Helmholtz was rather reserved and could not easily be approached by his students, unless they had some physical problem or a question which was unquestionably worthy of his attention. I made up my mind to ask him, when suitable opportunity presented itself, why Kirchhoff in his lectures paid so little attention to Faraday and Maxwell. It was a very significant sign of those days and I did not understand its meaning. Professor Koenig threw up his hands in holy horror when I informed him of my intention, and prophesied that all kinds of dire consequences would result from my daring proposition; pointing out that such a question would betray a lack of respect on my part both for Kirchhoff and for Helmholtz. Koenig himself could not answer my question except to say that he did not see why the German school of physics should worry much about the English school, particularly when there was a radical difference between the two in the realm of the theory of electromagnetic phenomena. I admitted that if Kirchhoff was the spokesman of the German school then there was a radical difference, intimating however, in the mildest possible way that, in my humble opinion, the difference counted in favor of the English school. I really did not know enough to express that opinion, but I did it under provocation. Koenig flushed up and there would have been quite a lively verbal contest if Helmholtz had not entered my room at that very moment, like a deus ex machina. He was making his customary round of visits to the rooms of his research students, in order to find out how their work was moving along. Both Koenig and I looked somewhat perplexed, betraying the fact that we had been engaged in a heated argument, and Helmholtz noticed it. We confessed that we had had a lively discussion; when he learned the subject of our discussion he smiled and referred us both to an address which he had delivered before the Chemical Society of London, five years before. It is entitled, “Recent Developments in Faraday’s Ideas Concerning Electricity.” The same day saw me with two volumes of Helmholtz’s addresses in my hands analyzing his Faraday address. I felt as I went on with this study as if the heavy mist were lifting which had prevented me from seeing a clear view of Faraday’s and Maxwell’s ideas. Tyndall’s fame for clearing up obscure points in physical science was deservedly great, but when I compared Helmholtz’s interpretation of Faraday and Maxwell with that which Tyndall gave me in his book entitled “Faraday as Discoverer,” I marvelled at Helmholtz’s superiority. It must also be remembered that Tyndall was for many years in almost daily contact with Faraday; and, as I pointed out before, he must also have had close personal relations with Maxwell during the period of 1860–1865. To me it seemed a miracle that Helmholtz, a German, saw so much more clearly what was in the minds of two great English philosophers, although he never had met them personally, than did another great English physicist, Tyndall, who knew Faraday and Maxwell personally, and one of them at least intimately. In the article in Nature, to which Tyndall first referred me and which Maxwell had written, will be found the following closing paragraph:
Helmholtz is now in Berlin, directing the labors of able men of science in his splendid laboratory. Let us hope that from his present position he will again take a comprehensive view of the waves and ripples of our intellectual progress, and give us from time to time his idea of the meaning of it all.
Helmholtz’s address on Faraday was one of those comprehensive views of which Maxwell spoke in 1874. Now what did Helmholtz see in Faraday and Maxwell which other physicists, like Tyndall, and even so famous a mathematical physicist as Kirchhoff, failed to see? It was, I thought after a careful study of Helmholtz’s address, the simplest thing in the world, particularly for one who, like myself, had been wrestling with Faraday’s lines of force, and with the hypothetical powers with which Faraday had endowed them. So simple, indeed, that I venture to describe it here. But in order to make the description as brief and as simple as possible I must go back again to the charged spherical conductor which always rendered good service in those days when I was trying to solve the riddle of Faraday’s new physical concepts.
By means of an electrical force generated by an electrical machine we can increase or diminish the charge on the surface of the conducting sphere. Now, the charge on the sphere increases or diminishes because the electrical force generated by the machine drives through a suitable conducting wire additional electrical charge to the sphere, or takes it away from it. This motion of the electrical charge through the conducting wire to or from the sphere is the electrical current. Here comes now the historic question: Does the electrical current stop at the surface of the charged sphere? The old electrical theories said “Yes,” but Maxwell, interpreting the ideas of Faraday, said, “No.” Helmholtz was the first to tell me that clearly and distinctly, and I understood him.
