Atomic Clock

The first atomic clock was an ammonia absorption line device at 23870.1 MHz built in 1949 at the U.S.

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 the National Physical Laboratory in the UK.

Louis Essen (right) and Jack Parry (left) standing next to the world’s first caesium-133 atomic clock.

A brief history of timekeeping

Innovation in Laboratory Time and Frequency, Neuchâtel

Norman Ramsey, in the origin of atomic clocks (French)

What are atomic clocks used for?

What is an atomic clock? with Pierre Thomann, RTS archive

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