Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/40

30 FARADAY ferring the reader to the Life of Faraday, by Dr Bence Jones. We may remark, however, that although the fact of the tangential force between an.. electric current and a magnetic pole was clearly stated by Orsted, and clearly apprehended by Ampere, Wollaston, and others, the realization of the continuous rotation of the wire and the magnet round each other was a scientific puzzle requiring uo mean ingenuity for its original solution. For on the one hand the electric current always forms a closed circuit, and on the other the two poles of the magnet have equal but opposite properties, and are inseparably connected, so that whatever tendency there is for one pole to circulate round the current in one direction is opposed by the equal tendency of the other pole to go round the other way, and thus the one pole can neither drag the other round and round the wire nor yet leave it behind. The thing cannot be done unless we adopt in some form. Faraday s ingenious solution, by causing the current, in some part of its course, to divide into two channels, one on each side of the magnet, in such a way that during the revolution of the magnet the current is transferred from the channel in front of the magnet to the channel behind it, so that the middle of the magnet can pass across the current without stopping it, just as Cyrus caused his army to pass dryshod over the Gyndes by divert ing the river into a channel cut for it in his rear. We must now go on to the crowning discovery of the in duction of electric currents. In December 1824 he had attempted to obtain an electric current by means of a magnet, and on three occasions he " had made elaborate but unsuccessful attempts to produce a current in one wire by means of a current in another wire or by a magnet. He still persevered, and on the 29th August 1831 he obtained the first evidence that an electric current can induce another in a different circuit. On September 23 he writes to his friend R. Phillips " I am busy just now again on electromagnetism, and think I have got hold of a good thing, but can t say. It may be a weed instead of a fish that, after all my labour, I may at last pull up." This was his first successful experiment. In nine more days of experimenting he had arrived at the results described in his first series of " Experimental Researches" read to the Royal Society, November 24-, 1841. By the intense application of his mind he had brought the new idea, in less than three months from its first develop ment, to a state of perfect maturity. The magnitude and originality of Faraday s achievement may be estimated by tracing the subsequent history of his discovery. As might be expected, it was at once made the subject of investiga tion by the whole scientific world, but some of the most experienced physicists were unable to avoid mistakes in stating, in what they conceived to be more scientific language than Faraday s, the phenomena before them. Up to the present time the mathematicians who have rejected Faraday s method of stating his law as unworthy of the precision of their science have never succeeded in devisino 1 any essentially different formula which shall fully express tho phenomena without introducing hypotheses about the mutual action of things which have no physical existence, such as elements of currents which flow out of nothing, then along a wire, and finally sink into nothing again. After nearly half a century of labour of this kind, we may say that, though the practical applications of Faraday s discovery have increased and are increasing in number and value every year, no exception to the statement of these laws as given by Faraday has been discovered, no new law has been added to them, and Faraday s original statement remains to this day the only one which asserts no more than can be verified by experiment, and the only one by which the theory of the phenomena can be expressed in a manner which is exactly and numerically accurate, and at the same time within the range of elementary methods of exposition. During his first period of discovery, besides the induc tion of electric currents, Faraday established the identity of the electrification produced in different ways ; the law of the definite electrolytic action of the current ; and the fact, upon which he laid great stress, that every unit of positive electrification is related in a definite manner to a unit of negative electrification, so that it is impossible to produce what Faraday called " an absolute charge of elec tricity" of one kind not related to an equal charge of the opposite kind. He also discovered the difference of the capacities of different substances for taking part in electric induction, a fact which has only in recent years been admitted by con tinental electricians. It appears, however, from hitherto unpublished papers that Henry Cavendish had before 1773 not only discovered that glass, wax, rosin, and shellac have higher specific inductive capacities than air, but had actu ally determined the numerical ratios of these capacities. This, of course, was unknown both to Faraday and to all other electricians of his time. The first period of Faraday s electrical discoveries lasted ten years. In 1841 he found that he required rest, and it was not till 1845 that he entered on his second great period of research, in which he discovered the effect of magnet ism on polarized light, and the phenomena of diamagnetism. Faraday had for a long time kept in view the possibility of using a ray of polarized light as a means of investigating the condition of transparent bodies when acted on by elec tric and magnetic forces. Dr Bence Jones (Life of Fara day, vol. i. p. 3G2) gives the following note from his laboratory book, 10th September 1822 : " Polarized a ray of lamp-light by reflexion, ami endeavoured to ascertain whether any depolarizing action (was) exerted on it l>y water placed between the poles of a voltaic battery in a glass cistern ; one "VYollaston s trough used ; the iluids decomposed were pure water, weak solution of sulphate of soda, and strong sulphuric acid ; none of them had any effect on the polarized light, either when out of or in the voltaic circuit, so that no particular arrange ment of particles could be ascertained in this way." Eleven years afterwards we find another entry in his notebook on 2d May 1833 (Life, by Dr Bence Jones, vol. ii. p. 29). lie then tried, not only the effect of a steady current, but the effect on making and breaking contact. " I do not think, therefore, that decomposing solutions or sub stances will be found to have (as a consequence of decomposition or arrangement for the time) any effect on the polarized ray. Should now try non-decomposing bodies, as solid nitre, nitrate of silver, borax, glass, &c., whilst solid, to see if any internal state induced, which by decomposition is destroyed, i.e., whether, when they can not decompose, any state of electrical tension is present. My borate of glass good, and common electricity better than voltaic. On May G he makes further experiments, and con cludes " Hence I see no reason to expect that any kind of structure or tension can be rendered evident, either in decomposing or non-decomposing bodies, in insulating or conducting states." Experiments similar to the last-mentioned have recently been made by Dr Kerr of Glasgow, who considers that he has obtained distinct evidence of action on a ray of polarized light when the electric force is perpendicular to the ray and inclined 45 to the plane of polarization. Many physicists, however, have found themselves unable to obtain Dr Kerr s result. At last, in 1845, Faraday attacked the old problem, but this time with complete success. Before we describe this result we may mention that in 18G2 he made the relation between magnetism and light the subject of his very last experimental work. He endeavoured, but in vain, to de tect any change in the lines of the spectrum of a flame when the flame was acted on by a powerful magnet.