The principle of the Foucault Pendulum was first demonstrated in Paris in 1851 by the noted French Physicist Jean Bernard Leon Foucault. The Foucalt Pendulum is a simple device conceived as an experiment to demonstrate the rotation of the Earth. While it had long been known that the Earth rotated, the introduction of the Foucault pendulum in 1851 was the first simple proof of the rotation in an easy-to-see experiment. Today Foucault pendulums are popular displays in science museums and universities.
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 center 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 shaft supporting the electromagnetic ring bears the following message from Queen Juliana: "It is a privilege to live this day and tomorrow. Juliana".