Foucault, revisited: Scientists show how to build a pendulum for any classroom

Nov 09, 2010

Walk into nearly any science museum worth its salt and you're likely to see a Foucault pendulum, a simple but impressive device for observing the Earth's rotation. Such pendulums have been around for more than 150 years, and little about how they work remains a mystery today.

The only problem, according to Argentinean researcher Horacio Salva, is that the devices are generally large and unwieldy, making them impractical to install in places where space is at a premium. This limitation was something he and his colleagues at the Centro Atómico Bariloche in Argentina wanted to address.

Now in the American Institute of Physics journal Review of Scientific Instruments, Salva and colleagues report success in what he acknowledges was a fun side project -- building two pendulums precisely enough to make measurements of the spinning Earth yet compact enough to fit in a lobby or classroom of just about any science building.

By definition, Foucault pendulums -- which are named after the French physicist Léon Foucault who first conceived of one in the middle of the 19th century -- count as a simple technology. Generally, a metal orb is suspended by a wire and hung from a height that can be dozens and dozens of feet. The orb is pulled back and released, and as it swings back and forth over the course of a day, it appears to slowly rotate in a circle. In fact what's observed is the Earth moving underneath the pendulum, which swings back and forth in a fixed plane, like a gyroscope.

Or rather, it's more accurate to say that pendulums swing in a mostly fixed plane. That's because, as anyone who pushes a child in a swing can attest, it's tough to keep a pendulum swinging in a straight line. Over time, due to the vagaries of friction and other forces, a pendulum will start to travel in an ellipse, an effect that can easily garble evidence of the Earth's rotation, which for generations has been novel enough to astonish when first observed. Here's the beginning of a February 23, 1908 article in The New York Times describing a Foucault pendulum display in the Big Apple: "Perhaps you were one of the crowd of people who saw the great Foucault pendulum experiment last week at Columbia University. Probably you watched it like the rest with openmouthed wonder."

The heavier the suspended orb and the longer the wire, the more limited the elliptical drift. Similarly, older children on taller swings tend to fly straighter than younger children in the shorter toddler swings.

Consider the dimensions of an 80-year-old Foucault pendulum on display at Philadelphia's Franklin Institute: a 180-pound-orb hangs from a wire 85 feet long and swings back and forth once every 10 seconds. The two pendulums built by Salva are kiddie-sized in comparison. In the case of the first, a 27-pound weight swings back and forth on a 16-foot-long piano wire once every 4 and a half seconds. The second pendulum uses the same weight and an even shorter wire. Using a copper ring underneath each orb to damp down the drift, Salva was able to easily observe and measure precession, the technical name for the movement of the Earth relative to the fixed swinging of the pendulums. Indeed his jiggering of the pendulums was able to tune out all but one percent of the elliptical "noise," at least in the case of his longer pendulum.

Admittedly, says Salva, this new pendulum by no means has the precision necessary to make any groundbreaking new measurements. But the design, he says, is sophisticated enough to be a useful tool for teaching basic physics concepts to physics students and the general public.

"There's obviously no pressure to do work like this," said Salva, who in his day job studies far more sophisticated "pendulums" involving the elasticity of various materials. "It's mostly for fun, though I think it may well help students in the future, too."

Pointing to one possible application, the paper notes that the device was able to detect earthquakes of medium intensity that took place as far away as 765 km. "Some earthquakes can be seen, because the seismic wave moves the support of the increasing the ellipse of the moment and changing the precession speed," said Salva.

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More information: The article, "A Foucault's pendulum design" by Horacio R. Salva, Rubén E. Benavides, Julio C. Perez, and Diego J. Cuscueta appears in the journal Review of Scientific Instruments. See: rsi.aip.org/resource/1/rsinak/v81/i11/p115102_s1

Provided by American Institute of Physics

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