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- GPO Master clock type 36 Mark 3, by Gent and Co., UK. This clock was made with a synchronising mechanism, responsive to 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. (en)
- Master clock from synchronised school clock system, c. 1928. Electromechanical movement winds each minute, and impulses slave clocks each minute. Operates on 24-volt DC. (en)
- Diagram of electric time system used around 1910 to keep time in factories, schools, and other large institutions. The master clock , 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. (en)
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