TOC | Previous Section: G. Marconi's Method | Next Section: Appendix B
A History of Wireless Telegraphy (2nd edition, revised), J. J. Fahie, 1901, pages 262-276:
THE RELATION BETWEEN ELECTRICITY AND LIGHT--BEFORE AND AFTER HERTZ.
SUBSTANCE of a lecture by Prof. Oliver Lodge, London Institution, December 16, 1880. 1
Ever since the subject on which I have to speak to-night was arranged, I have been astonished at my own audacity in proposing to deal, in the course of sixty minutes, with a subject so gigantic and so profound that a course of sixty lectures would be inadequate for its thorough and exhaustive treatment. I must, therefore, confine myself to some few of the most salient points in the relation between electricity and light, and I must economise time by plunging at once into the middle of the matter without further preliminary.
What is electricity? We do not know. We cannot assert that it is a form of matter; neither can we deny it. On the other hand, we cannot certainly assert that it is a form of energy; and I should be disposed to deny it. It may be that electricity is an entity per se, just as matter is an entity per se. Nevertheless, I can tell you what I mean by electricity by appealing to its known behaviour.
Here is a voltaic battery. I want you to regard it, and all electrical machines and batteries, as kinds of electricity-pumps, which drive the electricity along through the wire very much as a water-pump can drive water along pipes. W hile this is going on, the wire manifests a whole series of properties, which are called the properties of the current.
[Here were shown an ignited platinum wire, the electric arc between two carbons, an electric machine spark, an induction coil spark, and a vacuum tube glow. Also a large nail was magnetised by being wrapped in the current, and two helices were suspended and seen to direct and attract each other.]
To make a magnet, then, we only need a current of electricity flowing round and round in a whirl. A vortex or whirlpool of electricity is in fact a magnet, and vice versâ, And these whirls have the power of directing and attracting other previously existing whirls according to certain laws, called the laws of magnetism. And, moreover, they have the power of exciting fresh whirls in neighbouring conductors, and of repelling them according to the laws of diamagnetism. The theory of the actions is known, though the nature of the whirls, as of the simple streams of electricity, is at present unknown.
[Here was shown a large electro-magnet and an induction-coil vacuum discharge spinning round and round when placed in its field.]
So much for what happens when electricity is made to travel along conductors--i.e., when it travels along like a stream of water in a pipe, or spins round and round like a whirlpool.
But there is another set of phenomena, usually regarded as distinct and of another order, but which are not so distinct as they appear, which manifest themselves when you join the pump to a piece of glass or any non-conductor and try to force the electricity through that. You succeed in driving some through, but the flow is no longer like that of water in an open pipe; it is as if the pipe were completely obstructed by a number of elastic partitions or diaphragms. The water cannot move without straining and bending these diaphragms, and if you allow it, these strained partitions will recover themselves and drive the water back again. [Here was explained the process of charging a Leyden jar.] The essential thing to remember is that we may have electrical energy in two forms, the static and the kinetic; and it is therefore also possible to have the rapid alternation from one of these forms to the other, called vibration.
Now we will pass to the second question: What do you mean by light? And the first and obvious answer is, Everybody knows. And everybody that is not blind does know to a certain extent. We have a special sense-organ for appreciating light, whereas we have none for electricity. Nevertheless, we must admit that we really know very little about the intimate nature of light--very little more than about electricity. But we do know this, that light is a form of energy; and, moreover, that it is energy rapidly alternating between the static and the kinetic forms--that it is, in fact, a special kind of energy of vibration. We are absolutely certain that light is a periodic disturbance in some medium, periodic both in space and time--that is to say, the same appearances regularly recur at certain equal intervals of distance at the same time, and also present themselves at equal intervals of time at the same place; that, in fact, it belongs to the class of motions called by mathematicians undulatory or wave motions.
Now how much connection between electricity and light have we perceived in this glance into their natures? Not much truly. It amounts to about this: That on the one hand electrical energy may exist in either of two forms--the static form, when insulators are electrically strained by having had electricity driven partially through them (as in the Leyden jar), which strain is a form of energy, because of the tendency to discharge and do work; and the kinetic form, where electricity is moving bodily along through conductors, or whirling round and round inside them, which motion of electricity is a form of energy, because the conductors and whirls can attract or repel each other and thereby do work.
