Shortt-Synchronome clock

The Shortt–Synchronome free pendulum clock was a complex precision electromechanical pendulum clock invented in 1921 by British railway engineer William Hamilton Shortt in collaboration with horologist Frank Hope-Jones, and manufactured by the Synchronome Co., Ltd. of London, UK. They were the most accurate pendulum clocks ever commercially produced, and became the highest standard for timekeeping between the 1920s and the 1940s, after which mechanical clocks were superseded by quartz time standards. They were used worldwide in astronomical observatories, naval observatories, in scientific research, and as a primary standard for national time dissemination services. The Shortt was the first clock to be a more accurate timekeeper than the Earth itself; it was used in 1926 to detect tiny seasonal changes in the Earth's rotation rate. Shortt clocks achieved accuracy of around a second per year, although a recent measurement indicated they were even more accurate (see below). About 100 were produced between 1922 and 1956.

Shortt clocks kept time with two pendulums, a master pendulum swinging in a vacuum tank and a slave pendulum in a separate clock, which was synchronized to the master by an electric circuit and electromagnets. The slave pendulum was attached to the timekeeping mechanisms of the clock, leaving the master pendulum virtually free of external disturbances.

The Shortt clock consists of two separate units: the master pendulum in a copper vacuum tank 26 cm diameter and 125 cm high attached to a wall, and a precision pendulum clock "slaved" to it, standing a few feet away. To prevent any possibility of coupling between the pendulums, the two units were either installed far apart in different rooms, or the units were oriented so the planes of swing of the two pendulums were ninety degrees apart. The slave clock was a modified version of a standard Synchronome precision regulator clock. The two components were linked by wires which carried electric pulses that operated electromagnets in the mechanisms to keep the two pendulums swinging in synchronism. The master pendulum rod and its 14-pound weight were made of the alloy invar to reduce thermal expansion and contraction of the pendulum, which causes the pendulum's period to vary with changes in temperature. The residual thermal expansion rate was compensated to zero with a metal insert under the bob. The vacuum tank was evacuated by a hand-operated pump to a pressure of around 30 mm Hg to prevent changes in atmospheric pressure from affecting the rate of the pendulum, and also to greatly reduce aerodynamic drag on the pendulum, which increased its Q factor from 25,000 to 110,000, thus increasing its accuracy by a factor of four. Experiments by Shortt showed that at 30 mm Hg the energy consumed by the flexing of the suspension spring just equalled the energy consumed by deflecting the residual air molecules and therefore a higher vacuum was not required.

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