During World War II, Essen worked on radar and developed a number of instruments, including the cavity resonance wavemeter. It was this work that suggested to Essen the possibility of a more precise measurement of the speed of light. In 1946, in collaboration with A.C. Gordon-Smith, he used a microwave cavity, of precisely known dimensions, and exploited his expertise in time-measurement to establish the frequency for a variety of its normal modes. As the wavelength of the modes was known from the geometry of the cavity and from electromagnetic theory, knowledge of the associated frequencies enabled a calculation of the speed of light. 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. Even NPL director Sir Charles Galton Darwin, while supporting the work, observed that Essen would get the correct result once he had perfected the technique. Moreover, W.W. Hansen at Stanford University had used a similar technique and obtained a measurement which was more consistent with the conventional wisdom. However, a combination of Essen's stubbornness, his iconoclasm and his belief in his own skill at measurement inspired him to refine his apparatus and to repeat his measurement in 1950, establishing a result of 299,792.5±1 km/s. This was the value adopted by the 12th General Assembly of the Radio-Scientific Union in 1957. Most subsequent measurements have been consistent with this value. In 1983, the 17th Conférence Générale des Poids et Mesures adopted the standard value, 299,792.458 km/s for the speed of light.
Atomic clocks
Essen earned his Ph.D. and Doctor of Science from the University of London before becoming interested in the possibility of using the frequency of atomic spectra to improve time measurement. The feasibility of measuring time using caesium as an atomic reference had been demonstrated by the US National Bureau of Standards. In 1955, he developed, in collaboration with Jack Parry, the first practical atomic clock by integrating the caesium atomic standard with conventional quartz crystal oscillators to allow calibration of existing time-keeping.
Time standards
This work led Essen to champion the caesium spectrum as an international time standard. The ammoniamolecule had already been proposed as such but Essen was convinced that caesium would prove more stable. However, the International Astronomical Union meeting in Rome in 1952 had adopted the ephemeris time scale, on a proposal by Gerald Clemence defining the time unit in terms of the Earth's motion round the sun. 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.
Later life
Essen spent all his working life at the National Physical Laboratory. In 1971 he published The Special Theory of Relativity: A Critical Analysis, questioning Special relativity, which apparently was not appreciated by his employers. Essen said in 1978: He retired in 1972 and died in Great Bookham, Surrey in 1997.