Since, according to Faraday, each particle of the charge on the sphere carries attached to it a definite number of filaments or lines of force, it is obvious that the rate at which the charge on the sphere increases is, as I described above, the same as the rate at which the number of these lines of force are crowded into the space surrounding the sphere. Motion of the charge to the surface of the sphere is accompanied by a motion of the Faraday lines of force through every surface which surrounds the charged sphere. Since, according to Faraday, electricity is everywhere where the lines of force are, it follows that the motion of the lines through any surface means motion of electricity (in the sense in which I use this word) through that surface. Maxwell said, according to my understanding of Helmholtz, that motion of electricity, as represented by the motion of Faraday’s lines of force, is an electrical current just as much as the motion of electrical charges is. Electrical charges are terminals, only, of the lines of force; and why should the motion of the terminals be endowed with a power which is denied to the remaining parts of the lines of force? The principal power is, according to Oerstedt’s discovery, the generation of magnetism; that is, magnetic lines of force. According to Maxwell, then, the electrical current (that is, the motion of electrical charges through conductors) does not stop at the surface of the conductor, but continues in the non-conducting space beyond as motion of Faraday’s lines of force, as motion of electricity. The extension of the meaning of the word electrical current, just described, was, according to Helmholtz, the cardinal difference between the old electrical theories and the Faraday-Maxwell electromagnetic theory, and Helmholtz declared in favor of the last. I applauded Helmholtz and took off my hat to his clear vision of things which other people, including myself, failed to see. But can any one blame ordinary mortals, who were always accustomed to look upon the electrical current as motion of electrical charges in conductors, when they failed to see that the electrical current can take place even in a vacuum where there are no electrical charges at all, and therefore no motion of them? That was the physical concept which found its way so slowly into minds polarized by preconceived notions even after Helmholtz’s lucid explanation. This is substantially all there is in the Faraday-Maxwell electromagnetic theory as I gathered it directly from the Helmholtz address. But there is another very important element which I ought to describe here.
A corollary of Maxwell’s extension of the meaning of electrical current, which Helmholtz did not mention explicitly but which I soon found in Maxwell, is this: Electrical charges move because a force acts upon them; similarly the number of Faraday’s lines of force, passing through any surface in space, increases or diminishes because there is a force acting upon them. Wherever there is an action there is an equal and opposite reaction, according to the most fundamental law of Newton’s dynamics. Hence space, including the vacuum, must react when Faraday’s lines of force (that is, when the electricity represented by them) move through it. But if this reaction really exists in space, how can it be expressed? Faraday and Maxwell devoted much thought and many experimental investigations in search for a definite answer to this question, and they found it.
Faraday showed by experiment that if the charged sphere is immersed in an insulating fluid, say an insulating mineral oil, or in a solid insulator like rubber, or even if a piece of an insulator is brought near it, then the reacting force for a given charge on the sphere is smaller than when the sphere is surrounded by a vacuum; or, in other words, liquid and solid insulators are more permeable to the electrical lines of force (that is, to electricity) than a vacuum is. Therefore, an electrical force which is acting in order to increase the charge on the sphere and, as a result, increase the number of lines of force through the surrounding space, will experience the less reaction the more permeable the surrounding medium is. The reaction of an insulator against the action of an electrical force appears therefore as a reaction against the passage of electricity, that is, of electrical lines of force, through it. That picture of the process has stayed with me ever since my Berlin days.
The same line of reasoning which I followed above, regarding electrical lines of force, leads to similar results with regard to the magnetic lines of force. The reaction of the medium against an increase of the electrical and of the magnetic lines of force through it was the second new physical concept introduced into the electrical science by Faraday and Maxwell.
The Faraday-Maxwell electromagnetic theory extended the well-known electrical and magnetic actions and reactions from conductors to non-conductors, including the vacuum. If this theory is correct, then electromagnetic disturbances will be propagated from their source to all parts of space, and not along conductors only, by definite waves travelling at a definite velocity.
Maxwell’s calculation showed that electromagnetic disturbances are propagated through insulators in the same manner as light is propagated, and that, therefore, light is in all probability an electromagnetic disturbance. This is the substance of Maxwell’s electromagnetic theory of light; it is his answer to the question: “What is Light?”