Learning from a Museum Exhibit: The Case of the 19th-Century Compensation “Gridiron” Pendulum

The compensation pendulum is a pendulum that is constructed in a way that its length does not vary by changes of temperature. As the temperature of the air around the clock goes up, the rod of a regular pendulum clock mechanism expands and the clock runs slower. Historically, it has proven easier to compensate for changes in the ambient temperature than to eliminate them. One of the most common pendulums is the “gridiron,” which was contrived by Harrison around the year 1726. It consists of alternate rods of steel and brass (five of which are made of steel and four of brass), connected by cross-pieces at the top and bottom. It is called a “gridiron” because of the arrangement of its rods (Fig. 1). The principle of its operation is simple and unexpectedly effective. The steel would push down, lengthening the pendulum, but the brass, being attached at the bottom, would push up. The two lengths were in the inverse ratio of their thermal expansion coefficients, in which case the upward expansions just equal those that are downward, and therefore the center of oscillation remains at the same distance from the point of suspension.