Experiment of The Month
Five Meter Foucault Pendulum
NOTE: THIS PROJECT HAS BEEN DROPPED. THE PISTONS USED TO DRIVE THE PENDULUM GAVE THE PENDULUM A PREFERRED DIRECTION, CAUSING THE PENDULUM TO STOP ROTATING AND "STICK." A BRAND NEW PISTON WOULD WORK FOR A WEEK OR TWO, THEN LOCK IN A PREFERRED DIRECTION.
The new Lancaster Science Factory asked us to install a large-scale Foucault pendulum in their new exhibit space.The rotation of the plane of oscillation of this pendulum is the only readily observable evidence for the rotation of the earth on its axis. Using the Foucault pendulum to demonstrate the earth's rotation is an implementation of the motto of the British Royal Society (for science)*: "Nullius in Verba" - Accept nothing on the basis of words alone.
The pendulum uses the same parametric drive method as the 1-meter pendulums that we have been building for several years to give to our graduating secondary education (physics) majors. The parametric drive mechanism was described as a textbook problem by Landau and Lifshitz (Mechanics, Pergamon Press, 1960, page 84, problem 3) and was implemented in 1988 by Sir Brian Pippard . Simply put, the pivot point of the pendulum is pulled up each time the pendulum swings through its low point. The process is the same as a child pumping a playground swing.
For the one meter pendulums, the vertical motion was a few millimeters, and could be produced by a loudspeaker facing down, with the pendulum fastened to the speaker cone. For the long pendulum, the vertical motion is about 8 centimeters. This motion is produced by an air cylinder piston. (See photo at the bottom of the page.)
For the one meter pendulums, the bob is a dog's chew-toy, chosen for two main reasons.
- The rough surface suppresses large scale turbulent air flows around the bob, which might otherwise impede the rotational motion.
- The low mass (150 grams) and 7 cm diameter of the toy lower the "Q" of the pendulum resonance, making it easier to establish the correct driver frequency.
For the 5 meter pendulum the bob is more massive: a 2 kg "exercise ball," 20 cm in diameter.
For the 5 meter pendulum, the pivot is simply the jaws of a pin vise, clamping a 1.6mm diameter nylon fishing line. The flexing of this fishing line provides the main energy loss (damping) for the pendulum and broadens the resonance sufficiently. The driver frequency is about 0.45Hz, and must be controlled to within .001Hz to work well. The amplitude of the pendulum is controlled by shifting the driver frequency relative to the pendulum resonance frequency.
The pendulum constructed by senior Drew Pulsifer, and began swinging on November 2, 2007. The ultimate aim is to suspend the pendulum from the 10 meter high ceiling, but the need for access dictates that the height be reduced for the present.
*Full name: "Royal Society of London for the Improvement of Natural Knowledge"