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1 Essen, Louis
SUBJECT AREA: Horology[br]b. 6 September 1908 Nottingham, England[br]English physicist who produced the first practical caesium atomic clock, which was later used to define the second.[br]Louis Essen joined the National Physical Laboratory (NPL) at Teddington in 1927 after graduating from London University. He spent his whole working life at the NPL and retired in 1972; his research there was recognized by the award of a DSc in 1948. At NPL he joined a team working on the development of frequency standards using quartz crystals and he designed a very successful quartz oscillator, which became known as the "Essen ring". He was also involved with radio frequency oscillators. His expertise in these fields was to play a crucial role in the development of the caesium clock. The idea of an atomic clock had been proposed by I.I.Rabbi in 1945, and an instrument was constructed shortly afterwards at the National Bureau of Standards in the USA. However, this device never realized the full potential of the concept, and after seeing it on a visit to the USA Essen was convinced that a more successful instrument could be built at Teddington. Assisted by J.V.L.Parry, he commenced work in the spring of 1953 and by June 1955 the clock was working reliably, with an accuracy that was equivalent to one second in three hundred years. This was significantly more accurate than the astronomical observations that were used at that time to determine the second: in 1967 the second was redefined in terms of the value for the frequency of vibration of caesium atoms that had been obtained with this clock.[br]Principal Honours and DistinctionsFRS 1960. Clockmakers' Company Tompion Gold Medal 1957. Physical Society C.V.Boys Prize 1957. USSR Academy of Science Popov Gold Medal 1959.Bibliography1957, with J.V.L.Parry, "The caesium resonator as a standard of frequency and time", Philosophical Transactions of the Royal Society (Series A) 25:45–69 (the first comprehensive description of the caesium clock).Further ReadingP.Forman, 1985, "Atomichron: the atomic clock from concept to commercial product", Proceedings of the IEEE 75:1,181–204 (an authoritative critical review of the development of the atomic clock).N.Cessons (ed.), 1992, The Making of the Modern World, London: Science Museum, pp.190–1 (contains a short account).See also: Marrison, Warren AlvinDV
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