Today, atomic clocks are the most accurate clocks in existence. They are considerably more accurate than quartz clocks as they can be accurate to within a few seconds over thousands of years.
Atomic clocks were first theorized by Lord Kelvin in 1879. In the 1930s the development of Magnetic resonance created practical method for doing this. A prototypeammonia maser device was built in 1949 at the U.S. National Bureau of Standards (NBS, now NIST). Although it was less accurate than existing quartz clocks, it served to demonstrate the concept.
The first accurate atomic clock, a caesium standard based on a certain transition of the caesium-133 atom, was built by Louis Essen in 1955 at the National Physical Laboratory in the UK. Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time (ET).
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