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| [[Image:Telechron clock 2H07-Br Administrator.JPG|thumb|Telechron synchronous electric clock manufactured around 1940. By 1940 the synchronous clock became the most common type of clock in the U.S.]]
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| 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 [[quartz clock]]s replaced them in the 1980s. The first experimental electric clocks were constructed around 1840, but they were not widely manufactured until mains [[electric power]] became available in the 1890s. In the 1930s the synchronous electric clock replaced mechanical clocks as the most widely used type of clock.
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| ==Types==
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| [[File:Reform clock movement.JPG|thumb|right|250px|Electromechanical self-winding clock movement from [[Switzerland]].]]
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| Electric clocks can operate by several different types of mechanism:
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| *''Electromechanical clocks'' These have a traditional mechanical [[movement (clockwork)|movement]], which keeps time with an oscillating [[pendulum]] or [[balance wheel]] powered through a [[gear train]] by a [[mainspring]], but use electricity to rewind the mainspring with an [[electric motor]] or [[electromagnet]]. This mechanism is found mostly in antique clocks.
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| *''Electric remontoire clocks'' In these, the gear train was turned by a small spring or weighted lever, called a [[remontoire]], which was wound up more frequently by an electric motor or electromagnet. This mechanism was more accurate than a mainspring, because the frequent winding averaged out variations in the clock's rate caused by the varying force of the spring as it unwound. It was used in precision pendulum clocks, and in automotive clocks until the 1970s.
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| *''Electromagnetic clocks'' keep time with a pendulum or balance wheel, but the pulses to keep it going are not provided by a mechanical movement and [[escapement]] linkage, but by [[magnetic force]] from an electromagnet ([[solenoid]]). This was the mechanism used in the first electric clocks, and is found in antique electric pendulum clocks. It is also found in a few modern decorative mantel and desk clocks.
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| *''Synchronous clocks'' rely on the 50 or 60 Hz [[utility frequency]] of the [[Alternating current|AC]] [[electric power grid]] as a timing source, by driving the clock gears with a [[synchronous motor]]. They essentially count cycles of the power supply. While the actual frequency may vary with loading on the grid, the total number of cycles per 24 hours is maintained rigorously constant, so that these clocks can keep time accurately for long periods, barring [[power cut]]s; over months they are more accurate than a typical quartz clock. This was the most common type of clock from the 1930s but has now been mostly replaced by quartz clocks.
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| *''[[Quartz clock]]s'' are electric clocks which keep time by counting oscillations of a vibrating [[quartz crystal]]. They use modern low-voltage DC-powered circuitry, which may be supplied by a battery or derived from mains electricity. They are the most common type of clock today. Quartz clocks and watches as supplied by the manufacturer typically keep time with an error of a few seconds per week, although sometimes more.<ref name=error>[http://sound.westhost.com/clocks/sync.html Elliott Sound Products: build a synchronous clock]</ref> Inexpensive quartz movements are often specified to keep time within 30 seconds per month (1 second per day, 6 minutes per year).<ref>[http://www.brimarc.com/products/Quartz-Clock-Movements-23072.htm Brimarc, typical quartz clock movement specified accurate to within ±30 seconds/month]</ref> Lower error can be achieved by individual calibration if adjustment is possible, subject to the stability of the oscillator, particularly with change in temperature. Higher accuracy is possible at higher cost. {{See also|Quartz clock}}
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| *''[[Radio-controlled clock]]s'' are quartz clocks which are periodically synchronized with the [[Coordinated Universal Time|UTC]] [[atomic clock]] time scale via [[radio time signal]]s broadcast by [[Radio clock#List of radio time signal stations|dedicated stations]] around the world. They are distinct from [[clock radio]]s.
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| ==History==
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| [[Image:Bain-clock.jpg|thumb|upright=0.60|One of Alexander Bain's early electromagnetic clocks, from 1840s]]
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| In 1814, [[Sir Francis Ronalds]] (1788) of London invented the forerunner of an electric clock, the [[electrostatic]] clock. His prototype was powered with a [[dry pile]] battery. It proved unreliable in timekeeping, however, because of a strong dependence on a stable room temperature and 'weather conditions'.
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| In 1815, [[Giuseppe Zamboni]] (1776-1846) of Verona invented and showed another electrostatic clock run with [[dry pile]] battery and an oscillating orb. Over the test of time Zamboni's clock was praised "the most elegant and at the same time the most simple movement yet produced by the electric column".<ref>Perpetual Electromotive</ref> Zamboni's clock had a vertical needle supported by a pivot and was so energy efficient that it could operate on one battery for over 50 years.
