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 When the magnet was withdrawn, the galvanometer needle swung in the opposite direction, showing another current, so that electrical currents were evidently induced by the relative motions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.