- French physicist
who used a pendulum to demonstrate the rotation of the Earth on its
axis, and invented the gyroscope 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 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 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 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.