Longcase precision regulator with equation of time indicator, made by George Graham

London, England, around AD 1745

By the early years of the eighteenth century it was realised that temperature change was the enemy of accurate timekeeping. On the pendulum clock (introduced in 1657), the length of the pendulum determines the rate at which it swings. This in turn controls the rate at which the clock runs and thus its accuracy. It is not helpful, therefore, that a pendulum gets longer when heated and shorter when cooled. George Graham FRS (1673–1751), one of the leading clock, watch and instrument makers in London, was the first to apply temperature compensation to a pendulum clock successfully, in about 1730. In this example of his work, the brass pendulum rod carries a glass jar containing mercury. As the brass rod expands and contracts, the mercury expands and contracts in the opposite direction and keeps the length of the pendulum constant.

This weight-driven, month-going, longcase regulator also has Graham's dead-beat escapement, invented by him in about 1725. This was another innovation that improved the accuracy of clocks. This clock also has a 'bolt-and-shutter' to maintain power and keep it going while being wound. Added refinements are the secondary minute hand that shows true solar minutes and the dial in the arch that, as well as being a calendar, also shows the equation of time in minutes and seconds for each day of the year.

D. Roberts, British longcase clocks (West Chester PA, Schiffer Publishing, 1990)

T. Robinson, The longcase clock (Woodbridge (Suffolk), Antique Collector's Club, 1981)