A clock is an instrument for measuring and indicating the time. The word "clock" is derived ultimately (via Dutch, Northern French, and Medieval Latin) from the
Celtic words clagan and clocca meaning "bell". For horologists and other specialists the term "clock" continues to mean exclusively a device with a
striking mechanism for announcing intervals of time acoustically, by ringing a bell, a
set of chimes, or a gong.[citation needed] A silent instrument lacking such a
mechanism has traditionally been known as a timepiece.[1] In general usage today, however, a "clock" refers to any device for measuring and displaying the time
which, unlike a watch, is not worn on the person.
History
The clock is one of the oldest human inventions, meeting the need to consistently measure
intervals of time shorter than the natural units, the year, the day,
and the lunar month. Such measurement requires devices. Devices operating on several
different physical processes have been used over the millennia, culminating in the clocks of today.
Sundials and other devices
The sundial, which measures the time of day by the direction of shadows cast by the sun, was
widely used in ancient times. A well-designed sundial can measure local solar time with reasonable accuracy, and sundials continued to be used to monitor the performance of clocks
until the modern era. However, its practical limitations - it requires the sun to shine and
does not work at all during the night - encouraged the use of other techniques for measuring time.
Candle clocks and sticks of incense that burn down at, approximately, predictable speeds
have also been used to estimate the passing of time. In an hourglass, fine sand pours through a tiny hole at a constant rate and indicates a predetermined passage of an arbitrary period of
time.
Water clocks
-
Water clocks, along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the
gnomon and day-counting tally stick.[2] Given their great antiquity, where and when they first existed are not
known and perhaps unknowable. The simplest form of water clocks, the bowl-shaped outflow type, are known to have existed in
Babylon and in Egypt around the 16th century B.C. 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.[3]
The Greek and Roman civilizations are credited for initially advancing the water clock design to include complex gearing,
which was connected to fanciful automata and improved accuracy. These advances were
passed on through Byzantium and Islamic times, which
eventually made their way on to Europe. Independently, China developed its own advanced water
clocks, passing on their ideas to Korea and Japan.
Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. It is
important to point out that the need for the common person to 'know what time it is' largely did not exist until the
Industrial Revolution, when it became important to keep track of hours worked. In
the earliest of time, however, the purpose for using a water clock was for astronomical and astrological reasons. These early
water clocks were calibrated with a sundial. Through the centuries, water clocks were used for
timing lawyer's speeches during a trial, sermons and Masses in church, night watches of guards,
and even the labor of prostitutes, to name only a few. While never reaching the level of
accuracy based on today's standards of timekeeping, the water clock was the most accurate and commonly used timekeeping device
for millennia, until it was replaced by the more accurate pendulum clock in 17th century
Europe.
Early clocks
In 797 (or possibly 801), the Abbasid caliph of
Baghdad, Harun al-Rashid, presented Charlemagne with an Asian Elephant named Abul-Abbas together with a mechanical[citation needed] clock, out of which came a mechanical bird to announce the hours. This
indicates that the early mechanical clocks were probably made in Asia.
None of the first clocks survive from 13th century 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 ὡρα, hour, and λεγειν, 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 [citation needed].
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 clocca, "bell"), which gradually supersedes "horologe", suggests that it was
the sound of bells which also characterized the prototype mechanical clocks that appeared during the 13th century in Europe.
Between 1280 and 1320, there is an increase in the number of references to clocks and horologes in church records, and this
probably indicates that a new type of clock mechanism had been devised. Existing clock mechanisms that used water power were being adapted to take their driving power from falling weights. This power was controlled by
some form of oscillating mechanism, probably derived from existing bell-ringing or alarm devices. This controlled release of
power - the escapement - marks the beginning of the true mechanical clock.
Outside of Europe, the escapement mechanism had been known and used in medieval China, as the Song Dynasty horologist and engineer Su Song (1020 - 1101) incorporated it into his astronomical
clock-tower of Kaifeng in 1088 [citation needed]. However, his astronomical clock and rotating armillary sphere still relied on the use of flowing water (ie. hydraulics), while European clockworks of the following centuries shed this old habit for a more efficient
driving power of weights, in addition to the escapement mechanism.
