   #copyright

Clock

2007 Schools Wikipedia Selection. Related subjects: Engineering

   A clock (from the Latin cloca, " bell") is an instrument for measuring
   time. The clock in its most common, modern form (in use since at least
   the 14th century) displays the time in hours, minutes, and often
   seconds during a 12- or 24-hour period.

   Clocks used for technical purposes, of very high accuracy, are usually
   called chronometers. A common portable timekeeping instrument for
   personal used is the pocket or wrist watch.

   True clocks also have an announcing or striking mechanism that sounds
   after each set interval of time, usually by ringing a bell (which, as
   previously indicated, originally gave the clock its name), chimes, or
   gong. A silent clock without a striking mechanism is traditionally
   known as a timepiece, a term sometimes used by horologists and other
   specialists to describe ordinary wrist watches and other timekeeping
   devices lacking a striking mechanism (see Baillie et al., p. 307;
   Palmer, p. 19; Zea & Cheney, p. 172).

History

   A replica of an ancient Chinese incense clock
   Enlarge
   A replica of an ancient Chinese incense clock

   The clock is one of the oldest human inventions, requiring a physical
   process that will proceed at a known rate and a way to gauge how long
   that process has run. As the seasons and the phases of the moon can be
   used to measure the passage of longer periods of time, shorter
   processes had to be used to measure off hours and minutes. The sundial,
   which measures the time of day by the direction of shadows cast by the
   sun, was widely known in ancient times.

   Candles and sticks of incense that burn down at, approximately,
   predictable speeds were also used as to estimate the passage of time.
   In an hourglass fine sand pours through a tiny hole at a constant rate
   and indicates a predetermined passage of an arbitrary time.
   The massive clock on Big Ben, London, England. The 5 foot 4 inch (1.63
   m) person "holding on" to the six-o'clock marking has been inserted
   into the picture at correct scale. The hour hand is 9 feet (2.7 m)
   long, and the minute hand is 14 feet (4.3 m) long.
   Enlarge
   The massive clock on Big Ben, London, England. The 5 foot 4 inch (1.63
   m) person "holding on" to the six-o'clock marking has been inserted
   into the picture at correct scale. The hour hand is 9 feet (2.7 m)
   long, and the minute hand is 14 feet (4.3 m) long.

Water clocks

   The historian Vitruvius reported that the ancient Egyptians used a
   clepsydra, a time mechanism using flowing water. Herodotus had
   mentioned an ancient Egyptian time-keeping device that was based on
   mercury. By the 9th century AD a mechanical timekeeper had been
   developed that lacked only an escapement mechanism. Later years saw the
   rise of automated water clocks in Arabia, China, and Korea.

Early mechanical clocks

   No clocks survive from medieval Europe but various mentions in church
   records reveal some of the early history of the clock.

   Medieval religious institutions required clocks to measure and indicate
   the passing of time because, for many centuries, daily prayer and work
   schedules had to be strictly regulated. This was done by various types
   of time-telling and recording devices, such as water clocks, sundials
   and marked candles, probably used in combination. Important times and
   durations were broadcast by bells, rung either by hand or by some
   mechanical device such as a falling weight or rotating beater.

   The word 'horologia' (from the Greek hora, hour, and legein, 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. 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.

   These early clocks may not have used hands or dials but “told” the time
   with audible signals.

A new mechanism

   The word 'clock' (from the Latin word for "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.

   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 the 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 modelling 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 philosopy 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 dials 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 technicians for two years.

Early astronomical clocks

   The clocks constructed by Richard of Wallingford in St Albans by 1336,
   and by Giovanni de'Dondi in Padua from 1348 to 1364, 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.

Elements of the mechanical clock

   These 14th century clocks show the four key elements common to all
   clocks in subsequent centuries, at least up to the digital age:
     * the power, supplied by a falling weight, later by a coiled spring
     * the escapement, a periodic repetitive action that allows the power
       to escape in small bursts rather than drain away all at once
     * the going train, a set of interlocking gear wheels that controls
       the speed of rotation of the wheels connected between the power
       supply and the indicators
     * indicators, such as dials, hands, and bells

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 were developed during the 15th century, and this
   gave the clockmakers many new problems to solve, such as how to
   compensate for the changing power supplied as the spring unwound.

   The first record of a minute hand on a clock is 1475, in the Almanus
   Manuscript of Brother Paul.

   During the 15th and 16th centuries, clockmaking flourished,
   particularly in the metalworking towns of Nuremberg and Augsburg, and,
   in France, Blois. Some of the more basic table clocks have only one
   time-keeping hand, with the dial between the hour markers being divided
   into four equal parts making the clocks readable to the nearest 15
   minutes. Other clocks were exhibitions of craftsmanship and skill,
   incorporating astronomical indicators and musical movements. The
   cross-beat escapement was developed in 1585 by Jobst Burgi, who also
   developed the remontoire. Burgi's accurate clocks helped Tycho Brahe
   and Johannes Kepler to observe astronomical events with much greater
   precision than before.

