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and became the highest standard for timekeeping between the 1920s and the 1940s, after which mechanical clocks were superseded by quartz time standards.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 was 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 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.
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.
Reason for accuracy
A pendulum swinging in a vacuum without friction, at a constant amplitude free of external disturbances, theoretically keeps perfect time. However, pendulums in clocks have to be linked to the clock's mechanism, which disturbs their natural swing, and this was the main cause of error in precision clocks of the early 20th century. An ordinary clock's mechanism interacts with the pendulum each swing to perform two functions: first, the pendulum must activate some kind of linkage to record the passage of time. Second, the clock's mechanism, triggered by the linkage, must give the pendulum a push (impulse) to replace the energy the pendulum loses to friction, to keep it swinging. These two functions both disturb the pendulum's motion.
The advantages of the Shortt clock are first, it reduced the disturbance of the master pendulum due to the impulse by only giving the pendulums an impulse once every 30 seconds exactly (30 pendulum swings), and second, it eliminated all other interaction with the master pendulum by generating the necessary precise timing signal to control the slave clock (and record the passage of time) from the impulse mechanism itself, leaving the pendulum to swing "free" of interference.
How it works
The master and slave pendulums were linked together in a feedback loop which kept the slave synchronized with the master. The slave clock had a mechanical escapement using a 15-tooth count wheel which was moved forward each pendulum swing by a pawl attached to the pendulum. Every 15 swings (30 seconds), this released a gravity lever which gave the slave pendulum a push. This simultaneously closed a switch which reset the gravity lever and sent a pulse of current to an electromagnet which released a second gravity lever in the master unit to give the master pendulum a push. The impulse was provided by the weight of the gravity lever (acting as a remontoire) rolling off a wheel attached to the pendulum, this mechanism ensuring that the pendulum received an identical impulse, at precisely the same part of its stroke, every 30 seconds. The falling gravity lever closed another pair of contacts which reset the lever and provided an electrical pulse back to the slave unit.
Hit and miss synchronizer
The pulse from the master pendulum was used to keep the slave pendulum in phase with it through a device called a "hit and miss synchronizer". Every 30 swings, after the master pendulum was impulsed, the position of the two pendulums was compared. The pulse from the master unit powered an electromagnet which moved a vane into the path of a leaf spring attached to the slave pendulum. If the slave pendulum lagged behind the master, the spring would catch on the vane (called a "hit"). The spring would give the pendulum a push, which shortened the time for that swing. If it was ahead (a "miss") it would make its normal swing, without acceleration. The slave pendulum was set to a slightly slower rate than the master, so the slave would lag behind the master more each interval, until it received a "hit" which set it ahead again. Typically the acceleration resulting from a "hit" would be adjusted to be about twice the normal loss, so that "hit" and "miss" cycles would roughly alternate, hence the name of the mechanism. This cycle, repeated over and over, kept the slave precisely in step with the master over the long term. This feedback loop functioned as an electromechanical version of a phase locked loop, later used in electronics and quartz and atomic clocks.
Recent accuracy measurement
In 1984 Pierre Boucheron studied the accuracy of a Shortt clock preserved at the US Naval Observatory. Using modern optical sensors which detected the precise time of passage of the pendulum without disturbing it, he compared its rate to an atomic clock for a month. He found that it was stable to 200 microseconds per day, equivalent to an error rate of one second in 12 years, far more accurate than the 1 second per year that was previously measured. His data revealed the clock was so sensitive it was detecting the slight changes in gravity due to tidal distortions in the solid Earth caused by the gravity of the Sun and Moon.
- Shortt clock animation Detailed, high resolution animation of the entire mechanism, giving a good understanding of how it works.
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