Léon Foucault

Jean Bernard Léon Foucault (September 18, 1819February 11, 1868) was a French physicist best known for Foucault's pendulum, a device demonstrating the effect of the Earth's rotation. He also made an early measurment of the speed of light, invented the gyroscope, and discovered eddy currents.

Foucault was the son of a publisher at Paris, where he was born on September 18, 1819. After an education received chiefly at home, he studied medicine, which, however, he speedily abandoned for physical science. He first directed his attention to the improvement of L. J. M. Daguerre's photographic processes. For three years he was experimental assistant to Alfred Donné (18011878) in his course of lectures on microscopic anatomy.

With A. H. L. Fizeau he carried on a series of investigations on the intensity of the light of the sun, as compared with that of carbon in the arc lamp, and of lime in the flame of the oxyhydrogen blowpipe; on the interference of infrared radiation, and of light rays differing greatly in lengths of path; and on the chromatic polarization of light. In 184g [?] he contributed to the Comptes Rendus a description of an electromagnetic regulator for the electric arc lamp, and, in conjunction with Henri Victor Regnault, a paper on binocular vision. In 1850, by the use of a revolving mirror similar to that used by Sir Charles Wheatstone for measuring the speed of electric currents, he was able to demonstrate the greater speed of light in air than in water, and to establish that the speed of light in different media is inversely as the refractive indices of the media (see Fizeau-Foucault Apparatus).

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 and heavy pendulum exhibited by him at the Panthéon in Paris, and again in the following year by means of his invention 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 observatory at Paris.

In September of that year he discovered that the force required for the rotation of a copper disc becomes greater when it is made to rotate with its rim between the poles of a magnet, the disc at the same time becoming heated by the eddy or "Foucault currents" induced in the metal.

Foucault invented in 1857 the polarizer which bears his name, and in the succeeding year devised a method of giving to the mirror of reflecting telescopes the shape of a spheroid or a paraboloid of revolution. With Wheatstone’s revolving mirror he in 1862 determined the speed of light to be 298,000 km (about 185,000 mi) a second—10,000 km/s less than that obtained by previous experimenters and only 0.6% off the currently accepted value.

He was created in that year a member of the Bureau des Longitudes and an officer of the Legion of Honour. In 1864 he was made a foreign member of the Royal Society of London, and next year a member of the mechanical section of the Institute. In 1865 appeared his papers on a modification of Watt's governor, upon which he had for some time been experimenting with a view to making its period of revolution constant, and on a new apparatus for regulating the electric light; and in the following year (Compt. Rend. lxiii.) he showed how, by the deposition of a transparently thin film of silver on the outer side of the object glass of a telescope, the sun could be viewed without injuring the eye. Foucault died of paralysis on February 11, 1868 at Paris and was buried in the Cimetière de Montmartre.

From the year 1845 he edited the scientific portion of the Journal des Débats. His chief scientific papers are to be found in the Comptes Rendus, 18471869.


The original text for this article was based on the 1911 Encyclopaedia Britannica.

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