Clock



A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately (via Dutch, Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a 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 different physical processes have been used over the millennia, culminating in the clocks of today.

Sundials and other devices
The sundial, which measures the time of day by using the sun casting a shadow onto a cylindrical stone, was widely used in ancient times. A well-constructed sundial can measure local solar time with reasonable accuracy, and sundials continued to be used to monitor the performance of clocks until the modern era. However, its practical limitations—it requires the sun to shine and does not work at all during the night—encouraged the use of other techniques for measuring time.

Candle clocks and sticks of incense that burn down at approximately predictable speeds have also been used to estimate the passing of time. In an hourglass, fine sand pours through a tiny hole at a constant rate and indicates a predetermined passage of an arbitrary period of time.

Water clocks
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 are 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.

In the 13th century, Al-Jazari, an engineer who worked for Artuqid king of Diyar-Bakr, Nasir al-Din, made numerous clocks of all shapes and sizes. The book 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.

Early mechanical clocks
None of the first clocks survived from 13th century Europe, but various mentions in church records reveal some of the early history of the clock.

The word horologia (from the Greek ὡρα, hour, and λέγειν, to tell) was used to describe all these devices, but the use of this word (still used in several Romance languages) for all timekeepers conceals from us 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.

A new mechanism
The word clock (from the Latin word clocca, "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.

Outside of Europe, the escapement mechanism had been known and used in medieval China, as the Song Dynasty horologist and engineer Su Song (1020–1101) incorporated it into his astronomical clock-tower of Kaifeng in 1088. However, his astronomical clock and rotating armillary sphere still relied on the use of flowing water (i.e. hydraulics), while European clockworks of the following centuries shed this old habit for a more efficient driving power of weights, in addition to the escapement mechanism.

A mercury clock, described in the Libros del saber, a Spanish work from AD 1277 consisting of translations and paraphrases of Arabic works, is sometimes quoted as evidence for Muslim knowledge of a mechanical clock. The first mercury powered automata clock was invented by Ibn Khalafa al-Muradi

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.

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 interest 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. In 1292, Canterbury Cathedral installed a 'great horloge'. Over the next 30 years there are brief 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.

Early astronomical clocks
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.

Later developments
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 Peter 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 use 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 second hand on a clock dates back to about 1560 on a clock now in the Fremersdorf collection. However, this clock could not have been accurate, and the second hand was probably for indicating that the clock was working.

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 nearer 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.

A mechanical weight-driven astronomical clock with a verge-and-foliot escapement, a striking train of gears, an alarm, and a representation of the moon's phases was described by the Ottoman engineer Taqi al-Din in his book, The Brightest Stars for the Construction of Mechanical Clocks (Al-Kawākib al-durriyya fī wadh' al-bankāmat al-dawriyya), published in 1556-1559. Similarly to earlier 15th-century European alarm clocks, it was capable of sounding at a specified time, achieved by placing a peg on the dial wheel. At the requested time, the peg activated a ringing device. The clock had three dials which indicated hours, degrees and minutes. He later made an observational clock for the Istanbul observatory of Taqi al-Din (1577–1580), describing it as "a mechanical clock with three dials which show the hours, the minutes, and the seconds." This was an important innovation in 16th-century practical astronomy, as at the start of the century clocks were not accurate enough to be used for astronomical purposes.



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. In 1670, the English clockmaker William Clement created the anchor escapement, an improvement over Huygens' crown escapement. Within just one generation, minute hands and then second hands were added.

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. Many European governments offered a large prize for anyone who could determine longitude accurately; for example, Great Britain offered 20,000 pounds, equivalent to millions of dollars today. The reward was eventually claimed in 1761 by John Harrison, who dedicated his life to improving the accuracy of his clocks. His H5 clock was in error by less than 5 seconds over 10 weeks.

The excitement over the pendulum clock had attracted the attention of designers, resulting in a proliferation of clock forms. Notably, the longcase clock (also known as the grandfather clock) was created to house the pendulum and works. The English clockmaker William Clement is also credited with developing this form 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.



On November 17, 1797, Eli Terry received his first patent for a clock. Terry is known as the founder of the American clock-making industry.

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 electro-magnet and armature. In 1841, he first patented the electromagnetic pendulum.

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 vibration of a tuning fork, the behaviour of quartz crystals, or the quantum vibrations of atoms. Even mechanical clocks have since come to be largely powered by batteries, removing the need for winding.

How clocks work
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 repetitive oscillatory processes, like the swing of a pendulum or the vibration of a quartz crystal, which were more accurate. All modern clocks use oscillation.

Although the methods 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 added up in a chain of some type of counters to express the time in convenient units, usually seconds, minutes, hours, etc. Then finally some kind of indicator displays the result in a human-readable form.

Power source
This provides power to keep the clock going. Since clocks must run continuously, there is often a small secondary power source to keep the clock going temporarily during interruptions in the main power. In old mechanical clocks, a maintaining power spring kept the clock turning while the mainspring was being wound. In quartz 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.
 * In mechanical clocks, this is either a weight suspended from a cord wrapped around a pulley, or a spiral spring called a mainspring.
 * In electric clocks, it is either a battery or the AC power line.

