ELECTRIC OSCILLATIONS AND ELECTRIC WAVES.
In the previous chapters your attention has been directed to the
subject of waves on water and waves in air, and we shall now proceed
to discuss some of the more difficult matters connected with the
production of waves in the æther. We shall find that this portion of
our subject makes more demands upon our powers of comprehension, since
much that we have to consider is not directly the object of sense
perception, and the inferences which we have to make from observed
facts are less simple and easy to follow. Nevertheless, I trust that
if you have been able to grasp clearly the nature of a surface-wave on
water and of a compressional wave in air, you will not readily allow
yourselves to be discouraged from encountering a new class of ideas,
but will be able to advance still further, and gain a more or less
clear conception of the nature of an electric wave in the æther.
In the first place, we must consider the medium in which these waves
are created. We can see with our eyes a water-surface, and we are
able to understand without much difficulty that the surface can be
thrown into humps and hollows, or become wrinkled, and also that these
elevations and depressions can change their position, thus creating a
surface wave which moves forward. The movement of the water wave is,
therefore, only the result of a local elevation of the surface which
travels along or takes place progressively at different places on the
surface. Then, again, in the case of an air wave, although we cannot
see the air, we are able, with some little assistance from experiments,
to present to ourselves a clear mental picture of a progressive
movement through the air of a region of compression, that is to say, a
certain slice, layer, or zone of the air is more compressed than the
neighbouring portions, and this region of compression changes its place
progressively. It has been carefully explained that the production of
a wave of any kind implies, therefore, two things—first, a medium or
material in which the wave exists; and, secondly, some kind of periodic
change or movement which is experienced by the various portions of this
medium at different places successively.
If, therefore, we are given any medium, say water or air, and asked
to explain the production of a wave in it, we have first to consider
what kind of changes can take place in it, or on it, which can appear
progressively at different parts. In the case of the water-surface,
some parts may be heaped up higher than the rest, and the heaping up
may occur at successive places in such fashion that when it disappears
at one place it reappears at a contiguous or neighbouring place. In the
case of air, some portion may be compressed more than the rest, and
the place of compression may move forward, so that as the compression
is released in one place it makes its appearance in an adjacent one.
In the first case, we have a wave of elevation on water; in the second
case, a wave of compression in the air.
In the next place, let me carry you with me one step more. Here is a
glass bulb from which the greater part of the air has been removed. We
say, therefore, that there is a _vacuum_ in the bulb. It is impossible
for us to remove absolutely every trace of air from the bulb, and so
produce what would be called a _perfect vacuum_; but we can imagine
it to be accomplished, and we can picture to ourselves the glass bulb
absolutely deprived of every trace of air or other material substance.
The question then arises—Is the bulb really empty, or is there still
_something_ in its interior?
The same inquiry may be put in another way. The air we breathe forms
an atmosphere which surrounds our earth as a garment, but it decreases
rapidly in density as we ascend. At a height of about 50 miles above
the earth there is reason to believe the air is exceedingly rarefied
and, except for the presence of meteoric dust, the space between the
sun and the earth, and between the stars and the earth, is in all
probability a highly perfect vacuum, in the sense that it is empty of
generally diffused matter. The question then arises—Is interstellar
space absolutely and completely empty? We know perfectly well that
rays of light come to us from the sun and stars through this empty
space, and a fact of capital importance is, that these rays of light,
swift-footed though they are, take time to travel. It was long ago
suspected that this was the case, and the celebrated Galileo made the
first experimental attempt to determine the velocity of light. No real
knowledge on the subject was gained, however, until after he had made
his discovery that the planet Jupiter is accompanied by four moons (a
fifth moon has been discovered since), and that these rotate round
the planet in definite periods of time, constituting, therefore, the
“hands” of a perfect celestial clock. The sunlight, falling on the
great globe which forms the body of the planet Jupiter, casts behind
it a conical shadow; and the little moons, in their rotation, are
plunged into this shadow cone at intervals, and then for a time become
invisible, or eclipsed.
As soon, however, as these eclipses of Jupiter’s moons began to be
regularly observed, it was found that the intervals of time between two
eclipses of any one moon were not equal, but exhibited a progressive
variation in magnitude, and were longer by about 16 minutes and 26
seconds at one time of the year than at the other. The astronomer
Roemer, in the year 1675, correctly concluded that this difference must
be due to the fact that rays of light take time to traverse the earth’s
orbit, and not to any want of regularity in the operation of this
celestial timepiece. Hence, although the eclipses do happen at equal
intervals of time, our information about them is delayed by the time
taken for the ray of light to travel over the variable distance between
Jupiter and our earth. These observations, critically considered,
led, therefore, to the conclusion that the speed of light rays is
about 186,500 miles a second. By means which it would take too long to
describe here, experimental measurements of the velocity of light have
been made many times since by various investigators by methods which
do not involve astronomical observations, and the result has been to
confirm the above value, and to give us a very exact knowledge of the
speed with which rays of light travel through space. It is as shown in
the table below:—
THE VELOCITY OF LIGHT.
Miles per
second.
From observations on Jupiter’s satellites (Roemer) 186,500
” experimental measurements by Foucault (1862) 185,177
” ” ” ” Cornu (1874) 185,487
” ” ” ” ” (1878) 186,413
” ” ” ” Michelson (1879) 186,364
” ” ” ” ” (1882) 186,328
” ” ” ” Newcomb (1882) 186,333
When anything takes time to travel from one place to another, it can
only be one of two things. It must either be an actual object which is
transferred bodily from place to place, like a letter sent by post or
a bullet fired from a gun, or else it must be a wave-motion created in
a medium of some kind which fills all space. The illustrious Newton
suggested an hypothesis or supposition as to the nature of light, viz.
that it consists of small _corpuscles_ shot out violently from every
luminous body. It is a wonderful testimony to Newton’s exalted powers
of thought, that the most recent investigations show that hot and
luminous bodies, such as the sun and a lamp, are in fact projecting
small bodies called corpuscles into space, but there is abundant
proof that these are not the cause of light. Subsequently to the date
of Newton’s speculations on the nature of light, the alternative
hypothesis was developed, viz. that it consists in a wave-motion in a
universally diffused medium called the æther. A great gulf, however,
separates mere conjecture and speculation from that accumulation of
rigid proof which scientific investigation demands, and hence, although
this conception of an æther had arisen as an hypothesis in the minds of
Huyghens, Descartes, and many other philosophers, it was not accepted
by Newton, and the general assent of scientific investigators to the
hypothesis of a universal æther was long deferred. The philosopher
to whom we owe the crucial demonstration of the validity of, and
indeed necessity for, this assumption was Dr. Thomas Young, the first
Professor of Natural Philosophy in the Royal Institution of London.
Young was a man whose exalted intellectual powers were not properly
appreciated by the world until after his decease. His researches in
physical optics alone are, however, epoch-making in character. He it
was who first gave a proof that under some circumstances it is possible
for two rays of light to destroy each other, and thus produce darkness.
Briefly described, the experiment is as follows: If a beam of light of
one colour, say red, proceeding from a single source of light, falls
upon a screen in which are two small holes very near together, we
shall obtain from these holes two streams of light originating, as it
were, from closely contiguous sources. If we then hold a white screen
not far from these holes, and receive on it the light proceeding from
them, we shall find that the screen is marked with alternate bands
of red light and black bands. If we cover up one of the small holes,
the black bands vanish and the screen is uniformly illuminated. Young
pointed out that this effect was due to _interference_, and that the
difference of the distances from any black band to the two holes was an
exact odd multiple of a certain small distance called the wave-length
of the light. If light is a substance, no possible explanation can be
given which will enable us to account for the combination of two rays
of light producing darkness at their meeting-point. If, on the other
hand, rays of light consist of waves of some kind in a medium, then,
as we have seen in the case of water ripples and air waves, it is
quite possible for two wave-trains to annihilate each other’s effect
at a certain point, if a hollow of one wave-train reaches that place
coincidently with a hump belonging to the other.
