Louis Essen was born in 1908 in a small city in England called Nottingham. His childhood was typical of the time and he pursued his education with enjoyment and dedication. At the age of 20 Louis graduated from the University of Nottingham, where he had been studying. It was at this time that his career started to take off, as he was invited to join the NPL, or National Physics Laboratory. It was during Louis’s time at the NPL that he began working to develop a quartz crystal oscillator as he believed they were capable of measuring time as accurately as a pendulum based clock. Ten years after joining the NPL Louis had invented the Essen ring. This was an eponymous invention which took its name from the shape of the quartz which Louis had used in his latest clock and which was three times more accurate than the previous versions. Louis soon moved on to newer areas of research and began to study ways to measure the speed of light. During World War II he began to work on high frequency radar and used his technical ability to develop the cavity resonance wavemeter. From 1946 it was this wavemeter which he used, along with a colleague by the name of Albert Gordon-Smith, to make his lightspeed measurements. It has been acknowledged recently that Louis’s measurements were by far the most accurate to have been recorded up until that time. During the early part of the 1950’s Louis began to take an interest in research which was being carried out at the National Bureau of Standards (NBS) in the United States of America. He learnt that work was being carried out to invent a clock which was more accurate than any other. The American scientists were using the idea of maintaining a clock’s accuracy by using the radiation emitted or absorbed by atoms. At that time the Americans were using a molecule of ammonia but Louis felt that this was not working as well as if they were using different atoms, such as hydrogen or caesium, and so he began working on his own clock using these materials instead. 1953 saw Louis and a colleague, Jack Parry, receiving permission to develop an atomic clock at the NPL based on Louis’s existing knowledge of quartz crystal oscillators and other relevant techniques he had learned from the cavity resonance wavemeter he had previously designed. Only two years later Louis's first atomic clock was running, Caesium I, designed by the UK scientists. Development in the United States had all but stopped due to political difficulties. Louis continued to work on his atomic clock and by 1964 he had managed to increase the accuracy of the atomic clock from one second in 300 years to one second every 2000 years! The continued success of Louis’s work resulted in the definition of a second being changed from 1/864000 of a mean solar day to being calculated as the time it took for 9192631770 cycles of the radiation in an atomic clock. Louis Essen died in 1997 and before his death had been honoured with, amongst others, an OBE and the Tompion Gold Medal of the Clockmakers’ Company.