Watch and Clock

Clock and Watch Time, the fourth dimension, profoundly influences all aspects of life. People are constantly aware of the passing of time in their daily activities. Without the ability to synchronize comings and goings at school and work, complex societies would simply be unable to function. Until a few hundred years ago there was no way to tell time more accurately than the nearest hour. The introduction of accurate timepieces played a major part in the development of modern civilization

Watches and clocks are the most common devices for measuring time. The first portable timekeeper, the watch was developed shortly after 1500. Clocks are usually larger and stationary. With recent advances in automation and electronics, modern watches and clocks have become less expensive and more accurate. An especially accurate time-measuring device, called the chronometer, is a specialized clock. Some chronometers are capable of measuring time to a fraction of a trillionth of a second, which amounts to an error of one second every million years.
Clocks are made not just to tell time. They are also used for decoration or entertainment. An interesting example of early clock entertainment is the great astronomical clock in Prague, Czech Republic. It records not only the time but the day of the year and the positions of the sun and the moon. At the stroke of the hour a miniature performance occurs. A cock crows, figures beside the dial do a pantomime, and a replica of a skeleton tolls the hour.
A more familiar decorative clock is the cuckoo clock. Mechanical birds emerge from the clock on the hour to note the time of day with song. Clocks are also commonly used to announce particular times. An alarm clock or clock radio is in nearly every household. There are even multipurpose watches that function as mini-calculators, stopwatches, and radios.

Mechanical Timepieces

The mechanical watch and clock were major improvements over earlier time-measuring devices such as the hourglass, a narrow-waisted glass that measures an hour or other increment of time by the time in which sand, water, or mercury runs from an upper compartment into a lower compartment. Invented in the mid-1300s, the earliest mechanical clock was driven by falling weights attached to gear trains. Because it was regulated by friction, it was extremely inaccurate, by as much as two hours a day.

Pendulum.

The major scientific breakthrough that led to the development of accurate clocks occurred in 1583, when Galileo demonstrated that successive beats of a pendulum always take place in the same length of time, irrespective of the distance through which the pendulum swings. It was not until about 70 years later, however, that this discovery was applied to clock regulation. In 1656 Christiaan Huygens designed the first weight-driven clock in which motion was controlled by a pendulum. About a decade later the English physicist Robert Hooke added an escapement, producing the first accurate pendulum clock. By 1680 a minute hand was added to the hour hand, and a few years later the pendulum clock had a second hand.The large pendulum clock, later called the grandfather clock, has a long pendulum end enclosed in a tall case. It is driven by a heavy weight whose descent causes a drum to rotate. The motion of the drum is transmitted by gears to an escape wheel. The teeth on the escape wheel are caught by the detent, a rocking bar with two pallet arms and teeth, at the top of the pendulum rod. The detent and escape wheel together form the escapement. Each time the pendulum swings, the detent releases the teeth on the escape wheel. The drum, pulled by the weight, turns slightly until the other arm of the detent again catches a tooth on the wheel. When the pallet arm disengages from a tooth on the escape wheel, the pendulum is momentarily free to swing. The weight gives up energy that keeps the pendulum swinging. This motion is transferred by a sequence of gears to the hour, minute, and second hands. By a mechanism called the gravity escapement, the arms on each side of the pendulum are raised by the turning escape wheel. They are then allowed to fall against the pendulum, keeping it in motion.
The basic design of this mechanical clock remained unchanged for centuries, though accuracy was improved. The same design is used for clocks placed in the towers of churches and public buildings. The most famous tower clock in the world is the Westminster clock on the Victoria Tower of the Houses of Parliament in London. It is more popularly called Big Ben, after its giant bell on which the hours are struck. It has four dials, each 23 feet (7 meters) in diameter and located 180 feet (55 meters) above the ground. Big Ben's pendulum is 13 feet (4 meters) long and weighs 700 pounds (318 kilograms).

Mainspring.