On the other hand, light is the rapid alternation of energy from one of these forms to the other--the static form where the medium is strained, to the kinetic form when it moves. It is just conceivable then that the static form of the energy of light is electro-static--that is, that the medium is electrically strained--and that the kinetic form of the energy of light is electro-kinetic--that is, that the motion is not ordinary motion, but electrical motion--in fact, that light is an electrical vibration, not a material one.
On November 5 last year there died at Cambridge a man in the full vigour of his faculties--such faculties as do not appear many times in a century--whose chief work had been the establishment of this very fact, the discovery of the link connecting light and electricity, and the proof--for I believe that it amounts to a proof--that they are different manifestations of one and the same class of phenomena,--that light is, in fact, an electro-magnetic disturbance. The premature death of James Clerk-Maxwell is a loss to science which appears at present utterly irreparable, for he was engaged in researches that no other man can hope as yet adequately to grasp and follow out; but fortunately it did not occur till he had published his book on 'Electricity and Magnetism,' one of those immortal productions which exalt one's idea of the mind of man, and which has been mentioned by competent critics in the same breath as the 'Principia, itself.
The main proof of the electro-magnetic theory of light is this: The rate at which light travels has been measured many times, and is pretty well known. The rate at which an electro-magnetic wave disturbance would travel, if such could be generated (and Mr Fitzgerald, of Dublin, thinks he has proved that it cannot be generated directly by any known electrical means), can be also determined by calculation from electrical measurements. The two velocities agree exactly.
The first glimpse of this splendid generalisation was caught in 1845, five-and-thirty years ago, by that prince of pure experimentalists, Michael Faraday. His reasons for suspecting some connection between electricity and light are not clear to us--in fact, they could not have been clear to him; but he seems to have felt a conviction that if he only tried long enough, and sent all kinds of rays of light in all possible directions across electric and magnetic fields in all sorts of media, he must ultimately hit upon something. Well, this is very nearly what he did. With a sublime patience and perseverance which remind one of the way Kepler hunted down guess after guess in a different field of research, Faraday combined electricity, or magnetism, and light in all manner of ways, and at last he was rewarded with a result--and a most out-of-the-way result it seemed. First, you have to get a most powerful magnet, and very strongly excite it; then you have to pierce its two poles with holes, in order that a beam of light may travel from one to the other along the lines of force; then, as ordinary light is no good, you must get a beam of plane polarised light and send it between the poles. But still no result is obtained until, finally, you interpose a piece of a rare and out-of-the-way material which Faraday had himself discovered and made, a kind of glass which contains borate of lead, and which is very heavy or dense, and which must be perfectly annealed.
And now, when all these arrangements are completed, what is seen is simply this, that if an analyser is arranged to stop the light and make the field quite dark before the magnet is excited, then directly the battery is connected and the magnet called into action a faint and barely perceptible brightening of the field occurs, which will disappear if the analyser be slightly rotated. [The experiment was shown.] Now, no wonder that no one understood this result. Faraday himself did not understand it at all. He seems to have thought that the magnetic lines of force were rendered luminous, or that the light was magnetised; in fact he was in a fog, and had no idea of its real significance. Nor had any one. Continental philosophers experienced some difficulty and several failures before they were able to repeat the experiment. It was, in fact, discovered too soon, and before the scientific world was ready to receive it, and it was reserved for Sir William Thomson briefly, but very clearly, to point out, and for Clerk-Maxwell more fully to develop, its most important consequences.
This is the fundamental experiment on which Clerk-Maxwell's theory of light is based; but of late years many fresh facts and relations between electricity and light have been discovered, and at the present time they are tumbling in in great numbers.
It was found by Faraday that many other transparent media besides heavy glass would show the phenomenon if placed between the poles, only in a less degree; and the very important observation that air itself exhibits the same phenomenon, though to an exceedingly small extent, has just been made by Kundt and Röntgen in Germany.
