Pendulum Clock - Gravity-swing Pendulum

Gravity-swing Pendulum

The pendulum swings with a period that varies with the square root of its effective length. The rate of pendulum clocks is adjusted by moving the pendulum bob up or down on its rod, often by means of an adjusting nut under the bob. In some pendulum clocks, fine adjustment is done with an auxiliary adjustment, which may be a small weight that is moved up or down the pendulum rod. In some clocks, adjustment is accomplished by a small tray mounted on the rod where small weights are placed or removed to change the effective length, so the rate can be adjusted without stopping the clock.

The period of a pendulum increases slightly with the width (amplitude) of its swing; at an amplitude of 23° it loses 1% or about 1 minute 25 sec. per day, compared to its rate for small swings. The rate of error increases with amplitude, so when limited to small swings of a few degrees the pendulum is nearly isochronous; its period is independent of changes in amplitude. Therefore the swing of the pendulum in clocks is limited to 2° to 4°.