A master clock is a precision clock that provides timing signals to synchronize slave clocks as part of a clock network. The master clock in such installations is controlled by an accurate quartz crystal oscillator, usually referenced to an external frequency standard such as MSF, which is part of a world-wide timekeeping system.
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 installation at Rugby, 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.
A modern, atomic version of a Master Clock is the large clock ensemble found at the U.S. Naval Observatory
Master clock 36A, by Gent and Co., UK. These clocks were originally made with a synchronising mechanism, consisting of a heart shaped cam returnable to its zero position by the operation of a roller cam being pressed onto the cam by a solenoid, driven by an external signal sent by land line from the GPO time station at Rugby
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.
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