FOUCAULT, JEAN BERNARD LEON French physicist, was the son of a publisher at Paris, where he was born on Sept. 18, 1819. After studying medicine he became in terested in experimental physics. With A. H. L. Fizeau (q.v.) he carried on a series of investigations in light and heat. By the use of a revolving mirror, similar to that used by Sir Charles Wheatstone for measuring the rapidity of electric currents, he was able in 185o to establish that the velocity of light in different media varies inversely as the refractive indices of the media, and later to measure the velocity of light in air.
For his demonstration in 1851 of the diurnal motion of the earth by the rotation of the plane of oscillation of a freely suspended, long, heavy pendulum, and for his invention of the gyroscope, he received the Copley medal of the Royal Society in 1855, and in the same year he was made physical assistant in the imperial ob servatory at Paris. He discovered the existence of eddy or "Fou cault currents" induced in a copper disc moving in a strong mag netic field. Foucault invented the polarizer which bears his name, and devised a method of giving to the speculum of reflecting telescopes the form of a spheroid or a paraboloid of revolution. He also introduced improvements in the electric arc.
Foucault received many honours during his life ; he died of paralysis on Feb. II, 1868 at Paris. From the year 1845 he edited the scientific portion of the Journal des Debats. His chief scien tific papers are to be found in the Comptes Rendus, 1847-69.
See Revue tours scient. vi. (1869), PP. 484-489; Proc. Roy. Soc. xvii. (1869), pp. lxxxlll.—lxxxiv.; Lissajous, Notice historique sur la vie et les travaux de Leon Foucault (1875) ; P. Gilbert Leon Foucault, sa vie et son oeuvre scientifique (Brussels 1879).