Watches were developed later than clocks. The first portable timekeeper or watch was produced during the Renaissance. A weight and a crude escapement were suitable in clocks but could not be used in a portable watch. In the early 1500s a coiled metal spring that transmitted energy as it unwound was substituted for the weight. By the 1580s, around the time of Galileo's pendulum experiments, the first commercial watches were being produced in small quantities in Nuremberg, Germany. Known as Nuremberg eggs, these early watches were portable but had only an hour hand because the escapement mechanism was too inaccurate to show minutes.
Hooke, who introduced the basic pendulum and escapement mechanisms in clocks in the 1660s, applied the same idea to watches. He attached a small balance unit to the mainspring unit. A balance spring, or hairspring, was used to control the oscillations of the balance unit. This design formed the basis for all subsequent mechanical watches.
The mechanism of a mechanical watch is made up of six different parts. These are the mainspring, which is the source of energy; the gear train, which transmits the energy; the dial train, which governs the movement of the hands; the winding and setting mechanism; the escapement and balance unit, which controls the release of energy; and the plates, or framework, which enclose and protect the movement.
In operation the mainspring unwinds to turn the barrel, which drives the escape, or great, wheel through a gear train. The wheel moves in one direction until stopped by a pallet arm of the pallet and fork—the part analogous to the detent of a clock. The pallet and fork is attached to the balance wheel, which is then given an impulse from the escape wheel.
The escapement and balance assembly are essential to an accurate watch mechanism because they control the release of energy from the mainspring and the time in which each release of energy is made. The oscillations of the balance, the regularity of its movement, are controlled with a hairspring or escapement spring. When the balance wheel moves in one direction, its tiny hairspring is wound. As the escape wheel is stopped by one of the pallet arms, the hairspring uncoils. The balance wheel, in turn, is connected to the coiled mainspring, which winds as the balance wheel moves. When the pallet and fork releases the escape wheel, the wheel rotates one tooth until caught again by the other pallet arm. The balance wheel then turns in the opposite direction until the other pallet arm stops the escape wheel. This cycle continues and causes the familiar ticking sound of the watch. The escape wheel is stopped and released every one fifth of a second. Thus the balance wheel swings back and forth 300 times a minute, or 432,000 times a day. As in a clock, a series of gears transfers the motion of the escape wheel to the hour, minute, and second hands.
To keep friction to a minimum, special bearing surfaces are used for moving parts in watches. These surfaces are tiny, finely ground jewels, usually sapphires, rubies, or diamonds. They are used because they can be ground with great precision, are extremely hard, and do not corrode.
Synthetic jewels, made from fused powdered alumina (aluminum oxide), are also used. A watch may have from seven to 23 jewels. The value of a watch may be based on the number of jewels in the movement, provided the jewels serve as bearing surfaces. There are four common types of jewel bearings currently used in the manufacture of mechanical watches.

Electronic and Electrical Timepieces

Although mechanical clocks and watches are still manufactured in large quantities, they are increasingly being supplanted by electronic and electrical timekeepers. These newer devices are cheaper, easier to manufacture, and considerably more accurate.

Tuning fork.

Introduced in 1953, the tuning-fork watch was the first commercially successful electronic watch. Instead of a mechanical mainspring, it had a battery-driven tuning fork. The vibrations of the tuning fork at high frequencies changed the electrical current induced in tiny electromagnets. These changing currents were used to drive tiny electric motors that turned the hands of the watch face.

Quartz.

