Ruthie Robertson, age 13, of Johnson City, Tenn., for her question:
WHEN WAS THE FIRST WATCH MADE?
A watch is nothing more than a clock designed and made to be easily carried. Most watches have dials that show the passage of hours and minutes by the movement of two hands. The first watch is supposed to have been invented early in the 1500s in Germany, by a locksmith named Walter Henlein.
Henlein's watch was nothing more than a portable clock. It was so heavy that it had to be hung from a special belt around the waist.
Before Henlein invented his watch, time was told by clocks that used heavy weights. But Henlein invented a mainspring as the source of power to turn the wheels.
The manufacture of handmade watches quickly spread across Europe. Switzerland soon led all other countries in watch making and still ranks high.
Early watches had only an hour hand. The minute hand wasn't developed until 1687.
In the early 1800s, new machinery made it possible to produce accurate watches cheaply.
Until World War I, the large pocket watch was the most common type of watch for men. It often had what was called a hunting case a hinged cover that covered and protected the face.
The electric watch was introduced in the United States by Hamilton in 1957. In the electric watch, an energy cell or battery provides the power. The energy cell supplies electric power to a coil mounted on the balance wheel. As the coil passes through the magnetic field of two permanent magnets, an impulse from the balance wheel sends power to the train.
Digital watches became popular in the 1970. In the digital watch, power from the battery activates a quartz crystal. Electronic circuits translate vibrations from the crystal into hours, minutes and seconds. The information is then displayed by means of numbers that light up on the blank face of the watch.
Modern watches are extremely accurate. A good watch keeps time to within a half minute a day.
The best watches have moving parts with bearings made of jewels. At one time these were genuine jewels, but they are now mostly artificial. Watches usually contain from seven to 23 jewels. The average low priced wristwatch of today contains over 100 parts.
The frames of most high grade watches today are made from heavy metal plates. The frame of cheaper watches is usually made of two rather thin plates of metal. These two frames are held apart by means of pillars, which are merely posts of brass that help to serve as a framework.
The power unit of today's watch is the mainspring. This spring is a thin ribbon of highly tempered steel. The mainspring is inside a barrel which has teeth on its outer edge so that it can serve as a gear wheel.
Self winding wristwatches have a swinging weight geared to the mainspring. The movements of the wearer's arm causes the weight to swing and wind the mainspring.