A master clock is a precision clock that provides timing signals to synchronise slave clocks as part of a clock network. Networks of electric clocks connected by wires to a precision master pendulum clock began to be used in institutions like factories, offices, and schools around 1900. Modern radio clocks are synchronised by radio signals or Internet connections to a worldwide time system called Coordinated Universal Time (UTC), which is governed by primary reference atomic clocks in many countries.
A modern, atomic version of a master clock is the large clock ensemble found at the U.S. Naval Observatory.
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