Google Doodle Celebrates Léon Foucault With Interactive Pendulum: How Does French Physicist’s 1851 Invention Work?

By Philip Ross on September 18, 2013 11:30 AM EDT

Foucault pendulum
Foucault’s pendulum in the Pantheon in Paris. (Photo: Creative Commons)

You've probably noticed something different about Google's homepage today. Its familiar logo is replaced by an interactive Foucault pendulum, a device that demonstrates Earth's rotation. The pendulum is named for its inventor, the French physicist Leon Foucault, who was born Sept. 18, 1819. That makes today the 194th anniversary of his birth, a day Google felt deserves some consideration. Maybe because rotation, like Google, is something none of us can run from? 

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The Earth rotates on its axis, moving from west to east, at 1,037 mph. At that speed, it would take you about three hours to drive from New York to Los Angeles. At the same time, our planet orbits the sun at 67,000 mph - at which speed you could make a cross-country road trip in less than two and a half minutes.

Presumably, if you were to drop an apple down a mine shaft in a perfect vacuum, you'd see the apple land a bit west of its true vertical. The same would happen if you shot a gun from north to south; the bullet would have an eastern deflection.

Foucault designed the pendulum as an experiment to show that the earth is actually spinning on an axis. The device consists of a long wire with a heavy bob attached to the end. Once someone starts the device swinging, the bob, assuming it can swing in the absence of friction (a note on that in a minute), will slowly and surely start to swing in a different direction.

For example, if you were watching a Foucault pendulum rotate in San Francisco, Calif. - a latitude of 38 degrees North - you'd see the pendulum rotate 220 degrees in about 24 hours. If you took your Foucault pendulum closer to the North Pole, you'd see that rotation speed increase. Head South, and you'd see the pendulum complete less of that 360 degree rotation in 24 hours. Reach the equator, and the bob will swing on a straight path indefinitely.

It's actually the earth moving beneath the pendulum, not the pendulum itself that's moving.

So a note about friction. Anyone who's ever swung at a playground knows that nothing swings forever because of wind and air resistance. To remedy that, most Foucault pendulums today have an iron collar installed where the wire attaches to the ceiling. An electromagnet attracts the collar as the bob swings, but then shuts off automatically as it swings the other direction. This keeps the pendulum moving in the presence of wind and air resistance.

The Foucault pendulum interactive illustration sported in today's Google Doodle is a rudimentary version of the real deal, but the concept is firm in purpose. Here's how to use it:

First, click the small Earth icon to the right of the pendulum. The yellow line running through the Earth represents the current latitude. Move it up or down, and the pendulum will change direction more rapidly or more slowly, depending on how far you are from the "equator" (more power to you if you can locate the equator on such a small map). You'll see the pins knock down as the pendulum swings and rotates.

Then, click the clock icon and move the meter up or down to see the pendulum move in a 24 hour period. Voila! You've just demonstrated Earth's rotation.

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