HISTORY.] ELECTBIGITY jermo- By his discovery of thermo-electricity in 1822 (Fogg. 1 - Ann., vi.), Seebeck opened up a new department He city - found that when two different metals are joined in circuit there will be an electric current in the circuit if the junctions are not at the same temperature ; he arranged the metals in a thermo-electric series, just as Voltu and his followers had arranged them in a contact series. Camming (Annals of Phil., 1823) found that the order of the metals was not the same at different temperatures. This phenomenon has been called thermo electric inversion. In 1834 Peltier dis covered that if a current be sent round a circuit of two metals in the direction in which the thermo-electromotive force would naturally send it, then the hot junction is cooled, and the cool junction heated. This effect, which is reversible, and varies as the strength of the current, is called the Peltier effect. Sir W. Thomson made many experiments on thermo-electricity, and applied to the experimental results the laws of the dynamical theory of heat. His reasonings led him to predict a new thermo-electric phenomenon, the actual existence of which he afterwards verified by an elaborate, series of very beau tiful experiments (Phil. Trans., 1856). He hasgiven^a general theory of the thermo-electric properties of matter, taking into account the effect of structure, &c. His experi mental researches have been ably continued by Professor Tait, who, guided by theoretical considerations to the conjec ture that the curves in what Thomson called the " thermo electric diagram" 1 must be straight lines, made an extended series of experiments, and showed that they were in general very approximately either straight lines or made up of pieces of straight lines. Our knowledge of thermo-electri city has been advanced by Becquerel, Magnus, Matthiessen, Leroux, Avenarius, and others. Thermo-electric batteries of considerable power have been constructed by Markus, Noe, and Clamond, and employed more or less in the arts, ignet- In 1824 Arago (Ann. de Chim. et de Phys., t. xxvii. &c.) 1 of made a remarkable discovery, which led ultimately to re sults of the greatest importance. He found that when a magnetic needle is suspended over a rotating copper disc the needle tends to follow the motion of the disc. This phenomenon, which has been called the " magnetism of ro tation," excited great interest ; Barlow (Phil. Trans., 1825), Herschel, Seebeck (Pogg. Ann., vii., 1826), and Babbage (Phil. Tratis., 1825) mads elaborate researches on the sub ject; and Poisson (Mem. de I Acad., vii., 1826) attempted to give a theoretical explanation in his memoir on magnet ism in motion. The true explanation was not arrived at until Faraday took up the subject a little later. We may mention, here, however, the experiments of Pliicker, Matteucci, and Foucault on the damping of the motions of masses of metal between the poles of electromagnets. The damping of a compass needle suspended over a copper plate, observed by Seebeck (/. c.}, has been taken advan tage of in the construction of galvanometers. In 1831 Faraday began, with the discovery of theinduc- , ctric tion of electric currents, that brilliant series of experi ments mental researches which has rendered his name immortal. ara- The first experiment which he describes was made with two helices of copper wire wound side by side on a block of wood, and insulated from each other by intervening layers of twine. One of these helices was connected with a gal vanometer, and the other with a battery of a hundred plates, and it was found that on making and breaking the battery circuit a slight sudden current passed through the galvanometer in opposite directions in the two cases. He also discovered that the mere approach or removal of a circuit carrying a current would induce a current 1 A mode of representing the phenomena of thermo-electricity which hao been greatly developed and improved by Tait. in a neighbouring closed circuit, and that the motion of magnets produces similar effects. To express in a concise manner his discoveries, Faraday invented his famous conception of the lines of magnetic force, or lines the direction of which at any point of their course coincides with that of the magnetic force at that point. His discovery can be thus stated : Whenever the number of lines of force passing through a closed circuit is altered, there is an elec tromotive force tending to drive a cut-rent through the cir cuit, whose direction is such that it would itself produce lines of force passing through the circuit in the opposite direction. Nothing in the whole history of science is more remarkable than the unerring sagacity which enabled Faraday to disen tangle, by purely experimental means, the laws of such a com plicated phenomenon as the induction of electric currents. The wonder is only increased when we look to his papers, and find the first dated November 1831, 2 and another January 1832, in which he shows that he is in complete possession of all the general principles that are yet known on the subject. Faraday very soon was able to show that the current developed by induction had all the properties of the voltaic current, and he made an elaborate comparison of all the different kinds of electricity known, statical, dynamical or voltaic, magneto-, thermo-, and animal elec tricity, showing that they were identical so far as experi ment could show. In 1833 Lenz made a series of important Law of researches (Pogg. Ann., xxxi., 1834, xxxiv., 1835), which, Lenz. among other results, led him to his celebrated law by means of which the direction of the induced current can be pre dicted from the theory of Ampere, the rule being that the direction of the induced current is always such that its electromagnetic action tends to oppose the motion which produces it. This law leads to the same results as the prin ciples of Faraday. The researches of Putchie and Henry about this time, and of Dove a little later, are also of im portance. In 1845 F. E. Neumann did for magneto- Mathe- electric induction what Ampere did for electrodynamics, matical by developing from the experimental laws of Lenz the theory, mathematical theory of the subject (Abh. der Berl. Akad. der Wissenschaft, 1845-7). He discovered a function which has been called the "potential" (of one linear current on another or on itself), from which he deduced a theory of induction completely in accordance with ex periment. About the same time Weber deduced the mathematical laws of induction from his elementary law of electrical action, which, as we have already seen, he applied to explain electrostatic and electromagnetic action. In 1846 Weber, applying his improved instruments, arrived at accurate verifications of the laws of induction, which by this time had been developed mathematically by Neumann and himself. In 1 849 Kirchhoff determined experimentally in a certain case the absolute value of the current induced by one circuit in another; and in the same year Edlund made a series of careful experiments on the currents of self and mutual induction, which led to the firmer estab lishment of the received theories. Helmholtz gave the mathematical theory of the course of induced currents in various cases, and made a series of valuable experiments in verification of his theory (Pogg. Ann., Ixxxiii., 1851). Worthy of mention here are also the experiments and reasonings of Felici in 1852. In the Philosophical Maga zine for 1855, Sir W. Thomson investigated mathematically the discharge of a Leyden jar through a linear conductor, and predicted that under certain circumstances the dis charge would consist of a series of decaying oscillations. This oscillatory discharge was observed in 1857 by Fedder- sen (Pogg. Ann., cviii.) The law of Weber has been applied 8 The first experiment seems to have been actually made on the 29th
August 1831. See Bence Jones s Life of Faraday, vol. ii. p. 1.