# United Nations Visitors Centre

## Fact Sheet: Focault Pendulum

The Foucault Pendulum is a gift to the United Nations from the Government of the Netherlands. The Pendulum, on display in the Visitors’ Lobby of the General Assembly Building, is named after the French physicist Jean Bernard Léon Foucault and gives visual proof of the rotation of the earth.

The pendulum is a 200-pound gold plated sphere, 12 inches in diameter, partially filled with copper and suspended from the ceiling 75 feet above the ceremonial staircase by a stainless steel wire. A universal joint allows it to swing freely in any direction. The sphere passes directly over a raised metal ring at the centre that contains an electromagnet, which induces a current in the copper inside the ball. This supplies the necessary energy to overcome friction and air resistance and keeps it swinging uniformly.

The sphere takes approximately 36 hours and 45 minutes to complete its cycle. The time of rotation has been found mathematically to vary in accordance with the latitude of the location of the pendulum. At the North Pole, where the pendulum would be directly above the earth's axis and the latitude is 90 degrees, the time of rotation is 24 hours. At the equator, where the latitude is 0 degree, the plane of the pendulum would not shift at all. At other latitudes the Foucault effect varies, becoming more pronounced nearer the poles.

### DID YOU KNOW?

The principle of the rotation of the earth was first demonstrated in Paris in 1851 by the noted French physicist, Jean Bernard Léon Foucault, who suspended a cannon ball from the dome of the Pantheon by a wire about 250 feet long, and, at each swing, the pin swept over a circular ridge of sand, cutting into it and tracing the movement of the pendulum. The plane of swing was found to shift slowly in a clockwise direction. Actually, the floor underneath the pendulum was turning, since a free-swinging pendulum cannot change its plane.