A pendulum consisting of a heavy bob suspended by a long inextensible string from a fixed point, free to swing in any direction, designed to demonstrate the rotation of the Earth. The original experiment, in 1851, was conducted by the French physicist Jean‐Bernard‐Léon Foucault. If the experiment were set up at the north or south pole, the vertical plane of the swinging bob would appear to an observer fixed on the Earth to precess or rotate once a day. At a location of latitude λ‎ in the northern hemisphere, the vertical plane of the swinging bob would precess in a clockwise direction with an angular speed of ω‎ sin λ‎, where ω‎ = 7.29×10⁻⁵ rad s⁻¹, the angular speed of rotation of the Earth. The period of precession in Oxford, for example, would be about $30\frac{1}{2}$ hours.