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:⁠—

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—

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.

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.

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:⁠—

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.

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.

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.

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:⁠—