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Born in Nottingham, Essen earned his degree in physics from the University of London in 1928, having studied at University College Nottingham. He started work at the National Physical Laboratory (NPL) the following year, under D.W. Dye , investigating the potential of tuning forkA tuning fork is a simple metal two-pronged fork with the tines formed from a U-shaped bar of elastic material (usually steel). When set vibrating with a blow from a mallet, and after waiting a moment to allow some high overtones to die out, a tuning forks and quartzFor other uses of this word, see Quartz (disambiguation). Milk quartz rock Quartz is the most abundant mineral on Earth (about 12% vol. It has a hexagonal crystal structure made of trigonal-crystallized silica (silicon dioxide, SiO), with a hardness of 7 crystal oscillatorA crystal oscillator is an electronic device that uses the mechanical resonance of a physical crystal of piezoelectric material to create an electrical signal with a very precise frequency. It is an especially accurate form of an electronic oscillator.s for precise time measurement. His research led to his development of the quartz ring clock in 1938Events January -June January 3 The March of Dimes is established by Franklin Delano Roosevelt. January 11 Frances Moulton is the first woman to become president of a US national bank. January 20 Wedding of king Farouk I of Egypt and Farida Zulficar in Cai, the clock soon becoming a standard for time measurement at observatoriesObservatory of Strasbourg An observatory is a location utilised for the purpose of observing terrestrial and/or celestial events. Astronomy, climatology, geology, meteorology and volcanology are examples of disciplines for which observatories have been co throughout the world.
During World War IIWorld War II was the most extensive and costly armed conflict in the history of the world, involving the great majority of the world's nations, being fought simultaneously in several major theatres, and costing tens of millions of lives. The war was fough, Essen worked on radarThis article is about the device. For the fictional character in M A S H see Corporal Walter (Radar) O'Reilly. antenna (approximately 40m (130ft) in diameter) rotates on a track to observe activities near the horizon. Radar is an acronym for ra dio d etec 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 1946Events January January 4 Theodore Schurch becomes the last person to be executed for offences committed under the Treachery Act of 1940 January 7 Allied recognize Austrian republic with 1937 borders the country is divided into four occupation zones Januar, 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 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 (optical) wisdom. However, a combination of Essen's stubornness, his iconocalsm and his belief in his own skill at measurement (and a little help with calculations from Alan Turing) 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.