aerial, and the oscillations are controlled
and the signals made by short-circuiting one of the choking coils.

Another transmitting arrangement, which involves a slightly different
principle, and employs no oscillation transformer, is one due also to
Professor Braun. In this case, a condenser and inductance are connected
in series to the spark balls of an induction coil, and oscillations are
set up in this circuit. Accordingly, there are rapid fluctuations of
potential at one terminal of the condenser. If to this we connect a long
aerial, the length of which has been adjusted to be one quarter of the
length of wave corresponding to the frequency, in other words, to make
it a quarter-wave resonator, then powerful oscillations will be
accumulated in this rod. The relation between the height (H) of the
aerial and the frequency is given by the equation 3 × 10^{10} = 4_n_H,
where _n_ is the frequency of the oscillations and H the height of the
aerial in centimetres. The frequency of the oscillations is determined
by the capacity (C) and inductance (L) of the condenser circuit, and can
be calculated from the formula

    n = (5,000,000) / ([\sq]{C (in mfds.) × L (in cms.)}).

That is, the frequency is obtained by dividing into the number
5,000,000, the square root of the product of the capacity in
microfarads, and inductance in centimetres, of the condenser circuit.
It will be found, on applying these rules, that it is impossible to
unite together any aerial of a length obtainable in practice with a
condenser circuit of more than a very moderate capacity. It has been
shown that for an aerial two hundred feet in height the corresponding
resonating frequency is about one and a quarter million.[22] As we are
limited in the amount to which we can reduce the inductance of a
discharge circuit, probably to something like a thousand centimetres,
a simple calculation shows that the largest capacity we can employ is
about a sixtieth of a microfarad. This capacity, even if charged at
60,000 volts, would only contain thirty joules of energy, or about
22·5 foot-pounds, which is a small storage compared to that which can
be achieved when we are employing the above-described methods, which
involve the use of an oscillation transformer. In such a case,
however, it is an advantage to employ a spark-gap in compressed air,
because we can then raise the voltage to a much higher value than in
air of ordinary pressure without lengthening the spark so much as to
render it non-oscillatory.

When employing methods involving the use of an oscillation
transformer, it is possible to use multiple aerials having large
capacity, and hence to store up a very large amount of energy in the
aerial, which is liberated at each discharge. The most effective
arrangement is one in which the radiator draws off gradually a large
supply of energy from a non-radiating circuit, and so sends out a true
train of waves, and not mere impulses, into the ether, and as we shall
see later on, it is only when the radiation takes place in the form of
true wave trains that anything like syntony can be obtained.

There are a number of variants of the above methods of arranging the
radiator and associated energy-storing in circuit. Descriptions of
these arrangements will be found in patents by Mr. Marconi, Professor
Slaby and Count von Arco, Sir Oliver Lodge, Dr. Muirhead, Professor
Popoff, Professor Fessenden and others. In all cases, however, they
are variations of the three simple forms of radiator already
described.

Returning to the analogy with the air or steam siren suggested at the
commencement of this article, the reader will see in the light of the
explanations already given, that all parts of the air-wave producing
apparatus have their analogues in the electrical radiator as used in
Hertzian wave telegraphy. The object in the one case is to produce
rapid oscillations of air particles in a tube, which result in the
production of an air wave in external space; in the other case, the
arrangement serves to produce oscillations of electrons or electrical
particles in a wire, the movements of which create a disturbance in
the ether called an electrical wave. Comparing together, item by item,
it will be seen, therefore, that the induction coil or transformer
used in connection with electric-wave apparatus is analogous to the
air pump in the siren plant. In the electrical apparatus, this
electron pump is employed to put an electrical charge into a
condenser; in the air wave apparatus, the air pump is employed to
charge an air vessel with high pressure air. From the electrical
condenser the charge is released in the form of a series of electrical
oscillations, and in the air wave producing appliance, the compressed
air is released in the form of a series of intermittent puffs or
blasts. In the electrical wave producing apparatus, these electrical
oscillations in the condenser circuit are finally made to produce
other oscillations in an air wire or open circuit, just as the puffs
of air finally expend themselves in producing aerial oscillations in
the siren tube. Finally, in the one case we have a series of air waves
and in the other case, a series of electrical waves. These trains of
electric waves or air waves, as the case may be, are intermitted into
long and short groups, in accordance with the signals of the Morse
alphabet, and, therefore, the Hertzian wave transmitter, in whatever
form it may be employed, when operated by means of a Marconi aerial,
is in fact an electrical siren apparatus, the function of which is to
create periodic disturbances in the universal ether of the same
character as those which the siren produces in atmospheric air.

       *       *       *       *       *

We have to consider in the next place the arrangements of the
receiving station and the various forms of receivers that have been
devised for effecting telegraphy by Hertzian waves. Just as the
transmitting station consists essentially of two parts, viz., a part
for creating electrical oscillations and a part for throwing out or
radiating electric waves, so the receiving-station appliances may be
divided into two portions; the function of one being to catch up a
portion of the energy of the passing wave, and that of the other to
make a record or intelligible signal in some manner in the form of an
audible or visible sign.

Accordingly, there must be at the receiving station an arrangement
called a receiving aerial, which in general takes the form of a long
vertical wire or wires, similar in form to the transmitting aerial,
There is, however, a distinct difference in the function of the
transmitting aerial and the receiving aerial. The function of the
first is effective radiation, and for this purpose the aerial must
have associated with it a store of energy to be released as wave
energy; but the function of the receiving aerial is to be the seat of
an electromotive force which is created by the electric force and the
magnetic force of the incident electric wave.

In tracing out the mode of operation of the transmitting aerial, it
was pointed out that the electric waves emitted consisted of
alternations of electric force in a direction which is perpendicular
to the surface of the earth, and magnetic force parallel to the
surface of the earth. These two quantities, the electric force and the
magnetic force, are called the _wave vectors_, and they both lie in a
plane perpendicular to the direction in which the wave is travelling
and at right angles to one another, the electric force being
perpendicular to the surface of the earth. In optical language, the
wave sent out by the aerial would be called a plane polarised wave,
the plane of polarisation being parallel to the magnetic force. Hence,
if at any point in the path of the wave we erect a vertical conductor,
as the wave passes over it, it is cut transversely by the magnetic
force of the wave and longitudinally by the electric force. Both of
these operations result in the creation of an alternating
electromotive force in the receiving aerial wire.

As in all other cases of oscillatory motion, the principle of
resonance may here be brought into play to increase immensely the
amplitude of the current oscillations thereby set up in the receiving
aerial. As already explained, any vertical insulated wire placed with
its lower end near the earth has capacity with respect to the earth,
and it has also inductance, the value of these factors depending on
its shape and height. Accordingly, it has a natural electrical
time-period of its own, and if the periodic electromotive impulses
which are set up in it by the passage of the waves over it agree in
period with its own natural time-period, then the amplitude of the
current vibrations in it may become enormously greater than when there
is a disagreement between these two periods. Before concluding these
articles we shall return to this subject of electric resonance and
syntony, and discuss it with reference to what is called the tuning of
Hertzian wave stations. Meanwhile, it may be said that for the sake of
obtaining, at any rate in an approximate degree, this coincidence of
time-period, it is generally usual to make the receiving aerial as far
as possible identical with the transmitting aerial. If the receiving
aerial is not insulated, but is connected to the earth at its lower
end through the primary coil of an oscillation transformer, we can
still set up in it electrical oscillations by the impact on it of an
electric wave of proper period; and if the oscillation transformer is
properly constructed we can draw from its secondary circuit electric
oscillations in a similar period.

One problem in connection with the design of a receiving aerial is
that of increasing its effective length and capacity so as to increase
correspondingly the electromotive force or current oscillations in it.
It is clear that if we put a number of receiving wires in parallel so
that each one of them is operated upon by the wave separately,
although we can increase in this way the magnitude of the alternating
current which can be drawn off from the aerial, we cannot increase the
electromotive force in it except by increasing the actual height of
the wires. Unfortunately, there is a limit to the height of the
receiving aerial. It has to be suspended, like the transmitting
aerial, from a mast or tower, and the engineering problem of
constructing such a permanent supporting structure higher than, say,
two hundred feet is a difficult one.

Since any one station has to send as well as receive, it is usual to
make one and the same aerial wire or wires do double duty. It is
switched over from the transmitting to the receiving apparatus, as
required. This, however, is a concession to convenience and cost. In
some respects it would be better to have two separate aerials at each
station, the one of the best form for sending, and the other of the
best form for receiving.

In Mr. Marconi's early arrangements, the so-called coherer or
sensitive wave-detecting appliance, to be described more in detail
presently, was inserted between the base of the insulated receiving
aerial and the earth, but it was subsequently found by him to be a
great improvement to act upon the receiving device, not directly by
the electromotive force set up in the aerial, but by the induced
electromotive force of a special form of step-up oscillation
transformer he calls a "jigger," the primary circuit of which was
inserted in between the receiving aerial and the earth plate, and the
secondary circuit was connected to the sensitive organ of the
telegraphic receiving arrangements.[23] A suggestion to employ
transformed oscillations in affecting a coherer, had also been
described in a patent specification by Sir Oliver Lodge, in 1897, but
the essence of success in the use of this device is not merely the
employment of a transformer, but of a transformer constructed
specially to transform electrical oscillations.

Turning, then, to the consideration of the relation existing between
the transmitting and receiving aerials, we note that in their simplest
form these consist of two similar tall rods of metal placed upright,
with their feet in good connection with the earth at two places. We
may think of them as two identical lightning conductors, well earthed
at the bottom, and supported by non-conducting masts or towers. These
rods must be in good connection with the earth, and therefore with it
form, as it were, one conductor. If, as usual, these aerials are
separated by the sea, the intermediate portion of this circuit is an
electrolyte. The operations which take place when a signal is sent are
as follows:--

At the transmitting station, we set up in the transmitting aerial
electric oscillations, of which the frequency may be of the order of a
million, _i.e._, the oscillations as long as they last are at the rate
of a million a second. Each spark discharge at the transmitter
results, however, only in the production of a train of a dozen or two
oscillations, and these trains succeed each other at a rate depending
upon the transmitting arrangements used. Each oscillation in the
transmitting aerial is accompanied by the detachment from it of
semi-loops of electric strain, as already explained. The alterations
of electric strain directed perpendicularly to the earth, and of the
associated magnetic force parallel to the earth, constitute an
electric wave in the ether, just as the alternations of pressure and
motion of air molecules constitute an air wave. Associated with these
physical actions above ground, there is a propagation through the
earth of electric action, which may consist in a motion or atomic
exchange of electrons. Each change or movement of a semi-loop of
electric strain above ground has its equivalent below ground in
inter-atomic exchanges or movements of the electrons, on which the
ends of these semi-loops of electric strain terminate. The earth must
play, therefore, a very important part in so-called "wireless
telegraphy," and we might also say the earth does as much as the ether
in its production.

The function of the receiving aerial is to bring about a union between
these two operations above and below ground. When the electric waves
fall upon it, they give rise to electromotive force in the receiving
aerial, and, therefore, produce oscillations in it which, in fact,
are electric currents flowing into and out of the receiving aerial. We
may say that the transmitting aerial, the receiving aerial and the
earth form one gigantic Hertz oscillator. In one part of this system,
electric oscillations of a certain period are set up by the discharge
of a condenser and are propagated to the other part. In the earth,
there is a propagation of electric oscillations; in the space above
and between the aerials, there is a propagation of electric waves. The
receiving aerial _feels_, therefore, what is happening at the distant
aerial and can be made to record it.[24]

We have next to consider the question of the wave-detecting devices
which enable us to appreciate and record the impact of a wave or wave
train against the aerial. At the very outset it will be necessary to
coin a new word to apply generally to these appliances. Most readers
are probably familiar with the term "coherer," which was applied by
Sir Oliver Lodge, in the first instance, to an electric wave-detecting
device of one particular kind--viz., that in which a metal point was
lightly pressed against another metal surface and caused to stick to
it when an electric wave fell upon it. As our knowledge increased, it
was found that there were many cases in which the effect of the
electric radiation was to cause a severance and not a coherence, and
hence such clumsy phrases as "anticoherer" and "self-decohering
coherer" have come into use. Moreover, we have now many kinds of
electric wave detectors based on quite different physical principles.
At the risk of incurring reprobation for adding to scientific
nomenclature, the author ventures to think that the time has arrived
when a simple and inclusive term will be found useful to describe all
the devices, whatever their nature, which are employed for detecting
the presence of an electric wave. For this purpose the term
_kumascope_, from the Greek [Greek: kuma] (a wave), is suggested. The
scientific study of waves has already been called _kumatology_, and in
view of our familiarity with such terms as _microscope_,
_electroscope_ and _hygroscope_, there does not seem to be any
objection to enlarging our vocabulary by calling a wave-detecting
appliance a _kumascope_. We are then able to look at the subject
broadly and to classify kumascopes of different kinds.

We may, in the first place, arrange them according to the principle on
which they act. Thus, we may have electric, magnetic, thermal,
chemical and physiological operations involved; and finally, we may
divide them into those which are self-restoring, in the sense that
after the passage or action of a wave upon them they return to their
original sensitive condition; and those which are non-restoring, in
that they must be subjected to some treatment to bring them back again
to a condition in which they are fit to respond again to the action of
a wave.

We have no space to refer to the whole of the steps of discovery which
led up to the invention of all the various forms of the modern
electric kumascope or wave detector. Suffice it to say that the
researches of Hertz in 1887 threw a flood of light upon many
previously obscure phenomena, and enabled us to see that an electric
spark, and especially an oscillatory spark, creates a disturbance in
the ether, which has a resemblance in Nature to the expanding ripples
produced by a stone hurled into water. Scientific investigation then
returned with fresh interest to previously incomprehensible effects,
and a new meaning was read into many old observations. Again and again
it had been noticed that loose metallic contacts, loose aggregations
of metallic filings or fragments, had a mysterious way of altering
their conductivity under the action of electric sparks, lightning
discharges and high electromotive forces.

As far back as 1852, Mr. Varley had noticed that masses of powdered
metals had a very small conductivity, which increased in a remarkable
way during thunderstorms;[25] and in 1866, C. and S. A. Varley
patented a device for protecting telegraphic instruments from
lightning, which consisted of a small box of powdered carbon in which
were buried two nearly touching metal points, and they stated that
"powdered conducting matter offers a great resistance to a current of
moderate tension, but offers but little resistance to currents of high
tension."[26]

We then pass over a long interval and find that the next published
account of similar observations was due to Professor T.
Calzecchi-Onesti, who described in an Italian journal, _Il Nuovo
Cimento_ (see Vol. XVI., p. 58, and Vol. XVII., p. 38), in 1884 and
1885 his observations on the decrease in resistance of metal powders
when the spark from an induction coil was sent through them.[27] These
observations did not attract much attention until Professor E. Branly,
of Paris, in 1890 and 1891, repeated them on an extended scale and
with great variations, making the important observation that an
electric spark _at a distance_ had a similar effect in increasing the
conductivity of metallic powders.[28] Branly, however, noticed that in
some cases of conductors in powder, such as the peroxide of lead or
antimony, the effect of the spark was to cause a decrease of
conductivity.

To Professor E. Branly unquestionably belongs the honour of giving to
science a new weapon in the shape of a tube containing metallic
filings or powder rather loosely packed between metal plugs, and of
showing that when the pressure on the powder was adjusted such a tube
may be a conductor of very high resistance, but that the electrical
conductivity is enormously increased if an electric spark is made in
its neighbourhood. He also proved that the same effect occurred in the
case of two slightly oxidised steel or copper wires laid across one
another with light pressure, and that this loose or imperfect contact
was extraordinarily sensitive to an electric spark, dropping in
resistance from thousands of ohms to a few ohms when a spark was made
many yards away.

It is curious to notice how long some important researches take to
become generally known. Branly's work did not attract much attention
in England until 1892, when Dr. Dawson Turner described his own
repetition of Branly's experiments with the metallic filings tube at a
meeting of the British Association in Edinburgh. In the discussion
which followed, Professor George Forbes made an important remark. He
asked whether it was possible that the decrease in resistance could be
brought about by Hertz waves.[29]

This question shows that even in 1892 the idea that the effect of the
spark on the Branly tube was really due to Hertzian waves was only
just beginning to arise. The following year, however, Mr. W. B. Croft
repeated Branly's experiment with copper filings before the Physical
Society of London, and entitled his short Paper "Electric Radiation on
Copper Filings."[30] He exhibited a tube containing copper filings
loosely held between two copper plugs and joined in series with a
galvanometer and cell. The effect of an electric spark at a distance,
in causing increase of conductivity, was shown, and the return of the
tube to its non-conducting state when tapped was also noticed.

