Mercury pendulum A mercury pendulum is a temperature-compensated pendulum designed to overcome the limiting effects of temperature fluctuation on pendulums. In cold weather, a traditional pendulum would shorten, speeding up its rate of swing, and in hotter weather it would expand, slowing down the rate. The mercury pendulum had a bob that also functioned as a container for liquid metal mercury. The mercury would expand when the pendulum rod got longer as the temperature rose. This would cause the surface to rise, thus raising the centre of mass. When temperatures dropped, the reverse would happen. Thus, these two effects cancelled each other out, leaving the rate of the pendulum’s swing the same. George Graham invented the mercury pendulum in 1721. The gridiron pendulum that John Harrison invented in 1726 was also a temperature-compensated pendulum designed to solve the same problem.
