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A clock is an instrument to measure, keep, 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: the day, the lunar month, and the year. Devices operating on several physical processes have been used over the millennia. Some predecessors to the modern clock may be considered as "clocks" that are based on movement in nature: A sundial shows the time by displaying the position of a shadow on a flat surface. There is a range of duration timers, a well-known example being the hourglass. Water clocks, along with the sundials, are possibly the oldest time-measuring instruments. A major advance occurred with the invention of the verge escapement, which made possible the first mechanical clocks around 1300 in Europe, which kept time with oscillating timekeepers like balance wheels.[1][2][3][4] A silent instrument missing such a striking mechanism has traditionally been known as a timepiece.[5] In general usage today, a "clock" refers to any device for measuring and displaying the time. Watches and other timepieces that can be carried on one's person are often distinguished from clocks.[6] Spring-driven clocks appeared during the 15th century. During the 15th and 16th centuries, clockmaking flourished. The next development in accuracy occurred after 1656 with the invention of the pendulum clock. A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The electric clock was patented in 1840. The development of electronics in the 20th century led to clocks with no clockwork parts at all. The timekeeping element in every modern clock is a harmonic oscillator, a physical object (resonator) that vibrates or oscillates at a particular frequency.[2] This object can be a pendulum, a tuning fork, a quartz crystal, or the vibration of electrons in atoms as they emit microwaves. Clocks have different ways of displaying time, connected to its internal clockwork type: Analog clocks usually indicate time using angles. Digital clocks display a numeric representation of time. Two numeric display formats are commonly used on digital clocks: 24-hour notation and 12-hour notation. Most digital clocks use electronic mechanisms and LCD, LED, or VFD displays. For convenience, distance, telephony or blindness, auditory clocks present the time as sounds. There are also clocks for the blind that have displays that can be read by using the sense of touch. Some of these are similar to normal analog displays, but are constructed so the hands can be felt without damaging them. The evolution of the technology of clocks continues today. The study of timekeeping is known as horology.

Contents

1 History

1.1 Etymology

2 Time-measuring devices

2.1 Sundials 2.2 Devices that measure duration, elapsed time and intervals 2.3 Water 2.4 Early mechanical 2.5 Astronomical 2.6 Spring-driven 2.7 Pendulum 2.8 Hairspring 2.9 Marine chronometer 2.10 Mass production 2.11 Early electric 2.12 Quartz 2.13 Atomic

3 Operation

3.1 Power source 3.2 Oscillator

3.2.1 Synchronized or slave clocks

3.3 Controller 3.4 Counter chain 3.5 Indicator

4 Types

4.1 Time
Time
display methods

4.1.1 Analog 4.1.2 Digital 4.1.3 Hybrid (analog-digital) 4.1.4 Auditory 4.1.5 Word 4.1.6 Projection 4.1.7 Tactile 4.1.8 Multi-display

5 Purposes

5.1 Time
Time
standards 5.2 Navigation

6 Specific types 7 See also

7.1 Newsgroup

8 Notes and references 9 Bibliography 10 External links

History Main article: History
History
of timekeeping devices Etymology The word clock is derived (via Dutch, Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning "bell". Time-measuring devices Sundials

Simple horizontal sundial

Main article: Sundial The apparent position of the Sun
Sun
in the sky moves over the course of each day, reflecting the rotation of the Earth. Shadows cast by stationary objects move correspondingly, so their positions can be used to indicate the time of day. A sundial shows the time by displaying the position of a shadow on a (usually) flat surface, which has markings that correspond to the hours.[7] Sundials can be horizontal, vertical, or in other orientations. Sundials were widely used in ancient times.[8] With the knowledge of latitude, a well-constructed sundial can measure local solar time with reasonable accuracy, within a minute or two. Sundials continued to be used to monitor the performance of clocks until the modern era.[citation needed] However, practical limitations, such as that sundials only work well on relatively clear days, and never during the night, encouraged the development of other techniques for measuring and displaying time. The Jantar Mantar At Delhi and Jaipur are examples of sundials. They were built by Maharaja Jai Singh II. Devices that measure duration, elapsed time and intervals

The flow of sand in an hourglass can be used to keep track of elapsed time.

Many devices can be used to mark passage of time without respect to reference time (time of day, minutes, etc.) and can be useful for measuring duration or intervals. Examples of such duration timers are candle clocks, incense clocks and the hourglass. Both the candle clock and the incense clock work on the same principle wherein the consumption of resources is more or less constant allowing reasonably precise and repeatable estimates of time passages. In the hourglass, fine sand pouring through a tiny hole at a constant rate indicates an arbitrary, predetermined, passage of time. The resource is not consumed but re-used. Water Main article: Water clock

A scale model of Su Song's Astronomical
Astronomical
Clock
Clock
Tower, built in 11th century Kaifeng, China. It was driven by a large waterwheel, chain drive, and escapement mechanism.

Water clocks, also known as clepsydrae (sg: clepsydra), along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day counting tally stick.[9] Given their great antiquity, where and when they first existed is not known and perhaps unknowable. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon
Babylon
and in Egypt
Egypt
around the 16th century BC. Other regions of the world, including India
India
and China, also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, write about water clocks appearing as early as 4000 BC in these regions of the world.[10] Greek astronomer Andronicus of Cyrrhus
Andronicus of Cyrrhus
supervised the construction of the Tower of the Winds
Tower of the Winds
in Athens in the 1st century B.C.[11] The Greek and Roman civilizations are credited for initially advancing water clock design to include complex gearing, which was connected to fanciful automata and also resulted in improved accuracy. These advances were passed on through Byzantium
Byzantium
and Islamic
Islamic
times, eventually making their way back to Europe. Independently, the Chinese developed their own advanced water clocks(水鐘)in 725 A.D., passing their ideas on to Korea
Korea
and Japan. Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. Pre-modern societies do not have the same precise timekeeping requirements that exist in modern industrial societies, where every hour of work or rest is monitored, and work may start or finish at any time regardless of external conditions. Instead, water clocks in ancient societies were used mainly for astrological reasons. These early water clocks were calibrated with a sundial. While never reaching the level of accuracy of a modern timepiece, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by the more accurate pendulum clock in 17th-century Europe. Islamic
Islamic
civilization is credited with further advancing the accuracy of clocks with elaborate engineering. In 797 (or possibly 801), the Abbasid
Abbasid
caliph of Baghdad, Harun al-Rashid, presented Charlemagne
Charlemagne
with an Asian Elephant
Asian Elephant
named Abul-Abbas
Abul-Abbas
together with a "particularly elaborate example" of a water[12] clock. Pope Sylvester II
Pope Sylvester II
introduced clocks to northern and western Europe
Europe
around 1000AD[13]

An elephant clock in a manuscript by Al-Jazari
Al-Jazari
(1206 AD) from The Book of Knowledge of Ingenious Mechanical Devices.[14]

In the 13th century, Al-Jazari, an engineer from Mesopotamia (lived 1136–1206) who worked for Artuqid
Artuqid
king of Diyar-Bakr, Nasir al-Din, made numerous clocks of all shapes and sizes. A book on his work described 50 mechanical devices in 6 categories, including water clocks. The most reputed clocks included the Elephant, Scribe and Castle clocks, all of which have been successfully reconstructed. As well as telling the time, these grand clocks were symbols of status, grandeur and wealth of the Urtuq State.[citation needed] Early mechanical