Dr Kerr, of Glasgow, has extended the result to opaque bodies, and has shown that if light be passed through magnetised iron, its plane is rotated. The film of iron must be exceedingly thin, because of its opacity; and hence, though the intrinsic rotating power of iron is undoubtedly very great, the observed rotation is exceedingly small and difficult to observe; and it is only by very remarkable patience and care and ingenuity that Dr Kerr has obtained his result. Mr Fitzgerald, of Dublin, has examined the question mathematically, and has shown that Maxwell's theory would have enabled Dr Kerr's result to be predicted.
Another requirement of the theory is that bodies which are transparent to light must be insulators or non-conductors of electricity, and that conductors of electricity are necessarily opaque to light. Simple observation amply confirms this. Metals are the best conductors, and are the most opaque bodies known. Insulators such as glass and crystals are transparent whenever they are sufficiently homogeneous, and the very remarkable researches of Professor Graham Bell in the last few months have shown that even ebonite, one of the most opaque insulators to ordinary vision, is certainly transparent to some kinds of radiation, and transparent to no small degree.
[The reason why transparent bodies must insulate, and why conductors must be opaque, was here illustrated by mechanical models.]
A further consequence of the theory is that the velocity of light in a transparent medium will be affected by its electrical strain constant; in other words, that its refractive index will bear some close but not yet quite ascertained relation to its specific inductive capacity. Experiment has partially confirmed this, but the confirmation is as yet very incomplete.
But there are a number of results not predicted by theory, and whose connection with the theory is not clearly made out. We have the fact that light falling on the platinum electrode of a voltameter generates a current, first observed, I think, by Sir W. R. Grove; at any rate it is mentioned in his 'Correlation of Forces'--extended by Becquerel and Robert Sabine to other substances, and now being extended to fluorescent and other bodies by Professor Minchin. And finally--for I must be brief--we have the remarkable action of light on selenium. This fact was discovered accidentally by an assistant in the laboratory of Mr Willoughby Smith, who noticed that a piece of selenium conducted electricity very much better when light was falling upon it than when it was in the dark. The light of a candle is sufficient, and instantaneously brings down the resistance to something like one-fifth of its original value.
This is the phenomenon which, as you know, has been utilised by Professor Graham Bell in that most ingenious and striking invention, the photophone.
I have now trespassed long enough upon your patience, but I must just allude to what may very likely be the next striking popular discovery, and that is the transmission of light by electricity. I mean the transmission of such things as views and pictures by means of the electric wire. It has not yet been done, but it seems already theoretically possible, and it may very soon be practically accomplished.
THE RELATION BETWEEN ELECTRICITY AND LIGHT.
Substance of a lecture by Prof. Oliver Lodge, Ashmolean Society, Oxford, June 3, 1889. 2
For now wellnigh a century we have had a wave-theory of light; and a wave-theory of light is certainly true. It is directly demonstrable that light consists of waves of some kind or other, and that these waves travel at a certain well-known velocity, seven times the circumference of the earth per second, taking eight minutes on the journey from the sun to the earth. This propagation in time of an undulatory disturbance necessarily involves a medium. If waves setting out from the sun exist in space eight minutes before striking our eyes, there must necessarily be in space some medium in which they exist and which conveys them. Waves we cannot have unless they be waves in something.
No ordinary medium is competent to transmit waves at anything like the speed of light; hence the luminiferous medium must be a special kind of substance, and it is called the ether. The luminiferous ether it used to be called, because the conveyance of light was all it was then known to be capable of; but now that it is known to do a variety of other things also, the qualifying adjective may be dropped.
Wave motion in ether light certainly is; but what does one mean by the term wave? The popular notion is, I suppose, of something heaving up and down, or perhaps of something breaking on the shore in which it is possible to bathe. But if you ask a mathematician what he means by a wave, he will probably reply that the simplest wave is
y=a sin (p t -- n x),
and he might possibly refuse to give any other answer. And in refusing to give any other answer than this, or its equivalent in ordinary words, he is entirely justified; that is what is meant by the term wave, and nothing less general would be all-inclusive.
Translated into ordinary English, the phrase signifies "a disturbance periodic both in space and time." Anything thus doubly periodic is a wave; and all waves--whether in air as sound waves, or in ether as light waves, or on the surface of water as ocean waves--are comprehended in the definition.
