This is a replica of a Hewlett-Packard caesium-beam frequency standard, or atomic clock. The original was manufactured by the Santa Clara Division of the Hewlett Packard Company, USA (since 1999 a separate company called Agilent Technologies). The model HP 5061A atomic clock was used at observatories and timing laboratories around the world, including the Royal Greenwich Observatory, and is still in operation at some of them as part of the equipment contributing to that nation's timescale.

An HP 5061A clock was also the device used to time the British national radio time signals broadcast from Rugby, in turn checked by the clocks at the National Physical Laboratory, Teddington, the home of Britain's time service today.

One of the predictions of Einstein's theory of relativity was that gravity affects the passage of time: in other words, higher clocks run faster than lower clocks. In 1975 and 1977, two very important scientific experiments were carried out using HP 5061A atomic clocks in different positions on and above the Earth's surface, to test the predictions. Einstein was found to be correct, and it was only with the accuracy and portability of these atomic clocks that the experiments could be carried out.

The first practical atomic clock was developed in 1955 by Louis Essen and Jack Parry at the UK's National Physical Observatory. Atomic clocks are essentially quartz clocks which are automatically corrected for frequency changes by being locked into a fixed relationship with the frequency of caesium atomic transitions. Since 1967 the standard interval of time is the atomic second, with astronomical definitions of time being relegated to specialist uses only.