This was a more powerful electromagnet than that at the Institution. With this he sets to work with such energy that twelve pages of the laboratory book are filled in one day. His thoughts had ripened during the five days, and he advanced rapidly from point to point. The first experiment with the Woolwich magnet brings out another point, of which at once he grasped the significance:—
“Heavy Glass (original, or 174[48]) when placed thus produced a very fine effect. The brightness of the image produced rose gradually not instantly, due to this that the iron cores do not take their full intensity of magnetic state at once but require time, and so the magnetic curves rise in intensity. In this way the effect is one by which an optical examination of the electromagnet can be made—and the time necessary clearly shewn.”
He next ascertains definitely that the phenomenon is one of rotatory polarisation—that is to say, the action of the magnet is to twist and rotate the plane of polarisation through a definite angle depending on the strength of the magnet and the direction of the exciting current. He finds the direction of the rotation, and verifies it by comparison with the ordinary optical rotation produced by turpentine and by a solution of sugar, winding up with the words:—
“An excellent day’s work.”
For four days he went on accumulating proofs, and now succeeding with the very substances with which he formerly failed. On September 26th he tried the conjoint effect of a magnetic and an electric field. He also tried the effect of a current running along a transparent liquid electrolytically whilst the magnet was in operation. The only results appeared to be those due to the magnet alone. For six days in October the experiments were continued. He noted, as a desideratum, a transparent oxide of iron. “With some degree of curiosity and hope” he “put gold leaf into the magnetic lines, but could perceive no effect.” He was instinctively looking for the phenomenon which Kundt later discovered as a property of thin transparent films of iron. He entered amongst the speculative suggestions in his notebook the query: “Does this [magnetic] force tend to make iron and oxide of iron transparent?” On October 3rd he tried experiments on light reflected from the surface of metals placed in the magnetic field. He indeed obtained an optical rotation by reflection at the surface of a polished steel button, but the results were inconclusive owing to imperfection of the surface. It was reserved for Dr. Kerr to rediscover and follow up this effect. On October 6th he looked for mechanical and magnetic effects on pieces of heavy-glass and on liquids in glass bulbs placed between the poles of his magnet, but found none. He also looked for possible effects of rapid motion given to the diamagnetic while jointly subject to the action of magnetism and the light, but found none.
UNFULFILLED EXPECTATIONS.
On October 11th he thinks he has got hold of another new fact when experimenting on liquids in a long glass tube, the record of it filling three pages. But two days afterwards he finds it only a disturbing effect due to the communication of heat to the liquid from the surrounding magnetising coil. He seems to regret the loss of the new fact, but adds: “As to the other phenomenon of circular polarization, that comes out constant, clear, and beautiful.”
Then, with that idea of the correlation of forces always in his head, there recurs to him the notion that if magnetism or electric currents can affect a beam of light, there must be some sort of converse phenomenon, and that in some way or other light must be able to electrify or to magnetise. Thirty-one years before, when visiting Rome with Davy, he had witnessed the experiments of Morichini on the alleged magnetic effect of violet light, and had remained unconvinced. His own idea is very different. And October 14th being a bright day with good sunlight, he makes the attempt. Selecting a very sensitive galvanometer, he connects it to a spiral of wire 1 inch in diameter, 4·2 inches long, of 56 convolutions, and then directs a beam of sunlight along its axis. He tries letting the beam pass alternately through the coil while the outside is covered, and then along the exterior while the interior is shaded. But still there is no effect. Then he inserts an unmagnetised steel bar within the coil, and rotates it while it is exposed to the sun’s rays. Still there is no effect, and the sun goes down on another of the unfulfilled expectations. But had he lived to learn of the effect of light in altering the electric resistance of selenium discovered by Mayhew, of the photoelectric currents discovered by Becquerel, of the discharging action of ultra-violet light discovered by Hertz, of the revivifying effect of light on recently demagnetised iron discovered by Bidwell, he would have rejoiced that such other correlations should have been found, though different from that which he sought. On November 3rd he receives a new horseshoe magnet, with which he hoped to find some optical effect on air and other gases, but again without result. That the magnetism of the earth does actually rotate the plane of polarisation of sky light was the discovery of Henri Becquerel so late as 1878.
Faithful to his own maxim: “Work, finish, publish,” Faraday lost no time in writing out his research. It was presented to the Royal Society on November 6th, but the main result was verbally mentioned on November 3rd at the monthly meeting of the Royal Institution, and reported in the Athenæum of November 8th, 1845.
But even before the memoir was thus given to the world another discovery had been made. For on November 4th with the new magnet he repeated an experiment which a month previously had been without result. So preoccupied was he over the new event that he did not even go to the meeting of the Royal Society on November 20th, when his paper on the “Action of Magnets on Light” was read. What that new discovery was is well told by Faraday himself in the letter which he sent to Professor A. de la Rive on December 4th:—