What properties are essential to a medium capable of transmitting wave motion? Roughly we may say two--elasticity and inertia. Elasticity in some form, or some equivalent of it, in order to be able to store up energy and effect recoil; inertia, in order to enable the disturbed substance to overshoot the mark and oscillate beyond its place of equilibrium to and fro. Any medium possessing these two properties can transmit waves, and unless a medium possesses these properties in some form or other, or some equivalent for them, it may be said with moderate security to be incompetent to transmit waves. But if we make this latter statement one must be prepared to extend to the terms elasticity and inertia their very largest and broadest signification, so as to include any possible kind of restoring force and any possible kind of persistence of motion respectively.
These matters may be illustrated in many ways, but perhaps a simple loaded lath or spring in a vice will serve well enough. Pull aside one end, and its elasticity tends to make it recoil; let it go, and its inertia causes it to overshoot its normal position: both causes together cause it to swing to and fro till its energy is exhausted. A regular series of such springs at equal intervals in space, set going at regular intervals of time one after the other, gives you at once a wave motion and appearance which the most casual observer must recognise as such. A series of pendulums will do just as well. Any wave-transmitting medium must similarly possess some form of elasticity and of inertia.
But now proceed to ask what is this ether which in the case of light is thus vibrating? What corresponds to the elastic displacement and recoil of the spring or pendulum? What corresponds to the inertia whereby it overshoots its mark? Do we know these properties in the ether in any other way?
The answer, given first by Clerk-Maxwell, and now reiterated and insisted on by experiments performed in every important laboratory in the world, is--
The elastic displacement corresponds to electro-static charge (roughly speaking, to electricity).
The inertia corresponds to magnetism.
This is the basis of the modern electro-magnetic theory of light. Now let me illustrate electrically how this can be.
The old and familiar operation of charging a Leyden jar--the storing up of energy in a strained dielectric--any electro-static charging whatever--is quite analogous to the drawing aside of our flexible spring. It is making use of the elasticity of the ether to produce a tendency to recoil. Letting go the spring is analogous to permitting a discharge of the jar--permitting the strained dielectric to recover itself, the electro-static disturbance to subside.
In nearly all the experiments of electro-statics ethereal elasticity is manifest.
Next consider inertia. How would one illustrate the fact that water, for instance, possesses inertia--the power of persisting in motion against obstacles--the power of possessing kinetic energy? The most direct way would be to take a stream of water and try suddenly to stop it. Open a water-tap freely and then suddenly shut it. The impetus or momentum of the stopped water makes itself manifest by a violent shock to the pipe, with which everybody must be familiar. The momentum of water is utilised by engineers in the "water-ram."
A precisely analogous experiment in electricity is what Faraday called "the extra current." Send a current through a coil of wire round a piece of iron, or take any other arrangement for developing powerful magnetism, and then suddenly stop the current by breaking the circuit. A violent flash occurs if the stoppage is sudden enough, a flash which means the bursting of the insulating air partition by the accumulated electro-magnetic momentum.
Briefly, we may say that nearly all electro-magnetic experiments illustrate the fact of ethereal inertia.
Now return to consider what happens when a charged conductor (say a Leyden jar) is discharged. The recoil of the strained dielectric causes a current, the inertia of this current causes it to overshoot the mark, and for an instant the charge of the jar is reversed the current now flows backwards and charges the jar up as at first; again flows the current, and so on, discharging and charging the jar with rapid oscillations until the energy is all dissipated into heat. The operation is precisely analogous to the release of a strained spring, or to the plucking of a stretched string.
But the discharging body thus thrown into strong electrical vibration is embedded in the all-pervading ether, and we have just seen that the ether possesses the two properties requisite for the generation and transmission of waves--viz., elasticity, and inertia or density; hence, just as a tuning-fork vibrating in air excites aerial waves or sound, so a discharging Leyden jar in ether excites ethereal waves or light.
Ethereal waves can therefore be actually produced by direct electrical means. I discharge here a jar, and the room is for an instant filled with light. With light, I say, though you can see nothing. You can see and hear the spark indeed,--but that is a mere secondary disturbance we can for the present ignore--I do not mean any secondary disturbance. I mean the true ethereal waves emitted by the electric oscillation going on in the neighbourhood of this recoiling dielectric. You pull aside the prong of a tuning-fork and let it go: vibration follows and sound is produced. You charge a Leyden jar and let it discharge: vibration follows and light is excited.