Accordingly, the experiment of producing _interference_ between
two sets of light rays so that they destroy each other is a strong
argument in favour of the view that light must consist in some kind of
wave-motion existing in a medium susceptible of supporting it, filling
all space, and existing in all transparent bodies. This medium we call
the _luminiferous æther_.
The term “æther,” or “ether,” has been in use for many centuries to
express the idea of something more rare, tenuous, or refined than
ordinary matter.
The classical writers employed it to describe the space above the
higher regions of the atmosphere, which was, as they; supposed,
occupied by a medium less palpable or material than even air itself.
Thus Milton, speaking of the downfall of the enemy of mankind
(“Paradise Lost,” Book I. line 44), says—
“... Him the Almighty Power
Hurled headlong, flaming, from the _æthereal_ sky
With hideous ruin and combustion down.”
But although poets and philosophers had made free use of the notion
of an æther, or even assumed the existence of several æthers, the
conception did not become a serious scientific hypothesis until it was
experimentally shown by Young that the phenomena of optics imperatively
demand the assumption of such a medium in space which is not ordinary
matter, but possesses qualities of a special kind, enabling it to have
created in it waves which are propagated with the enormous velocity
of nearly one thousand million feet a second. The proofs which have
accumulated as to the validity of this hypothesis to explain optical
effects show that this medium or æther must exist, not only in free
space, but also in the interior of every solid, liquid, or gaseous
body, although its properties in the interior of transparent bodies are
certainly very different from those which it possesses taken by itself.
This æther fills every so-called vacuum, and we cannot pump it out from
any vessel as we can the air. It occupies, likewise, all celestial
space, and suns and stars float, so to speak, in an illimitable ocean
of æther. We cannot remove it from any enclosed place, because it
passes quite easily through all material solid bodies, and it is for
the same reason intangible, and it is not possessed of weight. Hence
we cannot touch it, see it, smell it, taste it, or in any way directly
appreciate it by our senses, except in so far as that waves in it of a
certain kind affect our eyes as light.
The fact that there is such a space-filling æther is, therefore, only
to be deduced by reasoning from experiments and observations, but it is
not directly the object of our sense-perceptions in the same way that
water or air can be. Nevertheless, there is abundant proof that it is
not merely a convenient scientific fiction, but is as much an actuality
as ordinary gross, tangible, and ponderable substances. It is, so to
speak, matter of a higher order, and occupies a rank in the hierarchy
of created things which places it above the materials we can see and
touch.
The question we have next to discuss is—What are the fundamental
properties of this æther? and what are the terms in which we must
describe its qualities? In order to answer these questions, we must
direct attention to some electrical effects, since it has been shown
that most of the electrical phenomena, like those of optics, point
to the necessity for the assumption of a similar universal medium,
different from ordinary matter. Abundant proof has been gathered in,
that the electro-magnetic medium and the luminiferous æther are one and
the same.
We are met at the very outset of our electrical studies by the term
_electric current_. Most of us know that the operation of electric
telegraphs and telephones, electric lamps and electric railways, depend
upon this employment of an agency called an electric current.
The question then arises—What _is_ an electric current? and the
answer to this question is not easy to give in a few words. We can,
however, begin by explaining what an electric current can do, and how
its presence can be recognized. Before me on the table is a spiral of
copper wire, and in this wire, by special means, I can create what we
call an electric current. I shall ask you to notice that when this is
done two effects are immediately produced. In the first place, the wire
becomes _hot_, and, secondly, it becomes _magnetic_. The fact that it
is hot is evident, because it is now nearly red hot, and is visibly
incandescent in the dark. If we dip the wire in iron filings, you will
see that these cling to the wire and are taken up by it, just as when
an ordinary steel magnet is substituted for the wire. The copper wire,
when traversed by the current, also attracts a compass needle, and we
thus demonstrate in another way its magnetic quality.
Whenever, therefore, we find these two states of heat and magnetism
present together in and round a wire, we may take it as an indication
that it forms part of a circuit through which an electric current is
flowing. An electric current is a physical state or condition which can
only exist in or all along a closed path which is called an electric
circuit. This electric circuit may consist of a metallic wire, or, as
we generally call it, a conductor, or, as we shall see, it may also in
part consist of what is usually called a non-conductor.
It is necessary, in the next place, to point out that an electric
current has a directive quality. It belongs to that category of things
like forces and movements, which have direction as well as magnitude.
It is not completely defined by the answer to the question—How much? We
must also ask—In what direction? The direction of an electric current
is settled by holding a small compass needle near to the conductor or
wire in which the current exists. The little magnet will set itself
with its north pole in one direction or in the opposite, _across_ the
wire. That is to say, the axis of the compass needle places itself at
right angles to that of the wire. The direction of the electric current
is decided in accordance with the following conventional rule: Imagine
yourself placed with your arms extended straight out like a cross,
and that the wire conveying the current is placed before your face in
a vertical position. Imagine, also, that the position in which the
compass needle naturally sets when held between you and the wire is
such that its North pole is on your right-hand side. Then the current
would be said to move _upwards_ in the wire. A current which is always
in one and the same direction in a wire is called a _continuous_,
_direct_, or _one-way_ current.
A current which periodically changes its direction so that it is first
in one direction and then in the other is called an _alternating_ or
_two-way_ current.
I can now show you two experiments, the employment of which will
enable us always to decide whether a current in a wire is a one-way
or a two-way current. In the first experiment you see a copper wire
stretched between the poles of a powerful horseshoe magnet. When a
one-way current is sent through the wire, it is pulled either up
or down, like a fiddle or harp string being plucked by the finger.
If, however, we send a two-way current through the wire, it moves
alternately up and down, and vibrates just like a harp-string when
plucked and left to itself.
The next experiment gives us, however, a more convenient method of
ascertaining the presence in a wire of an alternating or two-way
electric current. If two wire circuits are laid parallel to each
other, and we send through one of these an electric current, then,
in accordance with Faraday’s most notable discovery, we find that
the beginning or the ending of the one-way current in the first
wire gives rise at the moment to a transitory current in the second
wire. If, however, we pass through the first wire, which we call the
primary circuit, a two-way current, then, since this is, so to speak,
continually beginning and ending, we have a similar alternating or
two-way electric current produced in the secondary circuit.
This fact may be most neatly and forcibly illustrated by the employment
of the following pieces of apparatus: An insulated wire is wound many
times round a great bundle of iron wire, thus forming what is called
an electro-magnet. Through this wire is passed a strong alternating
electric current which reverses its direction 160 times a second.
Over the top of the electro-magnet we hold another coil of insulated
wire, the ends of which are connected to a small electric glow lamp
(see Fig. 64). When held near to the pole of the electro-magnet, we
find the little lamp in the secondary coil lights up brilliantly,
because there is created in that circuit a secondary or induced
alternating electric current by the action of the other current in the
primary or electro-magnet circuit. Thus we see that one alternating
electric current can, so to speak, give birth to another in a second
circuit held parallel to the first. In like manner this secondary
current can give rise to a third or tertiary current, and the third to
a fourth, and so on indefinitely.
[Illustration: FIG. 64.]