In 1929 a clock driven by a quartz crystal oscillator was developed as a scientific time-measuring device. Quartz crystals for watches were developed shortly afterward. By the 1960s these purely electrical oscillators were gradually replacing the tuning-fork watches. When a tiny quartz crystal is subjected to an alternating electric field, it produces an extremely steady fixed oscillation at frequencies around 100,000 Hertz (cycles per second). The oscillation is converted to an alternating current and reduced to a frequency to measure time. The current drives the clock motor.
In a quartz watch the oscillating electric current is fed to a miniaturized digital circuit that counts the oscillations and produces a small pulse of current approximately every thousand oscillations. This current is used either to drive the stopping motor that turns the hands on a conventional analog watch or to change the digits on a digital-display watch. The oscillator and the motors in the quartz watches are powered by a miniature replaceable battery.
Digital-display watches have no moving parts. Instead of relaying the time with gears and dial hands, a digital display uses an array of optical devices whose light-emitting or reflecting character can be changed by small electrical pulses. These pulses allow the formation of a lighted display of numerals for the time or date. Light-emitting diodes (LEDs) were the first digital-display device used in watches. A form of luminescent lamp, the LED device is a crystalline semiconductor diode. When current flows through the diode, electrons combine with localized positive charges and drop to a state of lower energy. Part of the released energy is emitted as a photon of light. The color of light emitted depends on the type of crystal used. In the 1970s, LEDs were supplanted by liquid-crystal devices (LCDs). These elements are tiny capsules filled with liquid crystals. When the molecules of the liquid are subjected to an electric field, the liquid crystals align, and light reflects off them. Without the field, their alignment reverts to its original, nonreflecting arrangement, so the elements appear dark. Combinations of LCDs are arranged to form patterns of reflected light that spell out the numerals of the time or date.
Production costs for accurate quartz watches with digital or analog faces have been greatly reduced. Many of the inexpensive watches are accurate to within seconds every year. This is rivaled by the more expensive and carefully crafted mechanical watches. The cost of modern electronic watches is more often determined by decorative, nonfunctional characteristics of the watch.
Electric clocks generally operate under the same principles as quartz watches. The major difference is that clocks can be powered by either an alternating house current or batteries. Electric clocks also usually have larger displays. The timing mechanisms in early electric clocks were controlled by the 60-cycle alternating current in homes.

Precision Time-Measuring Devices

Initially, the purpose of clocks and watches was primarily social—to coordinate the times that merchants and craftsmen would meet, come to work, or exchange goods. For this purpose extremely high accuracy was unnecessary. With the development of transatlantic commerce, however, and its expansion in the 17th and 18th centuries, accurate time measurements were needed to determine longitude at sea.
To determine longitude to within a mile, the time must be accurate to within about three seconds each day; for a voyage of a month, the watch or clock must be accurate to a tenth of a second each day. A precision timekeeper with this level of accuracy is called a chronometer.
The task of making a highly accurate mechanical clock or watch was not easy since temperature, humidity, and outside movements can affect the accuracy of the instrument. The rate at which a pendulum swings, for example, changes with even a slight variation in the pendulum's length. Materials expand when heated. Metals are especially sensitive to temperature changes because they are generally good heat conductors. Since pendulums made of metal lengthen and shorten with increases and decreases in temperature, the timing accuracy can be altered. Wood pendulums are similarly sensitive to humidity. For a ship-borne timekeeper even the motion of the ship can throw off the balance wheel of a watch or the pendulum of a clock.

Chronometer.

One of the earlier chronometers won a contest for its accuracy. After a disastrous 1707 shipwreck in which 2,000 lives were lost, the British government offered a reward of 20,000 pounds to anyone who could tell time to determine longitude with an error of no more than a mile. The prize was awarded in 1765 to John Harrison, an English carpenter and craftsman, who minimized all the possible sources of error by sheer mechanical skill. Harrison's timekeeper was an enormous 66-pound (30-kilogram) chronometer. His next machine was no bigger than a watch, but just as accurate. Almost simultaneously, Pierre LeRoy, a French scientist, used theoretical calculations and experiments to design another, more accurate, chronometer. This essentially solved the problem of longitude determination.
The development of electric and quartz clocks in the late 1800s and early 1900s led to chronometers with accuracies surpassing those of the best mechanical chronometers. In the mid-1950s, quartz chronometers were outmoded by chronometers whose oscillations derived from the vibrations of collections of atoms—the first atomic clocks.

Atomic clock.

The most precise timekeepers are atomic clocks. To regulate the electronic components in the clock, atomic clocks use masers, similar in principle to lasers, to amplify and count the microwaves— high-frequency radio waves—emitted by vibrating atoms . The first atomic clocks had errors of one second in 30 years—one part in a billion. By the mid-1990s, an atomic clock utilizing cesium atoms was being used as the fundamental standard of time. The cesium clock was guaranteed to neither gain nor lose one second in 1,000,000 years.

Other clocks.

Precise time measurement is possible by measuring the radio pulsations coming from collapsed stars, called pulsars. The periodic radio pulses, caused by the pulsar's rapid rotation in its own magnetic field, may yield accuracies that surpass those theoretically achievable with atomic clocks.
Since the 1970s, scientific timekeepers have succeeded not only in measuring time more accurately but also in measuring shorter intervals of time. Using lasers to produce extremely short and repeatable pulses of light, scientists are able to measure events happening in time intervals as short as 1,000 trillionth of a second.
Not all precision time-measuring devices are laboratory instruments. The familiar stopwatch, which can measure time to the nearest fifth, tenth, or even 100th of a second, is also a chronometer.

