© Science Museum/Science and Society Picture Library
This is the first successful atomic clock. In 1955, when it was developed, it proved more accurate than any other timekeeper in the world.
The use of the stable vibrations of caesium atoms as a time standard was first proposed by the physicist Isidor Rabi. This led to a design built at the US National Bureau of Standards in 1951, but it was not accurate enough to replace existing master clocks.
In 1953, Louis Essen and J. V. L. Parry, at the National Physical Laboratory, Teddington, began work on a parallel approach. The challenge was to provide a microwave radio signal that would be 'locked' to a natural radio frequency that excites caesium atoms to switch between energy states. This principle has been likened to an 'atomic pendulum'.
By 1955 they had developed a clock to provide an accuracy of 1 second in 300 years. This work led to the current international system whereby time is set by averaging results from atomic clocks in many laboratories across the world.
The move to atomic time has underlined the fact that the Earth itself is not a perfect timekeeper (as Immanuel Kant had postulated in 1754). As the Earth's motion varies, 'leap seconds' are added to keep our clocks in step with the solar day.