this is my clock怎么变一般疑问句
你大爷RsJs
Is this my clock?
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is this your clock?
is this my clock?
is this your clock
扫描下载二维码找出句子中的错误并改正：We see some bed in the room.Is this a alarm clock?Where is your grandparents?Can you take your pencil case to me?Oh,they aren,t know this English boy.根据汉语提示写出下面的句子：1.吉姆的电子游戏机在梳妆台上.2.我不知道姐姐在哪里.3.妈妈,我录像带在哪里?4.请把这些东西带给你外婆.5.布朗夫人需要两把椅子.
我了个0020D
Is this a alarm clock. 囧!这个还真不好改.就把a改为an吧.We see some bed（改为beds）in the room.Where is(改为are) your grandparents(为复数形式,要用are)?Can you take(我认为应该改这里,改成bring拿来,带来的意思.take ×× to ×× 是拿去的意思） your pencil case to me?Oh,they aren't(改为do not) know this English boy.根据汉语提示写出下面的句子：1.Jim 's electronic games(注意是games.整个词组意为电子游戏机,好像有这个说法) is on the dressing table.2.I don't know where my sister is.3.Mum, where is my videotape?4.Take these to your grandmother, please.5.Mrs.Brown needs two chairs.呵呵.我做出来是这样,可能会有一些错误,不过全对了也不好.要不然就算我是全对的吧.
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出门在外也不愁From Wikipedia, the free encyclopedia
For other uses, see .
"Timepiece" redirects here. For the Kenny Rogers album, see . For the 1965 short film by Jim Henson, see .
A clock is an instrument to indicate, keep, and co-ordinate . The word clock is derived ultimately (via , Northern French, and ) from the
words clagan and clocca meaning "". A silent instrument missing such a
mechanism has traditionally been known as a timepiece. In general usage today a "clock" refers to any device for measuring and displaying the time.
and other timepieces that can be carried on one's person are often distinguished from clocks.
The clock is one of the oldest human , meeting the need to consistently measure intervals of time shorter than the natural units: the , the , and the . Devices operating on several physical processes have been used over the millennia. A
shows the time by displaying the position of a shadow on a flat surface. There are a range of duration timers, a well-known example being the . , along with the sundials, are possibly the oldest time-measuring instruments. A major advance occurred in Europe around 1300 with the invention of the , which allowed construction of the first mechanical clocks, which used oscillating timekeepers like . Spring-driven clocks appeared during the 15th century. During the 15th and 16th centuries, clockmaking flourished. The next development in accuracy occurred after 1656 with the invention of the . A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The
was patented in 1840. The development of
in the 20th century led to clocks with no clockwork parts at all.
The timekeeping element in every modern clock is a , a physical object () that vibrates or
repetitively at a precisely constant . This object can be a , a , a , or the vibration of
as they emit . Analog clocks usually indicate time using angles. Digital clocks display a numeric representation of time. Two numeric display formats are commonly used on
clocks: 24-hour notation and 12-hour notation. Most digital clocks use electronic mechanisms and , , or
displays. For convenience, distance, telephony or , auditory clocks present the time as sounds. There are also clocks for the blind that have displays that can be read by using the sense of touch. Some of these are similar to normal analog displays, but are constructed so the hands can be felt without damaging them. The evolution of the technology of clocks continues today.
The study of timekeeping is known as .
Main article:
Simple horizontal sundial.
is shining, its apparent position in the sky moves during a day, reflecting the rotation of the Earth. Shadows cast by stationary objects move correspondingly, so their positions can be used to indicate the time of day. A
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 . With the knowledge of latitude, a well-constructed sundial can measure local
with reasonable accuracy, within a minute or two. Sundials continued to be used to monitor the performance of clocks until the .[] However, practical limitations, such as that sundials work only when the Sun shines, and never during the night, encouraged the use of other techniques for measuring and displaying time.
The flow of
can be used to keep track of elapsed time.
Many devices can be used to mark passage of time without respect to reference time (time of day, minutes, etc...) and can be useful for measuring duration and/or intervals. Examples of such duration timers are, ,
and the . 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
pouring through a tiny hole at a constant rate indicates an arbitrary, predetermined, passage of time, the resource is not consumed but re-used.
Main article:
Clock Tower, built in 11th century , . It was driven by a large , , and
mechanism.
