Clocks are basically instruments which maintain and measure oscillations to determine time. Early clocks measured the movement of a pendulum whose energy was allowed to escape through the appropriately named escapment. This allowed wheels to record the rate set by the pendulum through the hands on the clocks face.

The efficiency of these devices was measured by the degree with which the pendulum could swing unhindered by the devices attached to it as well as by reducing the effects of temperature and pressure changes.

By the 1700s observatories had adopted pendulums 994 millimetres in length as these beat at one second intervals. As clocks evolved mercury and other metals were used to make pendulums which were less affected by heat and this improved their accuracy.

The next major change in the development in observatory clocks was the introduction of electrically driven mechanisms. Early experimental work was done by Alexander Bain in the 1840s but it was probably the installation of Charles Shepard's clocks at the 1851 Great Exhibition and at Greenwich Observatory that marked the electric clocks first real success. These early clocks used pendulums to drive the clock but the electrical current allowed other clocks and time balls to be synchronised with its beats.

In 1891 a German, Dr. Siegmund Reifler introduced a new form of escapment which freed the pendulum from the clocks mechanism. These pendulums could be mounted in vacuum cylinders and their motions controlled and recorded using an electrical current which then powered 'slave' clocks.

In 1895 Hope-Jones and Bowell patented their first gravity impulse transmitters. Two years later they founded the Synchronome Company which began making electric clocks which were sold throughout Britain and the Colonies.

In the following decades a number of other electrical innovations challenged the Synchronome clock place in the British market. These included the 'Standard Time Company's' transmitters, the 'Pulsynetic' system and the 'Lowne' primary spring system.

In 1921 Mr. W. H. Shortt, who had worked closely with Hope-Jones, patented a new clock which allowed two pendulums to swing in precise sympathy. These took the load of the slave clock away from the master clock and were initiated by the master clock. These electric clocks, often referred to as 'Shortt clocks', brought a new level of accuracy to the world of timekeeping. From1925 Shortt clocks set the benchmark for electrical clock and they were installed in observatories, government departments and commercial businesses around the world.

Geoff Barker, Curatorial, March 2008

References
Stevenson, Roger, 'Mechanical and Electrical Clocks at Greenwich', appendix three in Howse, Derek, 'Greenwich Time and the Discovery of Longitutde', Oxford University Press, Oxford, England, 1980
-Jones, F., 'Observatory Time Installations', The Synchronome Company, London, 1925?
Haswell, J. E., 'Horology', The Empire Press, Norwich, 1947

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