In the discussion which followed the reading of this Paper, Professor
Minchin described the effects of electric radiation on his impulsion
cells. He followed up this by reading a Paper to the Physical Society
on November 24, 1893, on the action of Hertzian radiation on films
containing metallic powders, and expressed the opinion that the change
in resistance of the Branly tube was due to electric radiation.[31]

Thus, at the end of 1893, a few physicists clearly recognised that a
new means had been given to us for detecting those invisible ether
waves, the chief properties of which Hertz had unravelled with
surpassing skill six years before, by means of a detector consisting
of a ring of wire having a small spark-gap in it.

In June, 1894, Sir Oliver Lodge delivered a discourse at the Royal
Institution, entitled "The Work of Hertz," and at this lecture use was
made of the Branly tube as a Hertz wave detector. The chief object of
the lecture was to describe the properties of Hertzian waves and their
reflection, absorption and transmission, and many brilliant
quasi-optical experiments were exhibited. Although a Branly tube, or
imperfect metallic contact, then named by him a _coherer_, was
employed by Sir Oliver Lodge to detect an electric wave generated in
another room, there was no mention in this lecture of any use of the
instrument for telegraphic purposes.[32]

As we are here concerned only with the applications in telegraphy, we
shall not spend any more time discussing the purely scientific work
done with laboratory forms of this wave detector.

Without attempting to touch the very delicate question as to the
precise point at which laboratory research passed into technical
application, we shall briefly describe the forms of kumascope which
have been devised with special reference to Hertzian wave telegraphic
work. A very exact classification is at present impossible, but we may
say that telegraphic kumascopes may be roughly divided into six
classes. The first class includes all those that depend for their
action on the "coherer principle" or the reduction of the resistance
of a metallic microphone by the action of electromotive force. As they
depend upon an imperfect contact, they may be called _contact
kumascopes_. This class is furthermore subdivided into the
self-restoring and the non-self-restoring varieties. The second class
comprises the _magnetic kumascopes_ which depend upon the action of an
electrical oscillation as a magnetising or demagnetising agency. The
third class comprises the _electrolytic responders_, in which the
action of electric oscillations either promotes or destroys the
results of electrolysis. The fourth class consists of the
_electrothermal detectors_, in which the power of an electrical
oscillation as a high frequency electric current to heat a conductor
is utilised. The fifth class comprises the _electromagnetic_ or
_electro-dynamic_ instruments, which are virtually very sensitive
alternating-current ammeters, adapted for immensely high frequency.
The sixth class must be made to contain all those which cannot be well
fitted at present into any of the others, such as the sensitive
responder of Schäfer, the action of which is not very clearly made
out.

We may proceed briefly to describe the construction of the principal
forms of kumascope coming under the above headings. In the first
place, let us consider those which are commonly called the "coherers"
or, as the writer prefers to call them, the _contact kumascopes_. The
simplest of these is the crossed needle or single contact, which
originated with Professor E. Branly.[33] The pressure of the point of
a steel needle against an aluminium plate was subsequently found by
Sir Oliver Lodge to be a very sensitive arrangement when so adjusted
that a single cell sends little or no current through the contact.[34]
When an electric wave passes over it, good conducting contact ensues.
The point is, in fact, welded to the plate, and can only be detached
by giving the plate or needle a light shock or vibration. A variation
of the above form is a pair of crossed needles, one resting on the
other.

Professor Branly found, in 1891, that if a pair of slightly-oxidised
copper wires rest across one another the contact-resistance may fall
from 8,000 to 7 ohms by the impact of an electric wave. He has
recently devised a tripod arrangement, in which a light metal stool
with three slightly-oxidised legs stands on a polished plate of steel.
The contact points must be oxidised, but not too heavily, and the
stool makes a bad electrical contact until a wave falls upon it.[35]
The decoherence is effected by giving the stool a tilt by means of an
electromagnet.

These single or multiple-point kumascopes labour under the
disadvantage that only a very small current can be passed through the
variable contact when used as a relay arrangement, without welding
them together so much that a considerable mechanical shock is required
to break the contact and reset the trap.

The logical development of the single contact is, therefore, the
infinite number of contacts existing in the tube of metallic filings,
which has been the form of kumascope most used for many years. In its
typical form it consists of a tube of insulating material with
metallic plugs at each end, and between them a mass of metallic
powder, filings, borings, granules or small spheres, lightly touching
one another. Imperfect contact must be arranged by light pressure, and
in the majority of cases the resistance is very large until an
electric wave falls upon the tube, when it drops suddenly to a small
value and remains there until the tube is given a slight shake or the
granules disturbed in any way, when the resistance suddenly rises
again. This type of responder is a non-restoring kumascope, and
requires the continual operation of some external agency to keep it in
a condition in which it is receptive or sensitive to electric waves.

Much discussion and considerable research have taken place in
connection with the action and improvement of these metallic powder
kumascopes. As regards materials, the magnetic metals, nickel, iron
and cobalt, in the order named, appear to give the best results. The
noble metals, gold, silver and platinum, are too sensitive, and the
very oxidisable metals too insensitive, for telegraphic work, but an
admixture may be advantageously made.

Omitting the intermediate developments of invention, it may be said
that Mr. Marconi was the first to recognise that to secure great
sensibility in an electric wave detector of this type the following
conditions must be fulfilled: An exceedingly small mass of metallic
filings must be placed in a very narrow gap between two plugs, the
whole being contained in a vessel which is wholly or partly exhausted
of its air. Mr. Marconi devoted himself with great success to the
development of this instrument, and in a very short time succeeded in
transforming it from an uncertain laboratory appliance, capable of
yielding results only in very skilled hands, into an instrument
certain and simple in its operations as an ordinary telegraphic relay.
He did this, partly by reducing its size, and partly by a most
judicious selection of materials for its construction. As made at
present, the Marconi metallic filings tube consists of a small glass
tube, the interior diameter of which is not much more than one-eighth
of an inch, which has in it two silver plugs which are bevelled off
obliquely. These are placed opposite to each other, so as to form a
wedge-shaped gap, about a millimetre in width at the bottom and two,
or at most three, millimetres in width at the top (see Fig. 16). The
silver plugs exactly fill the aperture of the tube, and are connected
to platinum wires sealed through the glass. The tube has a lateral
glass tube fused into it, by which the exhaustion is made, which is
afterwards sealed off, and this tube projects on the side of the wider
portion of the gap between the silver plugs. The sensitive material
consists of a mixture of metallic filings, five per cent. silver and
ninety-five per cent. nickel, being carefully mixed and sifted to a
certain standard fineness. In the manufacture of these tubes, great
care is taken to make them as far as possible absolutely identical.
Each tube when finished is exhausted, but not to a very high vacuum.
The tube so finished is attached to a bone holder, by which it can be
held in a horizontal position. The object of bevelling off the plugs
in the Marconi tube is to enable the sensitiveness of the tube to be
varied by turning it round, so that the small quantity of filings lie
in between a wider or narrower part of the gap.[36]

[Illustration: FIG. 16.--MARCONI SENSITIVE TUBE OR METALLIC FILINGS
KUMASCOPE. PP, silver plugs; TT, platinum wires; F, nickel and silver
filings.]

Other ways of adjusting the quantity of the filings to the width of
the gap have been devised. Sometimes one of the plugs is made movable.
In other cases, such as the tubes devised by M. Blondel and Sir Oliver
Lodge, there is a pocket in the glass receptacle to hold square
filings, from which more or less can be shaken into the gap.

An interesting question, which we have not time to discuss in full, is
the cause of the initial coherence of the metallic filings in a Branly
tube. It does not seem to be a simple welding action due to heat, and
it certainly takes place with a difference of potential, which is very
far indeed below that which we know is required to produce a spark. On
the other hand, it seems to be proved that in a Banly tube, when acted
upon by electric waves, chains of metallic particles are produced. The
effect is not peculiar to electric waves. It can be accomplished by
the application of any high electromotive force. Thus Branly found
that coherence may be produced by the application of an electromotive
force of twenty or thirty volts, operating through a very high water
resistance, and thus precluding the passage of any but an excessively
small current. Again, the coherence seems to take place in some cases
when metallic particles are immersed in a liquid, or even in a solid,
insulator. Processor Branly has, therefore, preferred to speak of
masses of metallic granules as _radio-conductors_, and Professor Bose
has divided substances into positive and negative, according as the
operation of electromotive force is to increase the coherence of the
particles or to decrease it.

It has been asserted that for every particular Branly tube, there is a
critical electromotive force, in the neighbourhood of two or three
volts which causes the tube to break down and pass instantly from a
non-conductive to a conductive condition, and that this critical
electromotive force may become a measure of the utility of the tube
for telegraphic purposes. Thus, C. Kinsley (_Physical Review_, Vol.
XII., p. 177, 1901) has made measurements of this supposed critical
potential for different "coherers," and subsequently tested the same
as receivers at a wireless telegraph station of the U.S.A. Signal
Corps. The average of twenty-four experiments gave in one case 2·2
volts as the breaking down potential of one of these coherers or
Branly tubes, 3·8 volts for a second and 5·5 volts for the third.
These same instruments, tested as telegraphic kumascopes, showed that
the first of the three was most sensitive.

On the other hand, W. H. Eccles (_Electrician_, Vol. XLVII., pp. 682
and 715, 1901) has made experiments with Marconi nickel-silver
sensitive tubes, using a liquid potentiometer made with copper
sulphate, to apply the potential so that infinitesimal spark contacts
might be avoided and the changes in potential made without any
abruptness. He states that if the coherer tube is continuously tapped,
say at the rate of fifty vibrations per second, whilst at the same
time an increasing potential is applied to its terminals and the
current passing through it measured on a galvanometer, there is no
abrupt change in current at any point. He found that when the current
and voltage were plotted against each other, a regular curve was
obtained, which after a time becomes linear. A decided change occurs
in the conductivity of the mass of metallic filings when treated in
this manner at voltages lower than the critical voltage obtained by
previous methods. He ascertained that there was a complete
correspondence between the sensitiveness of the tubes used as
telegraphic instruments and the form of the characteristic curve of
current and voltage drawn by the above-described method.

In the same manner, K. E. Guthe and A. Trowbridge (_Physical Review_,
Vol. II., p. 22, 1900) investigated the action of a simple ball
coherer formed of half a dozen steel, lead or phosphor-bronze balls in
slight contact. They measured the current _i_ passing through the
series under the action of a difference of potential _v_ between the
ends, and found a relation which could be expressed in the form

    v = V(1 - e^{ki}),

where V and _k_ are constants.

The current through this ball coherer is, therefore, a logarithmic
function of the potential difference between its ends, of the form

    i = log(v - V)

and exhibits no discontinuity.

The inference was drawn that the "resistance" is due to films of water
adhering to the metallic particles through which electrolytic action
occurs.

A good metallic filings tube for use as a receiver in Hertzian wave
telegraphy should exhibit a constancy of action and should cohere and
decohere, to use the common terms, sharply, at the smallest possible
tap. It should not have a current passed through it by the external
cell of more than a fraction of a milliampere, or else it becomes
wounded and unsensitive.

The investigations which have already taken place seem to show pretty
clearly that the agency causing the masses of filings to pass from a
non-conductive to a conductive condition is electromotive force, and
that, therefore, it is the electromotive force set up in the aerial by
the incident waves which is the effective agent in causing the change in
the metallic filings tube, when this is used as a telegraphic kumascope.
This transformation of the tube from a non-conductor to a conductor is
made to act as a circuit-closer, completing the circuit, by means of
which a single cell of a local battery is made to send current through
an ordinary telegraph relay, and so by the aid of a second battery
operate a telegraphic printer or recorder of any kind. Hence it is clear
that after one impact, the metallic filings tube has to be brought back
to its non-conductive condition, and this may be achieved in several
ways. (1) By the administration of carefully-regulated taps or shocks or
by rotating the tube on its axis; (2) by the aid of an alternating
current; (3) in those cases where filings of magnetic metals are
employed, by magnetism.

The decoherence by taps was discovered by Branly,[37] and Popoff,
following the example of Sir Oliver Lodge, employed an electric bell
arrangement for this purpose.[38]

Mr. Marconi, in his original receiving instruments, placed an
electromagnet under the coherer tube with a vibrating armature like an
electric bell.[39] This armature carries a small hammer or tapper,
which, when set in action, hits the tube on the under side, and
various adjusting screws are arranged for regulating exactly the force
and amplitude of the blows. This tapper is actuated by the same
current as the Morse printer, or other telegraphic recorder, so that
when the signal is received and the metallic filings tube passes into
the conductive condition and closes the relay circuit, this latter in
turn closes the circuit of the Morse printer or other recorder, and at
the same time, a current passes through the electromagnet of the
tapper and the tube is tapped back. This sequence of operations
requires a certain time which limits the speed of receiving. The
tapper has to be so arranged that it is possible to receive and to
record not only the _dot_ but a _dash_ on the Morse system. The _dash_
is really a series of closely adjacent dots, which run together in
virtue of the inertia and inductance of the different parts of the
whole receiving apparatus. The adjustment has so to be made that,
whilst the _dash_ is being recorded and a continuous tapping is kept
up, yet, nevertheless the continued electromotive force in the aerial,
due to the continually arriving trains of waves, is able to act
against the tapping and to keep the filings in the tube in the
conductive condition. Hence, the successful operation of the
arrangement requires attention to a number of adjustments, but these
are not more difficult, or even as difficult, as those involved in the
use of many telegraphic receivers employed in ordinary telegraphy with
wires.

Mr. Marconi also introduced devices for preventing the sparks at the
contacts of the electromagnetic hammer from directly affecting the
tube, and also to prevent the electric oscillations which are set up
in the aerial from being partly shunted through other circuits than
that of the sensitive tube. We pass on to notice the remaining devices
for restoring the metallic filings tube to a condition of
sensitiveness or receptiveness.

A method for doing this by alternating currents is due to Mr. S. G.
Brown.[40] The pole pieces of the coherer tube are made of iron, and
they are enveloped in magnetising coils traversed by an alternating
electric current. Between these pole pieces is placed a small quantity
of nickel or iron filings, and under the action of the electromotive
force, due to an electric wave acting on them, may be made to cohere
in the usual fashion; but the moment that the wave ceases, the
alternating magnetism of the electrodes causes the filings to drop
apart or decohere. In place of the alternating current, Mr. Brown
finds that a revolving permanent magnet can be used to produce the
alternating magnetisation of the pole pieces of the sensitive tube or
coherer.

The third method of causing the decoherence of the filings is that due
to T. Tommasina. He found that when a Branly tube is made with filings
of a magnetic metal, such as iron, nickel and cobalt, the decoherence
of the filings can be produced by means of an electromagnet placed in
a suitable position under the tube.[41] The explanation of this fact
seems to be that, when an electric wave falls upon the tube or when
any other source of electromotive force acts upon it, chains of
metallic particles are formed, stretching from one electrode to the
other. Tommasina contends that he has proved the existence of these
chains of particles by experiments made with iron filings; and R.
Malagoli,[42] in referring to Tommasina's assertion, states that it
can be witnessed in the case of brass filings placed between two
plates of metal and immersed in vaseline oil, when a difference of
potential is made between the plates.

T. Sundorph[43] says he has confirmed Tommasina's discovery of the
formation of these chains of metallic particles in the coherer. The
filings do not all cling together, but certain chains are formed which
afford a conducting path for the current subsequently passed through
the coherer from an external source. Accordingly, Tommasina's method
of causing decoherence in the case of filings of magnetic metals is to
pull them apart by an external magnetic field; and he stated that
decoherence can be effected more easily and regularly in this way than
by tapping. Whilst on this point, it may be mentioned that C.
Tissot[44] says that he has found that the sensitiveness of a coherer
formed of nickel and iron filings can be increased by placing it in
the magnetic field, the lines of which are parallel to the axis of
the tube. According to MM. A. Blondel and G. Dobkevitch, this is
merely the result of an increased coherence of the particles.