In the 13th century, clock construction and engineering entered a new phase with the advancements made by Al-Jazari, a Muslim engineer from Diyar-Bakr in South East Turkey, who is thought to be behind the birth to
the concept of automatic machines[citation needed]. While working for Urtuq king of Diyar-Bakr, Nasir al-Din a son of the famous Sladin, al-Jazari made numerous clocks of all shapes and sizes. In 1206 he
was ordered by the king to document his inventions leading to the publication of an outstanding book on engineering called "The
Book of Knowledge of Ingenious Mechanical Devices” [citation needed]. This book became an invaluable resource for people of different
engineering backgrounds as it described 50 mechanical devices in 6 categories, including water clocks. The most reputed clocks
included the Elephant, the Castle and Scribe clocks, all of which were reconstructed by Muslim Heritage Consulting for Ibn
Battuta Shopping Mall in Dubai (UAE), where they are fully functional. As well as telling the time, these grand clocks were
symbols of status, grandeur and wealth of the Urtuq State.[4]
These mechanical clocks were intended for two main purposes: for signalling and notification (e.g. the timing of services and
public events), and for modeling the solar system. The former purpose is administrative,
the latter arises naturally given the scholarly interest in astronomy, science, astrology, and how these subjects integrated with
the religious philosophy of the time. The astrolabe was used both by astronomers and
astrologers, and it was natural to apply a clockwork drive to the rotating plate to produce a working model of the solar
system.
Simple clocks intended mainly for notification were installed in towers, and did not always require 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 [citation needed]. 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 [citation needed].
Early astronomical clocks
Besides the Chinese astronomical clock of Su Song in 1088 mentioned above, in Europe there were the clocks constructed by
Richard of Wallingford in St Albans by 1336,
and by Giovanni de Dondi in Padua from 1348 to 1364. They no
longer exist, but detailed descriptions of their design and construction survive [citation needed], while 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.
The Salisbury Cathedral clock, built toward the end of the 14th century, is
considered to be the oldest surviving mechanical clock in the world [citation needed].
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 17th century[5], 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
[citation needed].
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
[citation needed] was developed in 1585 by Jost Burgi, who also developed the remontoire. Burgi's accurate clocks helped Tycho Brahe 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 [citation needed]. 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 [citation needed], an improvement over Huygens'
crown escapement [citation needed]. 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 [citation needed].
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 [citation needed]. 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
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.
Alternative systems have been proposed. For example, the TWELV clock indicates the current hour
using one of twelve colors, and indicates the minute by showing a proportion of a circular disk, similar to a moon phase [citation needed].
Digital clocks
A digital clock outside
Kanazawa Station displays the time by controlling valves on a
fountain.
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
that are tuned to national atomic clocks. Since the release of digital clocks in the mainstream, the use of analog clocks has
dropped dramatically.
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). Most telecommunication companies also provide a Speaking clock service as well.
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.
- Crystal clocks use an electronic quartz
crystal oscillator and a frequency divider
or counter. Most battery-powered crystal clocks use a 215 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 kilometers away). The clock can decode the transmission and adjust its hands or display for near perfect accuracy.
The broadcast radio signals are generated by an atomic clock and typically have a data
rate of 1 bit/s.
- 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, mobile
phones and many MP3 players, including iPods.
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 [citation needed]. 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. An ideal clock
should too measure time in consistent, for example decimalized time units.
This leads to the following definitions [citation needed]:
- A clock is a recurrent process and a counter.
- A good clock is one which, when used to measure other recurrent processes, finds many of them to be periodic.
- An ideal clock is a clock (i.e., recurrent process) that makes the most other recurrent processes periodic.
The recurrent, periodic process (e.g. a metronome) is an oscillator and typically generates a clock signal. Sometimes that signal alone is (confusingly)
called "the clock", but sometimes "the clock" includes the counter, its indicator, and everything else supporting it.
This definition can be further improved by the consideration of successive levels of smaller and smaller error tolerances.
While not all physical processes can be surveyed, the definition should be based on the set of physical processes which includes
all individual physical processes which are proposed for consideration. Since atoms are so numerous and since, within current
measurement tolerances they all beat in a manner such that if one is chosen as periodic then the others are all deemed to be
periodic also, it follows that atomic clocks represent ideal clocks to within present
measurement tolerances and in relation to all presently known physical processes. However, they are not so designated by fiat.
Rather, they are designated as the current ideal clock because they are currently the best instantiation of the definition.
Navigation
Navigation by ships 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
See also
