Foucalt’s pendulum–a 28 kilogram bob suspended on a 67 metre wire– famously hangs in the Pantheon in Paris where Leon Foucalt himself demonstrated it in 1851.

The pendulum oscillates in a vertical plane which slowly rotates. The rotation is explained by the Earth’s motion which spins beneath the pendulum.

The length of the wire is important because it helps to iron out any irregularities that can always occur in the swing of a pendulum. In particular, it’s hard to set a pendulum in motion without imparting some ellipsoidal motion. The problem is that this ellipse always precesses, a motion that inevitably swamps the rotation caused by the Earth’s spin.

This is much less of a problem for long pendulums because the properties of the ellipsoidal motion are inversely related to the length of the wire.

If you have access to a 67 metre ceiling, you’re fine. But try it with a shorter pendulum and you’ll just never see the effect you want. That’s why Focault’s Pendulums are rather rare devices.

That may change thanks to some interesting work by Reinhard Schumacher and Brandon Tarbet at Carnegie Mellon University in Pittsburgh. What they’ve done is to work out how to drive a much shorter pendulum without allowing its ellipsoidal motion to precess.

They’ve done this with a motor that uses Faraday induction and magnetic repulsion to push a pendulum in a controlled fashion. The system is cleverly designed to cancel out any tendency to precess, whatever the size of the ellipse that the pendulum is describing.

But while the precession is suppressed, the rotation due to the Earth’s motion is not.

They’ve tested their idea on a 3 metre pendulum and say it works well.  They end their paper saying: “We plan to further test this method on even shorter pendula, since there is no lower limit at which the model given here should apply.”

Anybody want a desktop Focault Pendulum?

Ref: arxiv.org/abs/0902.1829: A Short Foucault Pendulum Free of Ellipsoidal Precession

This entry was posted on Friday, February 13th, 2009 at 7:35 am and is filed under Mean machines. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.