English: Diagram showing operation of temperature compensated "gridiron" pendulum, invented in 1726 by British clockmaker John Harrison. The pendulum uses rods of a high thermal expansion metal, zinc (yellow) to compensate for the expansion of rods of a low thermal expansion metal, iron (blue), so the overall pendulum doesn't change in length with temperature changes. Therefore the period of swing of the pendulum, and the rate of the clock, are constant with temperature.