Water clocks, also known as clepsydrae (sg: clepsydra), along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the vertical
and the day counting . 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
around the 16th century BC. Other regions of the world, including
and , 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.
astronomer
supervised the construction of the
in Athens in the 1st century B.C.
civilizations are credited for initially advancing water clock design to include complex , which was connected to fanciful
and also resulted in improved accuracy. These advances were passed on through
times, eventually making their way back to . Independently, the Chinese developed their own advanced water clocks（水鐘）in 725 A.D., passing their ideas on to
Some water clock designs were developed independently and some knowledge was transferred through the spread of trade.
societies do not have the same precise timekeeping requirements that exist in modern
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
reasons. These early water clocks were calibrated with a . 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
in 17th-century Europe.
Islamic civilization is credited with further advancing the accuracy of clocks with elaborate engineering. In 797 (or possibly 801), the
of , , presented
together with a "particularly elaborate example" of a water clock.
introduced clocks to northern and western Europe around 1000AD
in a manuscript by
(1206 AD) from The Book of Knowledge of Ingenious Mechanical Devices.
In the 13th century, , an engineer from Mesopotamia (lived ) who worked for
king of Diyar-Bakr, , made numerous clocks of all shapes and sizes. The book described 50 mechanical devices in 6 categories, including water clocks. The most reputed clocks included , Scribe and , 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.[]
The word horologia (from the Greek ?ρα, hour, and λ?γειν, to tell) was used to describe early mechanical clocks, but the use of this word (still used in several )
for all timekeepers conceals the true nature of the mechanisms. For example, there is a record that in 1176
installed a ‘’ but the mechanism used is unknown. According to , in 1198 during a fire at the abbey of St Edmundsbury (now ), the monks 'ran to the clock' to fetch water, indicating that their water clock had a reservoir large enough to help extinguish the occasional fire.
The word clock (from the Celtic words clocca and clogan, both meaning "bell"), which gradually supersedes "horologe", suggests that it was the sound of bells which also characterized the prototype mechanical clocks that appeared during the 13th century in .
The first mechanical clock was completed in China in AD 725 by
mechanism had been known and used in medieval China, as the
horologist and engineer
() incorporated it into his astronomical clock-tower of
in 1088.[] His astronomical clock and rotating
still relied on the use of flowing water (i.e. ), while European clockworks of the following centuries shed this method for a more efficient driving power of weights, in addition to the escapement mechanism.
A mercury clock, described in the Libros del saber, a Spanish work from 1277 consisting of translations and paraphrases of Arabic works, is sometimes quoted as evidence for Muslim knowledge of a mechanical clock. The first mercury powered automatic clock was invented by
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
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 —marks the beginning of the true mechanical clock.
These mechanical clocks were intended for two main purposes: for signalling and notification (e.g. the timing of services and public events), and for modeling the . 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
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
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 , canonical hours, and time as measured by astronomers at the time. Both styles of clock started acquiring extravagant features such as .
In 1283, a large clo its location above the
suggests that it was not a water clock.[] In 1292,
installed a 'great horloge'. Over the next 30 years there are mentions of clocks at a number of ecclesiastical institutions in England, Italy, and France. In 1322, a new clock was installed in , 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
for two years.[]
pointing to a clock, his gift to .
16th century clock machine ,Tomar, Portugal.
Besides the Chinese astronomical clock of Su Song in 1088 mentioned above, in Europe there were the clocks constructed by
by 1336, and by Giovanni
from 1348 to 1364. They no longer exist, but detailed descriptions of their design and construction survive, and modern reproductions have been made. They illustrate how quickly the theory of the mechanical clock had been translated into practical constructions, and also that one of the many impulses to their development had been the desire of astronomers to investigate celestial phenomena.
Wallingford's clock had a large astrolabe-type dial, showing the sun, the moon's age, phase, and node, a star map, and possibly the planets. In addition, it had a
and an indicator of the state of the tide at . 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 , 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 , built in 1386, is considered to be the world's oldest surviving mechanical clock that strikes the hours.
Clockmakers developed their art in various ways. Building smaller clocks was a technical challenge, as was improving accuracy and reliability. Clocks could be impressive showpieces to demonstrate skilled craftsmanship, or less expensive, mass-produced items for domestic use. The escapement in particular was an important factor affecting the clock's accuracy, so many different mechanisms were tried.
Spring-driven clocks appeared during the 15th century, although they are often erroneously credited to
watchmaker
(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 . Spring power presented clockmakers with a new problem: how to keep the clock
running at a constant rate as the spring ran down. This resulted in the invention of the
in the 15th century, and many other innovations, down to the invention of the modern going
Early clock dials did not indicate minutes and seconds. A clock with a dial indicating minutes was illustrated in a 1475 manuscript by Paulus Almanus, and some 15th-century clocks in
indicated minutes and seconds. An early record of a seconds hand on a clock dates back to about 1560 on a clock now in the Fremersdorf collection.:417–418
During the 15th and 16th centuries, clockmaking flourished, particularly in the metalworking towns of
and , and in , 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
was invented in 1584 by , who also developed the . 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
to observe astronomical events with much greater precision than before.[][]
Pendulum clock Ansonia. C.1904, Ansonia Clock Co., SANTIAGO, hanging oak gingerbread clock, 8-day time and strike.
