Slave clock

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In telecommunication and horology, a slave clock is a clock that depends for its accuracy on another clock, a master clock. Many modern clocks are synchronized, either through the Internet or by radio time signals, to a worldwide time standard called Coordinated Universal Time (UTC) based on a network of master atomic clocks in many countries. For scientific purposes, precision clocks can be synchronized to within a few nanoseconds by dedicated satellite channels. Slave clock synchronization is usually achieved by phase-locking the slave clock signal to a signal received from the master clock. To adjust for the transit time of the signal from the master clock to the slave clock, the phase of the slave clock may be adjusted with respect to the signal from the master clock so that both clocks are in phase. Thus, the time markers of both clocks, at the output of the clocks, occur simultaneously.[1]

Before the computer era, the term referred to satellite electrical clocks that are synchronized periodically by an electrical pulse issued by a master clock. From the late 19th to the mid 20th centuries, electrical master/slave clock systems were widely used in public buildings and business offices, with all the clocks in the building synchronized through electric wires to a central master clock.

These older styles of slave clocks either keep time by themselves, and are periodically corrected by the master clock, or require impulses from the master clock to advance. Many slave clocks of these types remain in operation, most commonly in schools.

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.


Mechanical slave clocks from the 1950s and 1960s era.


  1. ^  This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".

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