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A clock or a timepiece is a device used to
measure
measure
and indicate
time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...

time
. The clock is one of the oldest
human inventions
human inventions
, meeting the need to measure intervals of time shorter than the natural units: the
day The word day has a number of meanings, depending on the context it is used such as of astronomy, physics, and various calendar systems. As a term in physics and astronomy it is approximately the period during which the Earth completes one ro ...

day
, the
lunar month In lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's lunar phase, phases (Lunar month#Synodic month, synodic months, lunations), in contrast to solar calendars, whose annual cycles are based only directly o ...
,
year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the ...
and
galactic year The galactic year, also known as a cosmic year, is the duration of time required for the Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own g ...
. Devices operating on several physical processes have been used over the
millennia A millennium (plural millennia or millenniums) is a period of one thousand year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, the ...

millennia
. Some predecessors to the modern clock may be considered as "clocks" that are based on movement in nature: A
sundial A sundial is a horological device that tells the time of day (in modern usage referred to as civil time In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. M ...

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 An hourglass (or sandglass, sand timer, sand clock or egg timer) is a device used to measure the passage of time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irre ...

hourglass
.
Water clock A water clock or clepsydra (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximat ...
s, along with the sundials, are possibly the oldest time-measuring instruments. A major advance occurred with the invention of the
verge escapement The verge (or crown wheel) escapement is the earliest known type of mechanical escapement An escapement is a mechanical linkage in mechanical watches and clocks that gives impulses to the timekeeping element and periodically releases the gear ...
, which made possible the first mechanical clocks around 1300 in Europe, which kept time with oscillating timekeepers like
balance wheel A balance wheel, or balance, is the timekeeping device used in mechanical watch A mechanical watch is a watch that uses a Movement (clockwork), clockwork mechanism to measure the passage of time, as opposed to quartz watches which function electron ...
s., pp. 103–104, p. 31 Traditionally, in
horology Horology ("the study of time", related to Latin ''horologium'' from Greek language, Greek , "instrument for telling the hour", from ''hṓra'' "hour; time" and -o- interfix and suffix ''-logy''), . is the study of the measurement of time. Clo ...
, the term ''clock'' was used for a
striking clock A striking clock is a clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natur ...
, while a clock that did not strike the hours audibly was called a timepiece. This distinction is no longer made.
Watch A watch is a portable Clock, timepiece intended to be carried or worn by a person. It is designed to keep a consistent movement despite the motions caused by the person's activities. A wristwatch is designed to be worn around the wrist, attach ...

Watch
es and other timepieces that can be carried on one's person are usually not referred to as clocks. Spring-driven clocks appeared during the 15th century. During the 15th and 16th centuries,
clockmaking of early modern clockmakers, 1568 A clockmaker is an artisan who makes and/or repairs clocks. Since almost all clocks are now factory-made, most modern clockmakers only repair clocks. Modern clockmakers may be employed by jewellers, antique ...
flourished. The next development in accuracy occurred after 1656 with the invention of the
pendulum clock A pendulum clock is a clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the nat ...
by
Christiaan Huygens Christiaan Huygens ( , also , ; la, Hugenius; 14 April 1629 – 8 July 1695), also spelled Huyghens, was a Dutch mathematician, physicist, astronomer and inventor, who is regarded as one of the greatest scientists of all time and a major fig ...

Christiaan Huygens
. A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The mechanism of a timepiece with a series of gears driven by a spring or weights is referred to as
clockwork Clockwork refers to the inner workings of either mechanical machines called clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invention, human inventions, meeting th ...

clockwork
; the term is used by extension for a similar mechanism not used in a timepiece. The
electric clock An electric clock is a clock A clock is a device used to measure, verify, 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, t ...
was patented in 1840, and electronic clocks were introduced in the 20th century, becoming widespread with the development of small battery-powered
semiconductor device A device is an that relies on the properties of a material (primarily , , and , as well as s) for its function. Semiconductor devices have replaced s in most applications. They in the , rather than as free electrons across a (typically l ...
s. The timekeeping element in every modern clock is a
harmonic oscillator In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium List of types of equilibrium, the condition of a system in which all competing influences are balanced, in a wide variety of contexts. Equilib ...

harmonic oscillator
, a physical object (
resonator A resonator is a device or system that exhibits resonance Resonance describes the phenomenon of increased amplitude The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Pe ...
) that vibrates or oscillates at a particular frequency. This object can be a
pendulum A pendulum is a weight suspended from a pivot Pivot may refer to: *Pivot, the point of rotation in a lever A lever ( or ) is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or '':wikt:fulcrum, fulcrum''. A lever ...

pendulum
, a
tuning fork A tuning fork is an acoustic Acoustic may refer to: Music Albums * Acoustic (Bayside EP), ''Acoustic'' (Bayside EP) * Acoustic (Britt Nicole EP), ''Acoustic'' (Britt Nicole EP) * Acoustic (Joey Cape and Tony Sly album), ''Acoustic'' (Joey Cap ...

