Greenwich Observatory quartz clock equipment, 1942.

This equipment reveals the method used by Greenwich Observatory for measuring time to an accuracy of one-thousandth of a second per day. It includes three complete quartz crystal clocks, together with devices for inter-comparing the rates of the clocks and a dial showing the time indicated by one of them. The crystals themselves are in ovens not visible in the photograph. Quartz clocks were superseded by atomic clocks for determining standard time in the 1960s.