From Wikipedia, the free encyclopedia
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 use 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).
[edit] History
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. This primitive method of time measurement is not completely accurate, as it is of no use on cloudy or overcast days.
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 St. Stephen’s Tower, 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.
[edit] 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.
[edit] 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.
[edit] 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 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.
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 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. 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.
[edit] 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.
[edit] 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
[edit] 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.
Read the rest of this entry »
Your Cart