       *       *       *       *       *

Quite recently, Sir Oliver Lodge and Dr. Muirhead have employed as a
self-restoring coherer or kumascope a steel disc revolved by
clockwork, the edge of which just touches a globule of mercury covered
with a thin film of paraffin oil. The contact is made between the
mercury and the steel by the electric wave generating an electromotive
force in the aerial, sufficient to break through the thin film of oil.
When the wave stops, the circuit is again interrupted automatically.

This device is used without a relay to actuate directly a syphon
recorder as used in submarine telegraphy. The working battery employed
with it must only have an electromotive force of about a tenth of a
volt. It may be used also with a telephone in circuit and can,
therefore, be employed either for telegraphic or telephonic
reception.[45]

[Illustration: FIG. 17.--ITALIAN NAVY SELF-RESTORING KUMASCOPE. C,
carbon plug; I, iron plug; M, mercury globule; A, aerial; B, battery;
T, telephone; S, adjusting screw.]

One of the most sensitive of these self-restoring kumascopes is the
carbon-steel-mercury coherer, the invention of which has been
attributed to Castelli, a signalman in the Italian Navy,[46] but also
stated on good authority to have been the invention of officers in the
Royal Italian Navy, and has, therefore, been called the Italian Navy
coherer.[47] This instrument has been arranged in several forms, but
in the simplest of these it consists of a glass tube, having in it a
plug of iron and a plug of arc-lamp carbon, or two plugs of iron with
a plug of carbon between them. The plugs of iron, or of iron and
carbon, are separated by an exceedingly small globule of mercury, the
size of which should be between one and a-half and three millimetres.
The plugs closing the tube must be capable of movement, one of them by
means of a screw, as shown in the diagram (Fig. 17), taken from a
patent specification communicated to Mr. Marconi by the Marchese Luigi
Solari, of the Royal Italian Navy. One of the plugs of this tube is
connected to the aerial and the other to the earth, and they are also
connected through another circuit composed of a single dry cell and a
telephone. The arrangement then forms an extremely sensitive detector
of electric waves or of small electromotive forces, or, if a wave
falls on the aerial, the electromotive force at once improves the
contact between the mercury and the plugs, and therefore causes a
sudden increase in the current through the telephone, giving rise to a
sound; but when the wave ceases, or the electromotive force is
withdrawn, the resistance falls back again to its origin value, and
the arrangement is, therefore, self-acting, requiring no tapping or
other device for restoring it to receptivity.

A very ingenious form of combined telephone and coherer has been
devised by T. Tommasina.[48] In this instrument the diaphragm of an
ordinary Bell telephone carries upon it a very small carbon or
metallic coherer. This coherer is connected in between the aerial and
the earth, and is also in circuit with a battery and the electromagnet
of a telegraphic relay. When this relay operates it closes the circuit
of another battery which is placed in series with the telephone coil.
The moment the current passes through the telephone coil it attracts,
and therefore vibrates, the diaphragm and shakes up the metallic
filings. If an observer, therefore, places the telephone to his ear,
he hears a sound corresponding to every train of waves incident upon
the aerial. With this arrangement, one can obtain two different kinds
of results, according to the nature of the cohering powder placed in
the cavity in the diaphragm. First, if the powder consists of a
non-magnetic metal, gold, silver, platinum or the like, the receiver
will be very sensitive; and at the same time the current passing
through it when it is cohered will be sufficient to work a sensive
recording apparatus in series with the telephone coil. Secondly, if
the metallic powder placed in the cavity is a magnetic metal, the
receiver will be somewhat less sensitive, but will work with more
precision, because of the magnetic action of the magnet of the
telephone upon the cohering powder. If no recording apparatus is used,
the observer must write down the signals as heard in the telephone,
since corresponding to a short spark at the transmitting station, a
single tick or short sound is heard at the telephone, and
corresponding to a series of rapidly successive sparks, a more
prolonged sound is heard in the telephone. These two sounds, as
already explained, constitute the dot and the dash of the Morse
signals.

We may, in the next place, refer to that form of kumascope in which
the action of the wave or of electromotive force is not to decrease
the resistance of a contact, but to increase that of an imperfect
contact. As already mentioned, Professor Branly discovered long ago
that peroxide of lead acts in an opposite manner to metallic filings,
in that when placed in a Branly tube it increases its resistance under
the action of an electric spark, instead of decreasing it. Again,
Professor Bose has found that fragments of metallic potassium in
kerosene oil behave in a similar manner, and that certain varieties of
silver, antimony and of arsenic, and a few other metals, have a
similar property. Branly tubes, therefore, made with these materials,
or any arrangements which act in a similar manner, have been called
"anti-coherers." The most interesting arragement which has been called
by this name is that of Schäfer.[49] Schäfer's kumascope is made in
the following manner: A very thin film of silver is deposited upon
glass, and a strip of this silver is scratched across with a diamond,
making a fine transverse cut or gap. If the resistance of this divided
strip of silver is measured, it will be found not to be infinite, but
may have a resistance as low as forty or fifty ohms if the strip is
thirty millimetres wide. On examining the cut in the strip with a
microscope, it will be found that the edges are ragged and that there
are little particles of silver lying about in the gap. If, then, an
electromotive force of three volts or more is put on the two separated
parts of the strip, these little particles of silver fly to and fro
like the pith balls in a familiar electrical experiment, and they
convey electricity across from side to side. Hence a current passes
having a magnitude of a few milliamperes. If, however, the strip is
employed as a kumascope and connected at one end to the earth and at
the other end to an aerial, when electric waves fall upon the aerial,
the electrical oscillations thereby excited seem to have the property
of stopping this dance of silver particles and the resistance of the
gap is increased several times, but falls again when the wave ceases.
If, therefore, a telephone and battery are connected between two
portions of the strip, the variation of this battery current will
affect the telephone in accordance with the waves which fall upon the
aerial, and the arrangement becomes therefore a wave-detecting device.
It is said to have been used in wireless telegraph experiments in
Germany up to a distance of ninety-five kilometres.

We must next direct attention to those wave-detecting devices which
depend upon magnetisation of iron, and here we are able to record
recent and most interesting developments. More than seventy years ago
Joseph Henry, in the United States, noticed the effect of an electric
spark at a distance upon magnetised needles.[50] Of recent times, the
subject came back into notice through the researches of Professor E.
Rutherford,[51] who carried out at Cambridge, England, in 1896, a
valuable series of experiments on this subject. He found that if a
magnetised steel needle or a very small bundle of extremely thin iron
wires is magnetised and placed in the interior of a small coil, the
ends of which are connected to two long collecting wires, then an
electric wave started from a Hertz oscillator at a distance causes an
immediate demagnetisation of the iron. This demagnetisation he
detected by means of the movement of the needle of a magnetometer
placed near one end of the iron wire. Although Rutherford's wave
detector has been much used in scientific research, it was not, in the
form in which he used it, a telegraphic instrument, and could not
record alphabetic signals.

Not long ago Mr. Marconi invented, however, a telegraphic instrument
based upon his discovery that the magnetic hysteresis of iron can be
annulled by electric oscillations. In one form, Mr. Marconi's magnetic
receiver is constructed as follows[52] (see Fig. 18): An endless band
of thin iron wire composed of several iron wires about No. 36 gauge,
arranged in parallel, is made to move slowly round on two pulleys,
like the driving belt of a machine. In one part of its path the wire
passes through a glass tube, on which are found two coils of wire, one
a rather short, thick coil, and the other a very fine, long one. The
fine, long coil is connected with a telephone, and the shorter coil is
connected at one end to the earth and the other to the aerial. Two
permanent horse-shoe magnets are placed as shown n Fig. 18, with their
similar poles together, and, as the iron band passes through their
field, a certain length of it is magnetised, and owing to the
hysteresis of the material, it retains this magnetism for a short time
after it has passed out of the centre of the field. If then an
electric oscillation, coming down from the aerial, is passed through
the shorter coil, it changes the position of the magnetised portion of
the iron and, so to speak, brings the magnetised portion of iron back
into the position it would have occupied if the iron had had no
hysteresis. This action, by varying the magnetic flux through the
secondary coil, creates in it an electromotive force which causes a
sound to be heard in the telephone connected to it. If at a distant
place a single wave or train of waves is started and received by the
aerial, this will express itself by making an audible tick in the
telephone, and if several groups of closely adjacent wave trains are
sent, these will indicate themselves by producing a rapid series of
ticks in the telephone, heard as a short continuous noise and taken as
equivalent to the Morse _dash_.

[Illustration: FIG. 18.--MARCONI MAGNETIC RECEIVER. W_{1}W_{2}, wheels;
I, iron wire band; P, primary coil; S, secondary coil; T, telephone;
A, aerial; E, earthplate.]

It was by means of this remarkably ingenious instrument that Mr.
Marconi was able, in the summer of 1902, to detect the waves sent out
from Poldhu on the coast of Cornwall, and receive messages as far as
Cronstadt in the Baltic, in one direction, and as far as Spezzia in
the Mediterranean in another direction, and also to receive messages
across the Atlantic from the power stations situated in Glace Bay,
Nova Scotia, and from one at Cape Cod in Massachusetts, U.S.A., in
December, 1902.

There can be no question that this magnetic detector of Mr. Marconi's,
used in connection with a good telephone and an acute human ear, is
the most sensitive device yet invented for the detection of electric
waves and their utilisation in telegraphy without continuous wires. It
is marvellously simple, ingenious and yet effective, as a Hertzian
wave telegraphic receiver.

Whilst on the subject of magnetic wave detectors, the author may
describe experiments that he has been recently making to construct a
Hertzian wave detector on the Rutherford principle, which shall be
strictly quantitative. All the receivers of the coherer type and
electrolytic type give no indications that are at all proportional to
the energy of the incident wave. Their indications are more or less
accidental and depend upon the manner in which the receiver was last
left. There is a great need for a quantitative wave detector, the
indications of which shall give us a measure of the energy of the
arriving wave. It is only by the possession of such an instrument that
we can hope to study properly the sending powers of various
transmitters or the efficiency of different forms of aerial or devices
by which the wave is produced. This magnetic receiver is constructed
as follows:

A coil of fine wire is constructed in sections like the secondary coil
of an induction coil, and in the instrument already made, this coil
contains thirty or forty thousand turns of wire. In the interior of
this coil are placed a number of little bundles of fine iron wire
wound round with two coils, a fine wire coil which is a magnetising
coil, and a thicker wire coil which is a demagnetising coil. These
sets of coils are joined up, respectively, in series or in parallel.
Then, associated with this form of induction coil is a commutator of a
peculiar kind, which performs the following functions when a battery
is connected to it and when it is made to revolve by a motor or by
clockwork. First, during part of the revolution, the commutator closes
the battery circuit and magnetises the iron cores, and whilst this is
taking place the secondary circuit of the induction coil is
short-circuited and the galvanometer is disconnected from it.
Secondly, the magnetising current is stopped, and soon after that the
secondary coil is unshort-circuited and connected to the galvanometer,
and remains in this condition during the remainder of the revolution.
This cycle of operations is repeated at every revolution. If then an
electrical oscillation is sent into the demagnetising coils, and if it
continues longer than one revolution of the commutator, it will
demagnetise the iron core during that period of time in which the
battery is disconnected and the galvanometer connected. The
demagnetisation of the iron which ensues produces an electromotive
force in the secondary coil and causes a deflection of the
galvanometer, and this deflection will continue and remain steady if
the oscillation persists. Moreover, since this deflection is due to
the passage through the galvanometer of a rapid series of discharges,
it is large when the oscillations continue for a long time and are
powerful, and small when they continue for a short time or are weak.
We can, therefore, with this arrangement, receive on the galvanometer,
just as on the mirror galvanometer used in submarine cable work, a dot
or dash, and, moreover, the magnitude of these deflections is a
measure of the energy of the wave.

It is probable that when this arrangement is perfected it will become
exceedingly useful for making all kinds of tests and measurements in
connection with Hertzian telegraphy, even if it is not sensitive
enough to use as a long distance receiver.

Of late years a variety of wave-detecting devices have been brought
forward which depend upon electrolysis. One of the best known of these
is that by De Forest and Smythe.[53] In this arrangement, a tube
contains two small electrodes like plugs, which may be made of tin,
silver or nickel, or other metal. The ends of these plugs are flat and
separated from each other by about one two-hundredth of an inch.
Sometimes the end of one of these plugs is made cup shaped and the cup
or recess is filled with a mass of peroxide of lead and glycerine. In
the interval between the electrodes is placed an electrolyzable
mixture, which consists of glycerine or vaseline mixed with water or
alcohol, and a small quantity of litharge and metallic filings. These
metallic filings act as secondary electrodes. When a small
electromotive force is applied between the terminals of the electrodes
of this tube through a very high resistance of twenty or thirty
thousand ohms, an exceedingly small current passes through this
mixture, and it causes an electrolytic action which results in the
production of chains of metallic particles connecting the two
electrodes together. If, in addition to this, one terminal or
electrode of the arrangement is connected to an aerial wire and the
other terminal to the earth, then on the arrival of an electric wave
creating oscillations in the wire, these oscillations pass down into
the electrolytic cell, where they break up the chains of metallic
particles and thus interrupt the current passing through the telephone
quite suddenly, which is heard as a slight tick by an ear applied to
it. As soon as the wave ceases, the chain of metallic particles is
re-established, so that the appliance is always in a condition to be
affected by a wave. It is said that this breaking up and reformation
of the chains of metallic particles is so rapid that a short spark
made at the transmitting station is heard as a tick in the telephone,
but a rapid succession of oscillatory sparks is heard as a short
continuous sound; hence the two signals necessary for alphabetical
conversation can be transmitted.

Another receiver which has some resemblance to the above, although
different in principle, is that of Neugschwender.[54] In this
arrangement, which to a certain extent resembles the Schäfer detector,
a glass plate has upon it a deposit of silver in the form of a strip,
which is cut across at one place, thus interrupting it. If the cut is
breathed upon or placed in a moist atmosphere, a little dew is
deposited upon the glass, which bridges over the cut in the metal and
creates an electric continuity. Hence a small current can be passed
across the gap and through a telephone by one or two cells of a
battery. If, however, an electric oscillation passes across the gap on
its way from an aerial to the earth, then the continuity of the liquid
film is destroyed, and the current is interrupted and a sound created
in the telephone.

The opinion has been expressed by Sir Oliver Lodge that in this case
the interruption of the circuit which occurs is really due to the
coalescence of minute water particles into larger drops, as when
vapour is condensed into rain, and hence the continuity of the
material is interrupted.

We must then make a brief reference to other kumascopes which depend
upon the heating power of an electrical oscillation, which it
possesses in common with every other form of electric current.
Professor R. A. Fessenden[55] has constructed a very ingenious thermal
receiver in the following manner: An extremely fine platinum wire,
about 0·003 of an inch in diameter, is embedded in the middle of a
silver wire about one tenth of an inch in diameter, like the wick of
a candle. This compound wire is then drawn down until the diameter of
the silver wire is only 0·002 of an inch, and hence the platinum wire
in its interior, being reduced in the same ratio, will have been drawn
to a diameter of 0·00006 of an inch. A short piece of this drawn wire
is then bent into a loop and the ends fixed to wires. The tip of the
loop is then immersed in nitric acid and dissolved in the silver,
leaving an exquisitely fine platinum wire a few hundredths of an inch
in length and having a resistance of about thirty ohms. This little
loop is sealed into a glass bulb like a very small incandescent lamp,
or it may be enclosed in a small silver bulb and the air may be
exhausted. If an electrical oscillation is sent through this
exceedingly fine platinum wire it heats it and rapidly increases its
resistance. The electrical oscillations produced in an aerial are sent
through a number of these loops arranged in parallel, and the loops
are short-circuited by a telephone, joined in series with a source of
very small electromotive force produced by shunting a single cell or
opposing to one another two cells of nearly equal electromotive force.
Any variation of resistance of the little platinum loops due to the
heat produced by the oscillations, by suddenly altering the current
flowing through the telephone, will cause a sound to be heard in it.
The electrical oscillations when passing through the loops are
therefore detected by the heat which they generate in these
exquisitely fine platinum wires.

