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A physicist and mechanician, b. at Paris, 19 Sept., 1819; d. there 11 Feb., 1868. He received his early schooling at home and showed his mechanical skill by constructing a boat, a mechanical telegraph, and a working steam-engine. He passed the examinations for the B.A. and began to study medicine. Later, unable to bear the sight of blood, he abandoned medicine and worked for Donné as preparator in his course on medical microscopy. His elementary mathematical and scientific training had been very deficient and he supplemented it as he became interested in invention and experiment. In 1845 he succeeded Donné as scientific editor of the "Journal des Débats". In 1850 he was awarded the Copley medal, the highest honour of the Royal Society of London, for his work showing the relation between mechanical energy, heat, and magnetism. The position of physicist of the Paris Observatory was created for him in 1855. A member of the Bureau of Longitudes (1862), he was finally elected to Academy in 1865. Those of Berlin and St. Petersburg, and the Royal Society of London also honoured him.

Foucault worked along several lines. With Finch he experimented upon the interference of red rays and their influence on daguerrotype plates, while with Regnault he studied binocular vision. We are indebted to him for the crucial experiment overtuning the corpuscular or emission theory of light, defended by Kepler, Newton, and Laplace. Following Arago's suggestion he used the rotating mirror of Wheatstone to determine the difference between the velocities of light in various transparent media. Contrary to the emission theory he found that light travels faster in air than in the denser medium water (17 May, 1860). Light was reflected from a mirror through a tube, containing the medium to be studied, to a concave reflector and back again to the mirror. If the mirror was rotated the image was observed to shift by an amount depending on the speed of light through the particular medium in the tube. Exceedingly accurate meaurements were made of this enormous velocity (about 186,000 miles per second) with an apparatus occupying only twelve feet of space. Foucault invented an automatic regulator for the feed of the Davy electric arc lamp and thus made electric lighting practicable. The Foucault pendulum was invented to demonstrate visibly the rotation of the earth; the one exhibited at the Pantheon in Paris in 1851, was 220 feet long. The gyroscope with its intricate and puzzling movements was another device invented by him to show also the earth's motion around its axis. This gained for him the cross of the Legion of Honour. Foucault currents are heating currents of electricity developed in a disc of metal rotating between the poles of a strong magnet. He had observed and reported this effect in 1855. As physicist at the observatory he applied himself also to the improvement of large telescopic lenses and reflectors, devising a method for silvering the surface of a glass reflector. The mercury interrupter used the induction coil and an excellent form of engine governor are also due to him. Foucault at first appeared careless in the performance of his religious duties but in later years he was a practical Catholic. A stroke of paralysis put an untimely end to his useful work, just as he was about to enjoy the comforts of a well-equipped laboratory. His contributions to science are found in the "Comptes rendus", "Procés verbaux de la Société Philomathique", and "Bibliothèque d'Instruction populaire". His collected works have been put in order by C.M. Gabriel and published by his mother, "Recueil des Travaux Scientifiques de Léon Foucault" (Paris, 1878).

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