English: Gridiron pendulum from a precision regulator clock, in the book "Tidens naturlære" (Nature of time) 1903 by Poul la Cour. 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 with ordinary pendulums when their pendulum rods 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, R & S) and zinc (light rods, T). The rods are connected by a frame (a,b,f,g) 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 zinc compensates for the greater length of the steel rods, and the overall length of the pendulum stays constant with temperature. Zinc-steel gridirons like this example only required 5 rods, while brass-steel gridirons required 9 rods due to the smaller difference between the thermal expansion of brass and steel. It is a "seconds pendulum", with a period of 2 sec so each swing took one second, with a length from suspension to bob center of approximately one meter (39 inches).