This section needs attention from an expert in Time. See the talk page for details. WikiProject Time
Time
may be able to help recruit an expert. (August 2016)

The word horologia (from the Greek ὥρα, hour, and λέγειν, to tell) was used to describe early mechanical clocks,[15] but the use of this word (still used in several Romance languages) [16] for all timekeepers conceals the true nature of the mechanisms. For example, there is a record that in 1176 Sens Cathedral installed a ‘horologe’[17] but the mechanism used is unknown. According to Jocelin of Brakelond, in 1198 during a fire at the abbey of St Edmundsbury (now Bury St Edmunds), the monks 'ran to the clock' to fetch water, indicating that their water clock had a reservoir large enough to help extinguish the occasional fire.[18] The word clock (from the Celtic words clocca and clogan, both meaning "bell"), which gradually supersedes "horologe", suggests that it was the sound of bells which also characterized the prototype mechanical clocks that appeared during the 13th century in Europe. A water-powered cogwheel clock was created in China
China
in AD 725 by Yi Xing and Liang Lingzan. This is not considered an escapement mechanism clock as it was unidirectional, the Song dynasty
Song dynasty
polymath and genius Su Song
Su Song
(1020–1101) incorporated it into his monumental innovation of the astronomical clock-tower of Kaifeng
Kaifeng
in 1088.[19][page needed] His astronomical clock and rotating armillary sphere still relied on the use of either flowing water during the spring, summer, autumn seasons and liquid mercury during the freezing temperature of winter (i.e. hydraulics). A mercury clock, described in the Libros del saber, a Spanish work from 1277 consisting of translations and paraphrases of Arabic works, is sometimes quoted as evidence for Muslim knowledge of a mechanical clock. A mercury-powered cogwheel clock was created by Ibn Khalaf al-Muradi[20][21] In Europe, between 1280 and 1320, there is an increase in the number of references to clocks and horologes in church records, and this probably indicates that a new type of clock mechanism had been devised. Existing clock mechanisms that used water power were being adapted to take their driving power from falling weights. This power was controlled by some form of oscillating mechanism, probably derived from existing bell-ringing or alarm devices. This controlled release of power—the escapement—marks the beginning of the true mechanical clock, which differed from the previously mentioned cogwheel clocks. Verge escapement
Verge escapement
mechanism derived in the surge of true mechanical clocks, which didn't need any kind of fluid power, like water or mercury, to work. These mechanical clocks were intended for two main purposes: for signalling and notification (e.g. the timing of services and public events), and for modeling the solar system. The former purpose is administrative, the latter arises naturally given the scholarly interests in astronomy, science, astrology, and how these subjects integrated with the religious philosophy of the time. The astrolabe was used both by astronomers and astrologers, and it was natural to apply a clockwork drive to the rotating plate to produce a working model of the solar system. Simple clocks intended mainly for notification were installed in towers, and did not always require faces or hands. They would have announced the canonical hours or intervals between set times of prayer. Canonical hours
Canonical hours
varied in length as the times of sunrise and sunset shifted. The more sophisticated astronomical clocks would have had moving dials or hands, and would have shown the time in various time systems, including Italian hours, canonical hours, and time as measured by astronomers at the time. Both styles of clock started acquiring extravagant features such as automata. In 1283, a large clock was installed at Dunstable Priory; its location above the rood screen suggests that it was not a water clock.[citation needed] In 1292, Canterbury Cathedral
Canterbury Cathedral
installed a 'great horloge'. Over the next 30 years there are mentions of clocks at a number of ecclesiastical institutions in England, Italy, and France. In 1322, a new clock was installed in Norwich, an expensive replacement for an earlier clock installed in 1273. This had a large (2 metre) astronomical dial with automata and bells. The costs of the installation included the full-time employment of two clockkeepers for two years.[citation needed] Astronomical

Richard of Wallingford
Richard of Wallingford
pointing to a clock, his gift to St Albans Abbey.

16th-century clock machine Convent of Christ, Tomar, Portugal

Besides the Chinese astronomical clock of Su Song
Su Song
in 1088 mentioned above, in Europe
Europe
there were the clocks constructed by Richard of Wallingford in St Albans
St Albans
by 1336, and by Giovanni de Dondi in Padua from 1348 to 1364. They no longer exist, but detailed descriptions of their design and construction survive,[22][23] and modern reproductions have been made.[23] They illustrate how quickly the theory of the mechanical clock had been translated into practical constructions, and also that one of the many impulses to their development had been the desire of astronomers to investigate celestial phenomena. Wallingford's clock had a large astrolabe-type dial, showing the sun, the moon's age, phase, and node, a star map, and possibly the planets. In addition, it had a wheel of fortune and an indicator of the state of the tide at London
London
Bridge. Bells rang every hour, the number of strokes indicating the time.[22] Dondi's clock was a seven-sided construction, 1 metre high, with dials showing the time of day, including minutes, the motions of all the known planets, an automatic calendar of fixed and movable feasts, and an eclipse prediction hand rotating once every 18 years.[23] It is not known how accurate or reliable these clocks would have been. They were probably adjusted manually every day to compensate for errors caused by wear and imprecise manufacture. Water clocks are sometimes still used today, and can be examined in places such as ancient castles and museums. The Salisbury Cathedral clock, built in 1386, is considered to be the world's oldest surviving mechanical clock that strikes the hours.[24] Spring-driven Main article: Torsion pendulum clock

Renaissance Turret Clock, German, circa 1570

Spring driven Matthew Norman carriage clock with winding key

Clockmakers developed their art in various ways. Building smaller clocks was a technical challenge, as was improving accuracy and reliability. Clocks could be impressive showpieces to demonstrate skilled craftsmanship, or less expensive, mass-produced items for domestic use. The escapement in particular was an important factor affecting the clock's accuracy, so many different mechanisms were tried. Spring-driven clocks appeared during the 15th century,[25][26][27] although they are often erroneously credited to Nuremberg
Nuremberg
watchmaker Peter Henlein
Peter Henlein
(or Henle, or Hele) around 1511.[28][29][30] The earliest existing spring driven clock is the chamber clock given to Phillip the Good, Duke of Burgundy, around 1430, now in the Germanisches Nationalmuseum.[4] Spring power presented clockmakers with a new problem: how to keep the clock movement running at a constant rate as the spring ran down. This resulted in the invention of the stackfreed and the fusee in the 15th century, and many other innovations, down to the invention of the modern going barrel in 1760. Early clock dials did not indicate minutes and seconds. A clock with a dial indicating minutes was illustrated in a 1475 manuscript by Paulus Almanus,[31] and some 15th-century clocks in Germany
Germany
indicated minutes and seconds.[32] An early record of a seconds hand on a clock dates back to about 1560 on a clock now in the Fremersdorf collection.[33]:417–418[34] During the 15th and 16th centuries, clockmaking flourished, particularly in the metalworking towns of Nuremberg
Nuremberg
and Augsburg, and in Blois, France. Some of the more basic table clocks have only one time-keeping hand, with the dial between the hour markers being divided into four equal parts making the clocks readable to the nearest 15 minutes. Other clocks were exhibitions of craftsmanship and skill, incorporating astronomical indicators and musical movements. The cross-beat escapement was invented in 1584 by Jost Bürgi, who also developed the remontoire. Bürgi's clocks were a great improvement in accuracy as they were correct to within a minute a day.[35][36] These clocks helped the 16th-century astronomer Tycho Brahe to observe astronomical events with much greater precision than before.[citation needed][how?] Pendulum

The Dutch polymath and horologist Christiaan Huygens, the inventor of first precision timekeeping devices (pendulum clock and spiral-hairspring watch).[37]

From its invention in 1656 by Christiaan Huygens
Christiaan Huygens
until the 1930s, the pendulum clock was the world's most precise timekeeper, accounting for its widespread use.

