ELECTRICITY. 752 ELECTRICITY. are onlj* on the surfaces of metals, not in the in- terior. During the eighteenth century electric phe- nomena were studied very e.tensively, especially toward its close, when means of i)roducing elec- tric currents were discovered. Stephen Gray (q.v.). in the early part of the centun,-, observed the important fact that electric forces could be carried for a considerable distance by means of pack-thread and other bodies, thus discovering electric conduction, and he was then led to dis- tinguish between conductors and non-conductors. The possibility of two kinds of electricity — vitre- ous and resinous, or positive and negative — was established independently by du Fay in France and Kinnerslcy in Philadelphia. The former develoi)ed also a twolluid theory of elec- tricity which persisted for many years. The Leyden jar was discovered by accident in 1745, and from this time on public exhibitions of elec- tric ])henomena were most po|)ular. Most inter- esting and imjiortant work was being done also in America by Benjamin Franklin (q.v.) and a group of his friends. He proposed a one-fluid theory, which, after more than a centviry of neglect, has reappeared as a possible exjjlanation of the new phenomena concerning 'corpuscles' and ions. Having noticed the clVcct of points on conductors in discharging bodies, and believing that liglitning is an electric plicnnmenon. Frank- lin, in 1750, proposed an experiment to test his theory. It was to extend a long, pointed wire upward from a steeple, and to see if electrical charges could be observed at the lower end of the wire when a thunder-cloud passed overhead. The actual experiment was carried out according to these directions in 1752. in France, by Dalibard, and later by Delor. and in the same year Frank- lin performed his famous kite experiment. Soon after this lightning-rods, which had been sug- gested by Franklin, became popular both in America and Europe. The main phenomena of pyro-electricity were discovered by .-Epinvis, Berg- man, and Canton, about 1770. JIany of the most important facts in electricity which were made known during the nineteenth century by Coulomb. Ohm, and Faradav. were in realitv discovered by Henry Cavendish ( 17.31-1810) ." one of the world's greatest philosophers. He showed that the capacity of a condenser varied with its size and with the non-conductor used: he studied the conducting power of solutions; and he also dis- covered Avhat is now called Ohm's law. He saw, further, from mathematical considerations, that if electrical charges act on each other with a force varying directly as the amounts of. the charges and inversely as the square of their dis- tance apart, then a charged condvictor must have its charge entirely on its surfac^e: whereas, if the law of action were ditl'erent, there would be some charge on the interior. By a most in- genious experiment, performed before 1773, he showed that, to the limit of accuracy of his ap- paratus, there was absolutely no charge on the interior. These electrical papers of Cavendish were not published until 1S70. when they were edited by Clerk Maxwell. The law of electric action was proved independently and in a less ac- curate manner by CouloTub (q.v.). in 178.i. Many improvements in electrical apparatus were made: a circular glass plate and a rubber cushion coated wilh a tin amalgam were sub- stituted for the sulphur globe and the hand as used by von Guericke. The gold-leaf electro- scope was invented in 1780, by Abraham Bennet, and was improved by Volta (q.v.). The latter also, in 1775, invented the electrophorus. In 1780 Galvani (q.v.) made the historic ob- servation in regard to the twitching of the frog's legs, which was followed by the discovery in 17'J4, by Volta, of the correct explanation. This led at once to the invention of the so-called voltaic pile and voltaic cell, a description of which was conniiuiiicatcd bv Volta in a letter to Joseph Banks, F.K.S., written March 20. 1800. .Means were thus furnished for maintaining elec- tric currents. Within a few weeks after 'olta's letter, Nicholson and Carlisle in England con- structed a pile, and by means of it observed and studied the decomposition of water. The progress of the science of electricity dur- ing the nineteenth century was so ra|)id that it is impossible to do more than to note the most important steps. The investigation of the action of an electric current when passed through liquids was continued by Sir Humphry Davy (q.v.) and Jlichael Faraday (q.v.), to the latter of whom is due the statement of the laws of electrolysis. The theoretical ex- planation of the phenomena is the work of Oro- thuss, Faraday, Hittorf, Clausius. and Airbe- nius. Improved cells for the production' of cur- rents were made by Sturccon. Daniell, Grove. Bunsen, and others. So-called 'storage' cells, or secondary cells, were first described by Rittcr in 1803, and were improved by Plante and by Faure. In 1819 Oersted discovered the magnetic action of an electric current, and the exact mathcmat ical laws stating the action of a current upon a magnet and of one current upon another were deduced bv Biot and Savart (1820) and l>y Am- lidre (1821). Arago and Davy (1820) discovered independ- ently that a current passed throu.ffh a helical conductor magnetized a steel needle placed inside, ."sturgeon, in 182.5, made the first electro-ma.imet, using a single helix of wire wound round a bar of iron shaped like a horseshoe, varnish hciny used to insulate the coils of the wire. .Joseph Henry (q.v.) showed how this magnetic action of a current could be increased. He insulated the wire bv means of silk cloth, and wound it in layers like thread on a spool. Bv means of an electro-magnet made on this principle Henry con- structed a telegraph instrument. The galvanometer was invented by Schweigger. in 1820. and the astatic needle by Xobili. in 1825: but the modern forms of the instrument are due to the .genitis of Lord Kelvin. The phe- nomena of thermo-electrieitv were discovered bv Seebeck in 1821 : the Peltier effect, in 1834. The disooverv of induced currents was made independently by ITenry and Faraday in the years 1820-31 : and the latter described the prin- ciple of the dynamo which has been perfected in recent years by Wilde, Siemens. Gramme, and coinitless others. The first induction coil was made bv Charles Page, of Washington. D, C, in 1838. in 1842 Henry proved that the discharse i>f a Leyden jar w-as oscillatory — in which he had been anticipated by Savarv (1827) — and dis- covered the existence of waves in the ether pro- duced by such a discharge. The mathematical theorv of the discharge was given by lioth Kelvin and Helmholtz, and that of the waves by Clerk Maxwell. The properties of the waves were in-