The next development in accuracy occurred after 1656 with the invention of the .
had the idea to use a swinging bob to regulate the motion of a time-telling device earlier in the 17th century. , however, is usually credited as the inventor. He determined the mathematical formula that related pendulum length to time (99.38 cm or 39.13 inches for the one second movement) and had the first pendulum-driven clock made. The first model clock was built in 1657 in , but it was in
that the idea was taken up. The
(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
to utilize
as well as hand-painted ceramics.
In 1670, William Clement created the , 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 , a London clock-maker and others, and the Second Hand was first introduced.
In 1675, Huygens and
invented the , or the hairspring, designed to control the oscillating speed of the . This crucial advance finally made accurate pocket watches possible. The great English clockmaker, , was one of the first to use this mechanism successfully in his , and he adopted the minute hand which, after a variety of designs were trialled, eventually stabilised into the modern-day configuration.
invented the rack and snail striking mechanism for , which was a great improvement over the previous mechanism. The , that chimes the number of hours (or even minutes) was invented by either Quare or Barlow in 1676.
invented the
for clocks in 1720.
Drawings of Harrison's H4 chronometer of 1761, published in The principles of Mr Harrison's time-keeper, 1767.
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
to the value of 20,000 pounds, for anyone who could determine longitude accurately. , who dedicated his life to improving the accuracy of his clocks, later received considerable sums under the Longitude Act.
In 1735, Harrison built his first chronometer, which he steadily improved on over the next thirty years before submitting it for examination. The clock had many innovations, including the use of bearings to reduce friction, weighted balances to compensate for the ship's pitch and roll in the sea and the use of two different metals to reduce the problem of expansion from heat.
The chronometer was tested in 1761 by Harrison's son and by the end of 10 weeks the clock was in error by less than 5 seconds.
, the inventor of mass-produced clocks.
The British had predominated in watch manufacture for much of the 17th and 18th centuries, but maintained a system of production that was geared towards high quality products for the elite. Although there was an attempt to modernise clock manufacture with
techniques and the application of duplicating tools and machinery by the British Watch Company in 1843, it was in the
that this system took off. In 1816,
and some other Connecticut clock makers developed a way of mass-producing clocks by using .
started a factory in 1851 in
that also used interchangeable parts, and by 1861 was running a successful enterprise incorporated as the .
Main article:
published the
powered by
batteries. , Scottish clockmaker, patented the
in 1840. The electric clock's mainspring is wound either with an
or with an
and armature. In 1841, he first patented the
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
(DC), to rewind the spring or raise the weight of a mechanical clock would be classified as an . 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
Electric clocks that are powered from the
supply often use . The supply current alternates with a frequency of exactly 50  in many countries, and 60 hertz in others. The
of the motor rotates at a speed that is exactly related to the alternation frequency. Appropriate gearing converts this rotation speed to the correct ones for the hands of the analog clock.
The development of
in the 20th century led to clocks with no clockwork parts at all. Time in these cases is measured in several ways, such as by the alternation of the AC supply, vibration of a , the behaviour of
crystals, or the quantum vibrations of atoms. Electronic circuits divide these high-frequency oscillations to slower ones that drive the time display. Even mechanical clocks have since come to be largely powered by batteries, removing the need for winding.
properties of crystalline
were discovered by
in 1880. The first crystal oscillator was invented in 1917 by
after which, the first quartz crystal oscillator was built by
in 1921. In 1927 the first
was built by Warren Marrison and J. W. Horton at
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 , limited their practical use elsewhere. The National Bureau of Standards (now ) 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,
produced the world's first quartz , the . Their inherent accuracy and low cost of production resulted in the subsequent proliferation of quartz clocks and watches.
are the most accurate clocks in existence. They are considerably more accurate than
as they can be accurate to within a few seconds over thousands of years.
Atomic clocks were first theorized by
in 1879. In the 1930s the development of
created practical method for doing this. A prototype
device was built in 1949 at the U.S.
(NBS, now ). Although it was less accurate than existing , it served to demonstrate the concept.
The first accurate atomic clock, a
based on a certain transition of the
atom, was built by
in 1955 at the
in the UK. Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale
Today's most stable atomic clocks are
clocks, which are stable to within less than two parts in 1 quintillion (2×10-18).