Pendulum clock
Pendulum clock
Ansonia. C.1904, Ansonia Clock
Clock
Co., SANTIAGO, hanging oak gingerbread clock, 8-day time and strike.

The next development in accuracy occurred after 1656 with the invention of the pendulum clock. Galileo had the idea to use a swinging bob to regulate the motion of a time-telling device earlier in the 17th century. Christiaan Huygens, however, is usually credited as the inventor. He determined the mathematical formula that related pendulum length to time (about 99.4 cm or 39.1 inches for the one second movement) and had the first pendulum-driven clock made. The first model clock was built in 1657 in the Hague, but it was in England
England
that the idea was taken up.[38] The longcase clock (also known as the grandfather clock) was created to house the pendulum and works by the English clockmaker William Clement in 1670 or 1671. It was also at this time that clock cases began to be made of wood and clock faces to utilize enamel as well as hand-painted ceramics. In 1670, William Clement created the anchor escapement,[39] an improvement over Huygens' crown escapement. Clement also introduced the pendulum suspension spring in 1671. The concentric minute hand was added to the clock by Daniel Quare, a London
London
clockmaker and others, and the second hand was first introduced. Hairspring

Drawing of one of his first balance springs, attached to a balance wheel, by Christiaan Huygens, published in his letter in the Journal des Sçavants of 25 February 1675. The application of the spiral balance spring (spiral hairspring) for watches ushered in a new era of accuracy for portable timekeepers, similar to that which the pendulum had introduced for clocks.

In 1675, Huygens and Robert Hooke
Robert Hooke
invented the spiral balance spring, or the hairspring, designed to control the oscillating speed of the balance wheel. This crucial advance finally made accurate pocket watches possible. The great English clockmaker, Thomas Tompion, was one of the first to use this mechanism successfully in his pocket watches, and he adopted the minute hand which, after a variety of designs were trialled, eventually stabilised into the modern-day configuration.[40] The rack and snail striking mechanism for striking clocks, was introduced during the 17th century and had distinct advantages over the 'countwheel' (or 'locking plate') mechanism. During the 20th century there was a common misconception that Edward Barlow invented rack and snail striking. In fact, his invention was connected with a repeating mechanism employing the rack and snail.[41] The repeating clock, that chimes the number of hours (or even minutes) was invented by either Quare or Barlow in 1676. George Graham invented the deadbeat escapement for clocks in 1720. Marine chronometer

Drawings of Harrison's H4 chronometer of 1761, published in The principles of Mr Harrison's time-keeper, 1767.[42]

A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The position of a ship at sea could be determined with reasonable accuracy if a navigator could refer to a clock that lost or gained less than about 10 seconds per day. This clock could not contain a pendulum, which would be virtually useless on a rocking ship. In 1714, the British government offered large financial rewards to the value of 20,000 pounds,[43] for anyone who could determine longitude accurately. John Harrison, who dedicated his life to improving the accuracy of his clocks, later received considerable sums under the Longitude
Longitude
Act. In 1735, Harrison built his first chronometer, which he steadily improved on over the next thirty years before submitting it for examination. The clock had many innovations, including the use of bearings to reduce friction, weighted balances to compensate for the ship's pitch and roll in the sea and the use of two different metals to reduce the problem of expansion from heat. The chronometer was tested in 1761 by Harrison's son and by the end of 10 weeks the clock was in error by less than 5 seconds.[44] Mass production

Eli Terry, the inventor of mass-produced clocks.

The British had predominated in watch manufacture for much of the 17th and 18th centuries, but maintained a system of production that was geared towards high quality products for the elite.[45] Although there was an attempt to modernise clock manufacture with mass production techniques and the application of duplicating tools and machinery by the British Watch
Watch
Company in 1843, it was in the United States
United States
that this system took off. In 1816, Eli Terry
Eli Terry
and some other Connecticut clockmakers developed a way of mass-producing clocks by using interchangeable parts.[46] Aaron Lufkin Dennison
Aaron Lufkin Dennison
started a factory in 1851 in Massachusetts
Massachusetts
that also used interchangeable parts, and by 1861 was running a successful enterprise incorporated as the Waltham Watch
Watch
Company.[47][48] Early electric Main article: Electric clock In 1815, Francis Ronalds
Francis Ronalds
published the first electric clock powered by dry pile batteries.[49] Alexander Bain, Scottish clockmaker, patented the electric clock in 1840. The electric clock's mainspring is wound either with an electric motor or with an electromagnet and armature. In 1841, he first patented the electromagnetic pendulum. By the end of the nineteenth century, the advent of the dry cell battery made it feasible to use electric power in clocks. Spring or weight driven clocks that use electricity, either alternating current (AC) or direct current (DC), to rewind the spring or raise the weight of a mechanical clock would be classified as an electromechanical clock. This classification would also apply to clocks that employ an electrical impulse to propel the pendulum. In electromechanical clocks the electricity serves no time keeping function. These types of clocks were made as individual timepieces but more commonly used in synchronized time installations in schools, businesses, factories, railroads and government facilities as a master clock and slave clocks. Electric clocks that are powered from the AC supply often use synchronous motors. The supply current alternates with a frequency of 50 hertz in many countries, and 60 hertz in others. The rotor of the motor rotates at a speed that is related to the alternation frequency. Appropriate gearing converts this rotation speed to the correct ones for the hands of the analog clock. The development of electronics in the 20th century led to clocks with no clockwork parts at all. Time
Time
in these cases is measured in several ways, such as by the alternation of the AC supply, vibration of a tuning fork, the behaviour of quartz crystals, or the quantum vibrations of atoms. Electronic circuits divide these high-frequency oscillations to slower ones that drive the time display. Even mechanical clocks have since come to be largely powered by batteries, removing the need for winding. Quartz The piezoelectric properties of crystalline quartz were discovered by Jacques and Pierre Curie
Pierre Curie
in 1880.[50][51] The first crystal oscillator was invented in 1917 by Alexander M. Nicholson after which, the first quartz crystal oscillator was built by Walter G. Cady in 1921.[2] In 1927 the first quartz clock was built by Warren Marrison and J. W. Horton at Bell Telephone Laboratories
Bell Telephone Laboratories
in Canada.[52][2] The following decades saw the development of quartz clocks as precision time measurement devices in laboratory settings—the bulky and delicate counting electronics, built with vacuum tubes, limited their practical use elsewhere. The National Bureau of Standards
National Bureau of Standards
(now NIST) based the time standard of the United States
United States
on quartz clocks from late 1929 until the 1960s, when it changed to atomic clocks.[53] In 1969, Seiko produced the world's first quartz wristwatch, the Astron.[54] Their inherent accuracy and low cost of production resulted in the subsequent proliferation of quartz clocks and watches.[50] Atomic As of the 2010s, atomic clocks are the most accurate clocks in existence. They are considerably more accurate than quartz clocks as they can be accurate to within a few seconds over thousands of years.[55] Atomic clocks were first theorized by Lord Kelvin
Lord Kelvin
in 1879.[56] In the 1930s the development of Magnetic resonance created practical method for doing this.[57] A prototype ammonia maser device was built in 1949 at the U.S. National Bureau of Standards
National Bureau of Standards
(NBS, now NIST). Although it was less accurate than existing quartz clocks, it served to demonstrate the concept.[58][59][60] The first accurate atomic clock, a caesium standard based on a certain transition of the caesium-133 atom, was built by Louis Essen
Louis Essen
in 1955 at the National Physical Laboratory in the UK.[61] Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time (ET).[62] As of 2013, the most stable atomic clocks are ytterbium clocks, which are stable to within less than two parts in 1 quintillion (6982200000000000000♠2×10−18).[63] Operation

