National Physical Laboratory

The World's First Caesium Atomic Frequency Standard

Essen's Caesium Clock

Caesium Mk. 1, the atomic clock that represented a revolutionary advance in timekeeping, was designed by Louis Essen and fellow physicist Jack Parry and constructed at NPL in 1955. Although it was not the first machine to use atoms for timekeeping, it was the first to keep time better than any other clock in existence (including both pendulum and quartz clocks).

It was also the first clock whose timekeeping was much more constant than the rotation of the Earth. Caesium Mk.1 was so accurate that it would only gain or lose one second in three hundred years. In fact, it was so accurate that the SI second was eventually redefined in terms of atoms rather than the Earth's motion in 1967, based on measurements made using Essen's clock.

A history of Louis Essen

Louis Essen was born in Nottingham in 1908. After graduating from Nottingham University in 1928 with a first class honours degree, he was invited to join the National Physical Laboratory (NPL). Here, he worked on the development of quartz crystal oscillators, which could measure time as accurately as the best pendulum clocks.

During the Second World War, his work on high-frequency radar led him to develop the cavity resonance wavemeter, which he used from 1946 with Albert Gordon-Smith to measure the speed of light more accurately than any previous measurement.

In the early 1950s, Essen became interested in research being conducted in the USA about the possibility of producing a highly accurate clock based on atoms.

In 1953, Louis Essen and Jack Parry were given approval to produce an atomic clock at NPL. At the time, they had little experience of atomic clocks but Essen's knowledge of quartz oscillators and microwave resonators enabled them to have Caesium Mk.1 up and running by 1955. By 1967, atomic clocks had become so accurate that the second was redefined in terms of caesium atom vibrations. The original definition of the second as 1/86400 of a mean solar day was abandoned because the Earth's motion was not reliable enough.

Even more accurate time

Modern atomic clocks (including the NPL-CsF2 clock) are over 300 000 times more accurate than Caesium Mk.1, and scientists at laboratories around the world, including NPL, are working to define time even more precisely.

Last Updated: 14 Jan 2015
Created: 26 Oct 2011


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