Popular Science Monthly/Volume 38/December 1890/The Identity of Light and Electricity
|THE IDENTITY OF LIGHT AND ELECTRICITY.|
By HENRI HERTZ.
OUR first thought, when we speak of the relations of light and electricity, is of the electric light. That is not the subject of the present paper. The physicist thinks of the extremely delicate reciprocal actions of the two forces, such as the rotation by the current of the plane of polarization, or the variation under the influence of light of the resistance of a conductor. In these cases, however, the action is not direct, but a medium, ponderable matter, is interposed. There are other closer, more intimate relations between the two forces. It is my purpose to discuss the proposition that light is in its very essence an electrical phenomenon—whether it be the light of the sun, of a candle, or of a glow-worm. Suppress electricity in the universe, light would disappear; suppress the luminiferous ether, electric and magnetic forces would cease to act through space. This theory is not of to-day or of yesterday, but has a long and instructive history. My own experiments only mark one of the steps in its development; and it is my purpose to retrace its whole evolution, not one of its phases only. It is not easy in a matter of this kind to be clear without omitting something essential. The phenomena to be considered take place in space, in the ether itself, and are not perceptible to the touch or the hearing or the sight. Reflection and reasoning may permit us to grasp them, but it is hard to make an exact description of them. We shall endeavor, therefore, to connect them with ideas that are already known to us. We refer, therefore, first to what we already know concerning light and electricity.
We know of a certainty that light is an undulatory movement, and that the undulations are transversal; we have determined their length and their velocity; and all that follows from these facts is equally certain. It is, therefore, sure that all of space that is accessible to us is not void, but is filled with a substance capable of entering into vibration—the ether. But while we have clear notions of the geometrical conditions of the phenomena that occur in this matter, their physical nature is very obscure; and what we know of the properties of the substance is full of contradictions. Comparing the waves of light with those of sound, they were regarded as elastic. But only longitudinal waves have been observed in fluids, and under the conditions of matter transverse waves are impossible in them. We have been obliged, therefore, to assume that the ether acts as a solid body. But when we regard the motions of the stars and endeavor to determine their conditions, we have to affirm that ether behaves like a perfect fluid. Without endeavoring at present to explain the contradiction that presents itself here, we pass to electricity; it may throw some light on the problem.
Most of the persons who ask what electricity is have no doubts respecting its real existence, and only expect a description of the properties of the singular substance. With scientific man, the problem takes the form, Does electricity really exist? Do not electric phenomena, like the other ones, go back to properties of ether and ponderable matter? Our knowledge does not as yet permit us to answer this question affirmatively. Material electricity still has a place in our conceptions, and the old and familiar idea of two kinds attracting and repelling one another, to which are attributed actions at a distance resembling intellectual qualities, still persists in current language. This theory dated from the time when Newton's law of gravitation having been confirmed by astronomy, the idea of action at a distance without the intervention of a medium was familiar. Electric and magnetic attractions were thought to obey the same law as gravitation; and, admitting a similar action at a distance, the phenomenon was supposed to be explained in the simplest manner, and the limits of knowledge on the subject to have been reached. A different aspect was presented when in this century the reciprocal action of currents and magnets was discovered, an action infinitely variable, in which motion and time played a great part. In the necessity of increasing the number of actions at a distance to complete the theory, the simplicity which gave it its scientific probability disappeared. Simple formulas and general and elementary laws were then sought, of which Weber's law was the most important tentative. Whatever may be thought of the exactness of these essays, they formed an exceptional system and a seductive whole, a magic circle, which one could not leave after having once entered it. The road was one that could not lead to the truth. It required a fresh mind to resist the current, one that could enter upon, the study of the phenomena without preconceived opinions, and was capable of starting from what it observed, and not from what it had heard, read, or learned.