The invention of the mechanical clock in the 13th century initiated a change in timekeeping methods from
processes, such as the motion of the 's shadow on a
or the flow of liquid in a , to periodic
processes, such as the swing of a
or the vibration of a , which had the potential for more accuracy. All modern clocks use oscillation.
Although the methods they use vary, all oscillating clocks, mechanical and digital and atomic, work similarly and can be divided into analogous parts. They consist of an object that repeats the same motion over and over again, an , 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 , 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.
Keys of various sizes for winding up mainsprings on clocks.
This provides power to keep the clock going.
In mechanical clocks, the power source is typically either a weight suspended from a cord or chain wrapped around a , or a spiral
called a . Mechanical clocks must be wound periodically, usually by turning a knob or key or by pulling on the free end of the chain, to store
in the weight or spring to keep the clock running.
In , the power source is either a
or the . In clocks that use AC power, a small
is often included to keep the clock running if it is unplugged temporarily from the wall or during a power outage. Battery powered analog wall clocks are available that operate over 15 years between battery changes.
The timekeeping element in every modern clock is a , a physical object () that vibrates or
repetitively at a precisely constant .
In mechanical clocks, this is either a
In some early electronic clocks and watches such as the , it is a .
and watches, it is a .
In , it is the vibration of
as they emit .
In early mechanical clocks before 1657, it was a crude balance wheel or
which was not a harmonic oscillator because it lacked a . 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
to vibrate at a precise natural
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 , 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
for that purpose. Atomic clocks are , and their rate cannot be adjusted.
Some clocks rely for their accuracy on an that is, they are automatically
to a more accurate clock:
, used in large institutions and schools from the 1860s to the 1970s, kept time with a pendulum, but were wired to a
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.
do not have an internal oscillator, but count cycles of the 50 or 60
oscillation of the , 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
which rotates an exact fraction of a revolution for every cycle of the line voltage, and drives the gear train. Although changes in the
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.
keep time with a quartz crystal, but can be periodically (usually weekly) synchronized over the
to atomic clocks (), using the
(NTP). Sometimes computers on a
(LAN) get their time from a single local server which is maintained accurately.
keep time with a quartz crystal, but are periodically synchronized to
transmitted from dedicated
signals, which are set by .
This has the dual function of keeping the oscillator running by giving it 'pushes' to replace the energy lost to , and converting its vibrations into a series of pulses that serve to measure the time.
In mechanical clocks, this is the , 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
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 .
the controller is an evacuated
attached to a microwave
controlled by a . A thin gas of
atoms is released into the cavity where they are exposed to . A laser measures how many atoms have absorbed the microwaves, and an electronic
control system called a
tunes the microwave oscillator until it is at the exact frequency that causes the atoms to vibrate and absorb the microwaves. Then the microwave signal is divided by
to become the .
In mechanical clocks, the low
of the balance wheel or pendulum oscillator made them very sensitive to the disturbing effect of the impulses of the escapement, so the escapement had a great effect on the accuracy of the clock, and many escapement designs were tried. The higher Q of resonators in electronic clocks makes them relatively insensitive to the disturbing effects of the drive power, so the driving oscillator circuit is a much less critical component.
This counts the pulses and adds them up to get traditional time units of , , , 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 , known as the . The gear train also h 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
or dividers add the pulses up , using
logic. Often pushbuttons on the case allow the hour and minute counters to be incremented and decremented to set the time.
with mechanical
and sound producer striking on the 8th hour on the analog dial.
This displays the count of seconds, minutes, hours, etc. in a human readable form.
The earliest mechanical clocks in the 13th century didn't have a visual indicator and signalled the time
by striking . Many clocks to this day are
which strike the hour.
Analog clocks display time with an analog , which consists of a round dial with the numbers 1 through 12, the hours in the day, around the outside. The hours are indicated with an , which makes two revolutions in a day, while the minutes are indicated by a , which makes one revolution per hour. In mechanical clocks a gear tr in electronic clocks the circuit produces pulses every second which drive a
and gear train, which move the hands.
display the time in periodically changing
on a digital . 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.
services provided by telephone companies speak the time audibly, using either recorded or digitally .
Clocks can be classified by the type of time display, as well as by the method of timekeeping.
A linear clock at 's . 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
uses a fixed numbered dial or dials and moving hand or hands. It usually has a circular scale of 12 , which can also serve as a scale of 60 , and 60
if the clock has a second hand. Many other styles and designs have been used throughout the years, including dials divided into 6, 8, 10, and 24 hours. The only other widely used clock face today is the , because of the use of
organizations and timetables. The
was briefly popular during the , when the
was applied to time measurement, and an Italian 6 hour clock was developed in the 18th century, presumably to save power (a clock or watch striking 24 times uses more power).
