Master clock (at left) driving several slave clocks in an enthusiast's garage. Note that the third one from the left at the top is a radio-controlled radio 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.  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. Today many radio clocks are synchronised by radio signals or internet connections to a worldwide time system called Coordinated Universal Time (UTC) which is governed by master atomic clocks in many countries.
In the days before the availability of such highly accurate reference time many master clocks were an accurate electrically maintained pendulum clock. Thousands of such clocks were installed, in schools, offices, railway networks, telephone exchanges and factories all over the world; they resembled a longcase clock, but had a very robust mechanism and a less ornate case. The clock timing signals, generated by electrical contacts attached to the mechanism, were minute, half minute and sometimes one second electrical pulses, fed to the controlled equipment on pairs of wires. The devices driven could be wall clocks, tower clocks, factory sirens, school bells and occasionally clock chiming mechanisms. Some types, such as the Synchronome had optional extra mechanisms to compare the time of the clock with a standard received from the GPO time service which relayed the time signal from the Greenwich Observatory, which allowed small weights to be added or removed from the pendulum without interruption. Small weights could also be added or removed manually in the absence of this mechanism again without interruption.
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 three such manufacturers, all of whom made clocks to the same GPO specification and which used the Hipp Toggle impulse system; these were Gent and Co., of Leicester, Magneta Ltd of Leatherhead in Surrey, and Synchronome Ltd of Alperton, north-west London.
Diagram of electric time system used around 1910 to keep time in factories, schools, and other large institutions. The master clock (bottom center), controlled by a temperature-compensated mercury pendulum, is wired to slave clocks throughout the building. In addition to wall clocks, it also controls time stamps that are used to stamp documents with the time, and a turret clock used in a clock tower. The "program clock" is a timer that can be programmed with punched paper tape to ring bells or turn machines on and off at preprogrammed times.
GPO Master clock type 36 Mark 3, by Gent and Co., UK. This clock was made with a synchronising mechanism, consisting of a heart shaped cam returnable to its zero position by the operation of a roller bearing being pressed onto the cam by a solenoid, driven by an external signal relayed by land line from the GPO Chronopher  at St.Martin's le grand, which in turn, received it from the Greenwich Observatory.
Master clock from synchronised school clock system, c. 1928. Electromechanical movement winds each minute, and impulses slave clocks each minute. Operates on 24 volts DC.