Gridiron pendulum A gridiron pendulum is a temperature-compensated set of pendulum rods invented by John Harrison around 1720. With an increase in temperature, the downward expansion of three steel rods under tension was countered by the upward movement of two brass rods under compression so that the effective length of the pendulum rod in relation to the bob remained constant. The system was then made symmetrical with a central steel rod and two brass and two steel rods on either side, resembling a gridiron. The mercury pendulum invented by George Graham in 1721 was also a temperature-compensated pendulum designed to overcome the limitations of temperature fluctuation on pendulums.