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| In 1840, [[Alexander Bain (inventor)|Alexander Bain]] (1811-1877), a Scottish clock and instrument maker was the first to invent and patent the electric clock. His original electric clock patent is dated October 10, 1840. On January 11, 1841, Alexander Bain along with John Barwise, a chronometer maker, took out another important patent describing a clock in which an [[Electromagnetism|electromagnetic]] [[pendulum]] and an [[electric current]] is employed to keep the clock going instead of springs or weights. Later patents expanded on his original ideas.
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| Numerous people were intent on inventing the electric clock with electromechanical and electromagnetic designs around the year 1840, such as Wheatstone, Steinheil, Hipp, [[Louis-François-Clement Breguet|Breguet]], and Garnier, both in Europe and America.
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| [[Matthias Hipp]] (1813-1893), clockmaker born in [[Germany]], is credited with establishing the production series, mass marketable electric clock. Hipp opened a workshop in Reutlingen, [[Switzerland]], where he developed an electric clock to have the Hipp-Toggle, presented in Berlin at an exhibition in 1843. The Hipp-Toggle is a device attached to a pendulum or balance wheel that electro-mechanically allows occasional impulse or drive to the pendulum or wheel as its amplitude of swing drops below a certain level, and is so efficient that it was subsequently used in electric clocks for over a hundred years. Hipp also invented a small motor and built the chronoscope and the registering [[chronograph]] for time measurement.
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| The first electric clocks had prominent pendulums because this was a familiar shape and design. Smaller clocks and watches with a spiral-balance are made on the same principles as pendulum clocks.
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| [[Henry E. Warren|Henry Ellis Warren]] (1872-1957) invented the first synchronous electric clock which kept time from the oscillations of the power grid, in 1918.<ref name="WarrenPatent" >[http://www.google.com/patents/US1283434 U.S. patent #1283434] Warren, Henry E. ''Timing device'', filed February 26, 1917, issued October 29, 1918, on Google Patents</ref><ref name="AHS">{{cite web
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| | last =
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| | title = Famous Names in Electrical Horology
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| | work = Electrical Horology Group
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| | publisher = Antiquarian Horological Society, London, UK
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| | year = 2011
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| | url = http://www.ahsoc.demon.co.uk/ehg/electricalindex.html
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| | doi =
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| | accessdate = 2011-12-16}}</ref> The first commercial synchronous electric clock sold in the UK, the Synclock, was brought out in 1931.<ref name="AHS"/>
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| ==Electromechanical Clock==
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| [[File:Standard Electric Time Co. Electromechanical Master Clock 01.jpg|thumb|left|alt=photograph of Master Clock|Master clock from synchronized school clock system. c.1928 Electromechanical movement winds each minute and impulses slave clocks each minute. Operates on 24 Volts DC]]
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| A clock that employs electricity in some form to power a conventional clock mechanism is an electromechanical clock. Any spring or weight driven clock that uses electricity (either AC or DC) to rewind the spring or raise the weight of a mechanical clock then is an electromechanical clock. In electromechanical clocks the electricity serves no time keeping function. The timekeeping function is regulated by the pendulum. Near the end of the nineteenth century, the availability of the dry cell battery made it practical to use electric power in clocks. The use of electricity then led to many variations of clock and motor designs. Electromechanical clocks were made as individual timepieces but most commonly were used as integral parts of synchronized time installations. Experience in telegraphy led to connecting remote clocks (slave clocks) via wires to a controlling (master clock) clock. The goal was to create a clock system where each clock displayed exactly the same time. The master and the slaves are electromechanical clocks. The [[master clock]] has a conventional self-winding clock mechanism that is rewound electrically. The [[slave clock]] mechanism is not a conventional clock mechanism as it consists only of a ratchet wheel and time train. Slave clocks rely upon electrical impulses from the master clock to mechanically move the clock hands one unit of time. Synchronized time systems are made up of one master clock and any number of slave clocks. The slave clocks are connected by wires to the master clock. These systems are found in locations where multiple clocks would be used such as learning institutions, businesses, factories, transportation networks, banks, offices and government facilities. These self-winding clock systems were usually low voltage DC. They were installed thru the 1950s and by then systems with synchronous motor clocks were becoming the clock system of choice.
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| ==Electromagnetic clock==
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| [[Image:Pendule electrique l maitrier 05117.jpg|thumb|upright=0.6|Early French electromagnetic clock]]
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| The configuration of this device is comparatively very simple and reliable. The electrical current powers either a [[pendulum]] or an [[electromechanical]] [[oscillator]].
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| The electromechanical oscillator component has an attached [[magnet]] that passes two [[inductors]]. When the magnet passes the first inductor or sensor, the simple [[amplifier]] causes the current through the second inductor, and the second inductor works as an [[electromagnet]], providing an energy pulse to the moving oscillator. This oscillator is responsible for the accuracy of the clock. The electronic part would not generate electrical pulses if the oscillator was absent or did not move. The [[resonance|resonant]] [[frequency]] of the mechanical oscillator should be several times per second.