We can always make use of this test to ascertain and prove the
existence of an alternating current in any electric circuit. If we
provide a coil of insulated wire, having its ends connected to a small
incandescent lamp, and hold this lamp coil or secondary circuit near to
and parallel with any other circuit in which we suspect the existence
of an alternating electric current, and if the lamp in the secondary
circuit lights up, then we can say with certainty that there is an
alternating or two-way electric current in the first circuit.
Having, then, indicated briefly the effects which are produced by an
electric current when it exists in a conducting circuit, and the way in
which we can determine its presence and direction, we must pass on to
discuss some other facts connected with its production.
It is a maxim in philosophy that every effect must have a cause; hence
we must assign a name to the cause of the effect we call electric
current. This cause we call _electromotive force_.
Electromotive force may be created in many ways, and time will
not permit us to refer to these in detail, but it must be taken
that electrical machines, batteries, and dynamos are all of them
appliances for creating electromotive force, or, as it is sometimes
called, electric pressure, just as various kinds of force-pumps
are contrivances for creating pressure in fluids. We find that
electromotive force acts differently on various substances when they
are subjected to its operation. In some substances electromotive
force produces a continuous electric current, and in these cases the
material is called a _conductor_. In other cases electromotive force
creates what is called _electric strain_, or electric displacement,
and these substances are generally called _non-conductors_. The
difference between conductors and non-conductors can be illustrated by
a mechanical analogy. Consider, for instance, a force-pump consisting
of a cylinder with a tightly fitted piston; suppose the bottom of the
pump-tube to be closed by a pipe having in it a tap. If we open the
tap and apply pressure to the piston, we can force out of the pipe a
current of air which continues to flow as long as the piston is being
pressed down. In this case the pressure on the piston corresponds to
an electromotive force, and the current of air flowing out corresponds
with the electric current in the electrical circuit.
Supposing, however, that we shut the tap and then attempt to force down
the piston, we find at once an elastic resistance to motion. The piston
can be pressed down a little way, compressing the air and thus creating
a strain; but if the pressure is removed the piston flies up again, on
account of the compressional elasticity of the air. In this operation
we have a mechanical illustration of the action of electromotive force
on a material such as glass or air, which is called a non-conductor, or
sometimes a dielectric. In these bodies electromotive force produces
an electric strain, just as the mechanical force produces in the air
enclosed in the cylinder a mechanical strain. When the tap at the
bottom of the cylinder is closed, we can, by applying pressure, force
down the piston a little way, but that movement cannot be continued,
because we are building up an opposing pressure due to the elasticity
of the air.
It is possible to show you an electrical experiment which has a close
analogy with the above simple mechanical experiment. Here is a glass
tube which has platinum wires sealed into the two ends, and the tube
is partly exhausted of its air. Such a tube is called a _vacuum tube_,
and when an electrical current is passed through this rarefied air,
it causes it to become luminous, and, as you see when the room is
darkened, the tube is filled with a reddish light. A tube, therefore,
of this kind is very convenient in some experiments, because we can, in
effect, see the electric current passing through it. If I connect one
end of this tube with the earth, and the other with the terminal of an
electrical machine, and if then the handle of the electrical machine
is turned, the tube will continue to glow as long as the electrical
machine is rotated. The electrical machine must be regarded as a pump
which is forcing something called electricity through the vacuum tube,
and as long as the pressure is continued the current flows.
This corresponds with the case in which the tap at the bottom of the
force pump was open and a continuous current of air could be forced out
of it by pressing down the piston. Supposing, however, that I insert
between the vacuum tube and the electrical machine a plate of glass,
which is covered over with tinfoil on the two sides such an arrangement
constitutes what is called a condenser, or Leyden pane. We now repeat
the experiment, and begin to turn the handle of the electrical machine.
You will notice that the vacuum tube glows as before, and is filled
with a reddish light for a short time, but as we continue to turn the
handle this dies away, and after a few moments there is no further
evidence of an electric current passing through the vacuum tube.
You will understand, therefore, that an electric current cannot be
caused to flow for an indefinite time in one direction through a
glass plate, although, by the application of electromotive force, it
does evidently, as you see, pass through it for a short time. This is
analogous to the operation of the force-pump when the tap at the bottom
is closed. We then find that we can move the piston down a little way,
compressing or straining the enclosed air, but that its motion is
soon stopped by an opposing resistance. We therefore say that in the
glass plate we have created an _electric strain_ by the action of the
electromotive force, just as we describe the effect of the mechanical
pressure on the air by saying that we have created a compression in it.
But there is an additional resemblance between the electrically
strained glass and the mechanically compressed air. When any elastic
object has been strained, and is suddenly released, it regains its
position of equilibrium by a series of oscillations or vibrations.
Thus, for instance, if we take a strip of steel and fix one end of it
in a vice, and pull the other end on one side and then release it, the
steel regains its position of equilibrium only after having executed a
series of diminishing swings to and fro.
In the same way, if we place some mercury or water in a glass tube bent
in the shape of the letter ⋃, and displace the liquid by blowing
into the tube, then, on releasing the pressure suddenly, the liquid
will regain its position of equilibrium by a series of oscillations
which die gradually away. You will not have any difficulty in seeing
that this is really due to the inertia of the material, whether it be
steel or mercury or water which is displaced. In an exactly similar
manner, we find that when we have produced an electric strain in a
sheet of glass by the application of electromotive force, and if we
then remove the electromotive force and connect the two tinfoil or
metal surfaces by means of a piece of wire, the electric strain in the
glass disappears with a series of _electric oscillations_; that is to
say, the electric strain in the glass does not disappear or die away
gradually, but it is alternately reversed, at each reversal the strain
becoming less and less in magnitude. The result of this oscillatory
strain in the glass is to produce in the connecting wire an alternating
electric current.
[Illustration: FIG. 65.]
A very familiar and simple piece of electric apparatus is that known as
a Leyden jar (see Fig. 65). A Leyden jar consists of a glass vessel,
the outside and inside surfaces of which are respectively covered with
tinfoil. If we apply to these two surfaces an electromotive force,
we produce what is called an electric charge in the jar, which in
reality consists in a state of electric strain in the walls of the
vessel. When the jar is charged, if we connect together, by means of
a thick wire, the outside and the inside tinfoil surfaces, we have a
bright spark produced at the moment of making contact, and we have
a rapidly alternating electric current produced in the connecting
wire. If this connecting wire has a low resistance—in other words,
is a very good conductor—then this electric spark consists, not in a
discharge of electricity uniformly in one direction, but of a series of
rapidly succeeding sparks which are really discharges of electricity
or electric currents passing through the air alternately in opposite
directions. This can be demonstrated by taking a photograph of the
electric spark on a rapidly revolving photographic plate or strip. You
are probably all familiar with the sensitive photographic film which
is employed in hand cameras, such as the kodak. If a strip of this
sensitive film is bound round the edge of a wheel, and if the wheel is
set in very rapid rotation, and if we throw on the film, by means of a
lens, an image of an oscillatory electric spark, it will be clear to
you that, if the spark is continuous, it will produce upon the moving
photographic film an image which will be of the nature of a broad band.
If, however, the electric spark is intermittent, then this photographic
image will be cut up into a series of bars or patches, each one of
which will correspond to a separate image of one constituent of the
oscillatory spark.
Photographs of oscillatory electric sparks have in this way been taken
by many observers, and have afforded a demonstration that the electric
discharge of a Leyden jar, when taken through a wire of low resistance,
is not a continuous movement of electricity in one direction, but a
rapidly alternating electric current through the wire, forming the
oscillatory spark, and corresponding with an equally rapid alternating
electric strain in the glass, both strain and current dying gradually
away.
