Gridiron pendulum

The gridiron pendulum was a temperature-compensated clock pendulum invented by British clockmaker John Harrison around 1726, used in precision clocks. In ordinary clock pendulums, the pendulum rod expands and contracts with changes in temperature. The period of the pendulum's swing depends on its length, so pendulum clocks rate varied with changes in ambient temperature, causing inaccurate timekeeping. The gridiron pendulum consists of alternating parallel rods of two metals with different thermal expansion coefficients, such as steel and brass. The rods are connected by a frame in such a way that their different thermal expansions (or contractions) compensate for each other, so the overall length of the pendulum, and thus its period, stays constant with temperature.