tuning fork
, a
quartz crystal Quartz is a hard, crystalline mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), ro ...

quartz crystal
, or the vibration of
electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

electron
s in
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

atom
s as they emit
microwave Microwave is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies mat ...

microwave
s. Clocks have different ways of displaying the time. Analog clocks indicate time with a traditional
clock face A clock face is the part of an analog clock Analog or analogue may refer to: Computing and electronics * Analog signal An analog signal is any continuous signal for which the time-varying feature of the signal is a representation of some ...
, with moving hands. Digital clocks display a numeric representation of time. Two numbering systems are in use:
24-hour time The 24-hour clock, popularly referred to in the United States and some other countries as military time, is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hour ...
notation and 12-hour notation. Most digital clocks use electronic mechanisms and
LCD A liquid-crystal display (LCD) is a flat-panel display A flat-panel display (FPD) is an electronic display device s, LED display and Vacuum fluorescent display, VF display, top to bottom. A display device is an output device for presentation ...
,
LED An LED A light-emitting diode (LED) is a semiconductor A semiconductor material has an Electrical resistivity and conductivity, electrical conductivity value falling between that of a Electrical conductor, conductor, such as metallic ...
, or VFD displays. For the blind and for use over telephones,
speaking clock A speaking clock or talking clock is a live or recorded human voice service, usually accessed by telephone, that gives the correct time. The first telephone speaking clock service was introduced in France, in association with the Paris Observato ...
s state the time audibly in words. There are also clocks for the blind that have displays that can be read by touch. The study of timekeeping is known as
horology Horology ("the study of time", related to Latin ''horologium'' from Greek language, Greek , "instrument for telling the hour", from ''hṓra'' "hour; time" and -o- interfix and suffix ''-logy''), . is the study of the measurement of time. Clo ...
.


Etymology

The word ''clock'' derives from the medieval
Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in relation with") is "an appa ...

Latin
word for 'bell'——and has
cognate In linguistics Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed language, sign language) and writing. Most langu ...
s in many European languages. Clocks spread to England from the
Low Countries The term Low Countries, also known as the Low Lands ( nl, de Lage Landen, french: les Pays-Bas) and historically called the Netherlands ( nl, de Nederlanden), Flanders, or Belgica, refers to a coastal lowland region in Northwestern Europe ...
, so the English word came from the Middle Low German and Middle Dutch . The word derives from the
Middle English Middle English (abbreviated to ME) was a form of the English language spoken after the Norman conquest of England, Norman conquest (1066) until the late 15th century. The English language underwent distinct variations and developments following ...
, Old North French , or
Middle Dutch Middle Dutch is a collective name for a number of closely related West Germanic languages, West Germanic dialects whose ancestor was Old Dutch and was spoken and written between 1150 and 1500. The language served as a minority language in the Holy ...
, all of which mean 'bell', and stem from an
Old Irish Old Irish (''Goídelc''; ga, Sean-Ghaeilge; gd, Seann Ghàidhlig; gv, Shenn Yernish or ; Old Irish: ᚌᚑᚔᚇᚓᚂᚉ), sometimes called Old Gaelic, is the oldest form of the Goidelic The Goidelic or Gaelic languages ( ga, teangacha ...
root.


History of time-measuring devices


Sundials

The apparent position of the 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. Sundials can be horizontal, vertical, or in other orientations. Sundials were widely used in
ancient times Ancient history is the aggregate of past eventsWordNet Search – 3.0
"History"
from t ...

ancient times
. With the knowledge of latitude, a well-constructed sundial can measure local
solar time Solar time is a calculation of the passage of time Time is the continued of and that occurs in an apparently succession from the , through the , into the . It is a component quantity of various s used to events, to compare the dura ...
with reasonable accuracy, within a minute or two. Sundials continued to be used to monitor the performance of clocks until the 1830s, with the use of the telegraph and train to standardize time and time zones between cities.


Devices that measure duration, elapsed time and intervals

Many devices can be used to mark the passage of time without respect to reference time (time of day, hours, minutes, etc.) and can be useful for measuring duration or intervals. Examples of such duration timers are
candle clock A candle clock is a thin candle with consistently spaced marking that when burned, indicate the passage of periods of time. While no longer used today, candle clocks provided an effective way to tell time indoors, at night, or on a cloudy day. A ca ...
s,
incense clock The incense clock () is an India India (Hindi: ), officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by population, second-most populous country, the List of countries and d ...
s 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 Sand is a granular material composed of finely divided rock (geology), rock and mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer ...

