Big clocks

This photograph shows the clock made in 1795 for use at Scotland Yard in London. Its design is not much different to that of the clock made for King's College, Cambridge 124 years earlier.

In 1854, Lord Grimthorpe used a new kind of frame and escapement for the Westminster clock, often referred to as Big Ben, which was set running permanently in 1859. Strictly speaking, Big Ben is the name of the bell which strikes the hours, not the clock itself.

This photograph shows a turret clock mechanism made in 1939 by the company which built the Westminster clock. It has the same type of escapement and horizontal layout and chimes in the same way.

Like the one at Wells Cathedral, this clock mechanism has three trains of wheels, driven by separate weights but controlled by the same pendulum. In this picture, the part in the middle drives the hands, the part on the left controls the hour bell and the part on the right controls the bells which chime every fifteen minutes.

The clock uses a gravity escapement, which solves one of the problems of outdoor clocks. Changes in the weather can alter the force needed to turn the hands. This force feeds back through the mechanism. With the ordinary anchor escapement, it can affect the pendulum and alter the timekeeping. The design of the gravity escapement isolates the pendulum from such dangers.

Using a long, heavy pendulum also reduces the chance of inaccuracy. The pendulum at Westminster is 4.4 metres long and each half-swing takes 2 seconds.

References

John Watney, The mystery of time, Andover: Pitkin, 1999
J. Jespersen & J. Fitz-Randolph, From sundials to atomic clocks: understanding time and frequency, New York: Dover Publications, 1982
G. J. Whitrow, Time in history, Oxford: Oxford University Press, 1989
Eric Burton, The history of clocks and watches, London: Little, brown & Co., 1992
Gerhard Dohrn-van Rossam (Translated by Thomas Dunlap), History of the hour: clocks and modern temporal orders, Chicago & London: University of Chicago Press, 1996

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