Watch and Clock Production

Electronic watches and clocks are made by highly automated mass-production methods. The production of the main precision element in modern watches and clocks, the tiny quartz crystal, is entirely automated. Other major components produced by automation are the integrated circuits, or microchips, that drive the digital display, the small electric motor, and, in an analog timepiece, the gear train and hands of the watch or clock.
Assembly of these components into the finished timepiece is also automated. In Japan, a leader in watchmaking, watch companies were among the first to introduce assembly robots into factories.
The relatively small number of separate parts in electronic watches and clocks has been an important factor leading to total automation of assembly. For mechanical watches and clocks, automation is considerably more difficult, since a quality mechanical watch contains between 120 and 160 parts. Many of the parts are also very tiny—some watch screws are so small they look like metal filings unless viewed under strong magnification. Extreme accuracy is needed in the manufacture of tiny parts. Positional accuracy, for example, must exceed ³/₁₀₀ inch. Minute adjustments also are critical to an accurate mechanical timepiece. A balance wheel, for example, is carefully balanced using balance-wheel screws that weigh less than ¹/_60,000 ounce.
The pressure to keep mechanical watch prices within competitive bounds has forced leading Swiss manufacturers toward increasing automation. In some cases, watches have been redesigned to facilitate automated or semiautomated assembly by robotic equipment. The greater cost of human labor to produce mechanical watches has contributed to the development of their status as luxury items. The finest mechanical watches, often decorated with expensive gold cases and jewels, though less accurate than quartz watches, are valued for their appearance and workmanship as well as for their usefulness.

Early History of Timekeeping

The measurement of time dates back some 10,000 years and coincides with the development of agriculture. Farmers used timekeeping to determine the best planting periods. Early hunters used it to anticipate seasonal movement of game. Archaeologists have uncovered bones from this period inscribed with what are believed to be primitive lunar calendars.
Despite these early efforts, the first people to develop extensively a means of telling time with calendars and clocks were the Egyptians. By around 2800 BC they had established a 365-day calendar, based on their observations of the rising and setting of bright stars such as Sirius and of the periodic inundations of the Nile, upon which their agriculture relied. By 2100 BC the Egyptians had devised a way to divide the day into 24 hours. Around the same time, they made the first sundials, or shadow clocks, to measure time during the day. A sundial indicates the time of day by the position of the shadow of some object on which the sun's rays fall. The Egyptian shadow clock consists of a straight base with a raised crosspiece at one end. A scale with time divisions is inscribed on the base. The clock is set east-west and is reversed at midday. At night the Egyptians observed the positions of the stars, or they used burning ropes or candles.
By 1500 BC Egyptian scribes and priests had invented another, more accurate, way of telling time—the water clock. This device, called a clepsydra, uses the steady dripping of water from a vessel to drive a mechanical device that indicates the hour. The water clock remained in use until the development of mechanical clocks nearly 3,000 years later.
A major theoretical advance in time measurement was made when Babylonian astronomers determined that the length of the day, as measured from sunrise to sunrise, was not constant throughout the year but varied slowly with the seasons. This effect (termed equation of time) is caused by the varying distance between the Earth and the sun as the Earth moves in an elliptical orbit. Because of this effect, noon can be as much as a half hour before or after the time when the sun is highest in the sky.
Over the centuries the mechanisms driven by the water were improved and became more elaborate. Around 270 BC the Alexandrian engineer Ctesibios designed water clocks that rang bells, moved puppets, and caused mechanical birds to sing—foreshadowing the cuckoo clock. From AD 700 to 1000 Arab astronomers improved the sundial, using their knowledge of astronomy to correct for the varying motion of the sun during the course of a year. When mechanical clocks arrived in medieval Europe, timekeeping methods had not advanced much further than those of the Egyptians. Measurement of short time intervals, however, was possible with the hourglass. The search for accurate clocks began with the burgeoning late medieval commerce and the first fruits of the scientific revolution.