was a French physicist who, in 1850, measured the speed of light within about 0.6% of the currently accepted value. He invented the Foucault pendulum that demonstrates the rotation of the Earth by changing the plane of its oscillation as the earth rotated underneath it. Foucault also discovered the existence of eddy currents (Foucault currents) in a copper disk moving in a strong magnetic field. In 1859, he invented a simple but extremely accurate method of testing telescope mirrors for surface defects. He also invented the gyroscope, which is essentially a wheel on an axel that resists change in its orientation while spinning. The device is used to measure orientation based on the principle of conservation of angular momentum.