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.