It is light just as good as any other light. It travels at the same pace, it is reflected and refracted according to the same laws; every experiment known to optics can be performed with this ethereal radiation electrically produced, and yet you cannot see it. Why not? For no fault of the light; the fault (if there be a fault) is in the eye. The retina is incompetent to respond to these vibrations--they are too slow. The vibrations set up when this large jar is discharged are from a hundred thousand to a million per second, but that is too slow for the retina. It responds only to vibrations between 4000 billions and 7000 billions per second. The vibrations are too quick for the ear, which responds only to vibrations between 40 and 40,000 per second. Between the highest audible and the lowest visible vibrations there has been hitherto a great gap, which these electric oscillations go far to fill up. There has been a great gap simply because we have no intermediate sense-organ to detect rates of vibration between 40,000 and 4,000,000,000,000,000 per second. It was, therefore, an unexplored territory. Waves have been there all the time in any quantity, but we have not thought about them nor attended to them.
It happens that I have myself succeeded in getting electric oscillations so slow as to be audible. The lowest I have got at present are 125 per second, and for some way above this the sparks emit a musical note; but no one has yet succeeded in directly making electric oscillations which are visible, though indirectly every one does it by lighting a candle.
Here, however, is an electric oscillator which vibrates 300 million times a second, and emits ethereal waves a yard long. The whole range of vibrations between musical tones and some thousand millions per second is now filled up.
These electro-magnetic waves have long been known on the side of theory, but interest in them has been immensely quickened by the discovery of a receiver or detector for them. The great though simple discovery by Hertz of an "electric eye," as Sir W. Thomson calls it, makes experiments on these waves for the first time possible, or even easy. We have now a sort of artificial sense-organ for their appreciation--an electric arrangement which can virtually "see" these intermediate rates of vibration.
The Hertz receiver is the simplest thing in the world--nothing but a bit of wire, or a pair of bits of wire, adjusted so that when immersed in strong electric radiation they give minute sparks across a microscopic air-gap.
The receiver I have here is adapted for the yard-long waves emitted from this small oscillator; but for the far longer waves emitted by a discharging Leyden jar an excellent receiver is a gilt wall-paper or other interrupted metallic surface. The waves falling upon the metallic surface are reflected, and in the act of reflection excite electric currents, which cause sparks. Similarly, gigantic solar waves may produce auroræ and minute waves from a candle do electrically disturb the retina.
The smaller waves are, however, far the most interesting and the most tractable to ordinary optical experiments. From a small oscillator, which may be a couple of small cylinders kept sparking into each other end to end by an induction coil, waves are emitted on which all manner of optical experiments can be performed.
They can be reflected by plain sheets of metal, concentrated by parabolic reflectors, refracted by prisms, concentrated by lenses. I have at the College a large lens of pitch, weighing over 3 cwt., for concentrating them to a focus. They can be made to show the phenomenon of interference, and thus have their wave lengths accurately measured. They are stopped by all conductors, and transmitted by all insulators. Metals are opaque, but even imperfect insulators, such as wood or stone, are strikingly transparent, and waves may be received in one room from a source in another, the door between the two being shut.
The real nature of metallic opacity and of transparency has long been clear in Maxwell's theory of light, and these electrically produced waves only illustrate and bring home the well-known facts. The experiments of Hertz are in fact the apotheosis of that theory.
Thus, then, in every way Maxwell's brilliant perception of the real nature of light is abundantly justified; and for the first time we have a true theory of light, no longer based upon analogy with sound, nor upon a hypothetical jelly or elastic solid.
Light is an electro-magnetic disturbance of the ether. Optics is a branch of electricity. Outstanding problems in optics are being rapidly solved now that we have the means of definitely exciting light with a full perception of what we are doing, and of the precise mode of its vibration.
It remains to find out how to shorten down the waves--to hurry up the vibration until the light becomes visible. Nothing is wanted but quicker modes of vibration. Smaller oscillators must be used--very much smaller--oscillators not much bigger than molecules. In all probability--one may almost say certainly--ordinary light is the result of electric oscillation in the molecules of hot bodies, or sometimes of bodies not hot--as in the phenomenon of phosphorescence.