Another type of analog clock is the , which tracks the sun continuously, registering the time by the shadow position of its . Because the sun does not adjust to daylight savings times, users must add an hour during that time. Corrections must also be made for the , and for the difference between the longitudes of the sundial and of the central meridian of the
that is being used (i.e. 15 degrees east of the
for each hour that the time zone is ahead of ). 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 .
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 .
Main article:
Examples of digital clocks
Digital clock outside
displaying the time by controlling valves on a fountain
Basic digital clock radio
display with an analog-style clock (albeit generated by a digital computer) in the middle, and a digital-style in the top right corner
Diagram of a mechanical digital display of a
Digital clocks display a numeric representation of time. Two numeric display formats are commonly used on
with hours ranging 00–23;
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 , , many other display technologies are used as well (, , etc.). After a reset, battery change or power failure, these clocks without a backup
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
that are tuned to national . Since the advent of digital clocks in the 1960s, the use of analog clocks has declined significantly.
Some clocks, called '', 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 -.
Main article:
For convenience, distance, telephony or , auditory clocks present the time as sounds. The sound is either spoken , (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 ). Most telecommunication companies also provide a
service as well.
Software word clock
Word clocks are clocks that display the time visually using sentences. E.g.: "It’s about three o’clock." These clocks can be implemented in hardware or software.
Main article:
Some clocks, usually digital ones, include an optical
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
are also available.
Auditory and projection clocks can be used by people who are blind or have limited vision. There are also clocks for the blind that have displays that can be read by using the sense of touch. Some of these are similar to normal analog displays, but are constructed so the hands can be felt without damaging them. Another type is essentially digital, and uses devices that use a code such as
to show the digits so that they can be felt with the fingertips.
Some clocks have several displays driven by a single mechanism, and some others have several completely separate mechanisms in a single case. Clocks in public places often have several faces visible from different directions, so that the clock can be read from anywhere in the vicinity. Of course, all the faces show the same time. Other clocks show the current time in several time-zones. Watches that are intended to be carried by travellers often have two displays, one for the local time and the other for the time at home, which is useful for making pre-arranged phone calls. Some
have two displays, one showing
and the other , as would be shown by a sundial. Some clocks have both analog and digital displays. Clocks with Braille displays usually also have conventional digits so they can be read by sighted people.
Clocks are in homes, offices a smaller ones () are carried on the
larger ones are in public places, e.g. a
or . A small clock is often shown in a corner of ,
and many .
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 slee 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 fo they are sometimes called training clocks.
A clock mechanism may be used to control a device according to time, e.g. a
system, a , or a
(see: ). Such mechanisms are usually called . Clock mechanisms are also used to drive devices such as
and , which have to turn at accurately controlled speeds to counteract the rotation of the Earth.
depend on an internal signal at constant frequency to sy this is referred to as a . (A few research projects are developing CPUs based on .) Some equipment, including computers, also maintains time and date this is referred to as time-of-day clock, and is distinct from the system clock signal, although possibly based on counting its cycles.
Main articles:
For some scientific work timing of the utmost accuracy is essential. It is also necessary to have a standard of the maximum accuracy against which working clocks can be calibrated. An ideal clock would give the time to unlimited accuracy, but this is of course not realisable.
Many physical processes, in particular including some
between atomic , occur at exceedin counting cycles of such a process can give a very accurate and consistent time—clocks which work this way are usually called . Such clocks are typically large, very expensive, require a controlled environment, and are far more accurate than requir they are typically used in a .
Until advances in the late twentieth century,
depended on the ability to measure
and . Latitude can be the measurement of
requires accurate knowledge of time. This need was a major motivation for the development of accurate mechanical clocks.
created the first highly accurate
in the mid-18th century. The
still fires an accurate signal to allow ships to check their . Many buildings near major ports used to have (some still do) a large
mounted on a tower or mast arranged to drop at a pre-determined time, for the same purpose.
systems such as the
(GPS) require unprecedentedly accurate knowledge of time, this is supplied by equipme vehicles no longer need timekeeping equipment.
In determining the location of an , the arrival time of several types of
at a minimum of four dispersed observers is dependent upon each observer recording wave
according to a common clock.
A monumental conical pendulum clock by , 1867. Philadelphia, USA.
By mechanism
By function
(Lewis Mumford)
(digital circuits)
, largest clock in USA
, world's largest clock
(automata)
(watchclocks)
see Baillie et al., p. 307; Palmer, p. 19; Zea & Cheney, p. 172
. a device for measuring and showing time, which is usually found in or on a building and is not worn by a person
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