Obituary: Louis Essen

We live in a time-ordered world as witness the many thousands of clocks based on either caesium, rubidium or hydrogen, which combine to meet the nanosecond needs of telecommunications, navigation and defence systems. Apart from the infrequent introduction of leap seconds, time and time generation is now a matter for physicists rather than astronomers, and one of the most renowned of these was Louis Essen.

It is possible to identify quite positively the start of atomic timekeeping as the day in June 1955 when Essen and his collaborator J.V.L. (Jack) Parry achieved successful operation at the National Physical Laboratory (NPL), Teddington of the caesium beam resonator.

Theirs was not the first such equipment to operate; an earlier development at the US National Bureau of Standards in Washington, DC had demonstrated the potential of the caesium beam to provide a frequency reference. What distinguished Essen's achievement and transformed it into a time standard was the alliance of the caesium resonator with a number of existing clocks based on quartz crystal oscillators ( so-called "quartz clocks" ) and the ability provided thereby to compare atomic time with the available astronomical time scales.

It might be said that almost the whole of Essen's previous experience had uniquely fitted him to exploit this great leap forward in precise timekeeping which was henceforth to be based not on the Earth's motion but on the period appropriate to a small energy difference in the atom of caesium-133. He had joined NPL in 1929 from Nottingham University College and was immediately involved in the research under D.W. Dye, FRS on quartz crystal oscillators.

Dye had already developed an annular ring oscillator which Essen later transformed by substituting circumferential for radial oscillation into a practicable standard of high stability. Before the advent of atomic sources, Essen-ring oscillators in their commercially available form were to provide many laboratories and institutions throughout the world with a nearly constant frequency which was sufficiently stable over lengthy periods to reveal the extent of the seasonal variations in the Earth's rate of rotation.

The years of the Second World War involved a diversion of effort to other areas including the design and calibration of cavity resonator wave-meters at high radio frequencies. This provided the stimulus for a radically new approach to the determination of the speed of light based on measurements of the radio-frequency resonances in a cylindrical cavity of precisely known dimensions. The first results obtained in 1946 gave a value for the speed of light in vacuum some 16 km/s higher than the internationally accepted figure, equivalent to 1 part in 20,000.

Using improved equipment this result was confirmed with greater accuracy in 1950. Even with the support of the then Director of the NPL, Sir Charles Darwin, it required no little courage and self-assurance on Essen's part to challenge the scientific orthodoxy of the time. In later years, measurements by other workers using a variety of techniques continued to converge on Essen's result and also extended it to greater accuracy until in due course, in 1983, the speed of light was considered sufficiently well known to incorporate it in a redefinition of the metre in terms of the time taken for light to travel the metric distance.

Essen had kept in close touch, through several visits, with the efforts in the United States, especially at the National Bureau of Standards (NBS) to realise an atomic frequency reference. He had rightly disregarded the hyperbole attached to earlier devices based on the ammonia molecule and concluded that the future lay in accessing the relevant energy levels in atoms having a suitable electronic structure, such as those of hydrogen, rubidium and caesium.

On returning from his latest visit in 1953, Essen and Parry were allocated rather slender resources to produce an NPL equivalent of the NBS equipment. Despite little prior experience of the techniques of atomic spectroscopy they succeeded in a remarkably short time and the world's first source of atomic time became operational in June 1955 with an accuracy equivalent to about one second in 300 years. The extent of the advance can be seen from comparison with the behaviour of the Earth which might typically vary in its timekeeping by this amount in the period of one year.

Essen now faced a similiar situation to that encountered with his measurements of the speed of light in securing acceptance of his results, in this case in the form of a unit of atomic time. The initial calibration of the caesium frequency by reference to time from the Royal Greenwich Observatory enabled an atomic second to be defined and also disseminated through the UK radio broadcasts. However, the International Astronomical Union coincidentally meeting in Dublin in August 1955 had moved the goalposts and redefined time (Ephemeris Time) in relation to the Earth's motion round the Sun. This produced a unit which was constant by definition but available to useful accuracy only after several years. Essen was invited to present his results at Dublin but he was unable to influence events, although convinced by now after several months of operation of the caesium standard that the way forward in regard to both accuracy and availability was atomic and not astronomical time.

The second of Ephemeris Time was formally adopted by the international community in 1960 but its life was short and it was replaced by the atomic second in 1967. The value adopted was that obtained by Essen in collaboration with William Markowitz of the US Naval Observatory. They had met in Dublin and agreed to combine their efforts over three years to relate the atomic and ephemeris seconds. This was a good example of the happy relations which Essen was able to establish with colleagues in many parts of the world and the wide acceptance and esteem accorded him within the international scientific community.

It was inevitable that he should be the recipient of many honours and awards, arising mainly from the caesium standard but recognising also his work with rubidium and hydrogen standards. Notable among them was the A.S. Popov Gold Medal of the USSR Academy of Sciences awarded in 1959 for outstanding work during 1956-58 in the field of scientific radio, the first such award to a foreign scientist. In the same year Essen was appointed OBE and in the following year elected to a Fellowship of the Royal Society, while his contributions to horology were recognised by societies of clock- and watch-makers on both sides of the Atlantic. In the academic field he was awarded higher degrees of London University in 1941 and 1948.

Louis Essen retired from NPL in 1972, but developments in the field of caesium standards which he set in train have continued unabated with accuracies increasing by nearly a factor of ten in each decade. He would have been pleased to know that the fourth generation caesium apparatus at the Laboratory promises to continue that progression and provide some 10,000-fold advance on the original standard of 1955.

J. McA. Steele

Louis Essen, physicist; born Nottingham 6 September 1908; joined the National Physical Laboratory 1929, Principal Scientific Officer 1956-60, Deputy Chief Scientific Officer 1960-72; OBE 1959; FRS 1960; married 1937 Joan Greenhalgh (two daughters); died Great Bookham, Surrey 24 August 1997.