A pendulum is a weight suspended from a pivot or spring so that it swings freely to and fro, and in clockmaking it is the regulating element that controls the rate at which a clock runs. Because a pendulum of a given length swings at a nearly constant rate, it provides a far more accurate means of timekeeping than earlier devices.
The principle was studied by Galileo, and the Dutch scientist Christiaan Huygens designed the first pendulum clock in 1656, made for him by Salomon Coster of The Hague. English clockmakers quickly adopted the invention, and the long pendulum, typically beating seconds, combined with the anchor escapement, led to the development of the longcase clock. Shorter pendulums were used in bracket and mantel clocks. Changes in temperature alter the length of a pendulum, so clockmakers developed compensated pendulums, including George Graham's mercury pendulum and John Harrison's gridiron pendulum of brass and steel rods. Pendulum rods may be wood, steel or brass, and bobs range from simple lead discs to decorative gilt examples on French clocks. When buying, check that the pendulum is original or correct for the clock, since the wrong length affects timekeeping and replacement bobs are common.