Although this operation takes a long time to describe, yet,
nevertheless, an oscillatory spark consisting of 20 or 30 electric
oscillations may all be over in the ¹⁄₁₀₀₀₀ or even ¹⁄₁₀₀₀₀₀ second.
In the photograph now thrown upon the screen (see Fig. 66) you see
the image of an oscillatory electric spark, each oscillation of which
lasted ¹⁄₇₀₀₀ second. We can, however, give a still further proof that
the discharge of a Leyden jar or electric condenser is, under some
circumstances, oscillatory, in the following manner:—
[Illustration: FIG. 66.—A photograph of an oscillatory electric spark
(Hemsalech).]
You have already seen that an alternating or two-way electric current
existing in one circuit can produce another alternating or two-way
electric current in a neighbouring circuit. Before me, on the table, is
an arrangement by which a battery of six Leyden jars, L, is continually
being charged and discharged through a thick wire which is wound a
dozen times round a square wooden frame, P (see Fig. 67). In proximity
to this wooden frame there is another wooden frame, S, also having
on it a dozen or two turns of insulated wire; the circuit of this
last conductor is completed by a small incandescent lamp, G. You will
notice that when the Leyden jars are charged and discharged rapidly
through the primary conductor, the little glow-lamp of the secondary
circuit lights up brilliantly, and, in virtue of what has already
been explained, you will see that this experiment is a proof that the
discharge of the Leyden jars through the primary circuit must consist
in an alternating or two-way current; in other words, it must be
oscillatory.
[Illustration: FIG. 67.]
A still further proof may be given that the discharge of a Leyden jar
or condenser, when taking place through a low-resistance circuit, is
oscillatory in the following manner:—
We employ the vacuum tube that we brought to your notice a few moments
ago. When an electric current is sent always in the same direction
through such a tube, it is well known that the two ends of the tube are
not alike in appearance. The tube, as you have seen, is filled with a
luminous glow; but this glow is interrupted, forming what is called
a dark space near one terminal of the tube, this terminal being that
which is termed the negative pole. Accordingly, this unsymmetrical
appearance in the light in the tube is a proof that the electric
current is passing through it always in one direction. We can, however,
vary the experiment, and instead of illuminating the tube by means of a
direct-discharge or induction coil, which is always in one direction,
we are able to illuminate it by means of a rapid series of discharges
from a Leyden jar. You will then see that the glow-light in the tube is
symmetrical—the tube, in other words, is alike at both ends; and this
shows us that the discharge from the tube under these circumstances
must be alternating—that is, first in one direction and then in the
other.
Whilst this apparatus is in use, we can show you with it two other
very pretty experiments dependent upon the fact that the discharge
of a Leyden jar through a low-resistance circuit is alternating or
oscillatory. A moment ago we employed this oscillatory discharge in one
circuit to induce a secondary oscillatory discharge in another metallic
circuit, and this secondary oscillatory or alternating current was
made manifest by its power to illuminate a little incandescent lamp.
If, however, we place a large glass bulb, P, which has been partly
exhausted of its air, in the interior of the primary discharge coil,
you will see that this primary oscillatory discharge of the Leyden jar
is able to create in the glass bulb a brilliant luminous ring of light
(see Fig. 68). This is called an _electrodeless discharge_, and it is
due to the fact that the rapidly oscillatory current existing in the
wire wrapped round the bulb creates a similar oscillatory discharge in
the rarefied air in the interior of the bulb, this being a conductor,
and thus renders it luminous along a certain line.
The production of these electrodeless discharges in rarefied gases has
been particularly studied by Professor J. J. Thomson.
Another experiment illustrating what is called the inductive
transformation of electrical oscillations is in the arrangement
commonly called a Tesla coil. Such a coil is now before you. It
consists of a long coil of insulated wire which is placed in the
interior of a tall glass vessel, and on the outside of this glass
vessel is wound another insulated and much longer wire. If the
alternating or oscillatory discharge of a Leyden jar is allowed to take
place through the thicker wire in the interior of the glass cylinder,
it generates in the outer or secondary wire a very powerful alternating
or oscillatory electromotive force, and we see that this is the case
by connecting the ends of this secondary circuit to two insulated
brass balls, between which a torrent of sparks now passes. We may vary
the experiment by connecting the ends of the secondary circuit of the
Tesla coil to two insulated concentric rings of thin, bare, brass
wire, and then, when the room is darkened, we see the space between
these rings filled with a brilliant purple light, which is due to the
discharge taking place through the air under the action of the rapidly
oscillatory electromotive force generated in the secondary circuit.
[Illustration: FIG. 68.—An electrodeless discharge in an exhausted
bulb.]
I trust that these experiments will have produced a conviction in your
minds that the release of the electric strain in the glass _dielectric_
of a Leyden jar results in the production of electric oscillations
or rapidly alternating electric currents in the metallic circuit
connecting the two surfaces, just as the sudden release of a compressed
spring results in a series of mechanical oscillations.
We may here remark that any arrangement of two metallic plates with
a sheet of insulator or non-conductor between them is called a
_condenser_. Thus, a condenser can be built up by coating a sheeting
of glass on its two sides with tinfoil, or in place of glass we may
use mica, paraffin paper, or any other good non-conductor. We may
even use air at ordinary pressures; and thus, if two metal plates are
placed near to one another in air, the plates being both insulated—that
is, supported on non-conductors,—this arrangement constitutes what is
called an air condenser. An air condenser, therefore, is virtually only
a kind of Leyden jar in which the glass is replaced by air, and the
tinfoil by two stout metal plates.
[Illustration: FIG. 69.—Hertz oscillator.]
I must now proceed to describe and show you a particular kind of air
condenser which was invented by the late Professor Hertz, and, in
consequence, is called a Hertz oscillator (see Fig. 69). It consists
of two square or round metal plates which are carried on glass or
ebonite legs, and these plates have short, stout wires attached to
them, ending in brass knobs. If these plates are placed in line with
one another, they constitute an air condenser of a very peculiar
kind, the two brass plates correspond with the tinfoil surfaces of a
Leyden jar, and the air all round them corresponds with the glass of
the jar. Supposing the plates are so arranged that the brass knobs
are about ¹⁄₄ inch apart, or rather less, if then we connect these
two brass plates to the secondary terminals of an induction coil or
electrical machine capable of giving long sparks in air, we shall
find, when the electrical machine or induction coil is set in action,
that a very bright crackling spark passes between these little knobs,
and with proper experience it is easy to adjust the distance from
the knobs so that this spark is an oscillatory spark. Under these
circumstances, what is taking place is as follows: In the first place,
an electromotive force is acting between the two plates, and creating
an electric strain in the air all round them along certain lines, and
also between the two knobs. The air, and all other gases like it,
possess this peculiar property, that whilst at ordinary pressures
they are nearly perfect non-conductors, yet, nevertheless, if they
are subjected to more than a certain electric pressure, they pass
instantly into a condition in which they become very good conductors.
Accordingly, if we progressively increase the electromotive force
acting between the plates, up to a certain point the whole arrangement
acts like a Leyden jar; but there comes a moment when the air between
the knobs breaks down and passes from a non-conductive to a conductive
condition. The two plates then resemble at that moment the surfaces of
a charged Leyden jar which are connected together by a good conductor,
and, as we have already seen, under those circumstances the discharge
is oscillatory, and the electric strain in the non-conductor, or
dielectric, viz. the air around the plates, dies away by a series of
rapidly alternating electric strains in opposite directions.