   The first record of a second hand on a clock is 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.

   The next development in accuracy occurred after 1657 with the invention
   of the pendulum clock. Galileo had the idea to use a swinging bob to
   propel 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 that 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 is reported to have lost less
   than 5 seconds over 10 days.

   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 twentieth 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, the decay of radioactive elements or resonance of
   polycarbonates. Even mechanical clocks have since come to be largely
   powered by batteries, removing the need for winding.

Types

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

Time display methods

Analog clocks

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

   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 often also as a scale of 60 seconds – though
   many other styles and designs have been used throughout the years,
   including dials divided into 6, 8, 10, and 24 hours. Of these
   alternative versions, the 24 hour analog dial is the main type in use
   today. 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 chiming 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.

Digital clocks

   A digital clock outside Kanazawa Station displays the time by
   controlling valves on a fountain.
   Enlarge
   A digital clock outside Kanazawa Station displays the time by
   controlling valves on a fountain.

   Main Article: Digital clock

   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).

   Most digital clocks use an LCD or LED 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 00:00, or stay
   at 00:00, 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 which in turn are tuned to national atomic
   clocks.
   Basic digital clock radio.
   Enlarge
   Basic digital clock radio.

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 clock Big Ben).

Timekeeping methods

   Most types of clocks are built around some form of oscillator, an
   arrangement that goes through an endless sequence of periodic state
   changes, designed to provide a continuous and stable reference
   frequency. The periods of this oscillator are then counted and
   converted into the desired clock display.
     * Mechanical clocks use a pendulum as their oscillator, which
       controls the rotation of a system of gears that drive the clock
       display.

     * Electrical clocks use electrical current to run, rather than
       requiring manual winding and weights.

     * Crystal clocks use an electronic quartz crystal oscillator and a
       frequency divider or counter. Most battery-powered crystal clocks
       use a 2^15 Hz = 32.768 kHz oscillator.

     * Atomic clocks use a microwave oscillator ( maser) tuned by the
       energy transitions of elements such as caesium, rubidium or
       hydrogen. These are the most precise clocks available. Atomic
       clocks based on caesium are used as the official definition of time
       today.

     * Mains power clocks count the 50 or 60 hertz periods of their AC
       power.

     * Radio clocks receive time signal broadcasts from a radio
       transmitter (which may be hundreds of kilometres away). The clock
       can decode the transmission and adjust its hands or display for
       perfect accuracy. The broadcast radio signals received are
       generated by an atomic clock. These clocks are used extensively by
       mariners, especially short-wave radio clocks which use simultaneous
       bursts of time-signals, often encoded or encrypted – not to be
       confused with number stations.

     * Sundials observe the apparent rotation of the Sun around the Earth
       as their reference oscillation. They are observed with a solar
       tempometer.

Purposes

   Clocks are in homes and offices; smaller ones (watches) are carried;
   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 or mobile
   phones.

   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. Memory of this kind
   is often referred to as "non-volatile". 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.
   A desk clock
   Enlarge
   A desk clock

   This leads to the following definitions:
     * A clock is a recurrent periodic 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 (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 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.

Specific types of clocks

   A windup, mechanical, spring-driven alarm clock.
   Enlarge
   A windup, mechanical, spring-driven alarm clock.
     * Alarm clock
     * Analog clock with digital display
     * Astronomical clock
     * Atomic clock
     * Balloon clock
     * Binary clock
     * Bracket clock
     * Carriage clock
     * Cartel clock
     * Chiming clock
     * Clock network
     * Clock of the Long Now
     * Countdown clock
     * Cuckoo clock
     * Data clock for timescapes created with time-technology
     * Digital clock
     * Doll's head clock
     * Electric clock
     * Flip clock
     * Floral clock
     * Game clock
     * Hourglass
     * Japanese clock
     * Lantern clock
     * Lighthouse Clock
     * Longcase (or "grandfather") clock
     * Mantel clock
     * Master clock
     * Paper clock
     * Pedestal clock
     * Pendulum clock
     * Projection clock

   Digital clock display in an oven.
   Enlarge
   Digital clock display in an oven.
     * Quartz clock
     * Railroad chronometers
     * Reference clock
     * Rolling ball clock
     * Shelf clock
     * Sidereal clock
     * Skeleton clock
     * Slave clock
     * Stopwatch
     * Striking clock
     * Sundial
     * Tall-case clock
     * Tide clock
     * Time ball
     * Time clock
     * Tower clock
     * Torsion pendulum clock
     * Watch
     * Water clock
     * Wall clock
     * World clock

   Retrieved from " http://en.wikipedia.org/wiki/Clock"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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