The direct generation of visible light by electric means, so soon as we have learnt how to attain the necessary frequency of vibration, will have most important practical consequences.
For consider our present methods of making artificial light: they are both wasteful and ineffective.
We want a certain range of oscillation, between 7000 and 4000 billion vibrations per second,--no other is useful to us, because no other has any effect upon our retina; but we do not know how to produce vibrations of this rate. We can produce a definite vibration of one or two hundred or thousand per second--in other words, we can excite a pure tone of definite pitch; and we can command any desired range of such tones continuously by means of bellows and a keyboard. We can also (though the fact is less well known) excite momentarily definite ethereal vibrations of some millions per second, as I have explained; but we do not at present seem to know how to maintain this rate quite continuously. To get much faster rates of vibration than this, we have to fall back upon atoms. We know how to make atoms vibrate,--it is done by what we call "heating" the substance; and if we could deal with individual atoms unhampered by others, it is possible that we might get a pure and simple mode of vibration from them. It is possible, but unlikely; for atoms, even when isolated, have a multitude of modes of vibration special to themselves, of which only a few are of practical use to us, and we do not know how to excite some without also the others. However, we do not at present even deal with individual atoms; we treat them crowded together in a compact mass, so that their modes of vibration are really infinite.
We take a lump of matter, say a carbon filament or a piece of quicklime, and by raising its temperature we impress upon its atoms higher and higher modes of vibration, not transmuting the lower into the higher, but superposing the higher upon the lower, until at length we get such rates of vibration as our retina is constructed for, and we are satisfied. But how wasteful and indirect and empirical is the process! We want a small range of rapid vibrations, and we know no better than to make the whole series leading up to them. It is as though, in order to sound some little shrill octave of pipes in an organ, we are obliged to depress every key and every pedal, and to blow a young hurricane.
I have purposely selected as examples the more perfect methods of obtaining artificial light, wherein the waste radiation is only useless, and not noxious. But the old-fashioned plan was cruder even than this: it consisted simply in setting something burning, whereby not the fuel but the air was consumed; whereby also a most powerful radiation was produced, in the waste waves of which we were content to sit stewing, for the sake of the minute--almost infinitesimal--fraction of it which enabled us to see.
Every one knows now, however, that combustion is not a pleasant or healthy mode of obtaining light; but everybody does not realise that neither is incandescence a satisfactory and unwasteful method, which is likely to be practised for more than a few decades, or perhaps a century.
Look at the furnaces and boilers of a great steam-engine driving a group of dynamos, and estimate the energy expended; and then look at the incandescent filaments of the lamps excited by them, and estimate how much of their radiated energy is of real service to the eye. It will be as the energy of a pitch-pipe to an entire orchestra.
It is not too much to say that a boy turning a handle could, if his energy were properly directed, produce quite as much real light as is produced by all this mass of mechanism and consumption of material. There might, perhaps, be something contrary to the laws of nature in thus hoping to get and utilise some specific kind of radiation without the rest; but Lord Rayleigh has shown in a short communication to the British Association at York that it is not so, and that therefore we have a right to try to do it.
We do not yet know how, it is true, but it is one of the things we have got to learn.
Any one looking at a common glowworm must be struck with the fact that not by ordinary combustion, nor yet on the steam-engine and dynamo principle, is that easy light produced. Very little waste radiation is there from phosphorescent things in general. Light of the kind able to affect the retina is directly emitted; and for this, for even a large supply of this, a modicum of energy suffices.
Solar radiation consists of waves of all sizes, it is true; but then solar radiation has innumerable things to do besides making things visible. The whole of its energy is useful. In artificial lighting nothing but light is desired; when heat is wanted it is best obtained separately by combustion. And so soon as we clearly recognise that light is an electrical vibration, so soon shall we begin to beat about for some mode of exciting and maintaining an electrical vibration of any required degree of rapidity. When this has been accomplished, the problem of artificial lighting will have been solved.
1 Based on a report in 'Design and Work,' February 5, 1881.
2 Based on a report in the (London) 'Electrician,' September 6, 1889.
TOC | Previous Section: G. Marconi's Method | Next Section: Appendix B