Now, at this point I must recall to your recollection that, in speaking
about the production of air waves, I pointed out that one condition
essential to the production of an air wave was that there must be a
very sudden application or release of the air-pressure, such as is
caused by an explosion or escape of compressed air. We cannot produce
an air wave by moving any object such as a fan slowly to and fro
through the air. In order to produce an air wave we must strike the
air a very sudden blow, or, which comes to the same thing, we must
apply and remove a very sudden pressure to the air; and under these
circumstances we start into existence an air wave, which travels away
from the vibrating or rapidly moving body, and continues its journey
out into surrounding space.
I want to show you that, in the case of the Hertz oscillator, these
very sudden reversals of electric strain in the air or space round
about it, which take place at the moment when the oscillatory spark
passes between the knobs, creates in a similar manner what is called
an _electric wave_, which travels out into the space around. The point
you must appreciate is, that just as an air wave conveys away to
distant places a rapidly alternating compression made in the air by a
vibrating body at a particular place, so an electric wave conveys away
to distant places an alternating electric strain, which is originated
at some point in the medium by the oscillatory discharge of some
form of condenser. Before, however, we can demonstrate this fact, we
must have some means for detecting the influence of what we call an
electric wave. You will remember that, in the case of experiments with
air waves, I used a sensitive flame in order to make evident to you the
presence of waves in the air which you could not see, so here I must
use an appropriate detector for electric waves, the operation of which
will render evident to us the existence in the space round our electric
oscillator of the electric waves we cannot see.
Time will not permit me to discuss all the different forms of
electric-wave detector which have been invented. For our present
purposes we must limit ourselves to the description of one plan, which
depends on the remarkable fact that finely powdered dry metal or
metallic filings are non-conductors of the electric current until they
are subjected to an electromotive force exceeding a certain value, when
the metallic filings at once pass into a conductive condition.
If you recall the remarks made just now in connection with the special
electrical properties of air and other gases, you will notice that
there is a remarkable similarity between the electrical behaviour of
air at ordinary pressures to electromotive force, and that of a loose
mass of metallic filings. Both the air and the metallic filings are
non-conductors as long as the electromotive force acting on them does
not exceed a certain value, but if it exceeds this critical value,
they pass at once into a conductive condition. The fact that pieces
of metal in loose contact with one another behave in a similar manner
was discovered more than twenty years ago by the late Professor D. E.
Hughes, who, as you may perhaps know, was the inventor of a printing
telegraph, the microphone, and many other most important electrical
instruments. Professor Hughes was a great genius, and in many respects
in advance of his age. He it was who undoubtedly discovered that an
electric spark has the power of affecting at a distance the electric
conductivity of a metallic junction consisting of two metals in loose
contact.
The peculiar behaviour of metallic filings under electromotive
force, and under the influence of electric sparks at a distance, was
subsequently rediscovered by Professor Branly; and the effect of an
electric oscillatory spark in changing the conductivity of a light
metal contact was also rediscovered by Sir Oliver Lodge, and the
phenomena investigated by many other observers. I can show you the
experiment on a large scale in the following manner:—
[Illustration: FIG. 70.—A metal disc-coherer.]
I have here a number of aluminium discs, the size of sixpences, stamped
out of thin metal, and these are arranged in a sort of semi-cylindrical
trough between two terminal screws, so that the discs are very lightly
pressed together. Under these circumstances the pile of metal discs
is not a conductor, and it will not pass the electric current from a
battery which is joined up in series with an electric bell and the pile
of discs (see Fig. 70). Supposing, however, that I make an oscillatory
spark in proximity to this pile of metal discs, as I can do by taking
the discharge from a large Leyden jar near it; the pile of discs at
once becomes a conductor; the electric current from the battery can
then pass through it, and the bell rings. Such an arrangement has been
named by Sir Oliver Lodge a _coherer_, because, under the action of
the oscillatory spark, the discs cohere or stick together. We can
separate the discs by giving them a sharp rap, and then the operation
can be again repeated.
A much more sensitive arrangement can be made by taking a small box
of wood through the bottom of which pass two nickel wires which are
parallel to one another, but not in contact. In this box is placed
a small quantity of very finely powdered metallic nickel or nickel
filings, and if the quantity of these filings is properly adjusted,
it is possible to make an arrangement which possesses the property
that there is no conductivity between the two nickel wires under
ordinary circumstances, but that they become conductively connected to
one another the moment an oscillatory electric spark is made in the
neighbourhood. We shall speak of this contrivance as an _electric wave
indicator_, and we shall employ it in subsequent experiments to enable
us to detect the presence of an electric wave.
We must then return for a moment to the consideration of the production
of electrical oscillations in circuits of various kinds. I trust it has
been made plain to you that if two metallic surfaces, separated by a
non-conductor such as air or glass, are acted upon by an electromotive
force, the non-conductor becomes electrically strained. Another way of
stating this is to say that a positive charge of electricity exists
on one metal surface, and a negative charge on the other. The only
objection which can be raised to expressing the facts in this manner
is that it fastens attention rather upon the conductors than upon
the insulator, which is the real storehouse of the energy. If these
two metal surfaces are then connected together by a conductor of low
resistance, the charges disappear by a series of oscillations, and
the result is an electric current in the conducting circuit connecting
the plates, which rushes backwards and forwards in the circuit, but
gradually diminishes in strength until it completely dies away. You
may picture to yourselves the electrical effect as analogous to the
following experiment with two air-vessels: Supposing we have two
strong steel bottles, into one of which we compress a quantity of
air, and in the other we make a vacuum by pumping out nearly all the
air. These vessels would correspond with two conductors, one charged
with positive electricity and the other with negative. Imagine these
vessels connected by a wide pipe in which is placed a tap or valve,
which can be opened suddenly so as to permit the air to rush over from
the full vessel to the empty one. If this is done, it is a matter of
experience that the equality of pressure between the two vessels is not
at once established, but in virtue of the inertia quality of the air,
it only takes place after a series of oscillations of air in the pipe.
In rushing over from the full vessel to the empty one the air, so to
speak, overshoots the mark, and the state of the vessels as regards
air-pressure is exactly interchanged. The air then rushes back again,
and it is only after a series of to-and-fro movements of the air in the
pipe that an exact equality of pressure in the two vessels is attained.
The electrical actions which take place in connection with an electric
discharge between two conductors, one of which is charged positively
and the other negatively, are exactly analogous to the above-described
experiment with two air-vessels, one of which has air in it under
compression, and the other has had the air removed from it. You will
notice, however, that the oscillations of the air in the pipe in the
air-vessel experiment depend essentially upon the fact that air is a
substance which has _inertia_, or mass, and you will naturally ask
what is it which has inertia, or its equivalent, in the electrical
experiment? The answer to this question is as follows: Every electric
circuit has a quality which is called _inductance_, in virtue of which
an electric current cannot be started in it instantly, even under any
electromotive force, and conversely when the current is started it
cannot be immediately brought to rest. From the similarity of this
quality of the circuit to the inertia of ordinary material substances,
it has been sometimes called the _electric inertia_ of the circuit.
The word “inertia” really means inactivity, or laziness, but the term
as used in mechanics implies something more than mere inactivity. It
involves the notion of a persistence in motion when once the body is
set moving.
When a material substance is in motion it possesses energy, and has the
power of overcoming up to a certain point resistance to its motion.
This energy-holding power, or capacity for storing up energy of motion,
which is characteristic of all material substances, is a consequence
of their inertia. The fact is otherwise expressed by stating that the
_mass_ of a material substance is one element in the production of
energy of motion.
