File:Mercury regulator pendulum.png

English: Drawing of a mercury pendulum, a temperature-compensated pendulum used in precision pendulum clocks during the 19th century. Ordinary pendulums are inaccurate timekeepers because the supporting rod expands with increasing temperature and contracts with decreasing temperature, changing the period of swing. The most accurate "regulator" clocks, used mercury pendulums like this. The pendulum's weight (M) is a container of the liquid metal mercury. When the temperature rises, the pendulum rod (R) gets longer, but the mercury also expands and its level in the container rises slightly, moving the center of gravity upward toward the pivot. With the correct height of mercury in the container, these effects will cancel, and the center of gravity, and the pendulum's period, will remain constant with temperature. To adjust the rate of the clock, this pendulum has a tray (C) on the rod, to which small weights can be added and taken away, so the rate can be changed without stopping the clock.