Watch. How watches work

Watch is a small, portable clock. People use watches to tell time and also wear or carry them as personal accessories. More than 60 million watches are sold in the United States annually.

Portable clocks were first used during the 1500's by town watchmen in Europe. As the watchmen made their rounds, they carried the portable clocks on straps around their necks. As other people began carrying timepieces, the name was shortened to watch.

Kinds of watches. Most modern watches are worn on the wrist. Before the 1920’s, they were almost always carried in the pocket or purse. In the past, women sometimes used watches as decorative accessories, wearing them as necklaces, rings, or pins. Today's watches range from plain models costing less than $5 to ones decorated with precious stones that cost more than S50,000. Watches have traditionally displayed the time by means of hands pointing to numerals or markers on a dial. This method is known as analog display. Today, lighted numerals are also used to display time by a method called digital display.

Many watches give information in addition to the passing of hours and minutes. Most also show the passage of seconds. Many show the day of the week, the day of the month, and the year. Some watches sound an alarm at any desired time. Some novelty and special function watches show the wearer's pulse or body temperature. Some include electronic games or tiny calculators for solving mathematical problems.

How watches work. Every watch has two main parts, the case and the movement, or works, inside the case. The movement shows the time, provides power to run the watch, and regulates the speed of the watch. Watches differ according to how their movements perform these functions. This article divides watches into two groups — (1) mechanical watches and (2) electronic watches—based on how they are powered.

Mechanical watches are powered by a coiled spring called a mainspring. In many watches, the main- spring is wound by turning a knob, or crown, that is connected to a shaft inside the case. Other watches, called self-winding watches, contain a weight mechanism that winds the mainspring automatically when the watch is moved about. As the watch runs, the mainspring unwinds. The power that is supplied by the unwinding mainspring turns several tiny gear wheels that are connected in a series called the train. The hands of a watch are attached to individual gear wheels that turn at specific speeds. The speed of the gear wheels is partially determined by a mechanism that is called the escapement.

The escapement includes an escape wheel, a balance wheel, a balance spring, and a pallet lever The escape wheel is connected to the train and turns when the watch runs. It also transmits energy to the balance wheel, which is the time base, or timekeeping device, of the watch. The balance spring, also called the hair-spring, makes the balance wheel oscillate (swing back and forth) at a specific frequency.

Most balance wheels oscillate 5 or 6 times a second. The pallet lever has two pallets (hooks)—one at either end—that catch on the escape wheel. Each oscillation of the balance wheel causes the pallet lever to swing, thus enabling the escape wheel briefly to escape the grip of the pallets. The escape wheel then turns slightly before the pallets again catch on it.

This catching action stops the movement of the escape wheel and also produces the characteristic ticking sound of a mechanical watch. Each slight movement of the escape wheel is transmitted through the other wheels in the train to the hands of the watch. Because the oscillations of the balance wheel regulate the speed of the escape wheel, they are responsible for the accuracy of the timepiece.

Many mechanical watches have more than 100 parts. In the most expensive watches, some parts are finished by hand to assure accuracy and durability. In addition, the pallets and various other parts of such watches are made from tiny, hard jewels, such as natural or synthetic rubies, to reduce wear. Such timepieces contain 15 or more jewels.

One type of mechanical watch sold in the United States is called a pin-lever watch. In pin-lever watches, the parts are not finished by hand, and the pallets on the pallet lever are metal pins instead of jewels. These watches are inexpensive, but they wear out sooner than do finer watches. If a pin-lever watch is adjusted very carefully by a jeweler, it can be just as accurate as a more expensive jeweled watch.

A mechanical watch has hands that show the time on a dial. The movement of a mechanical watch, far right, includes a mainspring, which powers the watch. A balance wheel regulates the watch’s speed. It moves the pallet lever, which allows the escape wheel to move slightly. This action moves a train of gears that turn the hands on the watch face

Electronic watches contain tiny quartz crystals. Some are accurate to within 60 seconds a year. The time base for this type of watch is its vibrating quartz crystal. Most crystals vibrate 32,768 times a second. Quartz-based watches contain a battery-powered electronic integrated circuit on a tiny piece of silicon that is called a chip. This chip keeps the crystal vibrating, and it translates the vibrations into electric impulses. In electronic analog watches, these impulses activate a tiny motor that moves the watch hands at the correct speeds.

Another electronic watch, the solid-state watch, also uses quartz as its time base. However, it has no moving parts. Instead, the circuits of a solid-state watch translate the time information directly into a liquid crystal display (LCD) on the watch face. Most LCD's show the time in the form of digits. In a digital LCD, a thin layer of liquid crystal is sandwiched between two layers of glass.

Digital patterns are printed onto the glass with transparent conductive coatings. Normally, these patterns are invisible. However, when an electric charge is applied to the coatings, the liquid crystal becomes visible as a dark numeral. In analog LCD watches, the liquid crystal forms the pattern of watch hands, instead of digits, to indicate the time. A liquid crystal display requires little power from the battery and therefore appears continuously. But the display cannot be seen clearly in dim light. Some LCD watches have a light that can be turned on to illuminate the face.

A digital electronic watchdisplays the time in digits thatform when electric current

passes through patterns of liquid crystal. The movement of an electronic watch, far right, includes a battery, which makes a quartz crystal vibrate. An integrated circuit on a circuit board translates the vibrations into information for display on the face of the watch.