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

Water clocks, along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the day counting
tally stick A tally stick (or simply tally) was an ancient memory aid device used to record and document numbers, quantities, or even messages. Tally sticks first appear as animal bones carved with notches during the Upper Palaeolithic The Upper Paleol ...
. 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 was the capital city of the ancient Babylonian empire, which itself is a term referring to either of two separate empires in the Mesopotamian area in antiquity. These two empires achieved regional dominance between the 19th and 15th centu ...

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#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
astronomer Andronicus of Cyrrhus supervised the construction of the
Tower of the Winds The Tower of the Winds or the Horologion of Andronikos Kyrrhestes is an octagonal Pentelic marble clocktower in the Roman Agora in Athens that functioned as a ''horology, horologion'' or "timepiece". It is considered the world's first meteorologi ...

Tower of the Winds
in Athens in the 1st century B.C. The
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
and
Roman Roman or Romans most often refers to: *, the capital city of Italy *, Roman civilization from 8th century BC to 5th century AD *, the people of ancient Rome *', shortened to ''Romans'', a letter in the New Testament of the Christian Bible Roman ...
civilizations advanced water clock design with improved accuracy. These advances were passed on through
Byzantine The Byzantine Empire, also referred to as the Eastern Roman Empire, or Byzantium, was the continuation of the Roman Empire in its eastern provinces during Late Antiquity and the Middle Ages, when its capital city was Constantinople. It survi ...

Byzantine
and
Islamic Islam (; ar, اَلْإِسْلَامُ, al-’Islām, "submission o God Oh God may refer to: * An exclamation; similar to "oh no", "oh yes", "oh my", "aw goodness", "ah gosh", "ah gawd"; see interjection ''Oh, God!'' franchise * ''Oh, ...
times, eventually making their way back to Europe. Independently
the Chinese developed their own advanced water clocks
水鐘)in 725 AD, 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 Human history, also known as world history, is the description of humanity's past. It is informed by archaeology Archaeology or archeology is the study of human activity through the recovery and analysis of material culture. Archaeol ...
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 Astrology is a pseudoscience Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method The scientific method is an Empirical evidenc ...
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 A pendulum clock is a clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the nat ...
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 The Abbasid Caliphate ( or ar, اَلْخِلَافَةُ ٱلْعَبَّاسِيَّةُ, ') was the third caliphate A caliphate ( ar, خِلَافَة, ) is an Islamic state under the leadership of an Islam Islam (;There ar ...
caliph A caliphate ( ar, خِلَافَة, ) is an Islamic state {{Infobox war faction , name = Islamic State , anthem = '' Dawlat al-Islam Qamat'' {{small, ("My Ummah ' ( ar, أمة ) is an Arabic Arabic (, ' ...
of
Baghdad Baghdad (; ar, بَغْدَاد ) is the capital of and one of the in the , and compared to its large population it has a small area at just 673 square kilometers (260 sq mi). Located along the , near the ruins of the city of and the anc ...

Baghdad
,
Harun al-Rashid Harun al-Rashid (; ar, هَارُون الرَشِيد ''Hārūn Ar-Rašīd'', "Aaron the Just" or "Aaron the Rightly-Guided"; 17 March 763 or February 766 – 24 March 809 Common Era, CE / 148–193 Hijri year, AH) was the fifth Abbasid C ...
, presented
Charlemagne Charlemagne ( , ) or Charles the Great ( la, Carolus Magnus; 2 April 748 – 28 January 814) was King of the Franks The Franks—Germanic-speaking peoples that invaded the Western Roman Empire in the 5th century—were first led by i ...

Charlemagne
with an named
Abul-Abbas Abul-Abbas was an Asian elephant brought back to Carolingian emperor Charlemagne by his diplomat Isaac the Jew. The gift was from the Abbasid caliph Harun al-Rashid and symbolizes the beginning of Abbasid–Carolingian relations. The elephant ...
together with a "particularly elaborate example" of a water clock.
Pope Sylvester II Pope Sylvester II ( – 12 May 1003), originally known as Gerbert of Aurillac, was a French-born scholar and teacher who served as the bishop of Rome and ruled the Papal States from 999 to his death. He endorsed and promoted study of Arab and Grec ...

Pope Sylvester II
introduced clocks to northern and western Europe around 1000 AD.


Mechanical water clocks

The first known
gear A gear is a rotating A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation plane'', and the imaginary li ...

gear
ed clock was invented by the great mathematician, physicist, and engineer
Archimedes Archimedes of Syracuse (; grc, ; ; ) was a Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its popula ...

