Louis Essen

Louis Essen (1908 to 1997) was a British physicist and measurement engineer at the National Physical Laboratory in Teddington, best known for building the first practical caesium atomic clock. Born in Nottingham, the son of a cobbler, he joined the laboratory in 1929 and worked on time and frequency standards, developing a quartz ring clock in the 1930s that observatories used for astronomical timekeeping. During the Second World War he made special cables and measuring instruments for microwave radar, including the cavity resonance wavemeter, and used cavity resonators to measure the speed of light with new accuracy, giving a value close to the one accepted today. With his colleague Jack Parry he designed and built a caesium beam clock that first ran on 24 May 1955, and his work with American astronomers fed into the later atomic definition of the second and the idea of the leap second. He also took part in transatlantic frequency comparisons that led toward the Omega radionavigation system.