Heroes of Science: Physicists

by William Garnett

A copper disc was mounted so that it could be made to rotate rapidly. A wire was placed in connection with the centre of the disc, and the circuit completed by a rubbing contact on the circumference. A galvanometer was inserted in the circuit, and the large horseshoe magnet of the Royal Institution so placed that the portion of the disc between the centre and the rubbing contact passed between the poles of the magnet. A current flowed through the galvanometer as long as the disc was kept spinning. Then he found that the mere passage of a copper wire between the poles of the magnet was sufficient to induce a current in it, and concluded that the production of the current was connected with the cutting of the "magnetic curves," or "lines of magnetic force" which would be depicted by iron filings. Thus in the course of ten days' experimental work, in the autumn of 1831, Faraday so completely investigated the phenomena of electro-magnetic induction as to leave little, except practical applications, to his successors. A few weeks later he obtained induction currents by means of the earth's magnetism only, first with a coil of wire wound upon an iron bar in which a strong current was produced when it was being quickly placed in the direction of the magnetic dip or being removed from that position, and afterwards with a coil of wire without an iron core. On February 8, 1832, he succeeded in obtaining a spark from the induced current. Unless the electro-motive force is very great, it is not possible to obtain a spark between two metallic surfaces which are separated by a sensible thickness of air. If, however, the circuit of a wire is broken while the current is passing, a little bridge of metallic vapour is formed, across which for an instant the spark leaps. The induced current being of such short duration, the difficulty was to break the circuit while it was flowing. Faraday wound a considerable length of fine wire around a short bar of iron; the ends of the wire were crossed so as just to be in contact with one another, but free to separate if exposed to a slight shock. The ends of the iron bar projected beyond the coil, and were held just over the poles of the magnet. On releasing the bar it fell so as to strike the magnetic poles and close the circuit of the magnet. An induced current was generated in the wire, but, while this was passing, the shock caused by the bar striking the magnet separated the ends of the wire, thus breaking the circuit of the conductor, and a spark appeared at the gap. In this little spark was the germ of the electric light of to-day. Subsequently Faraday improved the apparatus, by attaching a little disc of amalgamated copper to one end of the wire, and bending over the other end so as just to press lightly against the surface of the disc. With this apparatus he showed the "magnetic spark" at the meeting of the British Association at Oxford.

Faraday supposed that when a coil of wire was in the neighbourhood of a magnet, or near to a conductor conveying a current, the coil was thrown into a peculiar condition, which he called the electro-tonic state, and that the induced currents appeared whenever this state was assumed or lost by the coil. He frequently reverted to his conception of the electro-tonic state, though he saw clearly that, when the currents were induced by the relative motion of a wire and a magnet, the current induced depended on the rate at which the lines of magnetic force had been cut by the wire. Of his conception of lines of force filling the whole of space, we shall have more to say presently. It is sufficient to remark here that, in the electro-tonic state of Faraday, Clerk Maxwell recognized the number of lines of magnetic force enclosed by the circuit, and showed that the electro-motive force induced is proportional to the rate of change of the number of lines of force thus enclosed.

It is seldom that a great discovery is made which has not been gradually led up to by several observed phenomena which awaited that discovery for their explanation. In the case of electro-magnetic induction, however, there appears to have been but one experiment which had baffled philosophers, and the key to which was found in Faraday's discovery, while the complete explanation was given by Faraday himself. Arago had found that, if a copper plate were made rapidly to rotate beneath a freely suspended magnetic needle, the needle followed (slowly) the plate in its revolution, though a sheet of glass were inserted between the two to prevent any air-currents acting on the magnet. The experiment had been repeated by Sir John Herschel and Mr. Babbage, but no explanation was forthcoming. Faraday saw that the revolution of the disc beneath the poles of the magnet must generate induced currents in the disc, as the different portions of the metal would be constantly cutting the lines of force of the magnet. These currents would react upon the magnet, causing a mechanical stress to act between the two, which, as stated by Lenz, would be in the direction tending to oppose the relative motion, and therefore to drag the magnet after the disc in its revolution. In the above figure the unfledged arrows show the general distribution of the currents in the disc, while the winged arrows indicate the direction of the disc's rotation. The currents in the semicircle A will repel the north pole and attract the south pole. Those in the semicircle B will produce the opposite effect, and hence there will be a tendency for the magnet to revolve in the direction of the disc, while the motion of the disc will be resisted. This resistance to the motion of a conductor in a magnetic field was noticed by Faraday, and, independently, by Tyndall, and it is sufficiently obvious in the power absorbed by dynamos when they are generating large currents.

Faraday's next series of researches was devoted to the experimental proof of the identity of frictional and voltaic electricity. He showed that a magnet could be deflected and iodide of potassium decomposed by the current from his electrical machine, and came to the conclusion that the amount of electricity required to decompose a grain of water was equal to 800,000 charges of his large Leyden battery. The current from the frictional machine also served to deflect the needle of his galvanometer. These investigations led on to a complete series of researches on the laws of electrolysis, wherein Faraday demonstrated the principle that, however the strength of the current may be varied, the amount of any compound decomposed is proportional to the whole quantity of electricity which has passed through the electrolyte. When the same current is sent through different compounds, there is a constant relation between the amounts of the several compounds decomposed. In modern language, Faraday's laws may be thus expressed:—

If the same current be made to pass through several different electrolytes, the quantity of each ion produced will be proportional to its combining weight divided by its valency, and if the current vary, the quantity of each ion liberated per second will be proportional to the current.

This is the great law of electro-chemical equivalents. The amount of hydrogen liberated per second by a current of one ampère is about ·00001038 gramme, or nearly one six-thousandth of a grain. This is the electro-chemical equivalent of hydrogen. That of any other substance may be found by Faraday's law.

From Faraday's results it appears that the passage of the same amount of electricity is required in order to decompose one molecule of any compound of the same chemical type, but it does not follow that the same amount of energy is employed in the decomposition. For example, the combining weights of copper and zinc are nearly equal. Hence it will require the passage of about the same amount of electricity to liberate a pound of copper from, say, the copper sulphate as to liberate a pound of zinc from zinc sulphate; but the work to be done is much less in the case of the copper. This is made manifest in the following way:—A battery, which will just decompose the copper salt slowly, liberating copper, oxygen, and sulphuric acid, will not decompose the zinc salt at all so as to liberate metallic zinc, but immediately on sending the current through the electrolyte, polarization will set in, and the opposing electro-motive force thus introduced will become equal to that of the battery, and stop the current before metallic zinc makes its appearance. In the case of the copper, polarization also sets in, but never attains to equality with the electro-motive force of the primary battery. In fact, in all cases of electrolysis, polarization produces an opposing electro-motive force strictly proportional to the work done in the cell by the passage of each unit of electricity. If the strength of the battery be increased, so that it is able to decompose the zinc sulphate, and if this battery be applied to the copper sulphate solution, the latter will be rapidly decomposed, and the excess of energy developed by the battery will be converted into heat in the circuit.

One important point in connection with electrolysis which Faraday demonstrated is that the decomposition is the result of the passage of the current, and is not simply due to the attraction of the electrodes. Thus he showed that potassium iodide could be decomposed by a stream of electricity coming from a metallic point on the prime conductor of his electric machine, though the point did not touch the test-paper on which the iodide was placed.