Archimedes
during the 3rd century BC. Archimedes created his astronomical clock that was also a cuckoo clock with birds singing and moving every hour. It is the first carillon clock as it plays music and simultaneously with a person blinking his eyes surprised by the singing birds. Archimedes clock works with a system of four weights, counter weights, and strings regulated by a system of floats in a water container with siphons that regulate the automatic continuation of the clock. The principles of this type of clocks are described by the mathematician and physicist Hero, who says that some of them work with a chain that turns a gear of the mechanism. Another Greek clock probably constructed at the time of Alexander was in Gaza, described by Procopius. The Gaza clock was probably a Meteoroskopeion, i.e. a building showing the celestial phenomena and the time. It had pointer for the time and some automations similar to the Archimedes clock. There were 12 doors opening one every hour with Hercules performing his labors, the Lion at one o'clock, etc, and at night a lamp becomes visible every hour, with 12 windows opening to show the time. Another
gear A gear is a rotating A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation plane'', and the imaginary li ...

gear
ed clock was developed in the 11th century by the Arab engineer
Ibn Khalaf al-Muradi Ibn Khalaf al-Murādī, (; 11th century) was an Andalusian engineer. Al-Murādī was the author of the technological manuscript entitled ''Kitāb al-asrār fī natā'ij al-afkār'' ('', The Book of Secrets in the Results of Thoughts'' or ''The B ...
in ; it was a water clock that employed a complex
gear train A gear train is a mechanical system A machine is any physical system with ordered structural and functional properties. It may represent human-made or naturally occurring device molecular machine that uses Power (physics), power to apply Forc ...

gear train
mechanism, including both segmental and
epicyclic gearing An epicyclic gear train (also known as a planetary gearset) consists of two gear Cast iron mortise wheel with wooden cogs (powered by an external water wheel) meshing with a cast iron gear wheel, connected to a pulley with drive belt. ...
, capable of transmitting high
torque In physics and mechanics, torque is the rotational equivalent of linear force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the na ...

torque
. The clock was unrivalled in its use of sophisticated complex gearing, until the mechanical clocks of the mid-14th century. Al-Muradi's clock also employed the use of mercury in its hydraulic linkages, which could function mechanical
automata automaton. An automaton (; plural: automata or automatons) is a relatively self-operating machine, or a machine or control mechanism designed to automatically follow a predetermined sequence of operations, or respond to predetermined instructi ...

automata
. Al-Muradi's work was known to scholars working under
Alfonso X of Castile Alfonso X (also known as the Wise, es, el Sabio; 23 November 1221 4 April 1284) was the king of Castile, León and Galicia from 30 May 1252 until his death in 1284. During the election of 1257, a dissident faction chose him to be king of Ge ...

Alfonso X of Castile
, hence the mechanism may have played a role in the development of the European mechanical clocks. Other monumental water clocks constructed by medieval Muslim engineers also employed complex gear trains and arrays of
automata automaton. An automaton (; plural: automata or automatons) is a relatively self-operating machine, or a machine or control mechanism designed to automatically follow a predetermined sequence of operations, or respond to predetermined instructi ...

automata
. Arab engineers at the time also developed a liquid-driven
escapement An escapement is a mechanical linkage in mechanical watches and clocks that gives impulses to the timekeeping element and periodically releases the gear train to move forward, advancing the clock's hands. The impulse action transfers energy to ...

escapement
mechanism which they employed in some of their water clocks. Heavy floats were used as weights and a constant-head system was used as an escapement mechanism, Hassan, Ahmad Y
Transfer Of Islamic Technology To The West, Part II: Transmission Of Islamic Engineering
''History of Science and Technology in Islam''
which was present in the hydraulic controls they used to make heavy floats descend at a slow and steady rate.
Donald Routledge Hill Donald Routledge Hill (6 August 1922 – 30 May 1994)D. A. King, “In Memoriam: Donald Routledge Hill (1922-1994)”, ''Arabic Sciences and Philosophy,'' Volume 5 / Issue 02 / September 1995, pp 297-302 was a British engineer Engineers, as prac ...
(1996), "Engineering", p. 794, in Rashed & Morelon (1996) pp. 751–95
A water-powered cogwheel clock was created in
China China (), officially the People's Republic of China (PRC; ), is a country in . It is the world's , with a of more than 1.4 billion. China spans five geographical and 14 different countries, the in the world after . Covering an area of ap ...
by
Yi Xing Yi Xing (, 683–727), born Zhang Sui (), was a Chinese astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects ...
and
Liang LingzanLiang Lingzan () was a Chinese artist, astronomer, inventor, mechanical engineer and politician of the Kaiyuan era during the Tang Dynasty The Tang dynasty (, ; ), or Tang Empire, was an imperial dynasty of China that ruled from 618 to 907, ...
. This is not considered an
escapement An escapement is a mechanical linkage in mechanical watches and clocks that gives impulses to the timekeeping element and periodically releases the gear train to move forward, advancing the clock's hands. The impulse action transfers energy to ...

escapement
mechanism clock as it was unidirectional, the
Song dynasty The Song dynasty (; ; 960–1279) was an imperial dynasty of China that began in 960 and lasted until 1279. The dynasty was founded by Emperor Taizu of Song Emperor Taizu of Song (21 March 927 – 14 November 976), personal name Zhao Kua ...
polymath A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a substantial number of subjects, known to draw on complex bodies of knowledge to solve specific prob ...

polymath
and genius
Su Song Su Song (; courtesy name A courtesy name (), also known as a style name, is a name bestowed upon one at adulthood in addition to one's given name. This practice is a tradition in the East Asian cultural sphere, including China, Japan, K ...
(1020–1101) incorporated it into his monumental innovation of the astronomical clock-tower of
Kaifeng Kaifeng () is a prefecture-level city in east-central Henan province, China. It is one of the Historical capitals of China, Eight Ancient Capitals of China, having been the capital seven times in history, and is best known for being the Chinese ...

