Foucault, (Jean Bernard) Léon (1819-1868)

French physicist who used a pendulum to demonstrate the rotation of the Earth on its axis, and invented the gyroscope in 1852. In 1862 he made the first accurate determination of the velocity of light.

Foucault investigated heat and light, discovered eddy currents induced in a copper disc moving in a magnetic field, invented a polarizer, and made improvements in the electric arc. In 1860, he invented high-quality regulators for driving machinery at a constant speed; these were used in telescope motors and factory engines.

Foucault was born and educated in Paris and became a physicist at the Paris Observatory in 1855. Until 1847 his scientific work was carried out in collaboration with Armand Fizeau. They took the first detailed photographs of the Sun's surface in 1845. In 1847 they found that the radiant heat from the Sun undergoes interference and that it therefore behaves as a wave motion. In 1850 Foucault succeeded in showing that light travels faster in air than in water, just beating Fizeau to the same conclusion; this also supported the wave theory.

A pendulum maintains the same movement relative to the Earth's axis and the plane of vibration appears to rotate slowly as the Earth turns beneath it. Foucault made a spectacular demonstration of this by suspending a pendulum from the dome of the Panthéon in Paris in 1851. The invention of the gyroscope followed from this, as Foucault realized that a rotating body would behave in the same way as a pendulum.