Louis Essen

Louis Essen OBE FRS[1](6 September 1908 – 24 August 1997) was an English physicist whose most notable achievements were in the precise measurement of time and the determination of the speed of light.

He started work at the National Physical Laboratory (NPL) the following year, under D. W. Dye, investigating the potential of tuning forks and quartz crystal oscillators for precise time measurement.

Their result, 299,792±3 km/s, was substantially greater than that from the prevailing sequence of optical measurements that had begun around the start of the 20th century and Essen had to withstand some fierce criticism and disbelief.

Essen earned his PhD (1941) and Doctor of Science (1948) from the University of London before becoming interested in the possibility of using the frequency of atomic spectra to improve time measurement.

The ephemeris second, based on a fraction of the tropical year derived from Simon Newcomb's expression for the mean solar motion, became a standard in 1960, but in 1967, at the 13th Conférence Générale des Poids et Mesures, the second was redefined in terms of a value for the ephemeris second that had been precisely measured by Essen in collaboration with William Markowitz of the United States Naval Observatory in terms of the frequency of a chosen line from the spectrum of caesium.