Microscope, crank winder and gyroscope

Foucault’s use of a pendulum to demonstrate the Earth’s rotation on its axis fascinated many of his contemporaries. Yet the experiment was imprecise and the result varied depending on the latitude: only at the poles can the pendulum make a complete rotation in a day, and in Paris its rotation is slower than the Earth’s. Foucault therefore refined his experimental protocol using a gyroscope. This instrument works on the same principle as a spinning top: a rotor (a bronze disc with a bulbous rim) is set in motion using a crank mechanism. Due to its very high rotation speed (between 150 and 200 revolutions/second), the rotor is freed from the constraints of gravity for ten to fifteen minutes. Thus it is enough to measure, with a needle or a microscope, the slow rotation of the gimbal (ring) surrounding the rotor to see the rotation of the Earth.