An electric current in one sense resembles a moving substance,
for it is an exhibition of energy in association with matter. The
current-energy is measured by the product of two factors: one is half
the square of the current-strength, and the other is the inductance
of the circuit. The analogy between the two cases may be more exactly
brought out by pointing out that the energy of motion of a moving body
is measured by the product of its mass and half the square of its
velocity. Hence it follows that the power of overcoming resistance, or,
in other words, of doing useful work or mischief, which is possessed
by a heavy body in motion is proportional, not simply to its speed,
but to the square of its speed. If a bullet, moving with a certain
speed, can just pass through one plank 1 inch thick, then, when moving
with twice the speed, it will pass through four such planks, and
if moving with three times the speed, through nine planks of equal
thickness. The energy of an electric current is similarly measured by
the product of the inductance of the circuit and half the square of
the current-strength. In the same or equal circuits two currents, the
strengths of which are in the ratio of 1 to 2, have energies in the
ratio of 1 to 4. The greater, therefore, the inductance of an electric
circuit, the greater is the tendency of an electric current set flowing
in it to run on after the electromotive force is withdrawn. The
inductance of a circuit is increased by coiling it into a coil of many
turns, and decreased by stretching it out in a straight line.
The important idea to grasp in connection with this part of the subject
is that, just as there are two forms of mechanical energy, viz. energy
of mechanical strain and energy of motion, so also there are two
forms of electrical energy, viz. energy of electro-static strain and
electric-current energy.
If, for instance, we bend a bow or extend a spring, this action
involves the expenditure of mechanical energy, or work, and the energy
so spent is stored up as energy of strain, or, as it is called,
distorsional energy in the distorted bow or spring. When, however,
the bow communicates its energy to the arrow or the spring to a ball,
and so sets these in motion, we have in the flying arrow or ball a
store of energy of motion. If a slip of steel spring is fixed at one
end, and then set in vibration, we have a continual transformation of
energy from the motional to the distorsional form. At one moment the
spring is moving violently, and at the next it is bent to its utmost
extent; and these states succeed each other. The store of energy in the
vibrating spring is, however, gradually frittered away, partly because
the continual bending of the steel heats it, and this heat dissipates
some of the energy; but also because the spring, if vibrating quickly
enough, imparts its energy to the surrounding air, and creates air
waves, which travel away, and rapidly rob the vibrating spring of its
stock of energy.
In a precisely similar manner all electrical oscillation effects depend
upon the fact that electric energy can exhibit itself in two forms. In
one form it is electro-static energy, or energy of electric strain. In
this form we have it when we charge a Leyden jar. The glass is then,
as explained, in a state of electrical strain, and its condition is
analogous to that of a stretched spring. The same holds good when we
have two conductors insulated from each other in air. We have then
an electrical strain in the air. It is important, however, to notice
that, since a perfect vacuum can support electric strain, it follows
that, in the cases where air or glass constitute this non-conductor, or
dielectric, of a condenser, the whole of the energy cannot be stored in
the material substance, the glass or the air. The real storehouse of
the energy is the æther, as modified by the presence of the ordinary
matter in the same place.
When we discharge the Leyden jar or condenser, the electro-static
energy in the dielectric disappears, and we obtain in its place an
electric current in the connecting conductor; and this, as described,
is an exhibition of energy in another form. If the resistance of the
connecting conductor is small, then we have electrical oscillations
established which consist in an alternate transformation of the energy
from an electro-static form to the electric-current form.
At each oscillation some energy is frittered away into heat in the
conductor, and if the conductor and condenser have a special form,
energy may be rapidly removed from the system by the electric waves
which are formed in the surrounding æther or dielectric. These waves
consist in the propagation through the medium of lines of electric
strain, just as an air wave consists in the propagation through the
air of regions of air-compression, or a water wave consists in the
propagation of an elevation on the surface.
Returning again to the discussion of the production of electrical
oscillations, it is necessary to consider a little more in detail the
manner in which we can create an electrical oscillation in what we have
called an _open electric circuit_. Let me begin with an experiment, and
it will then be easier for you to understand the particular points to
be explained.
[Illustration: FIG. 71.]
Before me are two long brass rods, each of them about 5 feet in length,
and the ends of these rods are provided with polished brass balls (see
Fig. 71). The rods are placed in one line and supported on pieces
of ebonite, and are so fixed that the two balls are separated from
one another by a space of about ¹⁄₄ inch. The two rods constitute,
therefore, two insulated conductors. These rods are connected by
coils of wire with the terminals of an instrument called an induction
coil, which I shall not stop to describe, but which you may regard
as a kind of electrical machine for producing electromotive force.
If we set the induction coil in action, it creates between its
terminals an intermittent but very powerful electromotive force, which
gradually increases up to a certain value, at which it breaks down
the conductivity of the air-gap between the two balls. Let us think
carefully what happens as the electromotive force of the induction coil
is increasing. One of the rods is in effect being electrified with
positive, and the other with negative, electricity, and these charges
are increasing in magnitude. The two rods constitute, as it were, the
two coated surfaces of a kind of Leyden jar, or condenser, of which
the surrounding air is the non-conductor. Accordingly, by all that has
been previously explained, you will easily understand that there is an
electric strain in the air which exists along certain lines, called
lines of electro-static strain, and this state in the air is exactly
similar to the condition in which the glass of a Leyden jar finds
itself when the jar is charged. If we were to delineate the direction
of this electric strain by lines drawn through the space around the
rods, we should have to draw them somewhat in the fashion represented
by the dotted lines in Fig. 71. As the electrical state of the rods
gradually increases in intensity, a point is reached at which the air
between the balls can no longer maintain this strain, and it breaks
down and passes into a conductive condition. The state of affairs round
the rods is then similar to that of a Leyden jar being discharged. An
electric current is produced across the air-gap, moving from one rod to
the other, and the intensely heated air in between the balls is visible
to us as an electric spark. This spark, if photographed, would be found
to be an oscillatory spark. The electric current in the rods cannot
continue indefinitely: it gradually falls off in strength, but as it
flows it creates in the space around the rods an electric strain which
is in the opposite direction to that which produced it, although taking
place along the same lines.
After a very short time, therefore, the electrical conditions which
existed at the moment before the air broke down are exactly reproduced,
only the direction of the strain is reversed. In other words, the rod
which was positively electrified is now negatively, and _vice versâ_.
Then this state of strain again begins to disappear, producing in
the rod an electric current, again in the reverse direction; and so
the energy, which was originally communicated to the space round the
rods in the form of an electric strain, continually changes its form,
existing at one moment as energy of the electric current passing across
the spark gap, and the next moment as energy of electric strain. We
may ask why this state of things does not continue indefinitely, and
the answer to that question is twofold. First because the rods possess
a property called electrical resistance, and this acts towards the
electric current just as friction acts towards the motion of material
substances; in other words, it fritters away the energy into heat. So
at each reversal of the electric current in the rod a certain quantity
of the original store of energy has disappeared, due to the resistance.
There is, however, a further and more important source of dissipation
of energy, and this is due to the fact that an electrical oscillation
of this kind taking place in a finite straight circuit, or, as it
is called, an open electric circuit, creates in the space around an
electric wave. The rapid reversal of the electric strain in the air
results in the production of an electric wave, just as in the case of
an explosion made in air, the rapid compression of the air results in
the production of an air wave. It is not easy for those who come to the
subject for the first time to fully grasp the notion of what is implied
by the term “an electric wave.”