Finally, one word must be said on the subject of electrodynamic
receivers, due to the same inventor. An exceedingly small silver ring
is suspended by a quartz fibre and has a mirror attached to it in the
manner of a galvanometer. This ring is suspended between two coils
joined in series, which are placed either in the circuit of the aerial
or in the secondary circuit of the small air core transformer inserted
between the aerial and the earth. When electrical oscillations travel
down the aerial they induce other electrical oscillations in the
silver ring, and if the ring is so placed that its normal position is
with its plane inclined at an angle of forty-five degrees to the plane
of the fixed coils, then the ring will be slightly deflected every
time an oscillation occurs in the aerial.

Omitting further mention of the details of the kumascopes in use and
the receiving aerial, we must next proceed to consider the receiving
arrangements taken as a whole.

In the original Marconi system, the sensitive tube or coherer was
inserted between the bottom of the receiving aerial and the earth.[56]
Accordingly, when the incident electric wave strikes the receiving
aerial and creates in it an oscillatory electromotive force, this last
will, if of sufficient amplitude, cause the particles of the coherer
to cohere and become conductive. This sudden change from a nearly
perfect non-conductivity to a conductive condition is made to act as a
switch or relay, closing or completing the circuit of a single cell,
and so sending a current through an ordinary telegraphic relay,
closing or completing the circuit of a single cell, which may in turn
actuate another recording telegraphic instrument, such as a Morse
printer. To prevent the oscillations from passing into the relay
circuit, small choking or inductance coils are inserted between the
ends of the sensitive tube and the relay and cell and serve to confine
the oscillations to the tube.

It has already been pointed out that in the transmitting aerial the
amplitude at the potential vibrations increases from the bottom to the
top, and when vibrating in its fundamental manner there is a potential
node at the earth connection and a potential loop or antinode at the
top. The same is true of the receiving aerial. Hence, if the kumascope
employed is a Branly metallic filings tube and is inserted near the
base of the aerial, the difference of potential between its two ends
will be small.

It has also been mentioned that a receiver of this type acts in virtue
of electromotive force or potential difference, and hence the proper
place to insert the coherer is not at the base of the aerial, but
between the top of the aerial and the earth. This, however, could not
be done by running up another wire from the earth, as that would
amount to putting the coherer between the tops of two identical
aerials, and between its ends there would be no difference of
potential. Professor Slaby, in conjunction with Count von Arco, has
given an ingenious solution of this problem. If we take two equal
lengths of wire, bent at right angles, and connect the point of
intersection with the earth, placing one of these wires vertically and
the other horizontally, we then have an arrangement which responds to
the impact of electric waves, and has electrical oscillations set up
in it in such fashion that the common point of the two wires has a
very small amplitude of potential, but the two extremities have equal
and large variations. If, then, we insert a coherer tube between the
earth and the outer extremity of the horizontal wire, it is influenced
in the same manner as it would be by the potential variations at the
top of the vertical wire. In other words, it is acted upon by a large
difference of potential instead of a small one. It is not found
necessary to stretch the horizontal wire out straight; it may be
coiled into a spiral with open turns, and the slight decrease in
capacity and increase in inductance resulting from this can be
compensated by cutting off a short piece of it.

[Illustration: FIG. 19.--SLABY RECEIVER. A, aerial; E, earth plate; F,
coherer; M, multiplier; C, condenser; R, relay; B, battery; E, earth
plate.]

In this way we have an arrangement (see Fig. 19) in which the outer
extremity of this open spiral experiences variations or potential which
exactly correspond with those at the summit of the vertical aerial. The
receiving arrangements are then completed as in Fig. 19, one end of the
coherer being attached to the outer end of the spiral and the other end
through a condenser to the earth, a relay and a voltaic cell being
arranged as shown in the diagram. The mode of operation of this receiver
is as follows: When the wave strikes the aerial it sets up in it
electrical oscillations with a potential antinode at the summit, and at
the same time a potential antinode is created at the outer end of the
spiral attached near the base of the aerial, this spiral being called by
Professor Slaby a _multiplicator_. As long as the coherer tube remains
non-conductive, the local cell cannot send a current through the relay,
but, as soon as the resistance is broken down by the impact of a wave,
the local cell sends a current through the coherer tube which, passing
down to the earth through the base of the aerial and up through the
earth connection to the condenser, completes its circuit through the
relay. Many variations of this arrangement have been made by Slaby and
Von Arco and by the Allgemeine Elektricitäts Gesellschaft of Berlin.

In 1898, Mr. Marconi made a great advance in the construction of his
receiving apparatus by the insertion of his "jigger" or oscillation
transformer in the aerial receiving circuit.[57] In this arrangement,
the primary coil of an air core transformer wound in a particular way
is inserted between the receiving aerial and the earth, and the
secondary circuit is cut in the middle and connected to the two
surfaces of a condenser, these surfaces being also connected through
the circuit of an ordinary telegraphic relay and a single cell (see
Fig. 20). The ends of the secondary circuit of this oscillation
transformer are also connected to the terminals of the coherer tube,
and these again are short-circuited by a small condenser.

[Illustration: FIG. 20.--MARCONI RECEIVER. A, aerial; J, jigger; CC,
condensers; F, filings tube; T, tapper; R, relay; B, battery; M, Morse
printer.]

The operation of this receiver is as follows: The oscillations set up
in the aerial pass through the primary circuit of the jigger, and
these induce other oscillations in the secondary circuit; the
electromotive force or difference of potential between the primary
terminals being transformed up in any desired ratio. It is this
exalted electromotive force which is made to act on the coherer tube,
and, inasmuch as the jigger operates in virtue of a current passing
through its primary circuit and this current is at a maximum at the
lower end of the aerial, the arrangement is exceedingly effective,
because it, so to speak, converts current into voltage. At the lower
end of the aerial, although the amplitude of the potential
oscillations is a minimum, the amplitude of the current oscillations
is a maximum, and the jigger transforms these large current
oscillations into large potential oscillations, _provided it is
constructed in the right manner_. We can also transform up or increase
the amplitude of the small potential variations near the bottom of the
aerial by employing the principle of resonance. Many devices of this
kind, due to Professor Slaby and others, have been suggested and tried
but the details are rather too technical to be fully described here.

It will be noticed that the receiving aerial may be arranged in one of
two ways--it may be either earthed at the lower end or it may be
insulated. It has been claimed that there is a great advantage in
earthing the receiving aerial directly in that it eliminates
atmospheric disturbances.

We shall allude to this point more particularly later on. Meanwhile it
may be mentioned that the receiving arrangements, as a whole,
constitute a sensitive arrangement, as shown by Popoff, Tommasina and
by all the large experience of Mr. Marconi himself for detecting
changes in the electrical condition of the atmosphere, which are
doubtless of the nature of electrical oscillations. On the other hand,
the receiving arrangements may be perfectly insulated and some
experimentalists have asserted that by this method the greatest
freedom is secured from atmospheric disturbances. Amongst the
non-earthed arrangements the system invented by Professor F. Braun, of
Strassburg, and worked by Messrs. Siemens, of Berlin, may be
mentioned.[58]

[Illustration: FIG. 21.--BRAUN'S NON-EARTHED RECEIVER. I, induction
coil; CC, condensers; S, spark gap; J, transmitting jigger; K,
receiving jigger; F, filings tube; R, relay; B, battery.]

Professor Braun's arrangements are indicated in the diagram in Fig.
21. In this case an induction coil is used to create a discharge
between two spark balls, and to these two balls are connected the two
outer coatings of two condensers, the inner coatings of which are
connected together through the primary coil of an air core
transformer. The secondary coil of this transformer is connected to
two extension wires forming a Hertz resonator, and the length of these
wires is so adjusted with reference to the time period of the primary
circuit that they resonate to it, the whole length from end to end of
the secondary circuit being half a wave-length. The receiver, as shown
in the diagram, consists of a pair of quarter wave-length receiving
wires connected through two condensers, which are short-circuited by
the primary coil of an oscillation transformer. The secondary circuit
of this last oscillation transformer has two extension wires to it,
turned in the same manner, to respond to the primary oscillator; and
in the circuit of one of these extension wires is placed a coherer
tube, short-circuited by a relay and a local battery.

It will thus be seen that there is an entire abolition of ground
connection, which, Professor Braun claims, practically avoids all
atmospheric disturbances.[59] The details of the receiving arrangement
are as follows:--The coherer tube consists of an ebonite tube
containing hard steel particles of a uniform size, placed in the
adjustable space between two polished steel electrodes. It is found
that with this steel coherer, a small amount of magnetism in the
particles increases its sensitiveness, and to obtain this, a ring
magnet is employed in connection with a coherer tube. Receiving
apparatus arranged on this system is said to have been used for
telegraphing between Heligoland and Cuxhaven, a distance of thirty-six
miles.

All the immense experience, however, gained by Mr. Marconi and those
who have worked with his system, is in favour of using the earth
connection. There is no doubt that Hertzian wave telegraphy can be
conducted over short distances by means of totally insulated aerials,
but for long distances the earth connection is essential, for the
reasons that have been explained previously.

There are many of the details of the receiving arrangements which
remain to be considered. If the communication is received by a
telegraphic instrument like the Morse printer, which requires a
current of anything like ten milliamperes to work it, then an
important element in the receiving arrangement is the relay. The relay
that is generally used is a modified form of the Siemens polarised
relay, which is so adjusted as to make a single contact. For marine
work on board ship, it is essential that this relay shall be balanced
so that variations in position shall not affect it. Sometimes the
relay is hung in gimbals like a compass, and at other times suspended
from a support by elastic bands, so as to avoid jolting. In any case,
the relay must be so adjusted that no change of position will cause it
to close the circuit of the telegraphic printer or recorder. Its
sensibility ought to be such that it is actuated by a tenth of a
milliampere, and, if possible, even by less. The alteration of
sensibility in the ordinary contact form of relay is the pressure that
is necessary to bring the platinum points of the circuit closer
together, so as to pass the minimum current which will work the
telegraph printer.

The important matter, however, in connection with the use of the relay
in Hertzian wave telegraphy, is that it should be capable of
adjustment without extraordinary skill. It is no use to put into the
hands of an operator a relay which requires abnormal dexterity to make
it work at all.

       *       *       *       *       *

It remains, then, to consider some of the questions connected with
practical Hertzian wave telegraphy and the problem of the limitation
of communication. These matters at the present moment very much occupy
the public attention, and many conflicting opinions are expressed
concerning them.

It may be observed at the outset that the difficulty of dealing with
the subject as freely as many desire is that Hertzian wave telegraphy
is no longer merely a subject of scientific investigation, but has
developed into a business and involves, therefore, other interests
than the simple advancement of scientific knowledge. We can, however,
discuss in a general manner some of the scientific problems which
present themselves for solution. The first of these is the
independence of communication between stations. It is desirable, at
the outset, to clear up a little misunderstanding. There is a great
difference between preventing the reception of communication when it
is not desired by the recipient, and preventing it when it is the
object of the latter to overhear if he can. It is, therefore,
necessary to distinguish between isolation and overhearing. We may say
that a station is _isolated_ when it is not affected by Hertzian waves
other than those it desires to receive; but that a station _overhears_
when it can, if it chooses, pick up communications not intended for
it, or cannot help receiving them against its will.

This distinction is a perfectly fair one. Any telegraph or telephone
wire can be tapped, if it is desired, but unless there is some fault
on the line, no station will receive a message against its desires.
Moreover, it may be noted that there are penalties attached to tapping
a telegraph wire, and at present there are none connected with the
misappropriation of an ether wave.

We shall, therefore, consider in the first place the methods so far
proposed for preventing any given receiver from being affected by
Hertzian waves sent out from other stations, except that of those from
which it is desired to receive them. The first method is that which
has been called the method of _electrical syntony_, and consists in
adjusting the electrical capacity and inductance of the various open
and closed circuits of the receiving and transmitting stations to be
put in communication so that they have the same electrical
time-period.[60]

In the Cantor Lectures before the Society of Arts in 1900, on
electrical oscillations and electric waves, the author has discussed
at length the conditions under which powerful electrical oscillations
can be set up in a circuit. It was there shown that every electric
circuit having capacity and inductance has a particular or natural
time-period of electrical oscillation depending on the product of
these qualities, and that, to accumulate powerful electrical
oscillations in it, the electromotive impulses on it must be delivered
at this rate. Illustrations were drawn from mechanics, such as the
examples furnished by vibrating pendulums and springs, and from
acoustics, as illustrated by the phenomena of resonance, to show that
small or feeble blows or impulses delivered at the proper time
intervals have a cumulative effect in setting up vibrations in a body
capable of oscillation. It is a familiar fact that if we time our
blows, we can achieve that which no single blow, however powerful, can
accomplish in throwing into vibration a body such as a pendulum, which
is capable of oscillation under the action of a restoring force.
Precisely the same is true of an electric circuit. We have already
seen that the receiving aerial has an alternating electromotive force
set up in it by the impact of the successive electric waves sent out
from the transmitter. It must, however, be remembered that the
transmitter sends out a series of trains of waves, not by any means a
continuous train, but one cut up into groups of probably ten to fifty
waves, each separated by intervals of silence, long, compared with the
duration of a single train of waves.

[Illustration: FIG. 22.--SEIBT'S APPARATUS FOR EXHIBITING ELECTRIC
RESONANCE. I, induction coil; S, spark gap; CC, condensers; L,
variable inductance; E, earth plate; WW, wire spirals; VV, vacuum
tubes.]

If, however, by a suitable adjustment of capacity and inductance, we
make the natural time-period of oscillation of the receiving aerial
circuits agree with those of the transmitting aerial, within certain
limits the former will only be receptive for waves of the frequency
sent out by the transmitter. It is quite easy to illustrate this
principle by numerous experiments. It can be done by means of an
apparatus devised by Dr. Georg Seibt for showing in an interesting
manner the syntonisation or tuning of two electric circuits. This
consists of two bobbins, each consisting of one layer of insulated
wire wound on a wooden rod (see Fig. 22). Each of these bobbins has a
certain electrical capacity with respect to the earth, when considered
as an insulated conductor, and it has also a certain inductance. If,
therefore, electromotive impulses are applied to one end of the bobbin
at regular intervals, electrical oscillations will be set up in it,
and, as already explained, if these are timed at a certain rate, the
bobbin will act like a closed organ-pipe to air impulses and
oscillations of potential will be accumulated at the opposite end,
which have much greater amplitude than the impressed oscillations at
the end at which they are applied. We can make the existence of the
amplitude oscillations of potential evident by attaching to one end of
the bobbin a vacuum tube, which will be illuminated thereby, or by
terminating it by a pointed piece of wire, so that an electrical brush
can be formed at the point, if the potential variations have
sufficient amplitude. We arrange also another closed oscillation
circuit, consisting of two Leyden jars and a variable inductance coil
and a pair of spark balls which are connected to an induction coil. In
this manner we can set up oscillations in the discharge circuit of
these Leyden jars, and we can vary the time period by altering the
inductance and the capacity. If we denote the capacity of the jars in
the microfarads by the letter C and the inductance in centimetres of
the discharge circuit of the jars by the letter L, it can then be
shown that the number of oscillations per second denoted by _n_ is
given by the expression--[61]

    n = (5,000,000,000) / ([\sq]{CL}).

If now we adjust the Leyden jar circuit to a particular rate of
oscillation, we have between the terminals of the jar or condenser an
alternating difference of potential or electromotive force. If we
connect one side of the jars to the earth and the other side to the
foot of one of the spirals or bobbins above described, we shall find
perhaps that the vacuum tube at the other end is not rendered
luminous. When, however, we adjust the inductance in the discharge
circuit of the jar to a certain value to make the frequency of the
condenser oscillations agree with the natural time period of the
bobbin terminated by the vacuum tube, this latter at once lights up
brilliantly. Again, if we connect both these bobbins at the same time
to the discharge circuit of the Leyden jar, we shall find that we can
make an adjustment of the inductance of that circuit, such that either
of the bobbins at pleasure can be made to respond and be set in
electrical vibration, as shown by the illumination of the vacuum tube
at its upper end or by an electrical brush being formed at the
terminal. In making this adjustment of inductance, we are _tuning_, as
it is called, the Leyden jar discharge circuit to the resonating
bobbin. A very small variation of the inductance of the jar circuit
causes the vacuum tube to change in luminosity. If, however, the
natural time periods of these bobbins do not lie very far apart, then
a faint luminosity will make its appearance in both the vacuum tubes.
Supposing, therefore, that we connect to the oscillating circuit of
the jar a number of bobbins having different time periods of
oscillation, like organ-pipes, and supply them all with one common
alternating electromotive force. These bobbins, whose natural time
period is very different to that of the osciilating circuit or
impressed electromotive force, will not respond, but those bobbins of
which the natural time period lies near to, even if not quite exactly
the same as, that of the impressed electromotive force will give
evidence of being set in oscillation. A very violent electromotive
force will cause them all to respond to some slight extent, no matter
whether the period of that impluse is tuned to their common period
precisely or not.