Faraday followed that course. He had heard that, in electrifying a body, something new was introduced into it; but he saw that the changes were external, and not within. He was told that the forces traversed space, but he remarked that the nature of the matter that filled the space had great influence on them. He had read that electricities existed, and that we only had to consider their properties; and yet he observed every day the effects of the forces without ever seeing the electricities themselves: in this way he reversed the proposition. The electric and magnetic forces became to him the only tangible reality, while electricity and magnetism fell to the rank of objects the existence of which is contestable. Considering these lines of forces, as he called them, independently of their cause, he regarded them under the form of states of space, tension, whorls, and currents, without occupying himself with what they might really be. He was satisfied with having established their existence, with observing their influence upon each other, their attractions for material bodies, and their propagation by the transmission of the excitation from one point of space to another. If it was objected that there could be no other state than absolute rest in empty space, he could answer: "Is space, then, empty? Does not the transmission of light force us to regard it as filled with matter? Can not the ether, which transmits the luminous waves, suffer modifications which we perceive under the form of electrical and magnetic actions? Is there not a relation between these modifications and these vibrations? Are not the luminous waves a kind of scintillation of these lines of force?" Such were the inductions and hypotheses which Faraday conceived. They were as yet only mental views; he applied himself earnestly to demonstrate them scientifically; and the relations of light, electricity, and magnetism became the favorite object of his studies.
The relation he found was not the one he sought. He continued his researches till age put an end to his labors. One of his principal questions was whether the transmission of electrical and magnetic forces is instantaneous. Is the magnetic field constituted at once to the limits of space whenever the current excites an electro-magnet? Or does the action first reach the nearer points and gradually propagate itself to the more remote ones? And is the sudden modification of the electric condition of a body felt simultaneously in identical variations, in all points of space, or is there a retardation augmented as the distance increases? In the latter case, the effect of the variation would be transmitted as a wave through space. Do such waves exist? Faraday obtained no answer to his questions, but the solution of them is directly related to his theories. If electric waves crossing space exist, the independence of the forces that produce them is demonstrated. We know that the forces do not traverse vacua instantaneously, for we can follow their propagation each instant from one point to another. Faraday's problems can, however, be solved by very simple experiments. If they had occurred to him, his theory would have triumphed at once. The relation of light and electricity would have been so clear that it could not have escaped even a less perspicacious eye than his own.
But so simple and speedy a way was not yet open to science. The first experiments brought no solution, and the current view was inconsistent with Faraday's ideas. In affirming that electric forces could exist independent of corresponding fluids, he contradicted the theory generally received at the time. A fundamental discussion of either hypothesis promised to be only a barren speculation. How much, then, should we admire the man who had the sagacity to co-ordinate these two hypotheses, apparently so distantly separated, so that they should eventually support one another, and a theory come out of them to which it should be impossible to deny probability! This man was Clerk Maxwell, whose Mathematical Theory of Light was published in 1865. We can not study the theory without feeling that mathematical formulas have a life of their own, and that they appear sometimes more intelligent than we ourselves, and even than the master who established them, giving out more than he looked for in them. Direction was given to Maxwell's researches by the fact that magnetic forces are produced from electricity in motion, and electric forces from magnetism in motion, but the effects were not appreciable except at great velocities. The idea of velocity, therefore, enters into the relation between electricity and magnetism, and the constant determining this relation, which is always found in it, is a velocity of enormous value. The velocity of electricity had been determined by delicate researches, and found equal to that of light. A disciple of Faraday could not fail to explain this coincidence by supposing that the same ether carried the electric forces and light. Hence the most important optical constant already existed in the electrical formulas. Maxwell labored to confirm this connection between the two orders of phenomena. He extended the electrical formulas so as to make them express, along with all the known phenomena, an entire class of hypothetical facts electrical undulations. He figured them as transversal waves, the length of which might have any value, but which propagated themselves through the ether at a constant velocity, that of light. It was then possible for Maxwell to demonstrate that there really exist in nature undulations possessing those properties, although, we were not in the habit of regarding them as electrical phenomena, and gave them the name of light. If Maxwell's electrical theory was rejected, there was no more reason for accepting his views concerning light. In like manner, if it was affirmed that light is a phenomenon of an elastic nature, his theory of electricity became impossible. But when his theory was studied without prepossession with the ideas that were current, all the parts could be seen to lend one another a mutual support, like the stones of a vault, and the whole resembled a gigantic arch thrown across the unknown, and uniting two known truths.