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. 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.
 * 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.

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. 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 don't have an internal oscillator, but rely on the 50 or 60 Hz oscillation of the AC power line, which is synchronized by the utility to a precision oscillator. This drives a synchronous motor in the clock which rotates once for every cycle of the line voltage, and drives the gear train.
 * Computer real time clocks keep time with a quartz crystal, but are periodically (usually weekly) synchronized over the internet to atomic clocks (UTC), using a system called Network Time Protocol.
 * Radio clocks keep time with a quartz crystal, but are periodically (often daily) synchronized to atomic clocks (UTC) with time signals from government radio stations like WWV, WWVB, CHU, DCF77 and the GPS system.

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, 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.
 * 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 exact 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.

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 by a knob on the back to set the time.
 * 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
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, including almost all mechanical and some electronic clocks, have a traditional dial or clock face, that displays the time in analog form with moving hour and minute hand. In quartz clocks with analog faces, a 1 Hz signal from the counters actuates a stepper motor which moves the second hand forward at each pulse, and the minute and hour hands are moved by gears from the shaft of the second hand.
 * Digital clocks display the time in periodically changing digits on a digital display.
 * 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.

Analog clocks


Analog clocks usually indicate time using angles. The most common clock face uses a fixed numbered dial or dials and moving hand or hands. It usually has a circular scale of 12 hours, which can also serve as a scale of 60 minutes, and 60 seconds if the clock has a second hand. Many other styles and designs have been used throughout the years, including dials divided into 6, 8, 10, and 24 hours. 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. The 10-hour clock was briefly popular during the French Revolution, when the metric system was applied to time measurement, and 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. 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 clocks


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


 * the 24-hour notation with hours ranging 00–23;
 * the 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 the United States and Canada).

Most digital clocks use an LCD, LED, or VFD display; many other display technologies are used as well (cathode ray tubes, nixie tubes, etc.). After a reset, battery change or power failure, digital clocks without a backup battery or capacitor either start counting from 12:00, or stay at 12:00, often with blinking digits indicating that time needs to be set. Some newer clocks will actually 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 analogue clocks has declined significantly.

Auditory clocks
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.

Purposes
Clocks are in homes, offices and many other places; smaller ones (watches) are carried on the wrist; larger ones are in public places, e.g. a train station or church. A small clock is often shown in a corner of computer displays, mobile phones and many MP3 players.

The purpose of a clock is not always to display the time. It may also be used to control a device according to time, e.g. an alarm clock, a VCR, or a time bomb (see: counter). However, in this context, it is more appropriate to refer to it as a timer or trigger mechanism rather than strictly as a clock.

Computers depend on an accurate internal clock signal to allow synchronized processing. (A few research projects are developing CPUs based on asynchronous circuits.) Some computers also maintain time and date for all manner of operations whether these be for alarms, event initiation, or just to display the time of day. The internal computer clock is generally kept running by a small battery. Many computers will still function even if the internal clock battery is dead, but the computer clock will need to be reset each time the computer is restarted, since once power is lost, time is also lost.

Ideal clocks
An ideal clock is a scientific principle that measures the ratio of the duration of natural processes, and thus will give the time measure for use in physical theories. Therefore, to define an ideal clock in terms of any physical theory would be circular. An ideal clock is more appropriately defined in relationship to the set of all physical processes.

This leads to the following definitions:
 * A clock is a recurrent process and a counter.
 * A good clock is one which, when used to measure other recurrent processes, finds many of them to be periodic.
 * An ideal clock is a clock (i.e., recurrent process) that makes the most other recurrent processes periodic.

The recurrent, periodic process (e.g. a metronome) is an oscillator and typically generates a clock signal. Sometimes that signal alone is (confusingly) called "the clock", but sometimes "the clock" includes the counter, its indicator, and everything else supporting it.

This definition can be further improved by the consideration of successive levels of smaller and smaller error tolerances. While not all physical processes can be surveyed, the definition should be based on the set of physical processes which includes all individual physical processes which are proposed for consideration. Since atoms are so numerous and since, within current measurement tolerances they all beat in a manner such that if one is chosen as periodic then the others are all deemed to be periodic also, it follows that atomic clocks represent ideal clocks to within present measurement tolerances and in relation to all presently known physical processes. However, they are not so designated by fiat. Rather, they are designated as the current ideal clock because they are currently the best instantiation of the definition.

Navigation
Navigation by ships and planes depends on the ability to measure latitude and longitude. Latitude is fairly easy to determine through celestial navigation, but the measurement of longitude requires accurate measurement 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.

Use of an atomic clock in radio signal producing satellites is fundamental to the operation of GPS (Global Positioning System) navigation devices.

Seismology
In determining the location of an earthquake, the arrival time of several types of seismic wave at a minimum of four dispersed observers is dependent upon each observer recording wave arrival times according to a common clock.

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