Gridiron pendulum from a precision clock, 1880s. The gridiron pendulum, invented 1726 by John Harrison, was a temperature compensated pendulum that stayed the same length with changes in temperature, preventing the variations in timekeeping that occurred when the rods of ordinary pendulums expanded or contracted with changes in temperature. In the gridiron the pendulum bob is supported by a "grid" of rods of two different metals, in this case steel "(dark rods)" and brass "(light rods)". The rods are connected by a frame so that expansion of the steel rods makes the pendulum longer, while expansion of the brass rods pushes the bob up, making the pendulum shorter. By making the rods of the correct length, the greater expansion of the brass compensates for the greater length of the steel rods, and the overall length of the pendulum stays constant with temperature. Zinc-steel gridirons only required 5 rods, while the smaller expansion coefficient of brass vs steel made 9 rods necessary in brass-steel gridirons, so this example is the latter type.