In the first lecture, you will perhaps remember, I pointed out that
the production of a wave in a medium of any kind can take place if
the medium possesses two properties. In the first place, it must
elastically resist some change or distortion, and, in the second place,
when that distortion is made it must tend to disappear if the medium
is left to itself, and in so doing the displacement of the medium must
overshoot the mark and be reproduced in the opposite direction, owing
to some inertia-like quality or power of persistence in the medium.
It would lead us into matters beyond the scope of elementary lectures
if we were to attempt to summarize all the evidence which exists
tending to show that the phenomena of electricity and magnetism
must depend upon actions taking place in some medium called the
_electro-magnetic medium_. All the great investigators at the beginning
of the last century, when electrical and magnetic phenomena were
beginning to be explored, came to this conclusion, and in the writings
of Joseph Henry, of Ampère, and of Faraday we find references again
and again to their conviction that the phenomena of electricity imply
the existence of a medium exactly in the same way as do the phenomena
of optics. It is only, however, in recent years that we have had
evidence before us, some of which will be reviewed in the next lecture,
which affords convincing proof that the luminiferous æther and the
electro-magnetic medium must be the same. The consideration of the
simplest electrical effects is sufficient to show that, if this medium
exists, it possesses at least two properties, one of which is that it
offers an elastic resistance to the production of electric strain in
it by means of electromotive force. A question which is sure to arise
in the minds of those who consider this subject carefully is, What is
the nature of an electric strain? And the only answer which we can give
at the present moment is that we must be content to leave the question
unanswered. We do not know enough yet about the mechanical structure
of the electro-magnetic medium, or æther, to be able to pronounce in
detail on the nature of the change we call an electric strain. It
may be a motion of some kind, it may be a compression or a twist, or
it may be something totally different and at present unthinkable by
us, but, whatever it is, it is some kind of change which is produced
under the action of electromotive force, and which disappears when the
electromotive force is removed.
Clerk-Maxwell, to whom we owe some of our most suggestive conceptions
of modern electricity, coined the phrase _electric displacement_ to
describe the change which we are here calling an electric strain.
One essential element in Maxwell’s theory of electricity is that an
electric strain or displacement, whilst it is being made or whilst
it is disappearing, is in effect an electric current, and it is for
that reason sometimes spoken of as a displacement current. We have
seen that every electric circuit possesses a quality analogous to
inertia, that is to say, when a current is produced in it it tends to
persist, and it cannot be created at its full value instantly by any
electromotive force.
Just as we cannot, at the present moment, pronounce in detail on the
real nature of electric strain, so we cannot say whether that quality
which we call inductance of a circuit is dependent upon a true inertia
of the electro-magnetic medium or on some entirely different quality
more fundamental.
It may be remarked, in passing, that there is a strong tendency in the
human mind to seek for and be satisfied with what we called mechanical
explanations. This probably arises from the fact that the only things
which we can picture to ourselves in our minds very clearly are
movements or changes in relative positions. If we can in imagination
reduce any physical operation to some kind of movement or displacement
taking place in some kind of material, we seem to arrive at a kind of
terminus of thought which is more or less satisfactory. We invariably
aim at being able to visualize an operation concerning which we are
thinking, and it requires some mental self-control to be able to
content ourselves with a general expression which does not lend itself
readily to visualization. There are plenty of indications, however,
that this mental method of procedure, and this endeavour to reduce all
physical operations to simple mechanics and to movements of some kind,
may in the end be found to be unjustifiable; and the time may arrive
when we may be more satisfied to explain mechanical operations in terms
of electrical phraseology rather than aim at dissecting electrical
effects into mechanical operations. Thus, for instance, instead of
speaking of electric inertia, it may be really more justifiable to
speak of the inductance of ordinary matter. The final terms in which
we endeavour to offer ourselves an explanation of physical events
are in all probability very much a matter of convenience and custom.
We may, however, for present purposes rest content by thinking of
the electro-magnetic medium as in some sense like a heavy elastic
substance which is capable of undergoing some kind of strain or
distortion, the said strain relieving itself as soon as the distorting
force is withdrawn; but, in addition, we must think of the medium
as possessing a quality analogous to inertia, so that as distortion
vanishes it overshoots the mark, and the medium only regains its
state of equilibrium at the particular point considered, by a series
of oscillations or alternate distortions, gradually decreasing in
amount. Any medium which possesses these two qualities has, in virtue
of explanations already given, the property of having waves created in
it, and what we mean by an electric wave is a state of electric strain
which is propagated through the æther with a velocity equal to that of
light, just as an air wave consists of a state of compression which is
propagated through the air with a velocity of 1100 feet a second.
[Illustration: FIG. 72.—Electric-radiation detector (Fleming).]
To sum up, we may then say that whenever rapid electrical oscillations
are created in open circuit, such as the two rods above described, the
arrangement constitutes a device for creating an impulse or effect
in the surrounding space called an _electric wave_ in the æther or
electro-magnetic medium; just as an organ-pipe or piano-string or other
musical instrument constitutes a device for creating waves in the air
by means of mechanical oscillations. The existence of these electric
waves, and their transference to distant places, can be rendered
evident by their action as already described upon finely powdered
metals. An apparatus which shows this effect very well is now arranged
before you. At one end of the table I have a pair of rods connected to
an induction coil, constituting a Hertz radiator, the action of which
has just been described. At the other end of the table are two similar
long rods, but their inner ends are connected to two small plates
of silver, which form the sides of a very narrow box, and between
these plates is placed a very small quantity of metallic powder. The
construction of this little box is as follows: A thin slip of ivory
has a little gap cut out of it (see Fig. 72), and on the two sides of
this slip of ivory are bound two silver plates bent in the shape of the
letter L, forming, therefore, a very narrow box with silver sides.
The two silver plates are connected to the two long rods. As already
explained, the metallic filings or finely powdered metal are not in
their ordinary condition an electric conductor. Accordingly, if we
connect to one of the silver plates one terminal of a battery joined in
series with an electric bell, the other end of the bell being connected
to the second silver plate, this battery cannot send a current through
the bell, because the circuit is interrupted by the non-conductive
metallic powder in the little box. Supposing, then, that we cause a
spark to pass between the balls of the radiator, and start an electric
wave. When this electric wave reaches the long rods connected to the
receiving arrangement, it sets up in these rods a sudden electromotive
force, and this electromotive force, as already explained, if of
sufficient magnitude, causes the loose mass of metallic filings to
pass from a non-conductive to a conductive condition. At that moment,
therefore, the battery is able to send an electric current through
the bell, and to cause it to ring. We can, however, stop the ringing
by giving the little box containing the metallic filings a tap,
which separates them from one another and interrupts the electric
conductivity. The function of the two rods connected with the receiver
is not quite the same as the function of the two rods connected to the
radiator. In order to create a vigorous electric wave, we must have a
radiator which possesses what is called considerable electric capacity,
and also considerable inductance, and we can only do this in general by
using long rods. On the other hand, at the receiving end the efficacy
of the rods is due to the fact that they, so to speak, add together the
electric strain taking place over a considerable distance; in other
words, the electromotive force which is set up in the receiving circuit
is dependent on the length of the rods. The longer, therefore, these
rods, the greater is the distance at which we can obtain the effect
which is shown to you with a given spark-length.
