Editing Clock (section)

====Synchronized or slave clocks====

[[Image:Greenwich clock 1-manipulated.jpg|thumb|The [[Shepherd Gate Clock]] at the [[Royal Observatory, Greenwich]] receives its timing signal from within the [[Royal Observatory, Greenwich]].]]

Some clocks rely for their accuracy on an external oscillator; that is, they are automatically synchronized to a more accurate clock:
* [[Slave clock]]s, used in large institutions and schools from the 1860s to the 1970s, kept time with a pendulum, but were wired to a [[master clock]] in the building, and periodically received a signal to synchronize them with the master, often on the hour.<ref>Milham, 1945, pp. 325–328</ref> Later versions without pendulums were triggered by a pulse from the master clock and certain sequences used to force rapid synchronization following a power failure.
[[File:Telechron clock 2H07-Br Administrator.JPG|thumb|Synchronous electric clock, around 1940. By 1940 the synchronous clock became the most common type of clock in the U.S.]]
* Synchronous electric clocks do not have an internal oscillator, but count cycles of the 50 or 60 [[Hertz|Hz]] oscillation of the AC power line, which is synchronized by the utility to a precision oscillator. The counting may be done electronically, usually in clocks with digital displays, or, in analog clocks, the AC may drive a [[synchronous motor]] which rotates an exact fraction of a revolution for every cycle of the line voltage, and drives the gear train. Although changes in the grid line frequency due to load variations may cause the clock to temporarily gain or lose several seconds during the course of a day, the total number of cycles per 24 hours is maintained extremely accurately by the utility company, so that the clock keeps time accurately over long periods.
* Computer [[real-time clock]]s keep time with a quartz crystal, but can be periodically (usually weekly) synchronized over the [[Internet]] to atomic clocks ([[Coordinated Universal Time|UTC]]), using the [[Network Time Protocol]] (NTP).
* [[Radio clock]]s keep time with a quartz crystal, but are periodically synchronized to [[radio time signal|time signals]] transmitted from dedicated [[Radio clock#List of radio time signal stations|standard time radio stations]] or [[satellite navigation]] signals, which are set by atomic clocks.