A chiming clock's mechanism.

The invention of the mechanical clock in the 13th century initiated a change in timekeeping methods from continuous processes, such as the motion of the gnomon's shadow on a sundial or the flow of liquid in a water clock, to periodic oscillatory processes, such as the swing of a pendulum or the vibration of a quartz crystal,[3][64] which had the potential for more accuracy. All modern clocks use oscillation. Although the mechanisms they use vary, all oscillating clocks, mechanical, digital and atomic, work similarly and can be divided into analogous parts.[65][66][67] They consist of an object that repeats the same motion over and over again, an oscillator, with a precisely constant time interval between each repetition, or 'beat'. Attached to the oscillator is a controller device, which sustains the oscillator's motion by replacing the energy it loses to friction, and converts its oscillations into a series of pulses. The pulses are then counted by some type of counter, and the number of counts is converted into convenient units, usually seconds, minutes, hours, etc. Finally some kind of indicator displays the result in human readable form. Power source

Keys of various sizes for winding up mainsprings on clocks.

In mechanical clocks, the power source is typically either a weight suspended from a cord or chain wrapped around a pulley, sprocket or drum; or a spiral spring called a mainspring. Mechanical clocks must be wound periodically, usually by turning a knob or key or by pulling on the free end of the chain, to store energy in the weight or spring to keep the clock running. In electric clocks, the power source is either a battery or the AC power line. In clocks that use AC power, a small backup battery is often included to keep the clock running if it is unplugged temporarily from the wall or during a power outage. Battery powered analog wall clocks are available that operate over 15 years between battery changes.

Oscillator The timekeeping element in every modern clock is a harmonic oscillator, a physical object (resonator) that vibrates or oscillates repetitively at a precisely constant frequency.[2]

In mechanical clocks, this is either a pendulum or a balance wheel. In some early electronic clocks and watches such as the Accutron, it is a tuning fork. In quartz clocks and watches, it is a quartz crystal. In atomic clocks, it is the vibration of electrons in atoms as they emit microwaves. In early mechanical clocks before 1657, it was a crude balance wheel or foliot which was not a harmonic oscillator because it lacked a balance spring. As a result, they were very inaccurate, with errors of perhaps an hour a day.[68]

The advantage of a harmonic oscillator over other forms of oscillator is that it employs resonance to vibrate at a precise natural resonant frequency or 'beat' dependent only on its physical characteristics, and resists vibrating at other rates. The possible precision achievable by a harmonic oscillator is measured by a parameter called its Q,[69][70] or quality factor, which increases (other things being equal) with its resonant frequency.[71] This is why there has been a long term trend toward higher frequency oscillators in clocks. Balance wheels and pendulums always include a means of adjusting the rate of the timepiece. Quartz
Quartz
timepieces sometimes include a rate screw that adjusts a capacitor for that purpose. Atomic clocks are primary standards, and their rate cannot be adjusted. Synchronized or slave clocks Some clocks rely for their accuracy on an external oscillator; that is, they are automatically synchronized to a more accurate clock:

Slave clocks, 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.[72] 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. Synchronous electric clocks do not have an internal oscillator, but count cycles of the 50 or 60 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 clocks keep time with a quartz crystal, but can be periodically (usually weekly) synchronized over the Internet
Internet
to atomic clocks (UTC), using the Network Time
Time
Protocol (NTP). Sometimes computers on a local area network (LAN) get their time from a single local server which is maintained accurately. Radio clocks keep time with a quartz crystal, but are periodically synchronized to time signals transmitted from dedicated standard time radio stations or satellite navigation signals, which are set by atomic clocks.

Controller This has the dual function of keeping the oscillator running by giving it 'pushes' to replace the energy lost to friction, and converting its vibrations into a series of pulses that serve to measure the time.

In mechanical clocks, this is the escapement, which gives precise pushes to the swinging pendulum or balance wheel, and releases one gear tooth of the escape wheel at each swing, allowing all the clock's wheels to move forward a fixed amount with each swing. In electronic clocks this is an electronic oscillator circuit that gives the vibrating quartz crystal or tuning fork tiny 'pushes', and generates a series of electrical pulses, one for each vibration of the crystal, which is called the clock signal. In atomic clocks the controller is an evacuated microwave cavity attached to a microwave oscillator controlled by a microprocessor. A thin gas of cesium atoms is released into the cavity where they are exposed to microwaves. A laser measures how many atoms have absorbed the microwaves, and an electronic feedback control system called a phase-locked loop tunes the microwave oscillator until it is at the frequency that causes the atoms to vibrate and absorb the microwaves. Then the microwave signal is divided by digital counters to become the clock signal.[73]

In mechanical clocks, the low Q of the balance wheel or pendulum oscillator made them very sensitive to the disturbing effect of the impulses of the escapement, so the escapement had a great effect on the accuracy of the clock, and many escapement designs were tried. The higher Q of resonators in electronic clocks makes them relatively insensitive to the disturbing effects of the drive power, so the driving oscillator circuit is a much less critical component.[2] Counter chain This counts the pulses and adds them up to get traditional time units of seconds, minutes, hours, etc. It usually has a provision for setting the clock by manually entering the correct time into the counter.

In mechanical clocks this is done mechanically by a gear train, known as the wheel train. The gear train also has a second function; to transmit mechanical power from the power source to run the oscillator. There is a friction coupling called the 'cannon pinion' between the gears driving the hands and the rest of the clock, allowing the hands to be turned to set the time.[74] In digital clocks a series of integrated circuit counters or dividers add the pulses up digitally, using binary logic. Often pushbuttons on the case allow the hour and minute counters to be incremented and decremented to set the time.

Indicator

Play media

A Cuckoo clock
Cuckoo clock
with mechanical automaton and sound producer striking on the 8th hour on the analog dial.

This displays the count of seconds, minutes, hours, etc. in a human readable form.

