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,