lu ITS0 (q.v.) made the historic ob servation in regard to the twitching of the rig's legs. Which was followed by the discovery in 17!14, 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 communicated by Volta in a letter to Joseph Bank:, F.R.S., written .Ma•ch 20. 1500. Alvan., were thus furnished for maintaining elec tric currents. Within a few weeks after Volta's letter, Nicholson and Carlisle in England OM 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 rapid 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 1111111phry Davy (q.v.) and Alichael 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 OTO thuss. Faraday, Hittorf, Clansins. and Arrhe nius. Improved cells for the production of cur rents were made by Sturgeon. Daniell, Grove. Bunsen, and others. So-called 'storage' or sceondary cells, were first described by Bitter in 1803, and were improved by Plante. and by Faure.
In 1s19 Oersted discovered the magnetic action of an electric current, and the exact mathemat ical laws stating the action of a currant upon a magnet and of one current upon another were deduced by Biot and Savart (H2))) and by pilre (1821).
Arago and Davy (1820) discovered indepond• ently that a current passed through a helical conductor magnetized a steel needle placed inside. Sturgeon, in 1825. made the first elect•o-magnet. using; a single helix of wire wound round a bar of iron shaped like a horseshoe. varnish being used to insulate the coils of the wire. Joseph Ilenry (q.v.) showed how this magnetic action of a current could he increased. lIc insulated the wire by of silk cloth, and wound it in layers like thread on a spool. By means of an eleetro-magnet made on this principle Henry COD a telegraph instrument.
The galvanometer was invented by Schwciver, in 1820. and the astatic needle by Nobili. in 1825: but the modern forms of the instrument are due to the of Lord Kelvin. The pile mullein) of thermo-clectrieity were discovered by Seebeck in 1821: the Peltier elfeel, in MI.
di.:•ove•y of induced currents was made independently by 'Henry and Faraday in the year: and the latter described the prin ciple of the dynamo which has been perfected in reeent years by Wilde, Siemens. t:raninw, and countless others. The first indnet ion coil was by of \Va.:hit-10cm, D. C.. in 1S3S. In 1542 lb-any that the discharge of a Leyden jar was oseillatory—in which the had been anticipated by Savary (15271—and dis covered the existence of in the ether Tam (1111.011 by such a diseharge. The mathematical
theory of the disehorge was given by both Kelvin and lielmliolt7. and that of the waves by Clerk Nlaxwell. The proiwrties of the waves were in.
vestigated by Hertz in ISSS, and more recently by _Marconi and others, who have made commer cial use of them.
In Faraday is due the realization of the impor tance of the pis ipert ies of the MIII-C1mduct ing media separating charged bodies. Ile proved that both electrostatic and magnetie actions take place through the media, not 'at a distance.' lle orig inated the idea of lines of induction, and estab lished their laws: he also explained the attrac tion and repulsion of bodies caused by electrical or magnet:e forces as due to the refatire prop erties of these bodies and of the surrounding medium. Clerk expressed Faraday's ex perimental discoveries in mathematical language and formul:e, and advanced the theory of elec tricity and magnetism which is the basis of all modern theories.
The idea of expressing all electrical and mag netic quantities in absolute units, based on sonic definite sy-tem of standards of length, time, and mars. is due to Gauss (q.v.) and \Veber (q.v.).
The most important recent advances in the knowledge of the connection between electricity and have come thrOngh the observation of the ionization (q.v.) of gases. Most of the researches along this line have been made under the direction of Prof. J. Thomson. at the Cav endish Laboratory. Cambridge. England.
To understand the fundamental principles of electricity, it is well to start with the experi ments that were performed by the early Greeks.
Ilene it is found that, if two bodies are brought into intimate contact, e.g. by pressure or by rubbing together. and then separated, they have certain new properties, the most marked of which is the power of attracting any small bits of matter. such as particles of dust or fragments of paper. Bodies possessing this power are said to be 'electrified' or 'charged with electricity.' It is easily observed that if certain bodies, such as glass. rubber. silk, are charged, the electrical effects are manifested only at the points where the contact was made with the other body: while with other bodies, such as pieces of metal, the electrical effects are manifested over the whole body regardless of where the contact was made. and the charge is said to spread over the surface. Bodies of the latter kind are called 'conductors': of the former, 'non-conductors.' It takes time— infinitesimally short—for a charge to distribute itself over the surface of a condu•to•; but in the end, unless there is further charging. conditions Collie into a steady state; the charge- are said to be at rest. The science of the distribution and other properties of electrical charges at rest is called 'electrostatics,' while the study of the phenomena involved in varying and moving charges belongs to 'electric currents.' or 'electro kinetics.'