A clock is an instrument to measure, keep, and indicate "time. The word clock is derived (via "Dutch, Northern French, and "Medieval Latin) from the "Celtic words clagan and clocca meaning ""bell". A silent instrument missing such a "striking mechanism has traditionally been known as a timepiece. 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.
The clock is one of the oldest human "inventions, meeting the need to consistently 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. 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. 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. This object can be a "pendulum, a "tuning fork, a "quartz crystal, or the vibration of "electrons in "atoms as they emit "microwaves. 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.
The apparent position of the "Sun in the sky moves over the course of a 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. Sundials can be horizontal, vertical, or in other orientations. Sundials were widely used in "ancient times. 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 work only when the Sun shines, and never during the night, encouraged the use of other techniques for measuring and displaying 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 and/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 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. 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 and in "Egypt around the 16th century BC. Other regions of the world, including "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.
"Greek astronomer "Andronicus of Cyrrhus supervised the construction of the "Tower of the Winds in Athens in the 1st century B.C. 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 and "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 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 civilization is credited with further advancing the accuracy of clocks with elaborate engineering. In 797 (or possibly 801), the "Abbasid "caliph of "Baghdad, "Harun al-Rashid, presented "Charlemagne with an "Asian Elephant named "Abul-Abbas together with a "particularly elaborate example" of a water clock. "Pope Sylvester II introduced clocks to northern and western Europe around 1000AD
In the 13th century, "Al-Jazari, an engineer from Mesopotamia (lived 1136–1206) who worked for "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]
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The word horologia (from the Greek ὡρα, hour, and λέγειν, to tell) was used to describe early mechanical clocks, but the use of this word (still used in several "Romance languages)  for all timekeepers conceals the true nature of the mechanisms. For example, there is a record that in 1176 "Sens Cathedral installed a ‘"horologe’ 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. 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 in AD 725 by "Yi Xing and "Liang Lingzan. This is not considered an "escapement mechanism clock as it was unidirectional, the "Song dynasty "polymath and "genius "Su Song (1020–1101) incorporated it into his monumental innovation of the astronomical clock-tower of "Kaifeng in 1088.["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
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 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 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 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]
Besides the Chinese astronomical clock of Su Song in 1088 mentioned above, in Europe there were the clocks constructed by "Richard of Wallingford in "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, and modern reproductions have been made. 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 Bridge. Bells rang every hour, the number of strokes indicating the time. 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. 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.
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, although they are often erroneously credited to "Nuremberg watchmaker "Peter Henlein (or Henle, or Hele) around 1511. 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. 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, and some 15th-century clocks in "Germany indicated minutes and seconds. An early record of a seconds hand on a clock dates back to about 1560 on a clock now in the Fremersdorf collection.:417–418
During the 15th and 16th centuries, clockmaking flourished, particularly in the metalworking towns of "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. These clocks helped the 16th-century astronomer "Tycho Brahe to observe astronomical events with much greater precision than before.["citation needed]["how?]
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 (99.38 cm or 39.13 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 that the idea was taken up. 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, 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 clockmaker and others, and the second hand was first introduced.
In 1675, Huygens and "Robert Hooke invented the "spiral balance, 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. The Rev. "Edward Barlow invented the rack and snail striking mechanism for "striking clocks, which was a great improvement over the previous mechanism. 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.
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, 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 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.
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. 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 Company in 1843, it was in the "United States that this system took off. In 1816, "Eli Terry and some other Connecticut clockmakers developed a way of mass-producing clocks by using "interchangeable parts. "Aaron Lufkin Dennison started a factory in 1851 in "Massachusetts that also used interchangeable parts, and by 1861 was running a successful enterprise incorporated as the "Waltham Watch Company.
In 1815, "Francis Ronalds published the "first electric clock powered by "dry pile batteries. "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 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.
The "piezoelectric properties of crystalline "quartz were discovered by "Jacques and "Pierre Curie in 1880. 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. In 1927 the first "quartz clock was built by Warren Marrison and J. W. Horton at "Bell Telephone Laboratories in Canada. 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 (now "NIST) based the time standard of the United States on quartz clocks from late 1929 until the 1960s, when it changed to atomic clocks. In 1969, "Seiko produced the world's first quartz "wristwatch, the "Astron. Their inherent accuracy and low cost of production resulted in the subsequent proliferation of quartz clocks and watches.
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. Atomic clocks were first theorized by "Lord Kelvin in 1879. In the 1930s the development of "Magnetic resonance created practical method for doing this. A prototype "ammonia "maser device was built in 1949 at the U.S. "National Bureau of Standards (NBS, now "NIST). Although it was less accurate than existing "quartz clocks, it served to demonstrate the concept. The first accurate atomic clock, a "caesium standard based on a certain transition of the "caesium-133 atom, was built by "Louis Essen in 1955 at the "National Physical Laboratory in the UK. Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale "ephemeris time (ET). As of 2013, the most stable atomic clocks are "ytterbium clocks, which are stable to within less than two parts in 1 quintillion (×10−18). 2
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, which had the potential for more accuracy. All modern clocks use oscillation.
Although the mechanisms they use vary, all oscillating clocks, mechanical and digital and atomic, work similarly and can be divided into analogous parts. 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.
This provides power to keep the clock going.
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, or quality factor, which increases (other things being equal) with its resonant frequency. 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 timepieces sometimes include a rate screw that adjusts a "capacitor for that purpose. Atomic clocks are "primary standards, and their rate cannot be adjusted.
Some clocks rely for their accuracy on an external oscillator; that is, they are automatically "synchronized to a more accurate clock:
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, 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.
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.
This displays the count of seconds, minutes, hours, etc. in a human readable form.
Clocks can be classified by the type of time display, as well as by the method of timekeeping.
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 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 the 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).
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 savings times, 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.
Digital clock outside "Kanazawa Station displaying the time by controlling valves on a fountain
"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:
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 "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.
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 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.
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.
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 to show the digits so that they can be felt with the fingertips.
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. Of course, 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 displays usually also have conventional digits so they can be read by sighted people.
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 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.
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 of course 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.
Until advances in the late twentieth century, "navigation depended on the ability to measure "latitude and "longitude. 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 in "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.
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a device for measuring and showing time, which is usually found in or on a building and is not worn by a person
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