Master clock

Master clock (at left) driving several slave clocks in an enthusiast's garage. The third one from the left at the top is a radio-controlled clock for reference.

The master atomic clock ensemble at the U.S. Naval Observatory in Washington, D.C., which provides the time standard for the U.S. Department of Defense.^[1] The rack-mounted units in the background are HP 5071A caesium beam clocks. The black units in the foreground are Sigma-Tau MHM-2010 hydrogen maser standards.

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.^[1]