Foucault, Jean Bernard Léon

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Foucault, Jean Bernard Léon

Foucault, Jean Bernard Léon (zhäN bĕrnärˈ lāôNˈ fo͞okōˈ), 1819–68, French physicist. Known especially for his research on the speed of light, he determined its velocity in air and found that its speed in water and other media diminished in proportion to the index of refraction. He originated the Foucault pendulum, with which he demonstrated the earth's rotation. He also improved astronomical instruments, especially the telescope; invented (1852) the gyroscope; and investigated the eddy current (known also as the Foucault current), an electric current induced in metal by a moving magnetic field. With Armand Fizeau he took the first clear photograph of the sun. From 1855 he was physicist at the Paris Observatory.
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Foucault, Jean Bernard Léon


Born Sept. 18, 1819, in Paris; died there Feb. 11, 1868. French physicist. Member of the Académie des Sciences (1865).

After receiving a medical education, Foucault became interested in experimental physics. He was made the scientific editor of the Journal des Débats in 1845, the physicist at the Imperial Observatory in Paris in 1855, and a member of the Bureau of Longitudes in Paris in 1862. Between 1844 and 1847, he carried out investigations in association with A. H. L. Fizeau. In 1850, implementing a suggestion of F. Arago, Foucault performed comparative measurements of the speed of light in water and in air by using a rapidly rotating mirror; the measurements conclusively confirmed the wave theory of light. In the same year, he proved experimentally that the earth rotates on its axis. In 1851, he used a 67-meter pendulum—the Foucault pendulum—to illustrate the earth’s rotation. Foucault was the first to draw attention to the heating of metallic bodies by eddy currents induced during the rapid rotation of the bodies in a magnetic field. He constructed a polarizing prism suitable for research in the ultraviolet region of the spectrum and invented a photometer, the gyroscope, a regulator for the electric arc lamp, and other devices.

Foucault became a corresponding member of the Imperial St. Petersburg Academy of Sciences in 1860. He was a member of the Prussian Academy of Sciences and a fellow of the Royal Society of London.


Lissajous, J. A. “Notice historique sur la vie et les travaux de Léon Foucault.” In Recueil des travaux scientifiques de Léon Foucault, vols. 1–2. Paris, 1875.
Gilbert, P. Léon Foucault, sa vie et son oeuvre scientifique. Brussels [1879].
Gliozzi, M. Istoriia fiziki. Moscow, 1970. (Translated from Italian.)


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.