Kaifeng
in 1088. His astronomical clock and rotating
armillary sphere An armillary sphere (variations are known as spherical astrolabe An astrolabe ( grc, ἀστρολάβος ; ar, ٱلأَسْطُرلاب ; persian, ستاره‌یاب ) is an ancient astronomical instrument that was a handheld model of the ...

armillary sphere
still relied on the use of either flowing water during the spring, summer, autumn seasons and during the freezing temperature of winter (i.e.
hydraulics Hydraulics (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ap ...
). 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 Ibn Khalaf al-Murādī, (; 11th century) was an Andalusian engineer. Al-Murādī was the author of the technological manuscript entitled ''Kitāb al-asrār fī natā'ij al-afkār'' ('', The Book of Secrets in the Results of Thoughts'' or ''The B ...
. In the 13th century,
Al-Jazari Badīʿ az-Zaman Abu l-ʿIzz ibn Ismāʿīl ibn ar-Razāz al-Jazarī (1136–1206, ar, بديع الزمان أَبُ اَلْعِزِ إبْنُ إسْماعِيلِ إبْنُ الرِّزاز الجزري,), ) was a Muslim polymath ...
, an engineer from Mesopotamia (lived 1136–1206) who worked for
Artuqid The Artuqids or Artuqid dynasty ( or ', or ', or ', sometimes also spelled as Artukid, Ortoqid or Ortokid; Turkish plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the gra ...
king of Diyar-Bakr,
Nasir al-Din Nasir al-Din ( ar, نصیر الدین or or , 'Defender of the Faith'), was originally a honorific title and is an Arabic masculine given name. There are many variant spellings in English. Notable people with the title or name include: Politic ...
, 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.


Fully mechanical

Clock that changed the world (H4, 1759) - Flickr - Tatters ❀.jpg, One mechanical clock
(was useful for sailing purposes) Mechanical digital clock.jpg, Mechanical digital clock
(with rolling numbers)
The word (from the Greek —'hour', and —'to tell') was used to describe early mechanical clocks, but the use of this word (still used in several
Romance languages The Romance languages, less commonly Latin or Neo-Latin languages, are the modern languages that evolved from Vulgar Latin Vulgar Latin, also known as Popular or Colloquial Latin is a range of informal sociolects of Latin Latin (, or , ) ...

Romance languages
) for all timekeepers conceals the true nature of the mechanisms. For example, there is a record that in 1176
Sens Cathedral Sens Cathedral (french: Cathédrale Saint-Étienne de Sens) is a Catholic The Catholic Church, often referred to as the Roman Catholic Church, is the List of Christian denominations by number of members, largest Christian church, with appro ...
installed an ' 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 Bury St Edmunds (), commonly referred to locally as Bury, is a historic market Market may refer to: *Market (economics) *Market economy *Marketplace, a physical marketplace or public market Geography *Märket, an island shared by Finland an ...
), 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'' (via Medieval Latin from
Old Irish Old Irish (''Goídelc''; ga, Sean-Ghaeilge; gd, Seann Ghàidhlig; gv, Shenn Yernish or ; Old Irish: ᚌᚑᚔᚇᚓᚂᚉ), sometimes called Old Gaelic, is the oldest form of the Goidelic The Goidelic or Gaelic languages ( ga, teangacha ...
, 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. In Europe, between 1280 and 1320, there was 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 Hour#Counting from sunset, 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 mentions of clocks at a number of ecclesiastical institutions in England, Italy, and France. In 1322, a Norwich cathedral astronomical clock, 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.


