As we have said, Faraday's principal work was accomplished in the domain of electricity. His supreme discovery, and, indeed, the most important practical discovery in the whole realm of electricity, was that of the induction effect of a current of electricity on a neighboring circuit. This was accomplished by experimental work of the highest order. Toward the end of 1824, when he was about thirty-three, he came to the definite conclusion that an electric current might be obtained by the motion of a magnet. His mind had been prepared for such a conclusion by Oersted's significant discovery in July, 1820, that an electric current acts somewhat like a magnet when the wire through which it flows is free to move. This discovery, definitely connecting electricity and magnetism, had been elaborated to an important degree by Ampère, and its sphere of application broadened by Wollaston. The curious though not unusual result in such cases, that it is not those who are in immediate touch with a great discoverer who develop or even apply his work, was illustrated by the fact that Ampère, the Frenchman, took up Oersted's discovery first, while Wollaston, working in England, had been the next one to follow successfully in the path thus opened up. It takes genius to go even a slight step farther into the unknown; the trained talent of disciples does not suffice. It was now Faraday, though not under Wollaston's influence, who was to continue successfully these labors.
In spite of his persuasion that a magnet would produce by induction an electric current, and the further step that a current in one wire could induce a current in another, experiments during seven years had brought him very little nearer the actual demonstration of this important principle. Those who think that great discoveries are made by accident and almost fall into the laps of their makers, as the apple upon Newton, should recall these seven years of unsuccessful labor on the part of Faraday. Finally, in 1831, he obtained the first definite evidence that an electric current can induce another in a different circuit. The discovery meant so much for him, that he hesitated to believe in his own success. Nearly a month after this first demonstration for himself, he wrote to his friend Phillips: "I am busy just now again on electro-magnetism, and think I have got hold of a good thing, but can't say. It may be a weed instead of a fish that, after all my labor, I may at last pull up."
He had long suspected, as we have said, that induction should occur, and he had tried currents of different strength, but without result. One day he noticed that, though he could not produce a permanent induced current, whenever the primary current started or stopped, there was a movement of the galvanometer connected with the secondary circuit, though the galvanometer remained at zero so long as the primary current flowed steadily. From this he proceeded to the demonstration that a bar magnet suddenly thrust into a helix of copper wire produced the same effect on the galvanometer, and evidently induced a transient current. When the magnet was withdrawn, the galvanometer needle swung in the opposite direction, showing another current, so that electrical currents were evidently induced by the relative motions of a magnet and a conductor. He continued his experiments in many different forms, and in the short space of a little more than a week, once the first definite hint was obtained, succeeded in so completely finding out the phenomena of electro-magnetic induction that scarcely more than practical applications in this subject were left for his successors.
Faraday's explanation of the induction of currents in the secondary circuit was probably quite as important a contribution to science as the series of experiments by which he demonstrated the occurrence of induced currents. His mind was not of the order that would accept action at a distance; that is, without some conducting medium through which the action took place. The old aphorism of the scholastics, "actio in distans repugnat"—action at a distance, that is, without a medium intervening, is absurd—would have appealed to him as a basic truth. The explanation that he outlined for induced currents was based on the lines of magnetic force, which he had so often delineated by means of iron filings. It was a favorite occupation of his, at moments of comparative leisure, to make varied pictures in iron filings of magnetic fields as they were exhibited under the influence of different combinations of magnets. He strewed iron filings over "gum paper," and then when the filings had arranged themselves in certain definite lines, he threw a jet of steam on the paper, which melted the gum and fixed the filings in position. He explained electrical action as the transmission of force along such lines as these, and he thought the whole electric field was filled with them.
Probably the best summary of Faraday's work on induction and its significance has been given us by Clerk Maxwell, in his article on Faraday, in the ninth edition of the Encyclopedia Britannica. There is no doubt but that Maxwell, above all men of the nineteenth century, was in a position to judge of the meaning of Faraday's work. He was not the sort of a man to say things in a panegyric mood, and his article on Faraday is indeed a model of well-considered judgment and critical illumination. Summing up the significance not only of Faraday's great discovery of induction, but also his theory in explanation of that discovery, he does not hesitate to say that his (Faraday's) opinion is the nearest approach to truth that has been advanced in this much-discussed subject.
"After nearly half a century of labor of this kind, we may say that, though the practical applications of Faraday's great discovery have increased and are increasing in number and value every year, no exception to the statement of these laws as given by Faraday has been discovered; no new law has been added to them; and Faraday's original statement remains to this day the only one which asserts no more than can be verified by experiment, and the only one by which the theory of the phenomena can be expressed in a manner which is actually and numerically accurate, and at the same time within the range of elementary methods of exposition."
With what eminent care and absolute truth Faraday's conclusions were reached may be judged from some further expressions of Clerk Maxwell's in the article just quoted, with regard to the attitude of certain mathematicians toward Faraday's work. In this matter, Clerk Maxwell, in talking on a theme that he had made especially his own, and in which his opinion must carry the greatest possible weight, said:
"Up to the present time, the mathematicians who have rejected Faraday's method of stating his law as unworthy of the precision of their science, have never succeeded in devising any essentially different formula which shall fully express the phenomena, without introducing the hypotheses about the mutual action of things which have no physical existence, such as elements of currents, which flow out of nothing, then along the wire, and finally sink into nothing again."
Faraday's results were described in papers afterwards incorporated in his first series of "Experimental Researches," which were read before the Royal Society, November 24th, 1841. These papers probably contain the best possible proof of Faraday's genius as an experimentalist and a leader in scientific observation. Within a few months after his first successful experiment, he had succeeded in bringing to perfection the whole doctrine of induction by currents and magnets, had laid down the fundamental ideas which were to constitute the formal basis of electro-magnetism for all time. Perhaps no better idea of the importance of the discovery thus made by Faraday can be given than will be found in Clerk Maxwell's compendious paragraph on this subject, in his sketch of Faraday, in the Encyclopedia Britannica. It may be said that no one in all the nineteenth century was more capable of appreciating properly the value of Faraday's work than this great electrical mathematician, who laid the firm foundation of mathematical electricity during the latter part of the nineteenth century. Clerk Maxwell says:
"This was of course a great triumph, and nobody appreciated this fact better than Faraday himself, who had been working at its problems for many years. One of the first problems that he had set himself in his note-book as a young man, was 'to convert magnetism into electricity,' and this he had now done. Within a month of the time that his first successful experiment was formed, he succeeded in obtaining induction currents by means of the earth's magnetism. Within a year he took the further immense step of obtaining a spark from the induced current. This would ordinarily have seemed quite impossible, since sparks occur only if the electromotive force is very high, and it was very low in his induced currents. He found, however, that if the circuit of wire in which a current was flowing is broken while the current is passing, a little bridge of metallic vapor is formed, across which the spark leaps. The difficulty with the experiment was to break the circuit during the extremely short period while the current is flowing. Faraday succeeded in doing this, and as a result obtained the first germ of the electric light. When he demonstrated this experiment by a very ingenious apparatus at the meeting of the British Association at Oxford, all were deeply interested, yet probably no one, even the most sanguine of the scientists present, thought for a moment that they saw the beginning of a far-reaching revolution of all the lighting of the world."