The difficulty of the theory did not permit it at first to acquire a large number of partisans. But after its inner sense was discerned it was followed out to its ultimate consequences, and then the value of its fundamental hypotheses was tested. Experiments were at first limited to a few propositions, the accessory parts of the theory. I have compared Maxwell's system to an arch traversing an abyss of the unknown. I might add that it was some time before the abutments could be connected. It was thus put in a position where it could support itself, but the span was too wide to permit any new structure to be built upon it. To accomplish that object pillars were needed, rising from the ground, to support the middle of the arch. The demonstration of the possibility of obtaining electrical or magnetic effects directly from light would constitute one of the pillars and confirm the theory; it would have immediately established the electrical part, and indirectly the optical part of it. The completion and symmetry of the structure demanded the building of both the pillars to which we compare these principles, but one was enough to begin with. The construction of the former pillar has not yet been undertaken; but after a multitude of researches a solid base has been found for the second, with sufficiently ample foundations, on which a part of the pillar has been raised. With the co-operation of many workers it will soon reach the top of the arch and afford support to the weight of the edifice which is to be raised upon it.
I have had the privilege of taking part in this portion of the work. To this fact I owe it that I am now laying my ideas before you; and I hope that I may be excused if I try at present to direct all attention to this part of the edifice. I shall unhappily be obliged, for want of time, to omit the labors of a large number of seekers, and shall be unable to show to what extent my experiments had been prepared for by my predecessors, and how near some of them had come to a definite result.
It does not at first seem so difficult to show whether propagation of electrical or magnetic forces is or is not instantaneous; to discharge a Leyden jar, and observe whether there is any delay in the response of an electroscope a little distance off; or to observe the needle while a remote electro-magnet is excited. But these experiments, and others like them, have been tried without any interval being determined between the cause and the effect. An upholder of Maxwell's theory understands that such failures are inevitable, and arise from the enormous rapidity of the transmission. We can only perceive the discharge of a Leyden jar, or the excitation of an electro-magnet, from a moderate distance, say, of ten metres. But light, and electricity as well, according to the theory, pass over such a space in a thirty-millionth of a second. So short an interval of time can be neither perceived nor measured directly. Furthermore, we have no signals by which to define that instant. We do not make a big chalk-mark when we want to tell off a tenth of a millimetre. It would be quite as absurd, in determining a duration of a thousandth of a second, to depend on the sound of a large bell to mark the beginning of the moment.
The time required for the discharge of a Leyden jar is, according to our common means of observation, infinitely short. That does not mean that it is not equal to the thirty-millionth of a second; and, for the present case, it would be more than a thousand times too long. But Nature furnishes us another resource. It has been long known that the Leyden discharge is not uniform, but is composed, like the sound of a bell, of a number of vibrations of partial discharges, which succeed one another at even intervals of time. Electricity is capable, then, of imitating elastic phenomena. The duration of each vibration is much less than that of the whole discharge; we might, therefore, try a vibration as a standard. Unfortunately, the shortest vibrations that have been observed are of a millionth of a second. While one of these vibrations is going on. its effect is propagated to three hundred metres; while within the limited space of a laboratory it will appear simultaneous with the vibration. Known phenomena, then, gave no aid, and it was necessary to look for another way. The difficulty was turned by applying the discovery that vibrations are produced in any conductor as well as by the discharge of the Leyden jar, and often much more rapidly. When the conductor of an electrical machine is discharged, vibrations are produced, the duration of which varies from the hundred-millionth to the millionth of a second. They are, it is true, only isolated vibrations that are extinguished rapidly a condition unfavorable for the experiment. But success would be possible even if we could observe only two or three of the vibrations. In the same way, in acoustics, we substitute, when we want to, brief signals sounded on wood for the lengthened sounds of whistles and cords.
