Space and Time

Shortt regulator no.99

Built 1937. Donated by the Tunstall Group plc of Whitley Bridge, Yorkshire in 1992.

For a quarter of a century the Shortt ‘free pendulum’ clock achieved previously unheard-of levels of precision time-keeping. Shortt clocks were so accurate that they could detect hitherto unknown variable changes in the speed of rotation of the earth. Its accuracy was only superseded by the development of the quartz clock and later the atomic clock.

The clock has been tested and achieves an accuracy of better than one second per year (1 in 32 million), which is approximately two-thousandths of a second per day. A Shortt clock installed at the Royal Greenwich Observatory in 1926 ran for nearly nine years without stopping.

 

How the clock works

The clock worked by linking a free-swinging pendulum (master) to a secondary pendulum (slave) by means of a frictionless electrical connection. The master pendulum operated in a vacuum chamber.

Only about 100 Shortt regulator clocks were ever made. It was invented by William Hamilton Shortt (1881-1971), a railway engineer. They were used by astronomers in observatories around the world. There is even a Shortt regulator clock in the Beijing Ancient Observatory.

Shortt patented his design for the Shortt regulator in 1922. Previous electric regulator clocks were less accurate. Such clocks produced an electrical signal, which could be transmitted for long distances, thereby synchronising clocks at different locations. Shortt (1881-1971) was a director of the company which manufactured clock systems for railway companies, for whom he worked as a consultant. After the First World War he became district engineer for the Southern Railway.

 

Why the clock was needed

Railways needed time to be standardised, whereas in the 19th century people generally regarded noon as the time when the sun was at its zenith in their particular location. This meant that railway timetables had to allow for this east-west variation, which caused considerable confusion. There was thus a pressing need to transmit a standard time across the railway network. As was the case with the One o’clock Gun fired at Bidston Observatory to allow mariners to calibrate their chronometers, the Shortt synchronisation signal sent by telegraph at noon was used to set clocks with an unprecedented degree of accuracy.

Liverpool was the base from which the Battle of the Atlantic was directed during the Second World War. Success in combating the submarine menace was partly achieved through improvements in echo-detection, known as ASDIC. Tunstall Telecom, the former owners of this clock, acquired the Synchronome Company of Alperton, Middlesex who built the machine to a Shortt design. This particular instrument was used for calibrating ASDIC equipment, which required a high degree of accuracy. ASDIC systems used echo-location to find underwater objects. The time taken for the echo ‘ping’ to return to the ship gave an indication of the distance, and precise timekeeping was necessary in order to compute the range with sufficient accuracy.