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).
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.
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.
[edit] Time display methods
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.
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 that are tuned to national atomic clocks.
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
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 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.
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.
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