Shortt Clock

Shortt Clock


(also free-pendulum clock), a highly accurate pendulum clock. The Shortt clock is named after the British inventor W. H. Shortt, who devised it in 1921.

A Shortt clock has two pendulums made of Invar. The first, called the master pendulum, is enclosed in a low-pressure chamber and placed in a constant-temperature cellar. The second, known as the working pendulum, is installed in an ordinary wooden clock case and kept in a convenient place. The working pendulum is connected to the indicating mechanism, which consists of hands, a clockface, and electric contacts. The clock is powered by an 8-volt DC battery.

In the Shortt clock, an impulse is released every second. The master pendulum, which swings freely, receives a mechanical impulse every 30 sec in order to overcome the damping associated with friction. Special electromechanical devices synchronize the oscillations of the working pendulum, which functions as both a drive mechanism and a counter. The length of the working pendulum may be adjusted so that the clock will keep either sidereal time or mean solar time. The mean-square clock rate of a Shortt clock is ±0.002–0.003 sec per day.

Shortt clocks became obsolete in the 1950’s because they did not provide the accuracy required in many astronomical investigations.


Bakulin, P. I., and N. S. Blinov. Sluzhba tochnogo vremeni. Moscow, 1968.