Astronomical

Besides the Chinese astronomical clock of Su Song in 1088 mentioned above, contemporary Islamic astronomy, Muslim astronomers also constructed a variety of highly accurate astronomical clocks for use in their mosques and Observatory, observatories, such as the water-powered astronomical clock by
Al-Jazari Badīʿ az-Zaman Abu l-ʿIzz ibn Ismāʿīl ibn ar-Razāz al-Jazarī (1136–1206, ar, بديع الزمان أَبُ اَلْعِزِ إبْنُ إسْماعِيلِ إبْنُ الرِّزاز الجزري,), ) was a Muslim polymath ...
in 1206, and the astrolabic clock by Ibn al-Shatir in the early 14th century. The most sophisticated timekeeping astrolabes were the
gear A gear is a rotating A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation plane'', and the imaginary li ...

gear
ed astrolabe mechanisms designed by Abū Rayhān Bīrūnī in the 11th century and by Muhammad ibn Abi Bakr in the 13th century. These devices functioned as timekeeping devices and also as calendars. A sophisticated water-powered astronomical clock was built by
Al-Jazari Badīʿ az-Zaman Abu l-ʿIzz ibn Ismāʿīl ibn ar-Razāz al-Jazarī (1136–1206, ar, بديع الزمان أَبُ اَلْعِزِ إبْنُ إسْماعِيلِ إبْنُ الرِّزاز الجزري,), ) was a Muslim polymath ...
in 1206. This castle clock was a complex device that was about high, and had multiple functions alongside timekeeping. It included a display of the zodiac and the solar and lunar paths, and a pointer in the shape of the Lunar phase, crescent moon which travelled across the top of a gateway, moved by a hidden cart and causing doors to open, each revealing a mannequin, every hour.Donald Routledge Hill, Routledge Hill, Donald, "Mechanical Engineering in the Medieval Near East", ''Scientific American'', May 1991, pp. 64–9 (cf.
Donald Routledge Hill Donald Routledge Hill (6 August 1922 – 30 May 1994)D. A. King, “In Memoriam: Donald Routledge Hill (1922-1994)”, ''Arabic Sciences and Philosophy,'' Volume 5 / Issue 02 / September 1995, pp 297-302 was a British engineer Engineers, as prac ...

Mechanical Engineering
It was possible to reset the length of day and night in order to account for the changing lengths of day and night throughout the year. This clock also featured a number of
automata automaton. An automaton (; plural: automata or automatons) is a relatively self-operating machine, or a machine or control mechanism designed to automatically follow a predetermined sequence of operations, or respond to predetermined instructi ...

automata
including falcons and musicians who automatically played music when moved by levers operated by a hidden camshaft attached to a water wheel. In Europe, there were the clocks constructed by Richard of Wallingford in St Albans by 1336, and by Giovanni Giovanni Dondi dell'Orologio, de Dondi in Padua, Italy, Padua from 1348 to 1364. They no longer exist, but detailed descriptions of their design and construction survive,North, John. God's Clockmaker: Richard of Wallingford and the Invention of Time. London: Hambledon and London (2005).King, Henry "Geared to the Stars: the evolution of planetariums, orreries, and astronomical clocks", University of Toronto Press, 1978 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 Rota Fortunae, 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 Moveable feast, 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.


Spring-driven

Matthew Norman carriage clock with winding key.jpg, Matthew Norman carriage clock with winding key 1908 Gilbert mantel clock decorated with Memento Mori decoupage.JPG, Decorated William Gilbert mantel clock 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 (clockwork), movement running at a constant rate as the spring ran down. This resulted in the invention of the ''stackfreed'' and the fusee (horology), fusee in the 15th century, and many other innovations, down to the invention of the modern ''going barrel (horology), 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,p. 529, "Time and timekeeping instruments", ''History of astronomy: an encyclopedia'', John Lankford, Taylor & Francis, 1997, . 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. 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 escapement#Cross-beat escapement, 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.


Pendulum

The next development in accuracy occurred after 1656 with the invention of the
pendulum clock A pendulum clock is a clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the nat ...
. Galileo Galilei, 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 Christiaan Huygens ( , also , ; la, Hugenius; 14 April 1629 – 8 July 1695), also spelled Huyghens, was a Dutch mathematician, physicist, astronomer and inventor, who is regarded as one of the greatest scientists of all time and a major fig ...

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 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 face A clock face is the part of an analog clock Analog or analogue may refer to: Computing and electronics * Analog signal An analog signal is any continuous signal for which the time-varying feature of the signal is a representation of some ...
s to use Vitreous enamel, 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.


Hairspring

In 1675, Huygens and Robert Hooke invented the spiral balance, spiral balance spring, or the hairspring, designed to control the oscillating speed of the
balance wheel A balance wheel, or balance, is the timekeeping device used in mechanical watch A mechanical watch is a watch that uses a Movement (clockwork), clockwork mechanism to measure the passage of time, as opposed to quartz watches which function electron ...
. 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 rack and snail striking mechanism for
striking clock A striking clock is a clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natur ...
s, 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 (priest), Edward Barlow invented ''rack and snail'' striking. In fact, his invention was connected with a repeating mechanism employing the rack and snail. The repeater (horology), repeating clock, that chimes the number of hours (or even minutes) on demand was invented by either Quare or Barlow in 1676. George Graham (clockmaker), George Graham invented the Escapement#Deadbeat escapement, deadbeat escapement for clocks in 1720.