The earliest mechanical clocks in the 13th century didn't have a visual indicator and signalled the time audibly by striking bells. Many clocks to this day are striking clocks which strike the hour. Analog clocks display time with an analog clock face, which consists of a round dial with the numbers 1 through 12, the hours in the day, around the outside. The hours are indicated with an hour hand, which makes two revolutions in a day, while the minutes are indicated by a minute hand, which makes one revolution per hour. In mechanical clocks a gear train drives the hands; in electronic clocks the circuit produces pulses every second which drive a stepper motor and gear train, which move the hands. Digital clocks display the time in periodically changing digits on a digital display. A common misconception is that a digital clock is more accurate than an analog wall clock, but the indicator type is separate and apart from the accuracy of the timing source. Talking clocks and the speaking clock services provided by telephone companies speak the time audibly, using either recorded or digitally synthesized voices.

Types Clocks can be classified by the type of time display, as well as by the method of timekeeping. Time
Time
display methods Analog See also: Clock
Clock
face

A linear clock at London's Piccadilly Circus tube station. The 24 hour band moves across the static map, keeping pace with the apparent movement of the sun above ground, and a pointer fixed on London
London
points to the current time.

A modern quartz clock with a 24-hour face

Analog clocks usually use a clock face which indicates time using rotating pointers called "hands" on a fixed numbered dial or dials. The standard clock face, known universally throughout the world, has a short "hour hand" which indicates the hour on a circular dial of 12 hours, making two revolutions per day, and a longer "minute hand" which indicates the minutes in the current hour on the same dial, which is also divided into 60 minutes. It may also have a "second hand" which indicates the seconds in the current minute. The only other widely used clock face today is the 24 hour analog dial, because of the use of 24 hour time
24 hour time
in military organizations and timetables. Before the modern clock face was standardized during the Industrial Revolution, many other face designs were used throughout the years, including dials divided into 6, 8, 10, and 24 hours. During the French Revolution the French government tried to introduce a 10-hour clock, as part of their decimal-based metric system of measurement, but it didn't catch on. An Italian 6 hour clock was developed in the 18th century, presumably to save power (a clock or watch striking 24 times uses more power).

A simple 24 hour clock showing the approximate position of the sun.

Another type of analog clock is the sundial, which tracks the sun continuously, registering the time by the shadow position of its gnomon. Because the sun does not adjust to daylight saving time, users must add an hour during that time. Corrections must also be made for the equation of time, and for the difference between the longitudes of the sundial and of the central meridian of the time zone that is being used (i.e. 15 degrees east of the prime meridian for each hour that the time zone is ahead of GMT). Sundials use some or part of the 24 hour analog dial. There also exist clocks which use a digital display despite having an analog mechanism—these are commonly referred to as flip clocks. Alternative systems have been proposed. For example, the "Twelv" clock indicates the current hour using one of twelve colors, and indicates the minute by showing a proportion of a circular disk, similar to a moon phase.[75]

Digital Main article: Digital clock

Examples of digital clocks

Digital clock
Digital clock
outside Kanazawa Station
Kanazawa Station
displaying the time by controlling valves on a fountain

Basic digital clock radio

Mobile phone
Mobile phone
display with an analog-style clock (albeit generated by a digital computer) in the middle, and a digital-style in the top right corner

Diagram of a mechanical digital display of a flip clock

Digital clocks display a numeric representation of time. Two numeric display formats are commonly used on digital clocks:

the 24-hour notation
24-hour notation
with hours ranging 00–23; the 12-hour notation
12-hour notation
with AM/PM indicator, with hours indicated as 12AM, followed by 1AM–11AM, followed by 12PM, followed by 1PM–11PM (a notation mostly used in domestic environments).

Most digital clocks use electronic mechanisms and LCD, LED, or VFD displays; many other display technologies are used as well (cathode ray tubes, nixie tubes, etc.). After a reset, battery change or power failure, these clocks without a backup battery or capacitor either start counting from 12:00, or stay at 12:00, often with blinking digits indicating that the time needs to be set. Some newer clocks will reset themselves based on radio or Internet
Internet
time servers that are tuned to national atomic clocks. Since the advent of digital clocks in the 1960s, the use of analog clocks has declined significantly. Some clocks, called 'flip clocks', have digital displays that work mechanically. The digits are painted on sheets of material which are mounted like the pages of a book. Once a minute, a page is turned over to reveal the next digit. These displays are usually easier to read in brightly lit conditions than LCDs or LEDs. Also, they do not go back to 12:00 after a power interruption. Flip clocks generally do not have electronic mechanisms. Usually, they are driven by AC-synchronous motors. Hybrid (analog-digital)

Hybrid real-time animated clock with seconds (12 hours)

Clocks with analog quadrants, with a digital component, usually minutes and hours displayed analogously and seconds displayed in digital mode. Auditory Main article: Talking clock For convenience, distance, telephony or blindness, auditory clocks present the time as sounds. The sound is either spoken natural language, (e.g. "The time is twelve thirty-five"), or as auditory codes (e.g. number of sequential bell rings on the hour represents the number of the hour like the bell, Big Ben). Most telecommunication companies also provide a speaking clock service as well. Word

Software word clock

Word clocks are clocks that display the time visually using sentences. E.g.: "It’s about three o’clock." These clocks can be implemented in hardware or software. Projection Main article: Projection clock Some clocks, usually digital ones, include an optical projector that shines a magnified image of the time display onto a screen or onto a surface such as an indoor ceiling or wall. The digits are large enough to be easily read, without using glasses, by persons with moderately imperfect vision, so the clocks are convenient for use in their bedrooms. Usually, the timekeeping circuitry has a battery as a backup source for an uninterrupted power supply to keep the clock on time, while the projection light only works when the unit is connected to an A.C. supply. Completely battery-powered portable versions resembling flashlights are also available. Tactile Auditory and projection clocks can be used by people who are blind or have limited vision. There are also clocks for the blind that have displays that can be read by using the sense of touch. Some of these are similar to normal analog displays, but are constructed so the hands can be felt without damaging them. Another type is essentially digital, and uses devices that use a code such as Braille
Braille
to show the digits so that they can be felt with the fingertips. Multi-display Some clocks have several displays driven by a single mechanism, and some others have several completely separate mechanisms in a single case. Clocks in public places often have several faces visible from different directions, so that the clock can be read from anywhere in the vicinity; all the faces show the same time. Other clocks show the current time in several time-zones. Watches that are intended to be carried by travellers often have two displays, one for the local time and the other for the time at home, which is useful for making pre-arranged phone calls. Some equation clocks have two displays, one showing mean time and the other solar time, as would be shown by a sundial. Some clocks have both analog and digital displays. Clocks with Braille
Braille
displays usually also have conventional digits so they can be read by sighted people. Purposes

Many cities and towns traditionally have public clocks in a prominent location, such as a town square or city center. This one is on display at the center of the town of Robbins, North Carolina.

