The History of the Foucault Pendulum

On the invention of the Foucault Pendulum
The invention of the pendulum that bears the name of its creator was the first direct demonstration of the Earth’s rotation, doubted by a few at that time, but not yet demonstrated at an experimental level. Foucault discovered the principle of the pendulum by chance: he was working on his lathe with a metal rod approximately one meter in length when, by accident, the tip of the rod began to vibrate in one direction. When he turned the spin chuck that held the rod, he observed that the direction of the vibration remained unchanged. Foucault deduced that the oscillation of the pendulum would also be independent of the rotation movement of the point of attachment to the ceiling. He proved this on January 6, 1851, in his laboratory located in the basement of his house, with a 5 kg mass and a 2 meter long string. The slow turn of the pendulum’s oscillation plane was the consequence of the Earth’s rotation.

The first public demonstration of this experiment was conducted in the Paris Observatory, on February 3, 1851. Foucault sent renowned Paris scientists the following invitation: “You are invited to see that the Earth rotates, tomorrow between 3 and 5, in the Meridian Room of the Paris Observatory”.

In March of that year, Foucault installed - in the Paris Pantheon - a pendulum consisting of a 28 kg sphere, 38 cm in diameter, hanging from a 67 meter long cable. Under the attachment point,
centred on the vertical axis, he placed a wooden circular base, 6 meters in diameter, with its edges divided into degrees and quarter degrees so that the public could clearly observe the movement of the pendulum’s oscillation plane. With each oscillation, lasting 16 seconds, a leftward movement of approximately 2.5 mm could be observed (in the Northern Hemisphere, the oscillation of the pendulum rotates clockwise). This demonstration became public on March 27 in the presence of Louis Napoleon, President of France.