Louis Essen

Born 6 September 1908, British physicist Louis Essen built the first practical atomic clock. Essen attended University College Nottingham and earned his physics degree from the University of London in 1928. The following year he joined England’s National Physical Laboratory in Teddington, where he began studying quartz oscillators. In 1938 he built a quartz ring clock, which measured time using the electrically induced oscillations of a quartz crystal. Thanks to Essen’s clock, it was determined that Earth’s speed of rotation varied over short timescales. During World War II, Essen worked on radar and other instruments, including the cavity resonance wavemeter, which he used in 1946 to achieve the most accurate measurement of the speed of light at that time: 299 792 km/s. He improved slightly on that finding in 1950, and that value wasn’t surpassed until 1975 when a more precise laser-based system was developed. In 1955, working with his assistant Jack Parry, Essen built an atomic clock that used the frequency of the atomic spectral lines of cesium to keep time more accurately. As a result, the standard unit of time—the second—was redefined in 1967 in terms of atomic frequencies rather than Earth’s motion. In 1960 Essen became deputy chief scientific officer at the National Physical Laboratory and was elected a fellow of the Royal Society. That year he wrote in Physics Today about his efforts to accurately measure time . He died in 1997 at age 88. (Photo credit: British Information Services, courtesy AIP Emilio Segrè Visual Archives, Physics Today Collection)

Date in History: 6 September 1908