Description
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. However, to prevent any possibility of coupling of the pendulums, the slave at the Lick Observatory was a considerable distance from the Master and in a different building; mounted so that the planes of swing of the slave and master were orthogonal . 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 eliminate 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.
Both pendulums were seconds pendulums, about 1 meter (39 in) long, with a period of 2 seconds; each swing of the master took exactly one second, with the slave's natural rate very slightly longer. The pendulums received a push from the mechanism once every 30 seconds to keep them swinging. The slave clock had two clock dials on it, showing the time kept by each pendulum, to verify that they were synchronized. It also had electrical terminals which produced a 1 Hz timing signal. Wires could be attached to these to transmit the clock's ultra-accurate time signal to clocks in other cities, or broadcast it by radio.
Read more about this topic: Shortt-Synchronome Clock
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