Louis Essen


Louis Essen
Louis Essen
Nottingham, England
Death date
Associated organizations
National Physical Laboratory
Fields of study
I I Rabi Award, IEEE / US Army Electronics R&D Command (1987)


British physicist, Louis Essen (1908-1997), played a pivotal role in the transformation of two of the basic measurements in physics: 'Time' from the solar system and astronomy to the atom and physics, and 'Length' from a metal bar to the speed of light multiplied by time. Essen also built and operated the first atomic clock that contributed to a national time and frequency standard.

Early Life and Career

Louis Essen was born in Nottingham, England, on 6 September 1908. The son of a cobbler, he was educated at High Pavement School, Nottingham, and Nottingham University College, graduating with a London University physics degree in 1928. Essen started his postgraduate studies under Dr Henry Brose during Britain’s economic recession but the financial burden on his parents became too much and he gave up his studies to find a job. Later, Essen received his PhD in 1941 and DSc in 1948 for research undertaken during the Second World War.

Essen joined the National Physical Laboratory (NPL), Teddington, Middlesex, England in 1929. Initially, he worked on time and frequency standards under the guidance of Dr David Dye. Dye was investigating two forms of standard: a tuning fork and a quartz oscillator. The tuning fork was delegated to Essen while Dye concentrated on the quartz oscillator. The fork was cut to a frequency of 1 kHz and, as a timekeeper, it was about as good as the pendulum itself. After Dye’s death in 1932, Essen concentrated on quartz oscillators and over the next four years developed the quartz ring clock (which vibrated at 100 kHz), an instrument of such stability that it was used in astronomical observatories including the Royal Observatory, Greenwich, and the US Naval Observatory.

During the Second World War, Essen’s expertise was applied to fixing the problems of instability in high frequency communications systems and developing special cables for use at microwave frequencies, as required for short wave radar. This was an uncharted area of technology so Essen developed new types of measuring instruments including the cavity resonance wavemeter, which became widely used in industry and in international calibration. During the war, Essen realised that cavity resonance devices might be used to measure the speed of light with greater accuracy than any earlier method. His first measurements in 1946 showed the speed of light to be 299,792 kilometers a second, 16 km/s greater than the value accepted at that time for calibrating radar systems. In 1950, using a different design of resonator, he derived an improved value of 299,792.5 km/s - which is very close to the currently agreed value.

In the post war years, Essen became interested in the proposals of American Nobel Laureate, I I Rabi, that the microwave frequency of atomic spectral lines might be used as a time standard of extraordinary accuracy. In the early 1950s, Essen visited American laboratories including the US National Bureau of Standards (now NIST) where ammonia and cesium clocks were under construction but they fell far short of the required standard. Back at Teddington, his experience with oscillators and resonance cavities gave him an insight into the problems and likely solutions. Within two years, Essen and his colleague, Jack Parry, designed and built a novel cesium clock of much greater stability and accuracy. Essen’s cesium-beam atomic clock was operated for the first time on 24 May 1955 and, in the next few weeks, radio time signals controlled by the atomic clock were being transmitted within the UK and, soon afterwards, around the world.

Over the next three years, Essen collaborated with the head of time services at the US Naval Observatory to determine the atomic frequency of cesium in terms of the ephemeris second of time. He played an important role in defining the ‘second’ (the unit of time in the metric system) which was adopted by the Interntional Bureau of Weights and Measures in 1967. Essen, jointly with the US astronomer, Gerald Clemence, also proposed the concept of the ‘leap second’ which was adopted internationally in 1972. Throughout the 1950s, Essen also collaborated with John Alvin ‘Jack’ Pierce (Harvard Cruft Laboratory, Cambridge, Massachusetts) on a series of transatlantic frequency comparison experiments which eventually led, through a number of stages, to the development of the OMEGA global radionavigation system.

Later in his career, Essen voiced controversial doubts about aspects of Einstein’s special theory. In 1971, as an Oxford Science research paper, he published, ‘The Special Theory of Relativity: A Critical Analysis’ which revealed contradictory assumptions and other flaws in some of Einstein’s ‘thought experiments’.

Awards & Honours

Essen was awarded the Charles Vernon Boys Prize, Physical Society, for his work on the velocity of light (1956), the Tompion Gold Medal of the Worshipful Company of Clockmakers for his work on quartz clocks (1957), the Wolfe Award, DSIR (1959), the A S Popov Gold Medal, USSR Acad. of Sciences, for his work on atomic clocks (1959), the Silvanus Thompson Prize, IEE (1960) and the Calendar Medal, Inst. of Measurement & Control (1972). Essen was the first recipient of the Wolfe Award and the first foreign recipient of the Popov Gold Medal.

Essen's work was recognised in the NPL by his promotion in 1960 to Deputy Chief Scientific Officer by special merit, one of the very few at that time in the whole Civil Service.

In addition, he was awarded the OBE (1959) and was elected a Fellow of the Royal Society of London on 3 March 1960. The International Union of Radio Science (URSI) appointed Essen as the Chair of Commission I (Radio Stds. & Measurements, now Commission A) in 1963. He became a Visiting Professor, Keele University, in 1968.

Essen was a member of the Institution of Electrical Engineers and an Honorary Fellow of the British Horological Institute. He was also a member of the International Astronomical Union.

Further Reading

Ray Essen, Revolutions in Time: the world of Louis Essen, clockmaker and father of atomic time, Amazon, independent publisher (2020)

NPL & Ray Essen, The memoirs of Louis Essen, National Physical Laboratory, UK (2015)