At this point questions arise of great practical importance. A matter
which has been in dispute in connection with practical Hertzian wave
telegraphy is how far this electrical tuning is a sufficient solution
of the practical problem of isolation. It is not denied that
experiments such as those made with Seibt's apparatus can be shown on
a small scale; and, on a still larger scale, Mr. Marconi gave to the
author in September, 1900, a demonstration in practical telegraphic
work of sending two independent Hertzian wave messages and receiving
them on two independent receivers attached to the same aerial.

Since that date much experience has been gained and large power
stations erected, and a statement has been frequently made that
syntony is no protection against interference when one of the stations
is sending out very powerful waves. The contention has been raised
that large power stations producing electric waves will therefore play
havoc with Hertzian wave telegraphy on a smaller scale, such as the
ship to shore and intermarine communication. Under these
circumstances, it appeared to the author important to subject the
matter to a special test, and Mr. Marconi, therefore, offered to give
a demonstration, with this object, in support of the opinion that he
has expressed positively that waves from his power stations do not
interfere with the working of his ship installations. This matter is
vital to the whole question of practical Hertzian wave telegraphy, for
the ship to shore communication is of stupendous importance; and if
Mr. Marconi had done nothing else except to render this possible and
effective, he would have earned, as he has done, the gratitude of
humanity for all time. Accordingly, the author embraced the
opportunity of making some careful tests to settle the question
whether the powerful waves sent out from a station such as Poldhu did
or did not affect the exchange of messages between ship and shore
stations in proximity, equipped with Marconi apparatus of a suitable
type.

These experiments were carried out on the eighteenth of March last, at
Poldhu, in Cornwall, and a programme was arranged by the author of the
following kind. Close to the Poldhu station is an isolated mast, which
was equipped by Mr. Marconi with a Hertzian wave apparatus, similar to
that he places on ships. Six miles from Poldhu is the Lizard receiving
station, with which ships proceeding up or down the English Channel
communicate. It was arranged that a series of secret messages, some of
them in cipher, should be delivered simultaneously at certain known
times, both to the power station at Poldhu and to the small adjacent
ship station; and it was arranged that these messages should be sent
off simultaneously, the operators being kept in ignorance up to the
moment of sending as to the nature of the messages. At the Lizard, Mr.
Marconi connected two of his receiving instruments to the aerial, one
of them tuned to the waves proceeding from the power station at
Poldhu, and the other to those proceeding from the small ship station.
At the appointed time, these two sets of messages were received
simultaneously in the presence of the author, each message being
printed down independently on its own receiver; and Mr. Marconi read
off and interpreted all these messages perfectly correctly, not having
known before what was the message that was about to be sent. In
addition to this, precautions were taken to prove that the power
station at Poldhu was really emitting waves sufficiently powerful to
cross the Atlantic and not being made to sing small for the occasion.
To assist in proving this, the messages sent out from the power
station were also received at a station at Poole, two hundred miles
away, and the assistant there was instructed to telegraph back these
messages by wire as soon as he received them. These messages came back
perfectly correctly, thus demonstrating that the power station was
sending out power waves. The whole programme was carried out with the
greatest care to avoid any mistakes on the part of the assistants, and
provided an absolute demonstration of the truth of Mr. Marconi's
assertion that the waves from one of his power stations, such as
Poldhu, do not in the least degree interfere with the transmission and
reception of messages between ship and shore, effected by means of
certain forms of Marconi apparatus for producing and detecting waves
of a different wave length.[62] This complete independence of
transmission, however, is entirely due to the employment of a
receiving circuit of a certain type in Mr. Marconi's receivers. It
does not at all follow that a receiving circuit of any kind, even a
Marconi receiver not especially arranged, set up in proximity to a
power station would not be affected. This, however, is not an
important matter. Far more important is it to show, as has been shown,
that practically perfect isolation can be achieved if it is desired.

It must be noted, however, that, although the fact that electric
circuits have a natural time-period of oscillation of their own is a
scientific principle which carries us a considerable way towards a
solution of what is called syntonic Hertzian wave telegraphy, it is
not in itself alone in every respect an entire solution of the
practical problem. The degree to which it is a solution depends to a
considerable extent upon the nature of the detecting device, or
kumascope, which we are employing. The coherer, or Branly filings
tube, has the peculiarity that its passage from a non-conductive to a
conductive condition follows immediately when the difference of
potential between its ends is made sufficiently great. In other words,
if the tube is acted upon by a sufficient electromotive force, it is
not necessary that electromotive force should be repeated at intervals
to make this particular form of kumascope responsive. Again, if we
consider the nature of the oscillations which are sent out from any
transmitting aerial, we find that each group of oscillations
corresponding to a single spark consists of waves gradually decreasing
in amplitude. In other words, the first wave of the group is the
strongest, and the decay in amplitude is often very rapid. Supposing,
then, we construct a simple receiver consisting of an aerial having
inserted in its circuit a sensitive Branly filings tube. Such a
receiver is almost entirely non-syntonic; that is to say, it is
affected by any wave passing over it which is sufficiently powerful.
We may look upon it that if the first wave of the series is
sufficiently powerful to affect the kumascope, the conductive change
takes place whether or not the first wave is followed by others.
Accordingly, it is perfectly certain that if a transmitter is sending
out trains of waves of any period, a simple combination of coherer and
aerial will be influenced, if it is placed near enough to the
transmitter. On the other hand, it is possible to combine a kumascope
of a certain type with a receiving aerial and other circuits in such a
manner that when the waves that reach it are feeble it shall not
respond at all unless those waves have very nearly a time period of a
certain value.

At this stage, it may be perhaps well to explain a little in detail
what is meant by an easily responsive circuit, and, on the other hand,
by an irresponsive circuit, or, as we may call it, a _stiff_ circuit.
Supposing that we consider an aerial consisting of a simple straight
wire having small capacity and small inductance, such a circuit admits
of being sent into electrical oscillation, not only by waves of its
own natural time-period, but by the sudden application of any violent
electromotive impulse. If, on the other hand, we bestow upon the
circuit in any way considerable inductance, we then obtain what may be
called a stiff or irresponsive circuit, which is one in which
electrical oscillations can be accumulated only by the prolonged
action of impulses tuned to a particular period.

A mechanical analogue of this difference may be found in considering
the different behaviour of elastic bodies to mechanical blows. Take,
for instance, a piece of elastic steel and fix the bottom end in a
vice. The steel strip may be thrown into vibration by deflecting the
upper end. It has, however, a very small mass, and therefore any
violent blow or blows, even although not repeated, will set it in
oscillation. If, however, we add mass to it by fixing at the other end
a heavy weight, such as a ball of lead, and at the same time make the
spring stiffer, we have an arrangement which is capable of being sent
into considerable oscillation only by the action of a series of
impulses or blows which are timed at a particular rate.

Returning then to the electrical problem, we see that in order to
preserve a kumascope or wave detector from being operated on by any
vagrant wave or waves having a period very different to an assigned
period, it must be associated with an electrical circuit of the kind
above called a stiff circuit.

We will now consider the manner in which the problem has been
practically attacked by Mr. Marconi, Dr. Slaby, Sir Oliver Lodge and
others, who have invented forms of receiver and transmitter, which are
syntonic or sympathetic to one another.

Some of the methods which Mr. Marconi has devised for the achievement
of syntonic wireless telegraphy were fully described by him in a Paper
read before the Society of Arts on May, 17, 1901.[63]

[Illustration: FIG. 23.--MARCONI TRANSMITTER AND RECEIVER. I,
induction coil; A, aerial; E, earth plate; HH, choking coils; S, spark
gap; J, transmitting jigger; K, receiving jigger; R, relay; C,
condenser; F, filings tube; B, battery. Many practical details are
omitted.]

On referring to his Paper, it will be seen that in one form his
transmitter consists of an aerial, near the base of which is inserted
the secondary circuit of an oscillation transformer or transmitting
jigger. One end of this secondary circuit is attached to the aerial
and the other end is connected to the earth through a variable
inductance coil. The primary circuit of this oscillation transformer
is connected in series with a condenser, consisting of a battery of
Leyden jars, and the two together are connected across to the spark
balls which close the secondary circuit of an induction coil, having
the usual make and break key in the primary circuit. Mr. Marconi so
adjusts the induction of the aerial and the capacity of the condenser,
or battery of Leyden jars, that the two circuits, consisting
respectively of this battery of Leyden jars and the primary circuit of
the transformer, and on the other hand of the capacity of the aerial
and the inductance in series with it, and that of the secondary
circuit of the transformer have the same time period. In other words,
these two inductive circuits are tuned together. At the receiving end,
the aerial is connected in series with a variable inductance and with
the primary circuit of another oscillation transformer, the second
terminal of which is connected to the earth. The secondary circuit of
this last oscillation transformer is cut in the middle and is
connected to the terminals of a small condenser. The outer terminals
of this secondary circuit are connected to the metallic filings tube
or other sensitive receiver and to a small condenser in parallel with
it (see Fig. 23). The terminals of the condenser which is inserted in
the middle of the secondary circuit of the oscillation transformer are
connected through two small inductance coils with a relay and a single
cell. This relay in turn actuates a Morse printer by means of a local
battery. The two circuits of the oscillation transformer are tuned or
syntonised to one another, and also to the similar circuit of the
transmitting arrangement. When this is the case, the transmitter
affects the co-resonant receiving arrangement, but will not affect any
other similar arrangement, unless it is within a certain minimum range
of distance. Owing to the inductance of the oscillation transformer
forming part of the receiving arrangements, the receiving circuit is,
as before stated, very stiff or irresponsive; the sensitive tube is
therefore not acted upon in virtue merely of the impact of the single
wave against the aerial, but it needs repeated or accumulated effects
of a great many waves, coming in proper time, to break down the
coherer and cause the recording mechanism to act. An inspection of the
diagram will show that as soon as the secondary electromotive force in
the small oscillation transformer or jigger of the receiving
instrument is of sufficient amplitude to break down the resistance of
the coherer, the local cell in circuit with the relay can send a
current through it and cause the relay to act and in turn make the
associated telegraphic instrument record or sound.

Mr. Marconi described in the above-mentioned Paper some other
arrangements for achieving the same result, but those mentioned all
depend for their operation upon the construction of a receiving
circuit on which the time-period of electrical oscillations is
identical with that of a transmitting arrangement. By this means he
showed experiments during the reading of his Paper, illustrating the
fact that two pairs of transmitting and receiving arrangements could
be so syntonised that each receiver responded only to its particular
transmitter and not to the other.

With arrangements of substantially the same nature, he made
experiments in the autumn of 1900 between Niton, in the Isle of Wight,
and Bournemouth, a distance of about thirty miles, in which
independent messages were sent and received on the same aerial.

Dr. Slaby and Count von Arco, working in Germany, have followed very
much on the same lines as Mr. Marconi, though with appliances of a
somewhat different nature. As constructed by the General Electric
Company, of Berlin, the Slaby-Arco syntonic system of Hertzian
telegraphy is arranged in one form as follows:--The transmitter
consists of a vertical rod like a lightning conductor, say, 100 or 150
feet in height. At a point six or nine feet above the ground, a
connection is made to a spark ball (see Fig. 24), and the
corresponding ball is connected through a variable inductance with one
terminal of a condenser, the other terminal of which is connected to
the earth. The two spark balls are connected to an induction coil, or
alternating current transformer, and by variation of the inductance
and capacity the frequency is so arranged that the wave-length
corresponding to it is equal to four times the length of that portion
of the aerial which is above the spark ball connection. The method by
which this tuning is achieved is to insert in the portion of the
aerial below the spark balls, between it and the earth, a hot wire
ammeter of some form. It has already been shown that in the case of
such an earthed aerial, when electrical oscillations are set up in it,
there is a potential node at the earth and a potential anti-node or
loop at the summit, if it is vibrating in its fundamental manner;
also, there is a node of current at the summit of the aerial and an
anti-node at the base. This amounts to saying that the amplitude of
the potential vibrations is greatest at the top end of the aerial, and
the amplitude of the current vibrations is greatest at the bottom or
earthed end. Accordingly, the inductance and capacity of the lateral
branch of the transmitter is altered until the hot wire ammeter in the
base of the aerial shows the largest possible current.

[Illustration: FIG. 24.--SLABY-ARCO SYNTONIC TRANSMITTER AND RECEIVER.
I, induction coil; M, multiplier; B, battery; A, aerial; F, filings
tube; R, relay; E, earth plate; C, condenser.]

The corresponding receiver is constructed in a very similar manner. A
lightning conductor or long vertical rod of the same height as the
transmitting aerial is set up at the receiving station, and at a point
six or nine feet from the ground a circuit is taken off, consisting of
a wire loosely coiled in a spiral, the length of which is nearly equal
to, although a little shorter than, the height of the vertical wire
above the point of connection. The outer end of this loose spiral is
connected to one terminal of the coherer tube, and the other terminal
of the coherer is connected to the earth through a condenser of rather
large capacity. The terminals of this last condenser are
short-circuited by a relay and a single cell. When the adjustments are
properly made, it is claimed that the receiver responds only to waves
coming from its own syntonised or tuned transmitter. In this case the
length of the receiving aerial above the point of junction with the
coherer circuit is one quarter the length of the wave. A variation of
the above arangements consists in making this lateral circuit equal in
length to one-half of a wave, and connecting the coherer to its centre
through a condenser to the earth. The outer end of this lateral
circuit is also connected to the earth (see Fig. 24).[64]

Dr. Slaby claims that this arrangement is not affected by atmospheric
electricity, and that the complete and direct earthing of the aerial
and also in the second arrangement, of the receiver of the outer end
of the lateral conductor, conduces to preserve the receiver immune
from any electrical disturbances except those having a period to which
it is tuned.

[Illustration: FIG. 25.--LODGE-MUIRHEAD SYNTONIC RECEIVER. I,
induction coil; S, spark gap; A, aerial; CC, condensers; E, earth
plate; R, relay; L, variable inductance; F, filings tube; B, battery.]

A method has also been arranged by him for receiving on the same
aerial two messages from different transmitting stations
simultaneously. In this case, two lateral wires of different lengths
are connected to the receiving aerial, and to the outer end of each of
these is connected a coherer tube, the other end of which is earthed
through a condenser. One of these lateral wires is made equal, or
nearly equal, in length to the aerial, and the other is made longer to
fulfil the following condition.[65] If we call H the height of the
receiving aerial above point of junction of the lateral wires, then
the length of one lateral wire is made equal to H, and the height of
the aerial is adjusted to be equal to one-quarter of the wave length
of one incident wave. The other lateral wire may then be made of a
length equal to one-third of H, and it will then respond to the first
odd harmonic of that wave, of which the fundamental is in syntony with
the vertical wire. By suitably choosing the relation between the
wave-lengths of the two transmitting stations, it is possible to
receive in this manner two different messages at the same time on the
same aerial. Subsequently to the date of the above-mentioned
demonstration of multiplex wireless telegraphy by Mr. Marconi an
exhibition of a similar nature was given by Professor Slaby in a
lecture given in Berlin on December 22, 1900.[66]

Both the above-described syntonic systems of Mr. Marconi and Dr. Slaby
are "earthed" systems, but arrangements for syntonic telegraphy have
been devised by Sir Oliver Lodge and Professor Braun which are
"non-earthed."

Sir Oliver Lodge and Dr. Muirhead have devised also syntonic systems.
According to their last methods, the systonic transmitting and
receiving arrangements are as shown in Fig. 25.[67] On examining the
diagrams it will be seen that the secondary terminals of the induction
coil are, as usual, connected to a pair of spark balls, and that these
spark balls are connected by a condenser and by a variable inductance.
One terminal of the condenser is earthed through another condenser of
large capacity, and the remaining terminal of the first condenser is
connected to an aerial. It should, therefore, be borne in mind in
dealing with electrical oscillations that a condenser of sufficient
capacity is practically a conductor, and an inductance coil of
sufficient inductance is practically a non-conductor. Hence the
insertion of a large capacity in the path of the aerial wire is no
advantage whatever and makes no essential difference in the
arrangement. In order to obtain any powerful radiation, the length of
the aerial, or sky wire, as they call it, must be so adjusted that its
length is one-quarter the wave-length corresponding to the oscillation
circuit, consisting of the condenser and variable inductance.