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| =={{Anchor|Synchronous clock}} Synchronous electric clock==
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| [[Image:Clock radio.jpg|thumb|Clock radio with synchronous clock, from 1950s]]
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| A synchronous electric clock does not contain a timekeeping oscillator like a pendulum, but instead counts the oscillations of the AC [[Mains power|utility current]] from its wall plug to keep time. It consists of a small AC [[synchronous motor]], which turns the clock's hands through a reduction [[gear train]].<ref name="Wise">{{cite book
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| | last = Wise
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| | first = S. J.
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| | authorlink =
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| | coauthors =
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| | title = Electric Clocks, 2nd Ed.
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| | publisher = Heywood & Co.
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| | year = 1951
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| | location = London
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| | pages = 95–100
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| | url = http://www.electricclockarchive.org/media/Books,%20documents/Electric%20Clocks%20(Wise).pdf
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| | doi =
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| | id =
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| | isbn = }}{{dead link|date=October 2013}}</ref> The motor contains [[electromagnet]]s which create a rotating [[magnetic field]] which turns an iron [[rotor (electric)|rotor]]. The rotation rate of the motor shaft is synchronized to the [[utility frequency]]; 60 cycles per second (Hz) in North and South America, 50 cycles per second in most other countries. The gear train scales this rotation so the minute hand rotates once per hour. Thus the synchronous clock can be regarded as not so much a timekeeper as a mechanical counter, whose hands display a running count of the number of cycles of alternating current.<ref name="Wise" />
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| One of the gears turning the clock's hands has a shaft with a sliding friction fitting, so the clock's hands can be turned manually by a knob on the back, to set the clock.
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| Synchronous motor clocks are rugged because they do not have a delicate pendulum or balance wheel. However, a temporary [[power outage]] will stop the clock, which will show the wrong time when power is restored. Some synchronous clocks have an indicator which shows if it has stopped and restarted.
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| ===Number of poles===
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| Some electric clocks have a simple two-pole synchronous motor which runs at one revolution per cycle of power, i.e., 3600 [[Revolutions per minute|RPM]] at 60 Hz and 3000 RPM at 50 Hz.<ref>[http://www.electricclockarchive.org/media/Books,%20documents/Electric%20Clocks%20(Wise).pdf Wise (1950) ''Electric Clocks'']{{dead link|date=October 2013}}, p.101-104</ref> However most electric clocks have rotors with more magnetic poles (teeth), consequently rotating at a smaller multiple of line frequency. This allows the gear train which turns the hands to be built with fewer gears, saving money.<ref>The speed of a synchronous motor '''''v''''' in revolutions per minute (RPM) is related to the number of poles by:
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| :<math>v = \frac {120f}{p}\,</math>
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| where '''''f''''' is the line frequency (50/60 Hz) and '''''p''''' is the number of poles on the rotor. Many designs have 30 poles, so that the motor runs at 240 RPM (at 60 Hz) or 200 RPM (at 50 Hz).</ref>
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| ===Accuracy===
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| {{unreferenced section|date=July 2013}}
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| [[Electric utilities]] keep the long-term average [[Utility frequency|frequency]] of their current very constant, calibrated by [[UTC]] atomic clock time, so synchronous clocks keep accurate time in the long term and do not accumulate error, although short-term fluctuations in the frequency caused by utility load variations may cause errors of a few seconds during the course of a day. For example, European utilities control the frequency of their grid once a day to make the total number of cycles in 24 hours correct. U.S. utilities correct their frequency once the cumulative error has reached 3-10 sec.
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| ===Spin-start clocks===
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| The earliest synchronous clocks from the 1930s were not self-starting, and had to be started by spinning a starter knob on the back.<ref name="Wise" /> An interesting flaw in these ''spin-start'' clocks was that the motor could be started in either direction, so if the starter knob was spun in the wrong direction the clock would run backwards, the hands turning counterclockwise. Later manual-start clocks had ratchets or other linkages which prevented backwards starting. The invention of the [[shaded-pole motor]] allowed self-starting clocks to be made, but since the clock would restart after a power interruption, the loss of time would not be indicated.
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| ==Notes==
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| {{Reflist}}
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| ==References==
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| *Viradez, Michel. History of Electric Clocks
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| *Katz, Eugenii. Alexander Bain Biography
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| *Perpetual Electromotive of Giuseppe Zamboni
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| *Chirkin, K. Electromechanical clocks. ''Radio,'' '''7''' (1968): p. 43.
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| ==See also==
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| * [[Master clock]]
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| * [[Shortt-Synchronome clock]]
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| [[Category:Clocks]]
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| [[Category:Horology]]
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| [[Category:Scottish inventions]]
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