Saturday, June 6, 2009

HOW TIME WAS MEASURED THEN CLOCK AND WATCHES INVENTED

INSIDE MECHANICAL CLOCK
Many mechanical clocks and watches are powered by a mainspring, which must be wound periodically to provide energy to drive the clock. The force from the wound mainspring drives the power wheel, which transmits motion through a series of pinion gears to the hour wheel and the minute wheel. The escapement wheel slows and regulates the motion of the power wheel. The motion of the escapement is regulated by the back and forth movement of the pivot. This motion also produces the familiar “tick-tock” of a clock and ensures that the hour and minute hands keep accurate time
LIQUID CRYSTAL DISPLAY (LCD) QUARTZ
Liquid crystal forms from various organic substances and takes its name from the crystal-like properties of liquids whose molecular orientation changes when a small electric current is applied. Sensitive to temperature and pressure, liquid crystal can appear as a solid at low temperatures and high pressures or as a liquid at high temperatures and low pressures. Liquid crystal is used to make clock displays, television receivers, and notebook computer screens.
POCKET WATCH
This engraving depicts a Nüremberg egg, a pocket watch designed by Peter Henlein in the early 16th century. Small, portable clocks like the Nüremberg egg became possible with the development of coiled springs as a power source for timekeeping devices.
WEIGHT-DRIVEN CLOCK
Most grandfather clocks are weight-driven, which means they are powered by the pull of a hanging weight. A mechanism called the escapement regulates the incremental release of the weight’s energy via toothed gears, which set other wheels in motion. As the pendulum swings back and forth, the escapement “walks” along the escape gear, the clock ticks, and the weight moves gradually downward, until it eventually needs to be reset. In other clocks, a wound spring serves the purpose of the weight.
JAPANESE LANTERN CLOCK
This Japanese clock was regulated by weights on a moving balance bar. Called a lantern clock because of its distinctive shape, it features the one-hand design common in most clocks until the 1650s. Minute hands appeared when Christiaan Huygens designed the more accurate pendulum clock.
POCKET SUNDIAL
This portable folding German sundial has a string gnomon (pointer), adjustable for accuracy at any latitude. As shadows fall across the sundial, the smaller dials show Italian and Babylonian hours. The dial also indicates the length of the day and the position of the sun in the zodiac.
SANDGLASSES
Ornate sandglasses like this one were once used to mark the passage of minutes and hours. Flipping the glass causes the fixed amount of sand to pass though its narrow central hole in a consistent length of time, creating a relatively accurate measure. Generally called hourglasses because an hour was their standard setting, almost any period of time could be set by altering the amount of sand or the size of the central opening.

HOW TIME MEASURED N ANCIENT TIME

Throughout history, time has been measured by the movement of the earth relative to the sun and stars. The earliest type of timekeeper, dating from as far back as 3500 bc, was the shadow clock, or gnomon, a vertical stick or obelisk that casts a shadow. An Egyptian shadow clock of the 8th century bc is still in existence. The first hemispherical sundial was described about the 3rd century bc by Chaldean astronomer Berossus. Ancient methods of measuring hours in the absence of sunlight included the notched candle and the Chinese practice of burning a knotted rope and noting the length of time required for the fire to travel from one knot to the next. Devices almost as old as the shadow clock and sundial include the hourglass, in which the flow of sand is used to measure time intervals, and the water clock, or clepsydra, in which the flow of water indicates passage of time. Clepsydras became more complicated, even to the inclusion of gearing in about 270 bc by Greek inventor Ctesibius of Alexandria. Eventually, a weight falling under the force of gravity was substituted for the flow of water in time devices, anticipating the mechanical clock.

THE MECHANICAL CLOCK

The historical origin of the mechanical clock is obscure. The first recorded examples are found in the 14th century. Until that time, a time-measuring instrument was known as a horologium, or hour teller. The name clock, which originally meant “bell,” was first applied in the present sense to the huge, mechanical time indicators installed in bell towers in the late Middle Ages.

Clockworks were initially heavy, cumbersome devices. A clock built in the 14th century by Henry De Vick of Württemberg for the royal palace (now the Palais de Justice) in Paris was powered by a 227-kg (500-lb) weight that descended a distance of 9.8 m (32 ft). The apparatus for controlling its rate of fall was crude and the clock inaccurate. Clocks of that period had dials with only one hand, which indicated the nearest quarter hour.

THE PENDULUM

A series of inventions in the 17th and 18th centuries increased the accuracy of clockworks and reduced the weight and bulk of the mechanisms. Galileo had described late in the 16th century the property of a pendulum, known as isochronism, stating that the period of the swing is constant. In 1657 Dutch physicist Christiaan Huygens showed how a pendulum could be used to regulate a clock. Ten years later English physicist Robert Hooke invented an escapement, which permitted the use in clocks of a pendulum with a small arc of oscillation. British clockmaker George Graham improved the escapement, and John Harrison developed a means of compensating for variations in the length of a pendulum resulting from changes in temperature.

WATCHES

Watchworks were developed when coiled springs were introduced as a source of power. This type of spring was used in Italy about 1450. About 1500 Peter Henlein, a locksmith in Nürnberg, Germany, began producing portable timepieces known popularly as Nürnberg eggs. In 1525 another artisan, Jacob Zech of Prague, invented a fusee, or spiral pulley, to equalize the uneven pull of the spring. Other improvements that increased the accuracy of watches included a spiral hairspring, invented about 1660 by Robert Hooke, for the balance wheel, and a lever escapement devised by British inventor Thomas Mudge about 1765.

Minute and second hands, and crystals to protect both the dial and hands, first appeared on 17th-century watches. Jeweled bearings to reduce friction and prolong the life of watchworks were introduced in the 18th century.

In the centuries that preceded the introduction of machine-made parts, craftsmanship of a high order was required to manufacture accurate, durable clocks and watches. Such local craft organizations as the Paris Guild of Clockmakers (1544) were organized to control the art of clockmaking and its apprenticeship. A guild known as the Clockmakers Company, founded in London in 1630, is still in existence. The Netherlands, Germany, and Switzerland also produced many fine artisans whose work was noted for beauty and a high degree of mechanical perfection.

The clock was often a decorative as well as a useful instrument. Early clocks were highly ornamented. Many bore sculptured figures, and clockworks were used in the towers of late medieval Europe to set in motion huge statues of saints or allegorical figures. Cuckoo clocks, containing carved wooden birds, which emerge and “sing” to tell the time, were made in the Black Forest of Germany as early as 1730 and are still popular. Some early English clocks were made in the form of lanterns or birdcages. The grandfather, or case, clock, which has the pendulum and weight exposed beneath a gear housing at the top of a tall cabinet, was designed before machine-cut gears were introduced, and it continues to be a popular ornamental clock.

Watches were originally shaped like drums or balls and were worn suspended from a belt or kept in a pocket. Wristwatches became popular as watchworks became smaller. Beginning in the 18th century, Switzerland became the center of a watchmaking industry, particularly in the villages of the Jura Mountains. At first a cottage industry, with families manufacturing watch parts at home to be assembled and sold by a master watchmaker, Swiss watchmaking by the 1850s had led to the development of a number of small factories and the foundation of a major industry. Some modern Swiss watchworks are tiny enough to fit into pencil ends or in earrings.

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