The receiving arrangement consists of a similar sky wire or aerial
earthed through a condenser of large capacity and having in the
portion above this last condenser another condenser of similar
capacity. At the earthed side of this last condenser a connection is
made to a resonant circuit, consisting of a variable inductance, and
another condenser and a sensitive metallic filings tube of the Branly
type; also a portion of this resonant circuit is shunted by another
consisting of a battery and telegraphic relay, as shown in the
diagram. The circuit, including the coherer, is tuned to its own
aerial and also to that of the transmitting circuit, and under these
circumstances trains of waves thrown off at the transmitting aerial
will sympathetically affect the receiving aerial.

There is nothing in the arrangement which specially calls for notice.
It is simply a variation of other known forms of syntonic transmitter
and receiver, and possesses all the advantages and disadvantages
attaching to such electrical syntonic methods.

Professor Braun's syntonic system, the receiver and transmitter of
which have been described, is also in one form a non-earthed system.
Innumerable other patentees have taken out patents for devices which
are modifications in small degree of the above arrangements.

It may be well to note at this point the disadvantages that are
possessed by any form of coherer as a telegraphic kumascope in
connection with proposed arrangements for the isolation of Hertzian
wave stations. All the detectors of the coherer type really depend for
their actuation upon electromotive force; that is to say, upon the
application to the terminals of the detector of a certain
electromotive force. Although there may be no sharp and defined
critical electromotive force, yet, nevertheless, as a matter of fact,
if the electromotive force applied exceeds a certain value, then the
detector passes suddenly from one state of conductivity to another. It
may be of great conductivity, as in the case of the Branly coherer, or
of lesser conductivity, as in the case of the so-called anti-coherers,
of which the Schäfer kumascope may be taken as a type. Accordingly,
when these instruments are subjected to a train of waves, each
individual group of which is damped, their operation is largely
governed by the fact that if the first wave or oscillation set up in
the receiving circuit is powerful enough to break down the coherer,
then the receiving mechanism acts, no matter whether the first impulse
is followed by others or not.

In comparison with so-called coherers, those depending upon the
changes in the magnetisation of iron by electrical oscillations
certainly have an advantage, because this is a process which requires
the application of alternating electric currents decreasing in
strength for a certain time; and it is found, therefore, that the
magnetic receivers do not require to be associated with such a stiff
or irresponsive resonant circuit to confine their indications to
oscillations or waves of one definite period, and that they lend
themselves much more perfectly to the work of "tuning" or syntonising
stations than do those kumascopes depending upon the contact or
coherer principle.

We may then glance at the alternative solutions of the problem offered
by other investigators. M. Blondel has proposed to effect the
syntonisation of two stations, not by syntonising the receiver for the
exceedingly high-frequency oscillations of the individual electric
waves, but to syntonise it for the much lower frequency, corresponding
to that of the intervals between the groups of waves. Thus, for
instance, if an ordinary simple transmitting aerial is set up, the
production of sparks between the spark balls results in the emission
of short trains of waves, each of which may consist of half a dozen or
more individual waves, the time of production of the whole group being
very small compared with the interval between the groups. M. Blondel
proposes, however, to syntonise the receiver, not for the
high-frequency period of the waves themselves, which may be reckoned
in millions per second, but for the low-frequency period between the
groups of waves, which is reckoned in hundreds per second. Thus, for
instance, if sparks are made at the rate of fifty or a hundred per
second, they can be made to actuate the telephone receiver and so
produce in the telephone a sound corresponding to a frequency of 50 or
100; in other words, to make a low musical note or hum. This
continuous sound can be cut up, by means of a key placed in the
primary circuit of the transmitting arrangement, into long or short
periods, and hence the letters of the alphabet signal.

M. Blondel's arrangements comprise a Mercadier's monotone telephone
and either a coherer or a particular form of vacuum tube as a
kumascope. On August 16, 1898, M. Blondel deposited with the Academy
of Sciences in Paris a sealed envelope containing a description of his
improvements in syntonic wireless telegraphy, which was opened on May
19, 1900.[68] The arrangement of the receiving apparatus was as
follows:--A single-battery cell keeps a condenser charged until the
kumascope is rendered conductive by the oscillations coming down the
aerial; and under these circumstances the condenser discharges through
the telephone and causes a tick to be heard in it. If the trains of
waves are at the rate of 50 or 100 per second, these small sounds run
together into a musical note, and this continuous hum can be cut up
into long and short spaces, in accordance with the Morse alphabet
signals. The telephone must not be an ordinary telephone, capable of
being influenced by any frequency, but be one which responds only to a
particular note, and under these conditions the receiving arrangement
is receptive only when the trains of waves arrive at certain regular
predetermined intervals, corresponding with the tone to which the
telephone is sensitive.

       *       *       *       *       *

A number of more or less imperfect arrangements, having the isolation
of communications for their object, have been devised or patented,
which are dependent upon the use of several aerials, each supposed to
be responsive only to a particular frequency; and attempts have been
made to solve the problem of isolation by MM. Tommasi, Tesla, Jegon,
Tissot, Ducretet and others.

We may then pass on to notice the attempts that have been made to
secure isolation by a plan which is not dependent on electrical
syntony. One of these, which has the appearance of developing into a
practical solution of the problem, is that due to Anders Bull.[69] In
the first arrangements proposed by this inventor, a receiver is
constructed which is not capable of being acted upon merely by a
single wave or train of waves or even a regularly-spaced train of
electric waves, but only by a group of wave trains which are separated
from one another by certain unequal, predetermined intervals of time.
Thus, for instance, to take a simple instance, the transmitting
arrangements are so devised as to send out groups of electric waves,
these wave trains following one another at time intervals which may be
represented by the numbers 1, 3 and 5; that is to say, the interval
which elapses between the second and third is three times that between
the first two, and the interval between the fourth and fifth is five
times that between the first two. This is achieved by making five
electric oscillatory sparks with a transmitter of the ordinary kind,
the intervals between which are settled by the intervals between holes
punched upon strips of paper, like that used in a Wheatstone automatic
telegraphic instrument. It will easily be understood that by a device
of this kind, groups of sparks can be made, say, five sparks rapidly
succeeding each other, but not at equal intervals of time. One such
group constitutes the Morse dot, and two or three such groups
succeeding one another very quickly constitute the Morse dash. These
waves, on arriving at the receiving station, are caused to actuate a
punching arrangement by the intermediation of a coherer or other
kumascope, and to punch upon a uniformly moving strip of paper holes,
which are at intervals of time corresponding to the intervals between
the sparks at the transmitting station. This strip of paper then
passes through another telegraphic instrument, which is so constructed
that it prints upon another strip a dot or a dash, according to the
disposition of the holes on the first strip. Accordingly, taken as a
whole, the receiving arrangement is not capable of being influenced so
as to print a telegraphic sign except by the operation of a series of
wave trains succeeding one another at certain assigned intervals of
time.

An improvement has been lately described by the same inventor,[70] in
which the apparatus used, although more complicated, performs the same
functions. At each station two instruments have to be employed; at the
transmitting station one to effect the conversion of Morse signals
into the properly arranged series of wave trains, and at the receiving
station an instrument to effect the re-conversion of the series of
wave trains into the Morse signals. These are called respectively the
dispenser and the collector. The details of the arrangements are
somewhat complicated, and can only be described by the aid of numerous
detailed drawings, but the inventor states that he has been able to
carry on Hertzian wave telegraphy by means of these arrangements for
short distances. Moreover, the method lends itself to an arrangement
of multiplex telegraphy, by sending out from different transmitters
signals which are based upon different arrangements of time intervals
between the electric wave trains. Although this method may succeed in
preventing a receiving arrangement from being influenced by vagrant
waves or waves not intended for it, yet an objection which arises is
that there is nothing to prevent any one from intercepting these wave
trains, and with a little skill interpreting their meaning. Thus, if
the record were received in the ordinary way on a simple receiver,
corresponding to a Morse dot would be printed five dots at unequal
intervals, and corresponding to a Morse dash would be printed two such
sets of five dots. A little skill would then enable an operator to
interpret these arbitrary signals. On the other hand, the inventor
asserts that he can overcome this difficulty by making intervals of
time between the impulses in the series so long that the latter become
longer than the intervals between each of the series of waves which
are despatched in continuous succession when the key is pressed for a
dash. In this case, when telegraphing, the series of dots would
overlap and intermingle with each other in a way which would make the
record unintelligible if received in the usual manner, but would be
perfectly legible if received and interpreted by a receiver adapted
for the purpose.

Another way of obliterating the record, as far as outsiders are
concerned, is to interpolate between the groups of signals an
irregular series of dots--_i.e._, of wave trains--which would affect
an ordinary coherer, and so make an unintelligible record on an
ordinary receiver, but these dots are not received or picked up by the
appropriate selecting instrument used in the Anders Bull system.

The matter most interesting to the public at the present time is the
long-distance telegraphy by Hertzian waves to the accomplishment of
which Mr. Marconi has devoted himself with so much energy of late
years. Everyone, except perhaps those whose interests may be
threatened by his achievements, must accord their hearty admiration of
the indomitable perseverance and courage which he has shown in
overcoming the immense difficulties which have presented themselves.
Five years ago he was engaged in sending signals from Alum Bay, in the
Isle of Wight, to Bournemouth, a distance of twelve or fourteen miles;
and to-day he has conquered twice that number of hundred miles and
succeeded in sending, not merely signals, but long messages of all
descriptions over three thousand miles across the Atlantic. Critics
there are in abundance, who declare that the process can never become
a commercial one, that it will destroy short-distance Hertzian
telegraphy, or that the multiplication of long-distance stations will
end in the annihilation of all Hertzian wave telegraphy. No one,
however, can contemplate the history of any development of applied
science without seriously taking to heart the lesson that the
obstacles which arise and which prove serious in any engineering
undertaking are never those which occur to armchair critics. Sometimes
the seemingly impossible proves the most easy to accomplish, whilst
difficulties of a formidable nature often spring up where least
expected.

The long-distance transmission is a matter of peculiar interest to the
author of these articles, because he was at an early stage in
connection with it invited to render Mr. Marconi assistance in the
matter.[71] The particular work entrusted to him was that of planning
the electrical engineering arrangements of the first power station
erected for the production of electric waves for long-distance
Hertzian wave telegraphy at Poldhu, in Cornwall. When Mr. Marconi
returned from the United States in the early part of 1900, he had
arrived at the conclusion that the time had come for a serious attempt
to accomplish wireless telegraphy across the Atlantic. Up to that date
the project had been an inventor's dream, much discussed, long
predicted, but never before practically taken in hand. The only
appliances, moreover, which had been used for creating Hertzian waves
were induction coils or small transformers, and the greatest distance
covered, even by Mr. Marconi himself, had been something like 150
miles over sea. Accordingly, to grapple with the difficulty of
creating an electric wave capable of making itself felt at a distance
of 3,000 miles, even with the delicate receiving appliances invented
by Mr. Marconi, seemed to require the means of producing at least four
hundred times the wave-energy that had been previously employed. The
author was, therefore, requested to prepare plans and specifications
for an electric generating plant for this purpose, which would enable
electrical oscillations to be set up in an aerial on a scale never
before accomplished.

This work involved, not merely the ordinary experience of an
electrical engineer, but also the careful consideration of many new
problems and the construction of devices not before used. Every step
had to be made secure by laboratory experiments before the
responsibility could be incurred of advising on the nature of the
machinery and appliances to be ordered. Many months in the year 1901
were thus occupied by the author in making small-scale experiments in
London and in superintendence of large-scale experiments at the site
of the first power station at Poldhu, near Mullion, in Cornwall,
before the plant was erected and any attempt was made by Mr. Marconi
to commence actual telegraphic experiments. As this work was of a
highly confidential nature, it is obviously impossible to enter into
the details of the arrangements, either as made by the writer in the
first instance, or as they have been subsequently modified by Mr.
Marconi. The design of the aerial and of the oscillation transformers
and many of the details in the working appliances are entirely due to
Mr. Marconi, but as a final result, a power plant was erected for the
production of Hertzian waves on a scale never before attempted. The
utilisation of 50 H.P. or 100 H.P. for electric wave production has
involved dealing with many difficult problems in electrical
engineering, not so much in novelty of general arrangement as in
details. It will easily be understood that Leyden jars, spark balls
and oscillators, which are quite suitable for use with an induction
coil, would be destroyed immediately if employed with a large
alternating-current plant and immensely powerful transformers.

[Illustration: FIG. 26.--WOODEN TOWERS SUPPORTING THE MARCONI AERIAL
AT POLDHU POWER STATION, CORNWALL, ENGLAND.]

In the initial experiments with this machinery and in its first
working there was very considerable risk, owing to its novel and
dangerous nature; but throughout the whole of the work from the very
beginning, no accident of any kind has taken place, so great have been
the precautions taken. The only thing in the nature of a mishap was
the collapse of a ring of tall masts, erected in the first place to
sustain the aerial wires, but which now have been replaced by four
substantial timber towers, 215 feet in height, placed at the corners
of a square, 200 feet in length. These four towers sustain a conical
arrangement of insulated wires (see Fig. 26) which can be used in
sections and which constitute the transmitting radiator or receiver,
as the case may be. Each of these wires is 200 feet in length and
formed of bare stranded wire.

At the outset, there was much uncertainty as to the effect of the
curvature of the earth on the propagation of a Hertzian wave over a
distance of many hundreds of miles. In the case of the Atlantic
transmission between the station at Poldhu in Cornwall and that at
Cape Cod in Massachusetts, U.S.A., we have two stations separated by
about 45 degrees of longitude on a great circle, or one-eighth part of
the circumference of the world. In this case, the versine of the arc
or height of the sea at the half-way point above the straight line or
chord joining the two places is 300 miles.

The question has recently attracted the attention of several eminent
mathematical physicists. The extent to which a free wave propagated in a
medium bends round any object or is diffracted depends on the relation
between the length of the wave and the size of the object. Thus, for
instance, an object the size of an orange held just in front of the
mouth does not perceptibly interfere with the propagation of the waves
produced by the speaking or singing voice, because these are from two to
six feet in length: but if arrangements are made by means of a Galton
whistle to produce air waves half an inch in length, then an obstacle
the size of an orange causes a very distinct acoustic shadow. The same
thing is true of waves in the ether. The amount of bending of light
waves round material objects is exceedingly small, because the average
length of light waves is about one-fifty-thousandth part of an inch. In
the case of Hertzian wave telegraphy, we are, however, dealing with
ether waves many hundreds of feet in length, and the waves sent out from
Poldhu have a wave-length of a thousand feet or more, say, one-fifth to
one-quarter of a mile. The distance, therefore, between Poldhu and Cape
Cod is only at most about twelve thousand wave-lengths, and stands in
the same relation to the length of the Hertzian wave used as does a body
the diameter of a pea to the wave-length of yellow light. There is
unquestionably a large amount of diffraction or bending of the electric
wave round the earth, and, proportionately speaking, it is larger than
in the case of light waves incident on objects of the same relative
size.

Quite recently Mr. H. M. Macdonald (see _Proc._ Roy. Soc., London,
Vol. LXXI., p. 251) has submitted the problem to calculation, and has
shown that the power required to send given electric waves 3,000 miles
along a meridian of the earth is greater than would be required to
send them over the same distance if the sea surface were flat in the
ratio of 10 to 3. Hence the rotundity of the earth does introduce a
very important reduction factor, although it does not inhibit the
transmission. Mr. Macdonald's mathematical argument has, however, been
criticised by Lord Rayleigh and by M. H. Poincaré (see _Proc._ Roy.
Soc., Vol. LXXII., p. 40, 1903).

The accomplishment of very long distances by Hertzian wave telegraphy
is, however, not merely a question of power, it is also a question of
wave-length. Having regard, however, to the possibility that the
propagation which takes place in Hertzian wave telegraphy is not that
simply of a free wave in space, but the transmission of a semi-loop of
electric strain with its feet tethered to the earth, it is quite
possible that if it were worth while to make the attempt, an ether
disturbance could be made in England sufficiently powerful to be felt
in New Zealand.

