Free pendulum and subsidiary, no. 47

In depth

On Christmas Day 1657, Christiaan Huygens first coupled a pendulum to a clock. His innovation brought clocks into previously unimaginable realms of accuracy, an oscillating pendulum being one of the most reliable methods then available to measure time. However, as soon as a pendulum is coupled to a gear train – essential if it is to release the escapement, receive further impulses and allow time to be shown on the dial – it is subject to varying amounts of interference, and loses a small, but significant, amount of accuracy. For this reason, almost from the pendulum clock’s inception, horologists dreamed of a ‘free pendulum’ that (the impossibility of perpetual motion notwithstanding) could be allowed to swing and keep time in maximum isolation from the clock movement.

The goal of the ‘free pendulum’ was effectively achieved in 1921, with this clock, designed by Wimbledon-born railway engineer William Hamilton Shortt. At a rate of a second a year, Shortt’s clock was (at the time) the most accurate pendulum clock ever commercially produced – capable of measuring small fluctuations in the earth’s rotation, as well as plotting star transits – and it remained a widely used time standard until the 1950s. Shortt’s innovation was to divide the requisite tasks of a clock between two pendulums that were separate but connected. One, dubbed the ‘master’, was the fabled free pendulum, swinging solitary and undisturbed in a cylindrical copper case sealed by a glass bell jar at the top, and a disk of plate glass at the bottom. This was the timekeeper, and the mechanism contains no countwheel – instead the pendulum is impulsed each thirty seconds using an electrically reset gravity arm. Crucially, the master clock does not initiate this impulsing process. It is the other clock, named the ‘slave’, which sets each thirty-second cycle in train, and it was the subtle and complex synchronisation between the two – facilitated by Shortt’s so-called ‘hit and miss synchroniser’ – that ultimately presented the time.

This clock is Shortt No. 47, one of 100 that are thought to have been manufactured between 1922-1956. All Shortt’s ‘free pendulums’ (apart from the test clock, Shortt No. 0) were manufactured by the Synchronome Company, headed by the electrical horology pioneer Frank Hope-Jones, who had provided use of the firm’s Clerkenwell offices for Shortt’s early experiments. The ‘slave’ clock is a standard Synchronome type – already a very accurate timekeeper – and the ‘slave’ for No. 45 is of the type advertised as ‘Type B’, mounted in a carved polished mahogany case with a 10-inch diameter regulator-type dial. This transmits seconds as well as half-minute impulses, and Synchronome boasted that it was used ‘as [a] standard cloc[k] in many Observatories, both for Sidereal and Mean Solar Time’. Indeed, apart from Shortt No. 7, installed in Shortt’s own home, the majority of the Shortt / Synchronome free pendulum clocks were purchased by the world’s astronomical and naval observatories. No. 45 was installed in the Naval Observatory in Washington, and seven clocks, including this one, were shipped to Russia (a market that abruptly closed on the outbreak of the Second World War, prompting Russian scientists to begin work on their own free pendulum experiments – [see TCW 1009 and 1011 and 1012]).

Shortt No. 47 was installed at the Sternberg Astronomical Institute (or ‘GAISh’), from whom it was purchased in the 1990s by William Scolnik, much of whose collection of precision clocks was purchased for the Clockworks in 2011.