![\"precession\"](\"http:\/\/arxivblog.com\/wp-content\/uploads\/2009\/02\/precession.jpg\" "\\"precession\\"")
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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.<\/p>\n
The pendulum oscillates in a vertical plane which slowly rotates. The rotation is explained by the Earth’s motion which spins beneath the pendulum.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
But while the precession is suppressed, the rotation due to the Earth’s motion is not.<\/p>\n
They’ve tested their idea on a 3 metre pendulum and say it works well.\u00a0 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.”<\/p>\n
Anybody want a desktop Focault Pendulum?<\/p>\n