Leaving, however, these hypothetical questions and matters of pure
conjecture, we may consider some of the facts which have resulted from
Mr. Marconi's long-distance experiments. One of the most interesting
of these is the effect of daylight upon the wave propagation. In one
of his voyages across the Atlantic, when receiving signals from Poldhu
on board the S.S. _Philadelphia_, he noticed that the signals were
received by night when they could not be detected by day.[72] In these
experiments Mr. Marconi instructed his assistants at Poldhu to send
signals at a certain rate from 12 to 1 a.m., from 6 to 7 a.m., from 12
to 1 p.m., and from 6 to 7 p.m., Greenwich mean time, every day for a
week. He has stated that on board the _Philadelphia_ he did not notice
any apparent difference between the signals received in the day and
those received at night until after the vessel had reached a distance
of 500 statute miles from Poldhu. At distances of over 700 miles, the
signals transmitted during the day failed entirely, while those sent
at night remained quite strong up to 1,551 miles, and were clearly
decipherable up to a distance of 2,099 miles from Poldhu. Mr. Marconi
also noted that at distances of over 700 miles, the signals at 6 a.m.,
in the week between February 23 and March 1, were quite clear and
distinct, whereas by 7 a.m. they had become weak almost to total
disappearance. This fact led him at first to conclude that the cause
of the weakening was due to the action of the daylight upon the
transmitting aerial, and that as the sun rose over Poldhu, so the wave
energy radiated, diminished, and he suggested as an explanation the
known fact of the dissipating action of light upon a negative charge.

Although the facts seem to support this view, another explanation may
be suggested. It has been shown by Professor J. J. Thomson that
gaseous ions or electrons can absorb the energy of an electric wave,
if present in a space through which waves are being transmitted.[73]
If it be a fact, as suggested by Professor J. J. Thomson, that the sun
is projecting into space streams of electrons, and if these are
continually falling in a shower upon the earth, in accordance with the
fascinating hypothesis of Professor Arrhenius, then that portion of
the earth's atmosphere which is facing the sun will have present in it
more electrons or gaseous ions than that portion which is turned
towards the dark space, and it will therefore be less transparent to
long Hertzian waves.[74] In other words, clear sunlit air, though
extremely transparent to light waves, acts as if it were a slightly
turbid medium for long Hertzian waves. The dividing line between that
portion of the earth's atmosphere which is impregnated with gaseous
ions or electrons is not sharply delimited from the part not so
illuminated, and there may be, therefore, a considerable penetration
of these ions into the regions which I may call the twilight areas.
Accordingly, as the earth rotates, a district in which Hertzian waves
are being propagated is brought, towards the time of sunrise, into a
position in which the atmosphere begins to be ionised, although far
from as freely as is the case during the hours of bright sunshine.

Mr. Marconi states that he has found a similar effect between inland
stations, signals having been received by him during the night between
Poldhu and Poole with an aerial the height of which was not sufficient
to receive them by day. It has been found, however, that the effect
simply amounts to this, that rather more power is required by day than
by night to send signals by Hertzian waves over long distances.

Some interesting observations have also been made by Captain H. B.
Jackson, R.N.,[75] on the influence of various states of the
atmosphere upon Hertzian wave telegraphy. These experiments were all
made between ships of the British Royal Navy, furnished with Hertzian
wave telegraphy apparatus on the Marconi system. Some of his
observations concerned the effect of the interpositon of land between
two ships. He found that the interposition of land containing iron
ores reduced the signalling distances, compared with the maximum
distance at open sea, to about 30 per cent. of the latter; whilst hard
limestone reduced it to nearly 60 per cent. and soft sandstone or
shale to 70 per cent. These results show that there is a considerable
absorption effect when waves of certain wave-length pass through or
over hard rocks containing iron ores. It would be interesting to know,
however, whether this reduction was in any degree proportional to the
dryness or moisture of the soil. Earth conductivity is far more
dependent upon the presence or absence of moisture than upon the
particular nature of the material which composes it other than water.

The observations of Captain Jackson, however, only confirm the already
well-known fact that Hertzian waves, as employed in the Marconi system
of wireless telegraphy, within a certain range of wave-length, are
considerably weakened by their passage through land, over land or
round land. In some cases he noticed that quite sharp electric shadows
were produced by rocky promontories projecting into the line of
transmission. His attention was also directed (_loc. cit._) to the
more important matter of the effect of atmospheric electrical
conditions upon the transmission. The effect of all lightning
discharges, whether visible or invisible, is to make a record on the
telegraphic receiver. On the approach of an atmospheric electrical
disturbance towards the receiving station on a ship, the first visible
indications generally are the recording of dots at intervals from a
few minutes to a few seconds on the telegraphic tape. Captain Jackson
states that the most frequent record is that of three dots, the first
being separated from the other two by a slight interval like the
letters E I on the Morse code, and this is the sign most frequently
recorded by distant lightning. But in addition to this, dashes are
recorded and irregular signs, which, however, sometimes spell out
words in the Morse code. He noted that these disturbances are more
frequent in summer and autumn than in winter and spring, and in the
neighbourhood of high mountains more than in the open sea. In settled
weather, if present, they reach their maximum between 8 p.m. and 10
p.m., and frequently last during the whole of the night, with a
minimum of disturbance between 9 a.m. and 1 p.m. Another important
matter noted by Captain Jackson is the shorter distance at which
signals can usually be received when any electrical disturbances are
present in the atmosphere, compared with the distance at which they
can be received when none are present. This reduction in signalling
distance may vary from 20 to 70 per cent, of that obtainable in fine
weather. It does not in any way decrease with the number of lightning
flashes, but rather the reverse, the loss in signalling distance
generally preceding the first indications on the instrument of the
approaching electrical disturbance. It is clear that these
observations fit in very well with the theory outlined above, viz.,
that the atmosphere when impregnated with free electrons or
negatively-charged gaseous ions is more opaque to Hertzian waves than
when they are absent. Captain Jackson gives an instance of ships whose
normal signalling distance was 65 miles, failing to communicate at 22
miles when in the neighbourhood of a region of electrical disturbance.
These effects in the case of wireless telegraphy have their parallel
in the disturbances caused to telegraphy with wires by earth currents
and magnetic storms.

Another effect which he states reduces the usual maximum signaling
distance is the presence of material particles held in suspension by
the water spherules in moist atmosphere. The effect has been noticed
in the Mediterranean Sea when the sirocco wind is blowing. This is a
moist wind conveying dust and salt particles from the African coast. A
considerable reduction in signalling distance is produced by its
advent.

Another interesting observation due to Captain Jackson is the
existence of certain zones of weak signals. Thus, for instance, two
ships at a certain distance may be communicating well; if their
distance increases, the signalling falls off, but is improved again at
a still greater distance. He advances an ingenious theory to show that
this fact may be due to the interference between two sets of waves
sent out by the transmitter having different wave-lengths.

Finally, in the Paper referred to, he emphasises the well-known fact
that long-distance signalling can only be accomplished by the aid of
an aerial wire and a "good earth." Summing up his results, he
concludes: (1) That intervening land of any kind reduces the practical
signalling distance between two ships or stations, compared with that
which would be obtainable over the open sea, and that this loss in
distance varies with the height, thickness, contour, and nature of the
land; (2) material particles, such as dust and salt, held in
suspension in a moist atmosphere also reduce the signalling distance,
probably by dissipating and absorbing the waves; (3) that electrical
disturbances in the atmosphere also act most adversely in addition to
affecting the receiving instrument and making false signals or
_strays_, as they are called; (4) that with certain forms of
transmitting arrangement, interference effects may take place which
have the result of creating certain areas of silence very similar to
those which are observed in connection with sound signals from a
siren.

It is clear, therefore, from all the above observations, that
Hertzian-wave telegraphy taking place through the terrestrial
atmosphere is not by any means equivalent to the propagation of a wave
in free or empty space; and that just as the atmosphere varies in its
opacity to rays of light, sometimes being clear and sometimes clouded,
so it varies from time to time in transparency to Hertzian waves, the
cause of this variation in transparency probably being the presence in
the atmosphere of negatively-charged corpuscles or electrons. If there
are present in the atmosphere at certain times "clouds of electrons"
or "electronic fogs," these may have the effect of producing a certain
opacity, or rather diminution in transparency to Hertzian waves, just
as water particles do in the case of sunlight.

We may, therefore, in conclusion, review a few of the outstanding
problems awaiting solution in connection with Hertzian wave wireless
telegraphy. In spite of the fact that this new telegraphy has not been
accorded a very hearty welcome by the representatives of official or
established telegraphy in Great Britain, it has reached a point,
unquestionably owing to Mr. Marconi's energy and inventive power, at
which it is bound to continue its progress. But that progress will not
be assisted by shutting our eyes to facts. Many problems of great
importance remain to be solved. We have not yet reached a complete
solution of all the difficulties connected with isolation of stations.
In the next place, the question of localising the source of the
signals and waves is most important. Our kumascopes and receiving
appliances at present are like the rudimentary eyes of the lower
organisms, which are probably sensitive to mere differences in light
and darkness, but which are not able to _see_ or _visualise_, in the
sense of locating the direction and distance of a radiating or
luminous body. Just as we have, as little children, to learn to see,
so a similar process has to be accomplished in connection with
Hertzian telegraphy, and the accomplishment of this does not seem by
any means impossible or even distant. We are dealing with
hemispherical waves of electric and magnetic force, which are sent out
from a certain radiating centre, and in order to localise that centre
we have to determine the position of the plane of the wave and also
the curvature of the surface at the receiving point. Something,
therefore, equivalent to a range finder in connection with light is
necessary to enable us to locate the distance and the direction of the
radiant point.

Lastly, there are important improvements possible in connection with
the generation of the waves themselves. At the present moment, our
mode of generating Hertzian waves involves a dissipation of energy in
the form of the light and heat of the spark. Just as in the case of
ordinary artificial illuminants, such as lamps of various kinds, we
have to manufacture a large amount of ether radiation of long wave
length, which is of no use to us for visual purposes--in fact,
creating ninety-five per cent, of dark and useless waves for every
five per cent. of luminous or useful waves--so in connection with
present methods of generating Hertzian waves, we are bound to
manufacture by the discharge spark a large amount of light and heat
rays which are not wanted, in order to create the Hertzian waves we
desire. It is impossible yet to state precisely what is the
efficiency, in the ordinary sense of the word, of a Hertzian wave
radiator; how much of the energy imparted to the aerial falls back
upon it and contributes to the production of the spark, and how much
is discharged into the ether in the form of a wave.

Nothing is more remarkable, however, than the small amount of energy
which, if properly utilised in electric wave making, will suffice to
influence a sensitive receiver at a distance of even one or two
hundred miles. Suppose, for instance, that we charge a condenser
consisting of a battery of Leyden jars, having a capacity of one
seventy-fifth of a microfarad, to a potential of 15,000 volts; the
energy stored up in this condenser is then equal to 1·5 joules, or a
little more than one foot-pound. If this energy is discharged in the
form of a spark five millimetres in length through the primary coil of
an oscillation transformer, associated with an aerial 150 feet in
height, the circuits being properly tuned by Mr. Marconi's method,
then such an aerial will affect, as he has shown, one of Mr. Marconi's
receivers, including a nickel silver filings coherer tube, at a
distance of over two hundred miles over sea. Consider what this means.
The energy stored up in the Leyden jars cannot all be radiated as wave
energy by the aerial, probably only half of it is thus radiated. Hence
the impartation to the ether at any one locality of about half a
foot-pound of energy in the form of a long Hertzian wave is sufficient
to affect sensitive receivers situated at any point on the
circumference of a circle of 200 miles radius described on the open
sea. Hertzian wave telegraphy is sometimes described as being
extravagant in power, but, as a matter of fact, the most remarkable
thing about it is the small amount of power really involved in
conducting it. On the other hand, Hertzian wave manufacture is not
altogether a matter of power. It is much more dependent upon the
manner in which the ether is struck. Just as half an ounce of dynamite
in exploding may make more noise than a ton of gunpowder, because it
hits the air more suddenly, so the formation of an effective wave in
the ether is better achieved by the right application of a small
energy than by the wrong mode of application of a much larger amount.
If we translate this fact into the language of electronic theory, it
amounts simply to this. It is the electron alone which has a grip of
the ether. To create an ether wave, we have to start or stop crowds of
electrons very suddenly. If in motion, their motion implies energy,
but it is not only their energy which is concerned in the wave making,
but the acceleration, positive or negative--_i.e._, the quickness with
which they are started or stopped. It is possible we may discover in
time a way of manufacturing long ether waves without the use of an
electric spark, but at present we know only one way of doing
this--viz., by the discharge of a condenser, and in the discharge of
large condensers of very high potentials it is difficult to secure
that extreme suddenness of starting the discharge which we can do in
the case of smaller capacities and voltages.

How strange it is that the discharge of a Leyden jar studied so
profoundly by Franklin, Henry, Faraday, Maxwell, Kelvin and Lodge
should have become an electrical engineering appliance of great
importance!

Whilst there are many matters connected with the commercial aspect of
Hertzian wave telegraphy with which we are not here concerned, there
is one on which a word may properly be said. The ability to
communicate over long distances by Hertzian waves is now demonstrated
beyond question, and even if all difficulties are not overcome at
once, it has a field of very practical utility, and may even become of
national importance. Under these circumstances, we may consider
whether it is absolutely necessary to place the signalling stations so
near the coast. The greater facility of transmission over sea has
already been discussed and explained, but in time of war, the masts
and towers which are essential at present in connection with
transmitting stations could be wrecked by shot or shell from an
enemy's battleship at a distance of five or six miles out at sea, and
would certainly be done within territorial waters. Should not this
question receive attention in choosing the location of important
signalling stations? For if they can, without prejudice to their use,
be placed inland by a distance sufficient to conceal them from sight,
their value as a national asset in time of war might be greatly
increased.

It has been often contended that whilst cables could be cut in time of
war no one can cut the ether; but wireless telegraph stations in
exposed situations on high promontories, where they are visible for
ten to fifteen miles out at sea and undefended by any forts, could
easily be destroyed. The great towers which are essential to carry
large aerials are a conspicuous object for ten miles out at sea; and a
single well-placed shell from a six-inch gun would wreck the place and
put the station completely out of use for many months. Hence if
oceanic telegraphy is ever to be conducted in a manner in which the
communication will be inviolable or, at any rate, not be capable of
interruption by acts of war, the careful selection of the sites for
stations is a matter of importance. A small station consisting of a
single 150-foot mast and a wooden hut can easily be removed or
replaced, but an expensive power station, the mere aerial of which may
cost several thousand pounds, is not to be put up in a short time.[76]

Meanwhile, whatever may be the future achievements of this new
_supermarine_ wireless telegraphy conducted over long distances, there
can be no question as to its enormous utility and present value for
intercommunication between ships on the ocean and ships and the shore.
At the present time, there are some forty or more of the transatlantic
ocean liners and many other ships equipped with this Hertzian wave
wireless telegraph apparatus on the Marconi system. Provided with this
latest weapon of applied science, they are able to chat with one
another, though a hundred miles apart on the ocean, with the ease of
guests round a dinner table, to exchange news or make demands for
assistance.

    Ships that pass in the night, and speak each other in passing--
    Only a signal shown, and a distant voice in the darkness;
    So, on the ocean of life, we pass and speak one another,
    Only a look and a voice, then darkness again, and a silence.

Abundant experience has been gathered to show the inexpressible value
of this means of communication in case of accident, and it can hardly
be doubted that before long the possession of this apparatus on board
every passenger vessel will be demanded by the public, even if not
made compulsory. Although the privacy of an ocean voyage may have been
somewhat diminished by this utilisation of ether waves, there is a
vast compensation in the security that is thereby gained to human life
and property by this latest application of the great energies of
nature for the use and benefit of mankind.

GEO. TUEKER, PRINTER, SALISBURY COURT, FLEET STREET, LONDON.

[1] This series of articles is based on the Cantor Lectures delivered
before the Society of Arts, London, in March, 1903. The lectures were
attended by many of the leading British scientific men and electrical
engineers, and attracted wide attention as the most complete and
authoritative statement hitherto made of wireless telegraphy. In writing
the articles for the "Popular Science Monthly," the author has omitted
advanced technicalities in order that the substance may be suitable for
the general reader.--EDITOR.