We now possess signals in comparison with which the thirty-millionth of a second is no longer a short interval. But they would be of little use if we were not able to compare them at that distance of about ten metres which we have proposed to ourselves. The means employed for this purpose are very simple. We fix a conductor—for instance, a straight metallic wire, having a slight interruption at one point—at the place where we desire to perceive the signal. When the electrical field is rapidly varied, a spark appears in the conductor.
The means of observation could be pointed out only by experiment. Theoretically it was hard to imagine it. The sparks are, in fact, microscopic, being hardly a hundredth of a millimetre long, and they continue less than a millionth of a second. It is extremely hard to conceive them as visible. Yet they can be seen, in a dark room and by an eye at rest. On so light a thread is hung the success of our undertaking. We had in the beginning a number of questions to answer. Under what conditions are the vibrations strongest? We must try to secure those conditions. What form should the conductor have? The phenomena will vary as we use straight or bent wires, or conductors of other forms. The form being determined upon, of what size should our conductor be? This is not a matter of indifference, for we shall see that we can not study all the vibrations with the same conductor. There are relations between the two elements like the phenomenon of resonance in acoustics. Lastly, in how many different positions can we arrange this conductor? We shall see the sparks at times increase in intensity, or become weaker, or disappear, I can not enter into these details; they are simply accessory to the theory as a whole. They are of importance only to the operator, and are simply properties of his instrument.
What the experimenter will educe from his process will depend on his knowledge of his means of action. The study of the instrument and the answers to the questions I have just mentioned therefore formed the most considerable part of my labor. This task having been disposed of, the solution of the problem was before me.
A physicist, given a number of diapasons and resonators, will find no difficulty in demonstrating that sound is not propagated instantaneously, even in the restricted space of a room. Having set the diapason in vibration, he goes with his resonator to different parts of the room and observes the intensity of the sound. He perceives that it becomes weak in some places, and infers from this that each vibration is annulled by another of later origin, which has reached the spot by a shorter route. If less time is taken in traversing the shorter road, propagation is not instantaneous, and the question is answered. But our physicist
will then show us that the points of silence succeed one another at equal intervals, and will deduce from this the length of the wave; and, if he knows the duration of the vibrations of the diapason, he will obtain, by these data, the velocity of the sound. We operate in the same way with our electrical vibrations. The conductor in which the vibrations are made fills the part of the diapason. The circuit, interrupted at a certain point, takes the place of the resonator, and may be called the electric resonator. We remark that sparks fly out at some points in the chamber, and quiet prevails in others. We notice that the spots inactive, electrically, follow in a regular order. We deduce from this, that the propagation is not instantaneous; and we can even measure the length of the wave. We are asked whether the waves are longitudinal or transversal. Let us place our metallic wire in two different positions in the same place in the room. It indicates an electrical excitation the first time, but not the second. Nothing more is needed to decide the question. The waves are transversal. If we are asked to give the velocity of propagation, we have only to multiply the length of wave which we have just measured by the duration of the vibration, which we can calculate. We find the velocity like that of light. If the correctness of this calculation is doubted, we have another resource. The velocity of electric waves in metallic wires is enormous, and quite equal to their velocity in the air. Further than this, it was directly measured a considerable time ago; for the problem was easily studied on wires kilometres long. We therefore have a purely experimental valuation of this velocity, and, although the result is only approximate, it does not contradict the one we have just got.
