page_title: National Physical Laboratory (United Kingdom)
section_title: Atomic clocks
hierarchical_section_title: National Physical Laboratory (United Kingdom) / Research / Atomic clocks
caption_attribution_description: English: Louis Essen and J. V. L. Parry standing next to the world's first caesium atomic clock, developed at the UK National Physical Laboratory in 1955. Deutsch: Louis Essen und J. V. L. Parry zeigen die erste Caesium-Atomuhr (1955)
context_page_description: The National Physical Laboratory is the national measurement standards laboratory for the United Kingdom. Founded in 1900, it is one of the oldest standardising laboratories in the world.
  Today, it is one of the most extensive government laboratories in the UK and has a "prestigious" reputation for its role in setting and maintaining physical standards for British industry. The heads of the NPL have included many individuals who were pillars of the British scientific establishment. Research work at NPL has contributed to the advancement of many disciplines of science, including the development of atomic clocks as well as packet switching, which is today one of the fundamental technologies of the Internet.
  NPL is based at Bushy Park in Teddington, England. It comes under the management of the Department for Business, Energy and Industrial Strategy.
context_section_description: The first accurate atomic clock, a caesium standard based on a certain transition of the caesium-133 atom, was built by Louis Essen and Jack Parry in 1955 at NPL. Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time (ET). This led to the internationally agreed definition of the latest SI second being based on atomic time.
caption: Louis Essen
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