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14 November 2003

A Pivot for a Foucault Pendulum

by C. L. Stong
Excerpted from "Scientific American's The Amateur Scientist", first published June, 1958.

Until recent years the problem of making a pendulum swing true resisted some of the world's best instrument makers. It seemed clear that any method of suspension must have radial symmetry such as one would expect of the suspension device depicted in Figure 3. To assure this Foucault and subsequent experimenters took great pains in procuring wire of uniform characteristics and in designing the fixture to which the wire was attached. Roger Hayward, the illustrator of this department, tells me that the wire for the Foucault pendulum in the Griffith Observatory in Los Angeles was specially drawn and tied to a long two-by-four beam for shipment from an eastern mill to the West Coast. The designers were afraid that coiling the wire would destroy its symmetry. The pivot to which the wire was attached at first consisted of a set of gimbals with two sets of knife-edges at right angles to each other. Despite these precautions the completed pendulum insisted on performing figure eights and ellipses. Hayward, who had designed other exhibits for the Observatory, suggested that the wire simply be held in rigid chuck. This invited a break at the junction of the wire and the chuck, which could cause the wire to lash into a crowd of spectators. To minimize this hazard a crossbar was clamped to the wire just above the ring-shaped driving magnet. Thus if the wire had broken, the crossbar would have been caught by the magnet ring. Clamping the wire in a chuck cured the difficulty. The wire has now been flexing for more than 20 years without any apparent ill effect.