These experiments are all very simple at the bottom, and yet they have most important consequences. They overthrow every theory that assumes that electrical forces traverse space instantaneously, and mark the triumph of Maxwell's system. It is no longer a simple thread of union between two orders of distinct phenomena. While his theory of light seemed at first to be probable, it is now hard not to regard it as true. But it may be that in approaching this end we shall be able to dispense with the support of the theory. Our experiments took place very near that neutral zone which, according to it, unites the domains of light and electricity. Only one step remains to be taken to land in this domain of optics, which is well known to us. It will not be superfluous. There are many friends of Nature interested in the problem of light who are capable of comprehending simple experiments, but to whom Maxwell's theory is still unintelligible. Moreover, the scientific method requires us to avoid roundabout ways when it is possible to follow a direct one. If, then, we succeed in producing phenomena like those of light by means of electric waves, all theorizing becomes superfluous; the identity of the two orders springs from the experiments themselves. Success in this way also is possible. Let us place the conductor that produces the variation of the electric condition in the focus of a large concave mirror. The electric waves will join, and will come forth from the mirror in the form of a rectilinear beam. We can, it is true, neither see nor touch this beam; but we know it is there, because we can see sparks pass from it to the conductors which it meets; and it becomes sensible when we arm ourselves with our electrical resonator. Its properties are all those of a luminous ray. We can, by turning the mirror, send it into different directions. Studying the path which it follows, we may see that it is propagated in a straight line. If we interpose conducting bodies in its way, they will not let it pass; they cast a shadow, but do. not destroy the ray; they reflect it, and we can follow the reflected beam and satisfy ourselves that it follows the laws of the reflection of light. We can also refract it as we do light; and, as we use a prism to study the refraction of light, so we do here. But the dimensions of the waves and of the beam force us to take a very voluminous prism. So we select a cheap substance—pitch or asphalt. Finally, we can study on our ray phenomena which we have heretofore observed only in light, those of polarization. If we place a kind of metallic grate in the track of the beam, we can observe our electric resonator emitting sparks or remaining quiescent in obedience to the same geometric laws as govern the variations in the glow of a ray of light in passing through a polarizing apparatus.
In making these experiments we have come into the domain of optics. In describing them we speak no longer of electricity, but use the language of optics. We do not say that the currents pass along the conductors, or that the electricities unite. We see nothing but undulations crossing one another in space, separating, combining, and re-enforcing or weakening one another. Having started from the domain of pure electricity, we have come step by step to purely optical phenomena. The passage is made for henceforth, and the road has become easy. The identification of light and electricity, which science suspected and theory predicted, has been definitely established, made perceptible to our senses and intelligible to the mind. From the heights we have attained, where the two orders of phenomena are blended, we look into the domains of optics and electricity. They seem more vast than we had supposed them to be. Optics is no longer limited to ethereal undulations of a few fractions of a millimetre, but includes waves the length of which is measured in decimetres, metres, and kilometres. But, enlarged as it is, it is still only an appendage to electricity. That gains yet more advantage. We shall hereafter see electricity in a thousand conditions in which we did not before suspect it. Every blaze, every luminous atom becomes an electrical phenomenon. Even if a body does not cast light, it is a center of electrical action if it radiates heat. The domain of electricity is therefore extended over all nature, and even possesses us; for is not the eye, in fact, an electrical organ? Such are the results which we obtain in these questions of detail; those that concern the philosophy of science are no less important.
One of our most difficult problems is that of actions at a distance. Are they real? Of all those which seemed indisputable to us, gravitation is the only one that is left. Will it also escape? The laws of its action themselves provoke the thought. The nature of electricity is another of these great Unknowns. It reverts to the question of the condition of electrical and magnetic forces in space. Behind this rises the most important problem of all—that of the nature and properties of the substance that fills space, of the ether, its structure, its movements, and its limits—if it has any. We see this question becoming more and more dominant over all the others. The knowledge of the ether seems destined not only to reveal to us the condition of the imponderable substance, but also the nature of matter itself and its inherent properties—weight and inertia.
The ancient systems of physics summarized everything as formed of water and fire. Modern physics will shortly be asking if all existing things are not modalities of the ether. Here lies the ultimate end of our knowledge, the culmination of all that we can hope to learn. Shall we ever reach it? Soon? We do not know. But we have reached a greater height than ever before, and we have gained a solid point of support which will make our upward progress and search for new truths easier. The way that is opening before us is not too steep, and the next step does not look inaccessible. There is a numerous company of seekers full of ardor and knowledge; and we wait with confident hope all the attempts that will be made in that direction.—Translated for The Popular Science Monthly from the Revue Scientifique.
- A communication to the Sixty-second Congress of German Naturalists and Physicians, at Heidelberg.