Marine chronometer

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 longitude prize, 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 marine chronometer, 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.


Mass production

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.. Reprinted by McGraw-Hill, New York and London, 1926 (); and by Lindsay Publications, Inc., Bradley, Illinois, ().


Early electric

In 1815, Francis Ronalds published the Electric clock#History, first electric clock powered by Voltaic pile#Dry pile, dry pile batteries. Alexander Bain (inventor), Alexander Bain, Scottish clockmaker, patented the
electric clock An electric clock is a clock A clock is a device used to measure, verify, 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, t ...
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 Electromagnetism, 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. Where an Alternating current, AC electrical supply of stable frequency is available, timekeeping can be maintained very reliably by using a synchronous motor, essentially counting the cycles. The supply current alternates with an accurate frequency of 50 hertz in many countries, and 60 hertz in others. While the frequency may vary slightly during the day as the load changes, generators are designed to maintain an accurate number of cycles over a day, so the clock may be a fraction of a second slow or fast at any time, but will be perfectly accurate over a long time. The Rotor (electric), 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. Time in these cases is measured in several ways, such as by counting the cycles of the AC supply, vibration of a
tuning fork A tuning fork is an acoustic Acoustic may refer to: Music Albums * Acoustic (Bayside EP), ''Acoustic'' (Bayside EP) * Acoustic (Britt Nicole EP), ''Acoustic'' (Britt Nicole EP) * Acoustic (Joey Cape and Tony Sly album), ''Acoustic'' (Joey Cap ...

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.


Quartz

The piezoelectric properties of crystalline quartz were discovered by Jacques Curie, 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 Guyton Cady, 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 at the time, 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 Watch, wristwatch, the Astron (wristwatch), Astron. Their inherent accuracy and low cost of production resulted in the subsequent proliferation of quartz clocks and watches.


Atomic

Currently, 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 trillions of years. Atomic clocks were first theorized by Lord Kelvin in 1879. In the 1930s the development of Nuclear magnetic resonance, 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 National Institute of Standards and Technology, 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, UK, 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 ().


Operation

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 Quartz is a hard, crystalline mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), ro ...

quartz crystal
, 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. 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


Oscillator

The timekeeping element in every modern clock is a
harmonic oscillator In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium List of types of equilibrium, the condition of a system in which all competing influences are balanced, in a wide variety of contexts. Equilib ...

harmonic oscillator
, a physical object (resonator) that vibrates or oscillates repetitively at a precisely constant frequency. * In mechanical clocks, this is either a pendulum or a
balance wheel A balance wheel, or balance, is the timekeeping device used in mechanical watch A mechanical watch is a watch that uses a Movement (clockwork), clockwork mechanism to measure the passage of time, as opposed to quartz watches which function electron ...
. * In some early electronic clocks and watches such as the Accutron, it is a
tuning fork A tuning fork is an acoustic Acoustic may refer to: Music Albums * Acoustic (Bayside EP), ''Acoustic'' (Bayside EP) * Acoustic (Britt Nicole EP), ''Acoustic'' (Britt Nicole EP) * Acoustic (Joey Cape and Tony Sly album), ''Acoustic'' (Joey Cap ...

tuning fork
. * In quartz clocks and watches, it is a
quartz crystal Quartz is a hard, crystalline mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), ro ...

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 Verge escapement, 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. 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 factor, 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 Standard (technical), 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. 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 Hertz, 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 to atomic clocks (Coordinated Universal Time, UTC), using the Network 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 radio time signal, time signals transmitted from dedicated Radio clock#List of radio time signal stations, 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, 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 resonator, cavity attached to a microwave Electronic oscillator, oscillator controlled by a microprocessor. A thin gas of caesium 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. In mechanical clocks, the low Q factor, 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.


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 A gear train is a mechanical system A machine is any physical system with ordered structural and functional properties. It may represent human-made or naturally occurring device molecular machine that uses Power (physics), power to apply Forc ...

gear train
, known as the wheel train (horology), 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. * In digital clocks a series of integrated circuit counter (digital), counters or dividers add the pulses up Digital data, digitally, using binary numeral system, 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 did not have a visual indicator and signalled the time audibly by striking bells. Many clocks to this day are
striking clock A striking clock is a clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natur ...
s which strike the hour. * Analog clocks display time with an analog clock face, which consists of a dial with the numbers 1 through 12 or 24, the hours in the day, around the outside. The hours are indicated with an hour hand, which makes one or two revolutions in a day, while the minutes are indicated by a Clock face, 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 A speaking clock or talking clock is a live or recorded human voice service, usually accessed by telephone, that gives the correct time. The first telephone speaking clock service was introduced in France, in association with the Paris Observato ...
services provided by telephone companies speak the time audibly, using either recorded or digitally Voice synthesis, synthesized voices.


Types

Clocks can be classified by the type of time display, as well as by the method of timekeeping.


