Foucault, Jean Bernard Léon [key], 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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