telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...

and

horology Horology (; related to Latin '; ; , interfix ''-o-'', and suffix ''-logy''), . is the study of the measurement of time. Clocks, watches, clockwork, sundials, hourglasses, clepsydras, timers, time recorders, marine chronometers, and atomic cl ...

, a slave clock is a

clock A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month and the ...

that depends on another clock, the

master clock Master or masters may refer to: Ranks or titles * Ascended master, a term used in the Theosophical religious tradition to refer to spiritually enlightened beings who in past incarnations were ordinary humans *Grandmaster (chess), National Master ...

. Modern clocks are synchronized through the

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...

or by radio time signals, to

Coordinated Universal Time Coordinated Universal Time or UTC is the primary time standard by which the world regulates clocks and time. It is within about one second of mean solar time (such as UT1) at 0° longitude (at the IERS Reference Meridian as the currently used ...

. UTC is based on a network of

atomic clocks An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...

in many countries. For scientific purposes, precision clocks can be synchronized to within

nanoseconds A nanosecond (ns) is a unit of time in the International System of Units (SI) equal to one billionth of a second, that is, of a second, or 10 seconds. The term combines the SI prefix ''nano-'' indicating a 1 billionth submultiple of an SI unit ( ...

by dedicated satellite channels. Slave clock synchronization is usually achieved by phase-locking the slave

clock signal In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits. A clock sign ...

to a signal received from the master clock. To adjust for the transit

time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, to ...

of the signal from the master clock to the slave clock, the phase of the slave clocks are adjusted so that both clocks are in phase. Thus, the time markers of both clocks, at the output of the clocks, occur simultaneously. The predecessors of

computer clock In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits. A clock signa ...

digital clock A digital clock is a type of clock that displays the time digitally (i.e. in numerals or other symbols), as opposed to an analogue clock. Digital clocks are often associated with electronic drives, but the "digital" description refers only to ...

s, these

electric clocks An electric clock is a clock that is powered by electricity, as opposed to a mechanical clock which is powered by a hanging weight or a mainspring. The term is often applied to the electrically powered mechanical clocks that were used before qua ...

were synchronized by an electrical pulse, wired to their master clock in the same facility. Thus the terms "master" and "slave." From the late 19th to the mid 20th centuries, electrical master/slave clock systems were installed, all clocks in a building or facility synchronized through electric wires to a central master clock. Slave clocks either kept time by themselves, and were periodically corrected by the master clock, or required impulses from the master clock. Many slave clocks of these types were in operation, most commonly in schools, offices, military bases, hospitals, railway networks, telephone exchanges and factories the world over.

School bell The ringing of a school bell announces important times to a school's students and staff, such as marking the beginnings and ends of the school day, class period, and breaks. In some schools it may take the form of a physical bell, usually e ...

s of

elementary schools A primary school (in Ireland, the United Kingdom, Australia, Trinidad and Tobago, Jamaica, and South Africa), junior school (in Australia), elementary school or grade school (in North America and the Philippines) is a school for primary ed ...

, high schools, and others were able to be sychronized across an entire campus, connected to the system. In schools, the master clock was in the principal's office, with slave units in classrooms which were in other buildings on campus. In factories, a system with a bell or horn could signal the end of a shift, lunchtime or break time. Very few relics of this electrical, analogue system operate in the

21st century The 21st (twenty-first) century is the current century in the ''Anno Domini'' era or Common Era, under the Gregorian calendar. It began on 1 January 2001 ( MMI) and will end on 31 December 2100 ( MMC). Marking the beginning of the 21st centur ...

. Most 21st century systems of the type are digital.

Pictures

Mechanical slave clocks from the 1950s and 1960s era. Image:Slave Clock.JPG, Slave clock Image:Slave Clock2.JPG, Synchronome slave clock Image:Clock number 6.jpg, Early G.P.O. type 6, manufactured by Synchronome Image:Slave Clock Inside.jpg, Slave clock drive mechanism Image:Slave Clock being driven by slave driver.JPG, A slave clock driven by a quartz clock module

References

External links

All about electric master and slave clocks
{{DEFAULTSORT:Slave Clock Telecommunications equipment Clocks Digital electronics