Shortt Hope Jones Synchronome Clock
The best mechanical timekeeper known is a pendulum swinging freely under gravity, but in order to convert such a pendulum into a practical clock, the pendulum must be sustained in motion so that the oscillations do not die down, and the swings must be counted. In ordinary clocks, the sustaining and counting function are both carried out by the escapement and the clock mechanism, but the free motion of the pendulum is considerably interfered with thereby, with a consequent loss of accuracy in timekeeping.
What is so special about the Shortt Free Pendulum Clock is that a free pendulum became a reality, and for a time during the period between the two world wars, it served as the pre-eminent time-keeper, and was to be found in all the worlds observatories.
It is a mechanism of exquisite delicacy. It relies for its extra-ordinary time keeping on the simple fact that a master pendulum is kept vibrating in a near vacuum, in a sealed room, bolted to a concrete wall, and with strict temperature control. The astonishing fact is that the pendulum has absolutely no work to do. It vibrates in solitary isolation and locks and unlocks no mechanisms. If this is so, how is time recorded, and to what does it owe its accuracy? It is a free pendulum in almost every sense of the word. Almost, because there is no such thing as a free pendulum, as there is no such thing as perpetual motion. In some way, it has to receive a maintaining force to keep it vibrating, and this is a gravity arm. If this force were arranged to be of a strictly uniform quantity, and was delivered in phase with the pendulum's vibration at zero position, it would have extra-ordinary time-keeping properties.
The success of the clock is due to the electro-mechanical age, which almost exactly lasted for 100 years during the 20th century. The clock is not an electric clock, because the impulse delivered to the pendulum does not rely on electricity but to gravity. But the reset force for the gravity arm is electric, and so are the signals that unlock the gravity arms, carry out phase comparison and generate the signal for time keeping. This is important, because the sealed pendulum chamber will only allow the passage of pairs of wires.
What is the nature of the slave, and how is the phase comparison achieved. To serve the master, Shortt enlisted the services of an established firm in London known as the Synchronome company. Its director was Frank Hope Jones, and his clock had gained wide acceptance for its ability to deliver an electrical signal on time to a large number of clock dials in series with its electro-magnets. Thus it found use in public buildings, and especially on the railways.
After much experimentation, Shortt approached Hope Jones and was allowed the use of his workshops in Clerkenwell, London to develop his ideas. Shortt wanted the synchronome to give the timed impulse for unlocking the master gravity arm, and become the slave for a higher master for the Synchronome was itself a highly accurate clock.
The first Shortt clock was tested in the Royal Observatory in Edinburgh in 1922, and the report on its behavior by the then Astronomer Royal for Scotland, Professor R. A. Sampson, was so favourable that others were soon installed at Greenwich (they subsequently had four), and in the principle observatories of the world. As an example of their performance, the maximum rate of change of Shortt No. 3 during its first year at Greenwich was .01 seconds and that of No. 41, during a continuous run of three years, from January 1943 to January 1946, in the Naval Observatory in Washington, was under 0.2 seconds.
The Shortt Clock has justly been described as the one of the most ingenious precision instruments ever made, dating from an age when imaginations were fired by electrical inventions. It has been regarded as the best timekeeper in its day, and has deservedly earned its place in the Horological Hall of Fame, being the first clock to allow accurate observation of the earth's nutation (the precession of the Earth's pole about the pole of the ecliptic). The Shortt clock turned out to be a great success. By 1956, the Synchronome company had made just about one hundred and exported them to observatories the world over. Over long periods, the variation of the Shortt clock might be as little as a second or two a year, which is probably close to the limit of what is possible for any clock whose resonant frequency depends on the uncertain forces of the Earth's gravity. It is a fitting climax to the era of the pendulum.
I was fascinated by this story when I saw the clock for the first time at the Royal Observatory, Greenwich, more especially as I could not work out how it functioned. I wondered whether parts of it could actually be built in Meccano and started experimenting with the gravity arm and its reset. I got it working and became very excited. Eventually I built both the Master Shortt Clock, and the Slave Hope Jones Clock and the time keeping electrical impulse counter clock and mounted it in a Grandfather size clock case in my home. I got the two clocks to function with each other in a phase locked loop and eventually found out what a truly extraordinary piece of engineering it is.