The Foucault Pendulum

The Foucault pendulum is named for its inventor, Jean Bernard Léon Foucault, a French physicist who created it in 1851 to visually demonstrate the Earth’s rotation on its axis. It was the first satisfactory proof of this rotation using Earth-based, rather than astronomical, observations.

While the pendulum’s arc appears to precess, or move ever-so-slightly clockwise, with each swing, Foucault demonstrated that it’s actually the Earth underneath that is moving. The pendulum precesses at varying rates north or south of the equator, making the experience of seeing it unique at each latitude.

The Foucault pendulum at The Cleveland Museum of Natural History was a gift from the National Association of Watch and Clock Collectors after the group held its 1986 annual meeting in Cleveland. Association member Everett R. Jones and colleague Jerome Griesmer, both engineers at the former Picker International in Cleveland, designed and built the device for the convention and later donated it to the Museum.

The word pendulum comes from the Latin word pendulus, which means “hanging.” A weight called a “bob” hangs on a wire that’s attached to a fixed point, high enough so the pendulum can swing freely in any direction.

When pulled back and released, the bob swings back and forth, propelled by gravity and inertia. Gravity is the force that, among other things, pulls objects to the ground, and inertia is the tendency of a body in motion to continue its motion unless acted upon by another force.

In the case of a pendulum, gravity pulls the bob downward to its lowest position, but the bob’s inertia keeps it swinging upward. When the pendulum reaches its highest position, gravity again pulls it downward. This process is continually repeated.

Minor forces, such as air resistance on the bob and friction within the suspension wire, will eventually slow a pendulum to a stop unless counteracted by some sort of “pulling” mechanism. The 270-pound bob on the Museum’s Foucault pendulum would swing freely for about two hours before these small forces brought it to a stop.

However, Jones and Griesmer invented unique electromagnetic and electronic circuitry that sits underneath the bob and gives it a slight “pull” every time the pendulum approaches its lowest position. A large iron plug is sealed inside the pendulum ball, and the electromagnet (located directly under the base) pulls the ball slightly during each sweep.

The electromagnet is not driving the pendulum, or causing it to precess about the base. The device simply counteracts the minor forces of air resistance and wire friction. Other pendulums throughout the world have mechanisms to compensate for these small forces, but most are mounted at the top of the wire near the ceiling.

The pendulum is not, in the strict sense of the word, a clock. But its swing is quite regular: 6.2 seconds for one swing, back and forth. How did Léon Foucault use the pendulum to prove the Earth’s rotation? If you view it for an extended period of time, the pendulum appears to swing in a different direction. The bob’s plane of oscillation (fancy words for the direction of swing) appears to move clockwise around the base at a rate of about 10 degrees per hour (about six inches).

What is causing the change in the direction of the pendulum's swing? The rotation of the Earth. The motion of a Foucault Pendulum is most easily envisioned at either of the Earth's Poles. At these locations, the direction of the swing remains fixed with respect to the background stars, while the Earth rotates beneath the pendulum. Because the observer is standing on the Earth, it appears that the direction of swing of the pendulum is changing. In 24 sidereal hours, the pendulum's direction of swing will go around a full 360 degrees.

The motion of a Foucault Pendulum at intermediate latitudes is more complicated, and the time required to go around a full 360 degrees is longer than 24 sidereal hours. At the latitude of Cleveland, it takes about 36 hours to go around a full 360 degrees. At the Earth's equator, the pendulum will not change its direction of swing.

Because it is really quite difficult to demonstrate that the Earth is turning, rather than the sky, the Foucault Pendulum, which was first demonstrated in Paris in 1851, became a popular means of showing the Earth's diurnal rotation.

The Museum’s Foucault pendulum has a 32-foot-long wire and 270-pound bob — a massive installation. When it was donated in 1986, the Lobby was the only space with a ceiling high enough to attach the bracket that holds the cable. However, the building’s engineers and architects determined that there were no beams above the ceiling upon which the pendulum could be hung. The roof was cut open and a support unit constructed so that the pendulum could be hung from the highest possible point.

After the cable was hung, the bob was attached and loaded with lead shot to bring its weight to 270 pounds. The electromagnet and its electronic circuitry were installed under the bob. The pendulum was put in motion in July 1986 and was formally dedicated in September 1987.