One point it is important to notice, and that is, that the rods of the
receiver _must_ be parallel to the rods of the radiator if we are to
obtain any effect at a distance. If we turn the rods of the radiator
round so that they are at right angles to those of the receiver, you
see that no sparks produced at the radiator balls cause the bell in
connection with the receiver to ring. The reason for this is because
the electric strain, which is propagated out into the space, exists
in a direction parallel to the radiator rods all along a line drawn
perpendicular to the rods through the spark-gap. The receiver rods
will not have electromotive force produced in them by this travelling
line of electric strain unless they are parallel to its direction.
It is to be hoped that the above explanations have afforded indications
of what is meant by an electric wave. On the other hand, there may be
many who find it exceedingly difficult to derive clear ideas when the
subject is presented to them clothed in such general terms as we have
been obliged to use.
It may assist matters, therefore, if, before concluding this chapter,
a word or two is said on the subject of recent investigation into the
inner mechanism of an electric current and an electric strain. It is
impossible to do this, however, without making mention, in the briefest
possible way, of modern researches into the constitution of matter. If
you can imagine yourselves furnished with a little crystal of ordinary
table salt, chemically called chloride of sodium, and the means of
cutting it up under an immensely powerful microscope, you might go on
dividing it up into smaller and smaller pieces. If this process could
be continued sufficiently far, we should ultimately obtain a very
small fragment of salt, which, if still further divided, would yield
two portions of matter not alike and not salt. This smallest possible
portion of salt is called a molecule of sodium chloride. Chemical facts
teach us that this molecule is made up of two still smaller portions of
matter, which are called respectively _atoms_ of chlorine and sodium.
We have good reason to believe that all solids, liquids, and gases are
composed of molecules, and these are built up of atoms, few or many.
In the case of some substances, such as _salt_, the molecule is very
simple and composed of two atoms. In other substances, such as albumen
or white of egg, the molecules are very complicated and composed of
hundreds of atoms. The word _atom_ means something which “_cannot be
cut_,” and until comparatively recent time the opinion was held that
atoms of matter were the smallest indivisible portions of matter which
could exist.
More than twenty-five years ago, Sir William Crookes showed, by
numerous beautiful experiments, that in a vacuum tube, such as you have
seen used to-day, a torrent of small particles is projected from the
negative terminal when an electric current is passed through the tube.
This stream of particles is called the cathode stream, or the cathode
radiator. Within recent times, Sir Joseph Thomson has furnished a proof
that this cathode stream consists of particles very much smaller than
chemical atoms, each particle being charged with negative electricity.
These particles are now called _corpuscles_, or _electrons_.
It has been shown that these electrons are constituents of chemical
atoms, and when we remove an electron from an atom we leave the
remainder positively electrified. An atom can, therefore, by various
means be divided into two portions of unequal size. First, a very
small part which is charged with negative electricity, and, secondly,
a remaining larger portion charged with positive electricity. These
two parts taken together are called ions, _i.e._ wanderers. The
negative ions, or electrons, or corpuscles, taken together constitute
what we call negative electricity, and up to the present no one has
been able to show that the corpuscle can be unelectrified. Hence the
view has been expressed that what we call electricity is a kind of
matter, atomic in structure, and that these negative ions or corpuscles
collectively are, in fact, the atoms of the electric fluid. These
corpuscles can move freely in the interior of some solids, moving
between the molecules of the solid just as little dogs can run about in
and amongst a crowd of people in a street. In these cases the substance
is called a conductor of electricity. In other substances the movement
of the corpuscles is more restricted, and these constitute the various
kinds of so-called non-conductors.
The corpuscle, being a small charge of negative electricity, creates
in all surrounding space a state called _electric force_. It is
impossible to expound this action more in detail without the use of
mathematical reasoning of a difficult character. Suffice it to say
that this electric force must be a particular condition of strain or
motion in the æther. If the corpuscle is in rapid motion, it creates
in addition another kind of strain or motion called _magnetic force_.
The electric force and the magnetic force are related to each other
in free space in such a manner that if we know the difference between
the values of the electric force at two very near points in space, we
are able to tell the rate at which the magnetic force is changing with
time in a direction at right angles to the line joining these near
points in space. We cannot specify in greater detail the exact nature
of these states or conditions which constitute magnetic force and
electric force, until we know much more than we do at present about the
real nature of the æther. The two fundamental qualities of the æther
are, however, its capacity to sustain these states we call the magnetic
force and the electric force.
The electrons of which we have spoken not only give rise to electric
and magnetic force when in movement, but they are themselves set
in motion by these forces. Thus electric force at any point moves
electrons placed at that spot, and an electron in motion is affected
and has its direction of motion changed when magnetic force acts on it.
Leaving further remarks on the relations of atoms, electricity, and
æther until the end of the last lecture, we may conclude the present
one by explaining the manner in which the observed facts connected with
a Hertz oscillator are interpreted in terms of this electron hypothesis
of electricity.
Take the simple case of two long insulated metal rods separated by a
spark-gap. The process of charging one rod positively and the other
negatively consists in forcing more corpuscles, or negative ions
or electrons, into one conductor and removing some from the other.
Any source of electromotive force, such as a dynamo or induction
coil, is, on this hypothesis, a sort of _electron-pump_, which pumps
electrons from one conductor and puts them into another. One conductor,
therefore, gains in electron-pressure, and the other loses.
The excess of electrons in one conductor endeavour to escape, and
a strain is produced on the electrons or atoms in the surrounding
dielectric or air, which may be looked upon as the effort of the
electrons, more or less tethered to the atoms, to escape. The air in
the spark-gap is subjected to the most intense strain, and when this
reaches a certain intensity some of the electrons are torn away from
their atoms, and the air in the gap then becomes a conductor. The
excess of electrons in one conductor rush through the channel thus
prepared, and this constitutes an electric current. The first rush
carries over too many electrons to equilibrate the electron-pressure,
and hence the first torrent of migrating electrons in one direction
is followed by a back-rush in the opposite one, this again in turn by
another in the original direction, and so the equality in the number
of electrons in each conductor is only established after a gradually
diminishing series of to-and-fro rushes of electrons across the
air-gap. This action constitutes a train of electrical oscillations.
At the same time that these operations are going on in and between
the conductors, the electrons attached to the atoms of the air or
other dielectric all around are being violently oscillated. These
oscillations may not proceed to such an extent as to detach electrons
from their atoms, but they are sufficient to create rapidly reversed
electric and magnetic forces. It appears that the very rapid movement
to and fro of an electron causes a wave in the æther, just as the rapid
movement of the hand through water causes a wave in water, or the
vibration of the prong of a tuning-fork creates a wave in the air.
The electron has some grip on the æther, such that the sudden starting
or stopping of the electron makes a disturbance which we may popularly
describe as a _splash in the æther_. Hence, if a large number of
electrons are suddenly started into motion in the same direction, the
effect on the æther is something like casting a multitude of stones on
the surface of still water, or the simultaneous action of a number of
small explosions in the air. Anything, therefore, which, so to speak,
lets the electrons go gradually, or softens the first rush, is inimical
to the production of a vigorous electric wave. On the other hand,
anything which causes the first rush of electrons from one conductor
to another across the air-gap to be very sudden is advantageous, and
results in a powerful wave. Experience shows that the nature of the
metal surfaces, whether polished or rough, has a great influence on
the wave-making power of the radiator. If the spark-balls or surfaces
are rough and not polished, it seems to tone down the violence of the
first electron rush, and the wave-making power of the oscillator is not
so great as if the balls are polished.
At this point, however, it will be best to withhold further discussion
on points of theory until we have considered the facts to be brought
before you in the next lecture, showing that the electric radiation
manufactured by means of electric oscillations is only one variety of a
vast range of æther waves, some forms of which are recognizable by us
as light and radiant heat.