Jean Bernard Leon Foucault born on 18 September 1819 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, invented the gyroscope, and discovered eddy currents. The Foucault crater on the Moon is named after him.
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 photographic processes. For three years he was experimental assistant to Alfred Donne in his course of lectures on microscopic anatomy.
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 1850, by the use of a revolving mirror 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.
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 Pantheon 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 invented 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 physicist at the imperial observatory at Paris.
In September of 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.
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.
Foucault died on February 11, 1868 at Paris .