Time display methods


Analog

Analog clocks usually use a
clock face A clock face is the part of an analog clock Analog or analogue may refer to: Computing and electronics * Analog signal An analog signal is any continuous signal for which the time-varying feature of the signal is a representation of some ...
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 decimal-based metric system of measurement, but it did not achieve widespread use. 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 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 Lunar phase, moon phase.


Digital

Kanazawa Station Water Clock.jpg, Digital clock displaying time by controlling valves on the fountain Digital-clock-radio-basic hf.jpg, Simplistic digital clock radio Analog clock with digital display.png, 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 data, 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 domestic environments). Most digital clocks use electronic mechanisms and
LCD A liquid-crystal display (LCD) is a flat-panel display A flat-panel display (FPD) is an electronic display device s, LED display and Vacuum fluorescent display, VF display, top to bottom. A display device is an output device for presentation ...
,
LED An LED A light-emitting diode (LED) is a semiconductor A semiconductor material has an Electrical resistivity and conductivity, electrical conductivity value falling between that of a Electrical conductor, conductor, such as metallic ...
, 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 Alternating current, AC-synchronous motors.


Hybrid (analog-digital)

Clocks with analog quadrants, with a digital component, usually minutes and hours displayed analogously and seconds displayed in digital mode.


Auditory

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 A speaking clock or talking clock is a live or recorded human voice service, usually accessed by telephone, that gives the correct time. The first telephone speaking clock service was introduced in France, in association with the Paris Observato ...
service as well.


Word

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

Some clocks, usually digital ones, include an optical Image projector, 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 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 Local mean time, mean time and the other Solar time#Apparent solar time, 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.


Purposes

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 tracker#Chronological tracker, solar trackers and Telescope mount#Equatorial mounts, 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, Homophonic puns in Mandarin Chinese#Gifts, giving a clock () is often taboo, especially to the elderly as the term for this act Faux pas derived from Chinese pronunciation#Clock, is a homophone with the term for the act of attending another's funeral (). A UK government official Susan Kramer gave a watch to Taipei mayor Ko Wen-je unaware of such a taboo which resulted in some professional embarrassment and a pursuant apology. This homonymic pair works in both Mandarin and Cantonese, although in most parts of 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 ("give") expression of the phrase.


Time standards

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 Electronic transition, 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 Metrology#Laboratories, standards laboratory.


Navigation

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 Time ball, 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 GPS require unprecedentedly accurate knowledge of time, this is supplied by equipment on the satellites; vehicles no longer need timekeeping equipment.


Specific types


See also

* 24-hour analog dial * Allan variance * Rockwell Automation Headquarters and Allen-Bradley Clock Tower, Allen-Bradley Clock Tower at Rockwell Automation Headquarters Building (Wisconsin) * American Watchmakers-Clockmakers Institute * BaselWorld * Circadian rhythm, Biological clock * Castle clock * Clockarium * Lewis Mumford#The clock as herald of the Industrial Revolution, Clock as herald of the Industrial Revolution (Lewis Mumford) * Clock face * Clock drift * Clock ident * Clock network * Clock of the Long Now * Clock signal (digital circuits) * Clockkeeper * Clockmaker * Colgate Clock (Indiana) * Colgate Clock (New Jersey), largest clock in USA * Corpus Clock * Cosmo 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 (U.S.) * Doomsday Clock * Earth clock * Equation clock * Federation of the Swiss 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 largest cuckoo clocks * Metrology * Mora clock * National Association of Watch and Clock Collectors * Projection clock * Replica watch * Rubik's Clock * Star clock * Singing bird box * System time * Time to digital converter * Timeline of time measurement technology * Timer *
Watch A watch is a portable Clock, timepiece intended to be carried or worn by a person. It is designed to keep a consistent movement despite the motions caused by the person's activities. A wristwatch is designed to be worn around the wrist, attach ...

Watch
* Watchmaker


Notes and references


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, NC (1984). pbk. 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. * * * Edey, Winthrop. ''French Clocks''. New York: Walker & Co. (1967). * Kak, Subhash, Babylonian and Indian Astronomy: Early Connections. 2003. * Kumar, Narendra "Science in Ancient India" (2004). . * 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), Daedalus Journal, Spring 2003. * Lloyd, Alan H. "Mechanical Timekeepers", ''A 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 in Western Life and Thought'', Archon Books, Hamden, Conn. (1980). * * North, John. ''God's Clockmaker: Richard of Wallingford and the Invention of Time''. London: Hambledon and 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 and the Mathematization of Nature''. New York: Cambridge University Press (1988). * Zea, Philip, & Robert Cheney. ''Clock Making in New England: 1725–1825''. Old Sturbridge Village (1992).


External links

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National Association of Watch & Clock Collectors Museum
{{Authority control Clocks, Time measurement systems Articles containing video clips