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A master clock is a precision clock that provides timing signals to synchronise 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 are synchronised by radio signals or Internet connections to a worldwide time system called Coordinated Universal Time (UTC), which is governed by primary reference in many countries.
A modern, atomic version of a master clock is the large clock ensemble found at the U.S. Naval Observatory.
In early networks, the slave clocks had their own timekeeping mechanism and were just corrected by the signals from the master clock every hour, 6, 12, or 24 hours. In later networks, the slave clocks were simply counters that used a stepper motor to advance the hands with each pulse from the master clock, once per second or once per minute. Some types, such as the Synchronome, had optional extra mechanisms to compare the time of the clock with a national time service that distributed time signals from astronomical regulator clocks in a country's naval observatory by telegraph wire. An example is the GPO time service in Britain which distributed signals from the Greenwich Observatory.
The British Post Office (GPO) used such master clocks in their electromechanical telephone exchanges to generate the call timing pulses necessary to charge telephone subscribers for their calls, and to control sequences of events such as the forcible clearing of connections where the calling subscriber failed to hang up after the called subscriber had done so. The UK had four such manufacturers, all of whom made clocks to the same GPO specification and which used the Electric clock impulse system; these were Gent and Co., of Leicester, Magneta Ltd of Leatherhead in Surrey, Synchronome Ltd of Alperton, north-west London, and Gillett and Johnson.
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