An instrument devised by Jean Foucault (1819–1868) in 1851 to prove that Earth rotates on an axis, the pendulum consisted of a heavy ball suspended by a very long fine wire. Sand beneath the pendulum recorded the plane of rotation of the pendulum over time.

A reconstruction of Foucault’s experiment is located in Portland, Oregon, at its Convention Center. It swings from a cable 90 feet (27.4 meters) long, making it the longest pendulum in the world.