Page:Dictionary of National Biography volume 18.djvu/201

Faraday showed how by cutting these lines, whether they belonged to an artificial magnet or to the earth, induced currents were generated. Causing, for example, a copper disk to spin across the earth's lines of force, he produced such currents, and described with precision the positions of the disk wherein no current could be produced by its motion. He played with the earth as with a magnetic toy. Placing an iron bar within a helix, he lifted the bar into the direction of the dipping needle. An induced current was instantly roused in the helix. On reversing the bar, a current in the opposite direction declared itself. Holding the helix in the line of dip, the introduction and withdrawal of an unmagnetised bar of iron produced currents in opposite directions. Barlow and Christie had experimented on iron shells and iron disks, but Faraday, with a brass globe and a copper disk, obtained all their effects. They had their eye upon the metal as capable of magnetism; he had his eye upon it as a conductor of electricity. His speculations and experiments on the possible action of the earth when water, whether tidal or fluvial, flowed over its surface, are deeply interesting. The following avowal and prediction, made in 1831, breathe the very spirit of the true investigator: 'I have rather been desirous of discovering new facts and new relations dependent on magneto-electric induction, than of exalting the force of those already obtained, being assured that the latter would find their full development hereafter.' The electric lighting of the present day is surely a splendid fulfilment of this prediction.

Every well-known experimenter is sure to be flooded with proposals and suggestions from outsiders. Crowds of such proposals came to Faraday, but one of them only, he declared, bore the slightest fruit. A young man named William Jenkin had observed a shock and spark of a peculiar character on the interruption of a voltaic current passing through a circuit containing a helix. He was anxious to follow the subject up, but his father, knowing that science was but a poor paymaster, dissuaded him from its pursuit. The examination of the facts noticed by Jenkin led Faraday to the discovery of the 'extra current,' his beautiful investigation on this subject being communicated to the Royal Society on 29 Jan. 1835. It bore the title 'On the Influence by Induction of the Electric Current upon itself.'

In 1831 Faraday had tapped new and inexhaustible sources of electricity. Pondering on the whole subject, he asked himself whether these various kinds of electricity were all alike. Are the electricities of the machine, the pile, the gymnotus and torpedo, magneto-electricity, and thermo-electricity, merely different manifestations of one and the same agent. He reviewed the knowledge of the time, turned upon the subject his power as an experimenter, and decided in favour of the 'identity of electricities." His investigation was read before the Royal Society on 10 and 17 Jan. 1833.

He now aimed at obtaining some knowledge of their relations as to quantity. Moistening bibulous paper with the iodide of potassium he decomposed the iodide by the electricity of the machine, producing a brown spot where the iodine was liberated. He then immersed two thin wires, the one of zinc, the other of platinum, to a depth of five-eighths of an inch in acidulated water. During eight beats of his watch he found that the electricity generated by this minutevoltaic arrangement produced the same effect on his galvanometer and on his moistened paper as thirty turns of his large electrical machine. The quantity of water here decomposed was immeasurably small, and still, if applied in the concentrated form which it assumes in the Leyden jar, it would, Faraday averred, be competent to kill a rat, and no man would like to bear it. He next determines the amount of electrical force involved in the decomposition of single grain of water. He is almost afraid to mention it, finding it equal to 800,000 discharges, not of the conductor, not of a single Leyden jar, but of the large Leyden battery of the Royal Institution. If concentrated in a single discharge, this amount of electricity would produce a great flash of lightning, while the chemical action of a single grain of water on four grains of zinc would yield a quantity of electricity equal to that of a powerful thunderstorm.

His next subject was the influence of the state of aggregation upon electric conduction. He found that the selfsame substance conducts, or refuses to conduct, according as it is liquid or solid. The current, for example, which passes through water cannot pass through ice. Oxides, chlorides, iodides, and sulphides were proved to be insulators when solid, and conductors when fused; the passage of the current through the fused mass being always accompanied by decomposition. Whether any trace of electricity could pass through a compound liquid without decomposing it was a disputed point. Faraday leaned to the idea that a small quantity might do so. Other investigators, foremost among whom was the celebrated De La Rive, contended that no trace of electricity can pass through a liquid compound without producing its equivalent decomposition. Faraday's paper on this 'New Law of