Clocks are in homes, offices and many other places; smaller ones (watches) are carried on the wrist or in a pocket; larger ones are in public places, e.g. a railway station or church. A small clock is often shown in a corner of computer displays, mobile phones and many MP3 players. The primary purpose of a clock is to display the time. Clocks may also have the facility to make a loud alert signal at a specified time, typically to waken a sleeper at a preset time; they are referred to as alarm clocks. The alarm may start at a low volume and become louder, or have the facility to be switched off for a few minutes then resume. Alarm clocks with visible indicators are sometimes used to indicate to children too young to read the time that the time for sleep has finished; they are sometimes called training clocks. A clock mechanism may be used to control a device according to time, e.g. a central heating system, a VCR, or a time bomb (see: digital counter). Such mechanisms are usually called timers. Clock
Clock
mechanisms are also used to drive devices such as solar trackers and astronomical telescopes, which have to turn at accurately controlled speeds to counteract the rotation of the Earth. Most digital computers depend on an internal signal at constant frequency to synchronize processing; this is referred to as a clock signal. (A few research projects are developing CPUs based on asynchronous circuits.) Some equipment, including computers, also maintains time and date for use as required; this is referred to as time-of-day clock, and is distinct from the system clock signal, although possibly based on counting its cycles. In Chinese culture, giving a clock (送鍾/送钟, sòng zhōng) is often taboo, especially to the elderly as the term for this act is a homophone with the term for the act of attending another's funeral (送終/送终, sòngzhōng).[76][77][78] A UK government official Susan Kramer
Susan Kramer
gave a watch to Taipei
Taipei
mayor Ko Wen-je
Ko Wen-je
unaware of such a taboo which resulted in some professional embarrassment and a pursuant apology.[79] It is undesirable to give someone a clock or (depending on the region) other timepiece as a gift. Traditional superstitions regard this as counting the seconds to the recipient's death. Another common interpretation of this is that the phrase "to give a clock" (simplified Chinese: 送钟; traditional Chinese: 送鐘) in Chinese is pronounced "sòng zhōng" in Mandarin, which is a homophone of a phrase for "terminating" or "attending a funeral" (both can be written as 送終 (traditional) or 送终 (simplified)). Cantonese people consider such a gift as a curse.[80] This homonymic pair works in both Mandarin and Cantonese, although in most parts of China
China
only clocks and large bells, and not watches, are called "zhong", and watches are commonly given as gifts in China. However, should such a gift be given, the "unluckiness" of the gift can be countered by exacting a small monetary payment so the recipient is buying the clock and thereby counteracting the '送' ("give") expression of the phrase. Time
Time
standards Main articles: Time
Time
standard and Atomic clock For some scientific work timing of the utmost accuracy is essential. It is also necessary to have a standard of the maximum accuracy against which working clocks can be calibrated. An ideal clock would give the time to unlimited accuracy, but this is not realisable. Many physical processes, in particular including some transitions between atomic energy levels, occur at exceedingly stable frequency; counting cycles of such a process can give a very accurate and consistent time—clocks which work this way are usually called atomic clocks. Such clocks are typically large, very expensive, require a controlled environment, and are far more accurate than required for most purposes; they are typically used in a standards laboratory. Navigation Until advances in the late twentieth century, navigation depended on the ability to measure latitude and longitude. Latitude
Latitude
can be determined through celestial navigation; the measurement of longitude requires accurate knowledge of time. This need was a major motivation for the development of accurate mechanical clocks. John Harrison created the first highly accurate marine chronometer in the mid-18th century. The Noon gun
Noon gun
in Cape Town
Cape Town
still fires an accurate signal to allow ships to check their chronometers. Many buildings near major ports used to have (some still do) a large ball mounted on a tower or mast arranged to drop at a pre-determined time, for the same purpose. While satellite navigation systems such as the Global Positioning System (GPS) require unprecedentedly accurate knowledge of time, this is supplied by equipment on the satellites; vehicles no longer need timekeeping equipment. Specific types

A monumental conical pendulum clock by Eugène Farcot, 1867. Philadelphia, USA.

By mechanism By function By style

Astronomical
Astronomical
clock Atomic clock Candle clock Congreve clock Conical pendulum
Conical pendulum
clock Digital clock Electric clock Flip clock Hourglass Incense clock Mechanical watch Oil-lamp clock Pendulum
Pendulum
clock Pipe organ clock Projection clock Pulsar clock Quantum clock Quartz
Quartz
clock Radio clock Rolling ball clock Spring drive watch Steam clock Sundial Torsion pendulum clock Water clock

10-hour clock Alarm clock Binary clock Chiming clock Chronometer watch Cuckoo clock Equation clock Game clock Japanese clock Master clock Musical clock Railroad chronometer Slave clock Speaking clock Stopwatch Striking clock Talking clock Tide clock Time
Time
ball Time
Time
clock World clock

American clock Automaton
Automaton
clock Balloon clock Banjo clock Bracket clock Carriage clock Cartel clock Cat clock Chariot clock Clock
Clock
tower Cuckoo clock Doll's head clock Floral clock French Empire mantel clock Granddaughter clock Grandfather clock Grandmother clock Lantern clock Lighthouse clock Longcase (Tall-case) clock Mantel clock Skeleton clock Tower clock Turret clock Watch

See also

Allan variance American Watchmakers-Clockmakers Institute BaselWorld Biological clock Castle clock Clockarium Clock
Clock
as herald of the Industrial Revolution
Industrial Revolution
(Lewis Mumford) Clock
Clock
face Clock
Clock
drift Clock
Clock
ident Clock
Clock
network Clock
Clock
of the Long Now Clock signal (digital circuits) Clockkeeper Clockmaker Colgate Clock
Clock
(Indiana) Colgate Clock
Clock
(New Jersey), largest clock in USA Corpus Clock Cosmo Clock
Clock
21, world's largest clock Cox's timepiece Cuckooland Museum Date and time representation by country Debt clock Le Défenseur du Temps (automata) Department of Defense master clock
Department of Defense master clock
(U.S.) Doomsday Clock Earth
Earth
clock Equation clock Federation of the Swiss Watch
Watch
Industry FH Guard tour patrol system (watchclocks) Iron Ring Clock Jens Olsen's World Clock Jewel bearing List of biggest clock faces List of clocks List of international common standards List of world's largest cuckoo clocks Metrology Mora clock National Association of Watch
Watch
and Clock
Clock
Collectors Replica watch Rubik's Clock Star clock Singing bird box System time Time
Time
to digital converter Timeline
Timeline
of time measurement technology Timer Watchmaker

