Page:Dictionary of National Biography volume 18.djvu/200

Faraday before the Royal Society at the close of 1829. Three successive sittings of the society were taken up by this lecture. The glass, however, did not turn out to be of important practical use, but it afterwards proved to be the foundation of two of Faraday's greatest discoveries. In 1831 he published a paper 'On a Peculiar Class of Optical Deceptions,' to which, the chromatrope owes its origin. In the same year he made a communication on vibrating surfaces, wherein be explained the gathering up of light powders at the places of most intense vibration, while heavy powders like sand, as beautifully shown by Chladni, arrange themselves along the nodal lines.

Faraday had now reached the threshold of a career of discovery unparalleled in the history of pure experimental science. Towards the end of 1831 he discovered and subdued the domain of magneto-electricity. The inductive action of an electrified body on an adjacent unelectrified body was familiar to him; and he thought that something similar — he knew not what — ought to occur when a wire curbing an electric current was brought near another wire carrying no current. He went thus to work. Two wires overspun with silk were wound side by side over the same wooden cylinder. The two ends of one of the wires were connected with a voltaic battery, and the two ends of the other with a galvanometer. Faraday was never satisfied until he had applied the greatest force at his command, and in the present instance a battery power varying from 10 to 130 cells was called into play. But no matter how powerful he made his currents in the one wire, the other wire remained absolutely quiescent, while the electricity was flowing through its neighbour, The attention of the keen-eyed experimenter was, however, soon excited by a small motion of his galvanometer needle which occurred at the moment the current from the battery first started through its wire. After this first slight impulse the needle came to rest; but on interrupting the battery circuit another feeble motion was observed, opposite in direction to the former one. This result, and many others of a similar kind, led him to the conclusion that the battery current through the one wire did in reality induce a similarcurrent through the other, but that it continued for an instant only, and partook more of the nature of the electric wave from a common Leyden jar, than of the current from a voltaic battery.' The momentary currents thus generated as if by a kind of kick, or reaction, he called 'induced currents.'

Faraday next showed that the mere approach of a wire forming a closed curve to another wire through which a current was flowing, aroused in the former an induced current. The withdrawal of the wire also excited a current in the opposite direction. These currents existed only during the time of approach and withdrawal, and vanished when the motion ceased. Prior to these experiments magnetism had been evoked by electricity. He now aimed at exciting electricity by magnetism. Round a welded iron ring he wound two coils of insulated copper wire, the coils occupying opposite halves of the ring. The ring, with its two coils, is represented in Foley's admirable statue as held in Faraday's hand. Through one of the two coils be sent a voltaic current, which powerfully magnetised the iron. During the moment of magnetisation a pulse was sent through the other coil strong enough to whirl round the needle of the galvanometer four or five times in succession. On interrupting the circuit a whirl of the needle in the opposite direction was observed. It was only during the moments of magnetisation and demagnetisation that these effects were produced. From his welded ring he passed on to straight bars of iron, and obtained with them the effects produced by his ring.

At that time the 'magnetism of rotation' excited universal attention. A non-magnetic metallic disk placed beneath a magnetic needle and set in rotation drew the needle after it. On reversing the motion of the disk the needle first stopped and then turned backwards, following the new rotation. Arago was the discoverer of this action, but he ventured on no explanation of it. Its solution was reserved for Faraday. The disk being a conductor of electricity, he clearly saw that his newly discovered induced currents must be excited in it by the adjacent needle. He forthwith established the existence of these currents, proving their direction to be such as must, in accordance with the laws of Œrsted, produce the observed rotation.

The well-known arrangement of iron filings round a magnet profoundly impressed Faraday from the first. By 'action at a distance,' coupled with the law of inverse squares, the position of these filings had been previously explained. Faraday never made himself at home with this idea, but visualised a something round the magnet which gave the filings their position. This conception, which he used for a long time as a mere 'representative idea,' fearing to commit himself to physical theory, lay at the root of his experiments. He called the lines along which the iron filings ranged themselves 'lines of force,' and his