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The first atomic clock, Caesium I, was designed by Louis Essen and built at the National Physical Laboratory in Teddington in 1955. Although it was not the first machine to use atoms for timekeeping, it was the first to keep time better than the best pendulum or quartz clocks.
It was also the first clock whose timekeeping was significantly more constant than the rotation of the Earth. Modern atomic clocks are even more accurate than Caesium I and time is now defined in terms of atoms rather than the Earth's motion.
All mechanical clocks work by counting the vibrations of something which has a constant frequency such as a pendulum. Unfortunately, the frequency of a pendulum is not perfectly constant. It is affected by changes in temperature, air pressure and the strength of gravity. This causes the clock to run too quickly or too slowly. The frequencies measured by atomic clocks are much higher than those of a pendulum but vary much less, so atomic clocks keep time much better. Caesium I was so accurate that it would only gain or lose one second in three hundred years. Modern atomic clocks are even more accurate.
It is difficult to imagine that we need to measure time so accurately but it is essential for many aspects of modern life which we take for granted. Modern navigation systems, mobile telephones, digital television and many industrial and commercial activities need this level of accuracy.
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). There, he worked on the development of quartz crystal oscillators, which could measure time as accurately as the best pendulum clocks. By 1938, he had developed the Essen ring. This referred to the shape of the piece of quartz used in his new clock, which was three times as accurate as the earlier versions.
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. Recent work has shown that Essen's measurement was much more accurate than earlier attempts had been.
In the early 1950s, Essen became interested in research being conducted in the USA, espesially at the National Bureau of Standards (NBS). This was concerned with the possibility of producing a highly accurate clock based on the radiation emitted or absorbed by atoms. Essen realised that the ammonia molecule, which the NBS had used in a clock in 1948, was not ideally suited for this purpose and based his clocks instead on atoms such as hydrogen, rubidium and caesium.
In 1953, Louis Essen and Jack Parry were given approval to produce an atomic clock at the 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 I running in 1955. Political difficulties in the USA had almost stopped research into atomic clocks at the NBS, so Caesium I became the world's first source of atomic time. By 1964, Essen had reduced the timekeeping errors from one second in 300 years to about one second in 2000 years. In 1967, atomic clocks had become so accurate that the second was redefined as the time taken for 9192631770 cycles of the radiation corresponding to the hyperfine transition of the ground state of caesium-133. 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.
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