Newsgroup

alt.horology

Notes and references

^ Dohrn-van Rossum, Gerhard (1996). History
History
of the Hour: Clocks and Modern Temporal Orders. Univ. of Chicago Press. ISBN 0-226-15511-0. , p.103-104 ^ a b c d e f Marrison, Warren (1948). "The Evolution of the Quartz Crystal Clock" (PDF). Bell System Technical Journal. 27: 510–588. doi:10.1002/j.1538-7305.1948.tb01343.x. Archived from the original (PDF) on November 10, 2014. Retrieved November 10, 2014.  ^ a b Cipolla, Carlo M. (2004). Clocks and Culture, 1300 to 1700. W.W. Norton & Co. ISBN 0-393-32443-5. , p.31 ^ a b White, Lynn, Jr. (1962). Medieval Technology and Social Change. UK: Oxford Univ. Press. p. 119.  ^ see Baillie et al., p. 307; Palmer, p. 19; Zea & Cheney, p. 172 ^ "Cambridge Advanced Learner's Dictionary". Retrieved 2018-01-29. a device for measuring and showing time, which is usually found in or on a building and is not worn by a person  ^ "How Sundials Work". The British Sundial
Sundial
Society. Retrieved 10 November 2014.  ^ "Ancient Sundials". North American Sundial
Sundial
Society. Retrieved 10 November 2014.  ^ Turner 1984, p. 1 ^ Cowan 1958, p. 58 ^ Tower of the Winds
Tower of the Winds
– Athens ^ James, Peter (1995). Ancient Inventions. New York, NY: Ballantine Books. p. 126. ISBN 0-345-40102-6.  ^ William Godwin (1876). "Lives of the Necromancers". p. 232.  ^ Ibn al-Razzaz Al-Jazari
Al-Jazari
(ed. 1974), The Book of Knowledge of Ingenious Mechanical Devices. Translated and annotated by Donald Routledge
Routledge
Hill, Dordrecht/D. Reidel. ^ Leonhard Schmitz; Smith, William (1875). A Dictionary of Greek and Roman Antiquities. London: John Murray. pp. 615‑617.  ^ Modern French "horloge" is very close; Spanish "reloj" and Portuguese "relógio" drop the first part of the word. ^ Bulletin de la société archéologique de Sens, year 1867, vol. IX, page 390, available at www.archive.org. See also fr:Discussion:Horloge ^ The Chronicle of Jocelin of Brakelond, Monk of St. Edmundsbury: A Picture of Monastic and Social Life on the XIIth Century. London: Chatto and Windus. Translated and edited by L. C. Jane. 1910.  ^ History
History
of Song 宋史, Vol. 340 ^ Mario Taddei. "The Book of Secrets is coming to the world after a thousand years: Automata
Automata
existed already in the eleventh century!" (PDF). Leonardo3. Retrieved 2010-03-31.  ^ Donald Routledge Hill
Donald Routledge Hill
(1991). "Arabic Mechanical Engineering: Survey of the Historical sources". Arabic Sciences and Philosophy. Cambridge University Press. 1 (2): 167–186 [173]. doi:10.1017/S0957423900001478.  ^ a b North, John. God's Clockmaker: Richard of Wallingford
Richard of Wallingford
and the Invention
Invention
of Time. London: Hambledon and London
London
(2005). ^ a b c King, Henry "Geared to the Stars: the evolution of planetariums, orreries, and astronomical clocks", University of Toronto Press, 1978 ^ Singer, Charles, et al. Oxford History
History
of Technology: volume II, from the Renaissance to the Industrial Revolution
Industrial Revolution
(OUP 1957)pg 650-1 ^ White, Lynn Jr. (1966). Medieval Technology and Social Change. New York: Oxford Univ. Press. pp. 126–127. ISBN 0-19-500266-0.  ^ Usher, Abbot Payson (1988). A History
History
of Mechanical Inventions. Courier Dover. p. 305. ISBN 0-486-25593-X.  ^ Dohrn-van Rossum, Gerhar (1997). History
History
of the Hour: Clocks and Modern Temporal Orders. Univ. of Chicago Press. p. 121. ISBN 0-226-15510-2.  ^ Milham, Willis I. (1945). Time
Time
and Timekeepers. New York: MacMillan. p. 121. ISBN 0-7808-0008-7.  ^ "Clock". The New Encyclopædia Britannica. 4. Univ. of Chicago. 1974. p. 747. ISBN 0-85229-290-2.  ^ Anzovin, Steve; Podell, Janet (2000). Famous First Facts: A record of first happenings, discoveries, and inventions in world history. H.W. Wilson. p. 440. ISBN 0-8242-0958-3.  ^ p. 529, " Time
Time
and timekeeping instruments", History
History
of astronomy: an encyclopedia, John Lankford, Taylor & Francis, 1997, ISBN 0-8153-0322-X. ^ Usher, Abbott Payson (1988). A history of mechanical inventions. Courier Dover Publications. p. 209. ISBN 0-486-25593-X.  ^ Landes, David S. (1983). Revolution in Time. Cambridge, Massachusetts: Harvard University Press. ISBN 0-674-76802-7.  ^ Willsberger, Johann (1975). Clocks & watches. New York: Dial Press. ISBN 0-8037-4475-7.  full page color photo: 4th caption page, 3rd photo thereafter (neither pages nor photos are numbered). ^ Lance Day; Ian McNeil, eds. (1996). Biographical dictionary of the history of technology. Routledge
Routledge
( Routledge
Routledge
Reference). p. 116. ISBN 0-415-06042-7.  ^ "Table clock c. 1650 attributed to Hans Buschmann that uses technical inventions by Jost Bürgi". The British Museum. Retrieved 2010-04-11.  ^ Macey, Samuel L. (ed.): Encyclopedia of Time. (NYC: Garland Publishing, 1994, ISBN 0815306156); in Clocks and Watches: The Leap to Precision by William J. H. Andrewes, p. 123–127 ^ " History
History
of Clocks".  ^ "The History
History
of Mechanical Pendulum
Pendulum
Clocks and Quartz
Quartz
Clocks". about.com. 2012. Retrieved 16 June 2012.  ^ " History
History
Of Clocks".  ^ Horological Journal, September 2011, pages 408-412. ^ The principles of Mr Harrison's time-keeper ^ John S. Rigden (2003). Hydrogen: The Essential Element. Harvard University Press. p. 185. ISBN 978-0-674-01252-3.  ^ Gould, Rupert T. (1923). The Marine Chronometer. Its History
History
and Development. London: J. D. Potter. p. 66. ISBN 0-907462-05-7.  ^ Glasmeier, Amy (2000). Manufacturing Time: Global Competition in the Watch
Watch
Industry, 1795–2000. Guilford Press. Retrieved 2013-02-07.  ^ " Eli Terry
Eli Terry
Mass-Produced Box Clock." Smithsonian The National Museum of American History. Web. 21 Sep. 2015. ^ Roe, Joseph Wickham (1916), English and American Tool Builders, New Haven, Connecticut: Yale University Press, LCCN 16011753 . Reprinted by McGraw-Hill, New York and London, 1926 (LCCN 27-24075); and by Lindsay Publications, Inc., Bradley, Illinois, (ISBN 978-0-917914-73-7). ^ Thomson, Ross (2009). Structures of Change in the Mechanical Age: Technological Invention
Invention
in the United States
United States
1790–1865. Baltimore, MD: The Johns Hopkins University Press. p. 34. ISBN 978-0-8018-9141-0.  ^ Ronalds, B.F. (2016). Sir Francis Ronalds: Father of the Electric Telegraph. London: Imperial College Press. ISBN 978-1-78326-917-4.  ^ a b "A Revolution in Timekeeping". NIST. Archived from the original on April 9, 2008. Retrieved 30 April 2008.  ^ "Pierre Curie". American Institute of Physics. Retrieved 8 April 2008.  ^ Marrison, W. A.; Horton, J. W. (February 1928). "Precision determination of frequency". I.R.E. Proc. 16 (2): 137–154. doi:10.1109/JRPROC.1928.221372.  ^ Sullivan, D.B. (2001). " Time
Time
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and Frequency
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Wristwatch, 1969". IEEE History
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Center. Retrieved 11 July 2015.  ^ Dick, Stephen (2002). Sky and Ocean Joined: The U.S. Naval Observatory, 1830–2000. Cambridge University Press. p. 484. ISBN 0-521-81599-1.  ^ Sir William Thomson (Lord Kelvin) and Peter Guthrie Tait, Treatise on Natural Philosophy, 2nd ed. (Cambridge, England: Cambridge University Press, 1879), vol. 1, part 1, page 227. ^ M.A. Lombardi; T.P. Heavner; S.R. Jefferts (2007). " NIST
NIST
Primary Frequency
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and frequency measurement at NIST: The first 100 years (PDF). 2001 IEEE International Frequency
Frequency
Control Symposium. NIST. pp. 4–17. Archived from the original (PDF) on September 27, 2011.  ^ " Time
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Time
and Frequency. New York: Courier Dover. p. 39. ISBN 0-486-40913-9.  ^ "How clocks work". InDepthInfo. W. J. Rayment. 2007. Retrieved 2008-06-04.  ^ Milham, Willis I. (1945). Time
Time
and Timekeepers. New York: MacMillan. p. 74. ISBN 0-7808-0008-7.  ^ Milham, 1945, p.85 ^ "Quality factor, Q". Glossary. Time
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Bibliography

