Léon Foucault

name = Léon Foucault|
image_width = 225px
caption = Léon Foucault (1819-1868)
birth_date = September 18 1819
birth_place = Paris, France
death_date = dda|1868|2|11|1819|9|18|
death_place = Paris, France
residence = France
nationality = French
field = Physics
work_institution = Paris Observatory
alma_mater =
doctoral_advisor =
doctoral_students =
known_for = Foucault Pendulum
author_abbreviation_bot =
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prizes =
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Jean Bernard Léon Foucault (IPA2|ʒɑ̃ bɛʁnaʁ leɔ̃ fu'ko) (18 September 1819 – 11 February 1868) was a French physicist best known for the invention of the Foucault pendulum, a device demonstrating the effect of the Earth's rotation. He also made an early measurement of the speed of light, discovered eddy currents, and although he didn't invent it, is credited with naming the gyroscope. The Foucault crater on the Moon is named after him.

Early years

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 due to a fear of blood. [CathEncy|url=http://www.newadvent.org/cathen/06156c.htm|title=Jean-Bertrand-Léon Foucault] 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 (1801–1878) 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.

Middle years

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 in the Panthéon in Paris, caused a sensation in both the learned and popular worlds, for it was the first dynamical proof of the Earth's rotation. In the following year he used (and named) the gyroscope as a conceptually simpler experimental proof. In 1855 he received the Copley Medal of the Royal Society for his 'very remarkable experimental researches'. Earlier in the same year he was made "physicien" (physicist) at the imperial observatory at Paris.

In September 1855 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 current or "Foucault currents" induced in the metal.

In 1857, Foucault invented the polarizer which bears his name, and in the succeeding year devised a method of testing the mirror of a reflecting telescope to determine its shape. The so-called "Foucault Test" allows the worker to tell if the mirror is perfectly spherical, or if it deviates from a sphere. Prior to Foucault's invention, testing reflecting telescope mirrors was a "hit or miss" proposition. With Charles Wheatstone’s revolving mirror he in 1862 determined the speed of light to be 298,000 km/s (about 185,000 mi./s) —10,000 km/s less than that obtained by previous experimenters and only 0.6% off the currently accepted value.

Later years

In that year, he was made a member of the Bureau des Longitudes and an officer of the Légion d'Honneur. In 1864 he was made a member of the Royal Society of London, and the next year a member of the mechanical section of the Institute. In 1865 his papers on a modification of Watt's governor appeared, 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 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. His chief scientific papers are to be found in the "Comptes Rendus", 1847—1869.

Death and afterwards

Foucault died of what was probably a rapidly-developing case of multiple sclerosis [W. Tobin, The Life and Science of Léon Foucault, Cambridge University Press (2003).] on February 11, 1868 in Paris and was buried in the Cimetière de Montmartre.

Further reading

William Tobin, "Perfecting the Modern Reflector". Sky & Telescope, October 1987.

William Tobin, "Léon Foucault". Scientific American, July 1998.

Amir D. Aczel, "Pendulum: Léon Foucault and the Triumph of Science", Washington Square Press, 2003, ISBN 0-7434-6478-8

William Tobin, "The Life and Science of Léon Foucault: The Man who Proved the Earth Rotates". Cambridge University Press, 2003. ISBN 0-521-80855-3

External links

Collected Works:
* [http://num-scd-ulp.u-strasbg.fr:8080/513/ Volume One - "Recueil des travaux scientifiques de Léon Foucault"] 1878.
* [http://num-scd-ulp.u-strasbg.fr:8080/527/ Volume Two - "Recueil des travaux scientifiques de Léon Foucault"] 1878.

* [http://www.magnet.fsu.edu/education/tutorials/java/foucaultdisk/index.html Foucault Disk - Interactive Java Tutorial] Foucault created this device showing how eddy currents work (National High Magnetic Field Laboratory)
* [http://web2.bium.univ-paris5.fr/livanc/?cote=00576&do=livre Donné & Foucault Atlas of medical micrographs] 1845


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