[2] For a more detailed account of this hypothesis, the reader is
referred to an article by the present writer, entitled "The Electronic
Theory of Electricity," published in the "Popular Science Monthly" for
May, 1902.

[3] See J. J. Thomson, "Recent Researches in Electricity and Magnetism,"
chap. I., p. 16.

[4] See O. Heaviside, "Electromagnetic Theory," Vol. I., p. 54.

[5] Wiedemann's _Annalen_, 36, p. 1, 1889; or in his republished Papers,
"Electric Waves," p. 137, English translation by D. E. Jones.

[6] The fraction 7/22 here denotes a stranded wire formed of seven
strands, each single wire having a diameter expressed by the number 22
on the British standard wire gauge.

[7] G. Marconi, "Syntonic Wireless Telegraphy," _Journal_ of the Society
of Arts, Vol. XLIX., p. 501, 1901.

[8] Instruction for the manufacture of large induction coils may be
obtained from a "Treatise on the Construction of Large Induction Coils,"
by A. T. Hare. (Methuen & Co., London.)

Also see Vol. II. of "The Alternate-Current Transformer," by J. A.
Fleming, chap. I. ("The Electrician" Printing and Publishing Co., 1, 2
and 3, Salisbury-court, Fleet-street, London, E.C.)

[9] See "The Alternate-Current Transformer," by J. A. Fleming. Vol. I.,
p. 184.

[10] Du Moncel states that MacGauley of Dublin independently invented
the form of hammer break as now used. See "The Alternate-Current
Transformer," Vol. II. chap. I. J. A. Fleming.

[11] See Professor J. Trowbridge, "On the Induction Coil" _Phil. Mag._,
April, 1902 Vol. III., Series 6, p. 393.

[12] See Dr. Wehnelt's article in the _Elektrotechnische Zeitschrift_,
January, 1899.

[13] See _The Electrician_, Vol. XLII., 1899, pp. 721, 728, 731, 732 and
841; communications from Mr. Campbell Swinton, Professor S. P. Thompson,
Dr. Marchant, the author and others; also p. 864, same volume, for a
leader on the subject; also p. 870, letters by M. Blondel and Professor
E. Thomson. See also _The Electrician_, Vol. XLIII., p. 5, 1899,
extracts from a Paper by P. Barry; _Comptes Rendus_, April, 1899. See
also the _Electrical Review_, Vol. XLIV., p. 235, 1899, February 17.

[14] See _The Electrician_, Vol. XLII., 1899.

[15] For a discussion of the function of the condenser in an ordinary
induction coil, see "The Alternate-Current Transformer," by J. A.
Fleming. Vol. II., p. 51.

[16] See Lord Rayleigh, _Phil. Mag._, December, 1901.

[17] It has sometimes been stated that the spark balls must be _solid_
metal and no hollow, but this is a fallacy, and has been disproved by
Mr. C. A. Chant. See "An Experimental Investigation into the Skin Effect
in Electrical Oscillators," _Phil. Mag._, Vol. III., Sec. 6, p. 425,
1902.

[18] See _Proc._ Roy. Soc., London, February 23 and April 12, 1860; or
reprint of Papers on electrostatics and magnetism, p. 247.

[19] See _Phil. Mag._, August, 1902, Vol. IV., p. 224, 6th Series. Mr.
Jervis-Smith has also described an experiment to show how much the use
of compressed air round a spark gap is of advantage in working an
ordinary Tesla coil. In his British specification, No. 12,039 of 1896,
Mr. Marconi had long previously mentioned the use of compressed air
round the spark gap.

[20] This energy storage is at the rate of 44 foot-pounds per cubic foot
of glass. This figure shows what a relatively small amount of energy is
capable of being stored up in the form of electric strain in glass. In
the case of an air condenser, it is only stored at the rate of 1
foot-pound per cubic foot.

[21] See British specification No. 7,777 of 1900.--G. Marconi.
"Improvements in Apparatus for Wireless Telegraphy."

[22] That this number really does represent the order of this
oscillation frequency in an aerial has been shown by C. Tissot, _Comptes
Rendus_, 132, p. 763, March 25, 1901, by photographs taken of the
oscillatory spark of a Hertzian wave telegraphic transmitter. (See
_Science Abstracts_, Vol. IV., Abs. 1,518.) He found frequencies from
0·5 million to 1·6 million.

[23] The term "jigger" is one of those slang terms which contrive to
effect a permanent attachment to various arts and crafts. Similarly, the
word "booster" is now used for a step-up or voltage-raising transformer
or dynamo, inserted in series with an electric supply main. The word
"boost" is a slang term signifying to raise or lift up. "To give a real
good boost" is an expression for lending a helping hand. The term
"jigger," in the same manner, is an adaptation of a seaman's term for
hoisting tackle or lift.

[24] The "earth" itself probably only conducts electrolytically. All
such materials as sand, clay, chalk, etc., and most surface soils are
fairly good insulators when very dry, but conduct in virtue of moisture
present in them.

[25] _The Electrician_, Vol. XL., p. 86 (leader).

[26] British Patent Specification, C. and S. A. Varley, No. 165, 1866.

[27] See also _Journal de Physique_, Vol. V., p. 573, 1886.

[28] See _Comptes Rendus_, Vol. CXI., p. 785; Vol. CXII., p. 112, 1891;
or _La Lumière Electrique_, Vol. XL., pp. 301, 506, 1891; or _The
Electrician_, Vol. XXVII., 1891, pp. 221, 448.

[29] See _The Electrician_, Vol. XXIX., 1892, pp. 397 and 432.

[30] Mr. W. B. Croft, _Proc._ Phys. Soc., Vol. XII., p. 421. Report of
meeting on October 27, 1893.

[31] See Professor Minchin, _Proc._ Phys. Soc., November 24, 1893; or
_The Electrician_, Vol. XXXII., 1893, p. 123. See also Professor
Minchin, _Phil Mag._, January, 1894, Vol. XXXVII., p. 90, "On the Action
of Electromagnetic Radiation on Films containing Metallic Powders."

[32] This lecture was afterwards published as a book, the first edition
bearing the same title as the lecture--viz., "The Work of Hertz and Some
of His Successors." In the second edition, published in 1898, an
appendix was added (p. 59) containing "The History of the Coherer
Principle," and the original title of the work had prefixed to it
"Signalling Without Wires."

[33] See _The Electrician_, Vol. XXVII., p. 222, 1891. E. Branly,
"Variations of Conductivity under Electrical Influence."

[34] See _The Electrician_, Vol. XL., p. 90. Sir Oliver Lodge, "The
History of the Coherer Principle."

[35] See Professor E. Branly, "A Sensitive Coherer," _Comptes Rendus_,
Vol. CXXXIV., p. 1,187, 1902; or _Science Abstracts_, Vol. V., p. 852,
1902.

[36] This device of making the inter-electrode gap in a tubular filings
coherer wedge-shaped has been patented again and again by various
inventors. See German patent No. 116,113, Class 21a, 1900. It has also
been claimed by M. Tissot.

[37] See _The Electrician_, Vol. XXVII., 1891, p. 448.

[38] _Journal_ of the Russian Physical and Chemical Society, Vol.
XXVIII., Division of Physics, Part I., January, 1896.

[39] See British Patent Specification No. 12,039, June 2, 1896.

[40] British Patent Specification No. 19,710 of 1899.

[41] _Comptes Rendus._, Vol. CXXVIII., p. 1,225, 1889; _Science
Abstracts_, Vol. II., p. 521.

[42] _Il Nuovo Cimento_, Vol. X., p. 279, 1899.

[43] _Wied Ann._, Vol. LXVIII., p. 594, 1899; _Science Abstracts_, Vol.
II., p. 757.

[44] _Comptes Rendus_, Vol. CXXX., p. 902, 1900; _Science Abstracts_,
Vol. III., p. 615.

[45] See _Proc._ Roy. Soc., London, Vol. LXXI., p. 402.

[46] See Report by Capt. Quintino Bonomo, "Telegrafia Senza Fili," Rome,
1902; _L'Elettricista_, Ser. II., Vol. I., pp. 118, 173.

[47] See Royal Institution, Friday evening discourse, by Mr. Marconi,
June 13, 1902; also _The Electrician_, Vol. XLIX., p. 490; also a letter
to _The Times_ of July 3, 1902, by the Marchese Luigi Solari.

[48] See U.S.A. Patent Specification No. 700,161, May 24, 1900.

[49] See E. Marx, _Phys. Zeitschrift_, Vol. II., p. 249; _Science
Abstracts_, Vol. IV., p. 471. See also German Patent Specification No.
121,663, Class 21a.

[50] See "The Scientific Writings of Professor Joseph Henry."

[51] _Phil. Trans._ Roy. Soc., London, 1897, Vol. CLXXXIX.A, p. 1.

[52] See _Proc._ Roy. Soc., London, June 12, 1902. "Note on a Magnetic
Detector for Electric Waves which can be employed as a Receiver for
Space Telegraphy," by G. Marconi.

[53] See U.S.A. Patent Specification No. 716,000, Application of July 5,
1901.

[54] See the _Electrical Review_, Vol. XLIV., 1899, May 26; _Wied Ann._,
Vol. LXVIII., p. 92; or German Patent Specification No. 107,843.

[55] U.S.A. Patent Specification No. 706,742, 1902.

[56] See British Patent Specification, G. Marconi, No. 12,039, June 2,
1896.

[57] See G. Marconi, British Patent Specification No. 12,326, of June 1,
1898.

[58] See the _Electrical Review_, September 26, 1902, Vol. LI., p. 543.

[59] There is a good deal of contradiction between various inventors on
this point, some saying that "earthed" aerials obviate atmospheric
electrical disturbances, and others that insulated aerials are in this
respect superior. The truth appears to be that, neither form is
absolutely free from risk of disturbance by this cause.

[60] The capacity of an electrical circuit corresponds to the elastic
pliability, or what is commonly called the elasticity, of a material
substance, and the inductance to mass or inertia. Hence capacity and
inductance are qualities of an electric circuit which are analogous to
the elasticity and inertia of such a body as a heavy spring.

[61] See Cantor Lectures, on "Electrical Oscillations and Electric
Waves," delivered before the Society of Arts, London, November 26,
December 4, 10, 17, 1900. Lecture I., p. 12, of reprint.

[62] A fuller account of these experiments was given by the author in a
letter to the London _Times_ published on April 14, 1903.

[63] See _Journal_ of the Society of Arts, Vol. XLIX., p. 505. "Syntonic
Wireless Telegraphy," by G. Marconi.

[64] See German Patent Specifications, Class 21a, No. 7,452 of 1900, and
also No. 8,087 of 1901.

[65] See German Patent Specification, Class 21a, No. 7,498 of 1900,
applied for November 9, 1900. The above-mentioned patent is subsequent
in date to Mr. Marconi's experiments on the same subject.

[66] See _The Electrician_, January 18, 1900, Vol. XLVI., p. 475. Also
reprint of a Paper of Professor A. Slaby, "Abgestimmte und mehrfache
Funkentelegraphie."

[67] See British Specification No. 11,348 of 1901.

[68] See _Comptes Rendus_, May 21, 1900; Rapports du Congrès
International d'Electricité, Paris, 1900, p. 341.

[69] See _The Electrician_, Vol. XLVI., p. 573, February 8, 1901.

[70] See _The Electrician_, Vol. L., p. 418, January 2, 1903.

[71] See Mr. Marconi's Friday evening discourse at the Royal
Institution, June 13, 1902; also _The Electrician_, Vol. XLIX., p. 390.

[72] See _Proc._ Roy. Soc., June 12, 1902. "A Note on the Effect of
Daylight upon the Propagation of Electromagnetic Impulses over Long
Distances," by G. Marconi.

[73] See _Phil. Mag._, Vol. IV., p. 253, Series 6, August, 1902. J. J.
Thomson, "On Some Consequences of the Emission of Negatively-electrified
Corpuscles by Hot Bodies."

[74] The opinion that ionisation of the air by sunlight is a cause of
obstruction to Hertzian waves propagated over long distances has also
been expressed by Mr. J. E. Taylor. See _Proc._ Roy. Soc., Vol. LXXI.,
p. 225, 1903. "Characteristics of Earth Current Disturbances and their
Origin."

[75] See _Proc._ Roy. Soc., May 15, 1902. "On Some Phenomena affecting
the Transmission of Electric Waves over the Surface of the Sea and
Earth," by Captain H. B. Jackson, R.N., F.R.S.

[76] Mr. Marconi has informed the writer that these strategic questions
have received attention in selecting the sites for large Marconi power
stations in Italy.

                  *       *       *       *       *

 [Detailed Transcriber's Notes

 The text has been made to match the original text as much as possible
 retaining all apparent printer's errors and inconsistencies. The
 following, detail the apparent printer's errors etc. identified in
 the original text.

 Variation in spelling, Strasburg and Strassburg for Strasbourg.

 There are a number of inconsistencies in hyphenation present in the
 original text. Those concerned with the variation between one word or
 a hyphenated word are detailed below. Those concerned with the
 variation between multiple words and hyphenated words are too
 numerous to detail individually.

 Inconsistent hyphenation of word, 'anti-node' and 'antinode' both
 present in original text.

 Inconsistent hyphenation of word, 'electro-dynamic' and
 'electrodynamic' both present in original text.

 Inconsistent hyphenation of word, 'horse-shoe' and 'horseshoe' both
 present in original text.

 Inconsistent hyphenation of word, 'over-blowing' and 'overblowing'
 both present in original text.

 Page 5, possible printer's error, a for at, 'consisting when a rest'.

 Page 6, printer's error, comma rather than full stop at end of
 sentence, 'ether constituting electric radiation,'.

 Page 10, printer's error, millmetre for millimetre, 'three thousand
 volts per millmetre,'.

 Page 13, possible printer's error, set for sets, 'there are three set
 of phenomena'.

 Page 13, printer's error, duplicate word, 'detached and and travel
 away.'.

 Page 13, brackets added to in-line equation to aid clarity, 'F =
 (3/8)CV^{2}/10^{6}.'.

 Page 13, both equations originally multi-line fraction, rendered into
 one line for clarity.

 Page 15, both equations originally multi-line fraction, rendered into
 one line for clarity.

 Page 22, printer's error, correponding for corresponding,
 'correponding to this frequency'.

 Page 22, printer's error, consist for consists, 'due to Braun, consist
 of attaching'.

 Page 24, printer's error, one-hundreth for one-hundredth, 'capacity of
 one-hundreth of a microfarad,'.

 Page 28, printer's error, missing full stop at end of sentence added,
 'in the case of the hammer break.'.

 Page 33, printer's error, supppse for suppose, 'Let us supppse'.

 Page 44, equation originally multi-line fraction, rendered into one
 line for clarity.

 Page 46, printer's error, comma rather than full stop at end of
 sentence, 'to the transmitting aerial,'.

 Page 48, possible printer's error, alterations for alternations,
 'alterations of electric strain'.

 Page 54, printer's error, Banly for Branly, 'proved that in a Banly
 tube,'.

 Page 56, variation in spelling, unsensitive for insensitive, 'wounded
 and unsensitive.'.

 Page 59, possible printer's error, sensive for sensitive 'to work a
 sensive recording apparatus'.

 Page 59, possible printer's error, arragement for arrangement, 'most
 interesting arragement'.

 Page 61, printer's error, missing letter i, 'as shown n Fig. 18,'.

 Page 70, equation originally multi-line fraction, rendered into one
 line for clarity.

 Page 71, printer's error, osciilating for oscillating, 'to that of the
 osciilating circuit'.

 Page 71, printer's error, impluse for impulse, 'the period of that
 impluse'.

 Page 74, possible printer's error, extra comma in date, 'on May, 17,
 1901.'.

 Page 76, printer's error, arangements for arrangements, 'variation of
 the above arangements'.

 Page 77, printer's error, systonic for syntonic, 'the systonic
 transmitting'.

 Page 86, printer's error, interpositon for interposition, 'effect of
 the interpositon of land'.

 Page 87, printer's error, signaling for signalling, 'the usual
 maximum signaling'.

 Footnote 17, printer's error, missing letter t, 'must be _solid_
 metal and no hollow,'.

 Footnote 31, printer's error, missing full stop after abbreviation,
 '_Phil Mag._'.

 Footnote 41, printer's error, extra full stop after reference,
 '_Comptes Rendus._'.
 ]