Baillie, G.H., O. Clutton, & C.A. Ilbert. Britten’s Old Clocks and Watches and Their Makers (7th ed.). Bonanza Books (1956). Bolter, David J. Turing's Man: Western Culture in the Computer Age. The University of North Carolina Press, Chapel Hill, N.C. (1984). ISBN 0-8078-4108-0 pbk. Very good, readable summary of the role of "the clock" in its setting the direction of philosophic movement for the "Western World". Cf. picture on p. 25 showing the verge and foliot. Bolton derived the picture from Macey, p. 20. Bruton, Eric (1982). The History
History
of Clocks and Watches. New York: Crescent Books Distributed by Crown. ISBN 978-0-517-37744-4.  Dohrn-van Rossum, Gerhard (1996). History
History
of the Hour: Clocks and Modern Temporal Orders. Trans. Thomas Dunlap. Chicago: The University of Chicago Press. ISBN 0-226-15510-2.  Edey, Winthrop. French Clocks. New York: Walker & Co. (1967). Kak, Subhash, Babylonian and Indian Astronomy: Early Connections. February 17, 2003. Kumar, Narendra "Science in Ancient India" (2004). ISBN 81-261-2056-8. Landes, David S. Revolution in Time: Clocks and the Making of the Modern World. Cambridge: Harvard University Press (1983). Landes, David S. Clocks & the Wealth of Nations, Daedalus Journal, Spring 2003. Lloyd, Alan H. "Mechanical Timekeepers", A History
History
of Technology, Vol. III. Edited by Charles Joseph Singer et al. Oxford: Clarendon Press (1957), pp. 648–675. Macey, Samuel L., Clocks and the Cosmos: Time
Time
in Western Life and Thought, Archon Books, Hamden, Conn. (1980). Needham, Joseph (2000) [1965]. Science & Civilisation in China, Vol. 4, Part 2: Mechanical Engineering. Cambridge: Cambridge University Press. ISBN 0-521-05803-1.  North, John. God's Clockmaker: Richard of Wallingford
Richard of Wallingford
and the Invention
Invention
of Time. London: Hambledon and London
London
(2005). Palmer, Brooks. The Book of American Clocks, The Macmillan Co. (1979). Robinson, Tom. The Longcase Clock. Suffolk, England: Antique Collector’s Club (1981). Smith, Alan. The International Dictionary of Clocks. London: Chancellor Press (1996). Tardy. French Clocks the World Over. Part I and II. Translated with the assistance of Alexander Ballantyne. Paris: Tardy (1981). Yoder, Joella Gerstmeyer. Unrolling Time: Christiaan Huygens
Christiaan Huygens
and the Mathematization of Nature. New York: Cambridge University Press (1988). Zea, Philip, & Robert Cheney. Clock
Clock
Making in New England: 1725–1825. Old Sturbridge Village (1992).

External links

Look up clock in Wiktionary, the free dictionary.

Wikimedia Commons has media related to Clocks.

Wikisource
Wikisource
has the text of the 1911 Encyclopædia Britannica article Clock.

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& Clock
Clock
Collectors Museum

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and fate deities Wheel of time Immortality

Philosophy of time

A-series and B-series B-theory of time Causality Duration Endurantism Eternal return Eternalism Event Multiple time dimensions Perdurantism Presentism Static interpretation of time Temporal finitism Temporal parts The Unreality of Time

Human experience and use of time

Accounting period Chronemics Fiscal year Generation time Mental chronometry Music Procrastination Punctuality Temporal database Term Time
Time
discipline Time
Time
management Time
Time
perception

Specious present

Time-tracking software Time-use research Time-based currency
Time-based currency
(time banking) Time
Time
value of money Time
Time
clock Timesheet Yesterday – Today – Tomorrow

Time
Time
in

Geology

Geological time

age chron eon epoch era period

Geochronology Geological history of Earth

Physics

Absolute time and space Arrow of time Chronon Coordinate time Imaginary time Planck epoch Planck time Proper time Rate Spacetime Theory of relativity Time
Time
dilation

gravitational

Time
Time
domain Time
Time
translation symmetry Time
Time
reversal symmetry

other subject areas

Chronological dating Chronobiology Circadian rhythms Dating methodologies in archaeology Time
Time
geography

Related topics

Carpe diem Clock
Clock
position Space System time Tempus fugit Time
Time
capsule Time
Time
complexity Time
Time
signature Time
Time
travel

Time
Time
portal Category

v t e

Time
Time
measurement and standards

Chronometry Orders of magnitude Metrology

International standards

Coordinated Universal Time

offset

UT ΔT DUT1 International Earth
Earth
Rotation and Reference Systems Service ISO 31-1 ISO 8601 International Atomic Time 6-hour clock 12-hour clock 24-hour clock Barycentric Coordinate Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate Time International Date Line Leap second Solar time Terrestrial Time Time
Time
zone 180th meridian

Obsolete standards

Ephemeris time Greenwich Mean Time Prime meridian

Time
Time
in physics

Absolute time and space Spacetime Chronon Continuous signal Coordinate time Cosmological decade Discrete time and continuous time Planck time Proper time Theory of relativity Time
Time
dilation Gravitational time dilation Time
Time
domain Time
Time
translation symmetry T-symmetry

Horology

Clock Astrarium Atomic clock Complication History
History
of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch Water clock Sundial Dialing scales Equation of time History
History
of sundials Sundial
Sundial
markup schema

Calendar

Astronomical Dominical letter Epact Equinox Gregorian Hebrew Hindu Intercalation Islamic Julian Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names

Archaeology and geology

Chronological dating Geologic time scale International Commission on Stratigraphy

Astronomical
Astronomical
chronology

Galactic year Nuclear timescale Precession Sidereal time

Other units of time

Flick Shake Jiffy Second Minute Moment Hour Day Week Fortnight Month Year Olympiad Lustrum Decade Century Saeculum Millennium

Related topics

Chronology Duration

music

Mental chronometry Metric time System time Time
Time
value of money Timekeeper

Authority control

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