Childrens Illustrated Encyclopedia

126

Clocks and watches

HAVE YOU EVER COUNTED how many times you

look at a clock in one day? Time rules everyday life.

To catch a bus, get to school, or meet a friend, you

need to be on time. Clocks and watches make this

possible. Clocks are timekeeping devices too large to

be carried; watches are portable. Some tell the time

with hands moving around a dial; others with numbers.

All clocks and watches use a controlling device, such as

a pendulum, that steadily keeps the time.

Early people relied on the passing of days, nights,

and seasons to indicate time. Later, they used other

methods, such as sundials,

water clocks, and candles

with marks on them.

Mechanical timepieces were developed

between the 15th and 17th centuries with the

invention of clockwork and the pendulum.

Springs or falling weights moved gearwheels

to drive the clocks. These clocks had hands

and a dial and could be made small enough to

allow the invention of the watch. Today, many

clocks and watches are electronic and rely on

the regular vibrations of a quartz crystal to

keep time accurately.

SUNDIAL
The sun’s shadow moves slowly
around a dial marked off in
hours. As the shadow moves, it
indicates the time. The sundial,
which was invented about 5,000
years ago in Egypt,
was one of the
earliest methods
of measuring time.

ATOMIC CLOCK

If it were to run for more than

one million years, this atomic

clock would be less than one

second off! The atomic

clock is the most accurate

of all clocks. It is controlled

by vibrating atoms and is

used in science to measure

intervals of time with

extraordinary accuracy.

DIGITAL WATCH

A battery powers a digital watch, and a

tiny quartz crystal regulates its speed.

Electricity from the battery makes the

crystal vibrate thousands of times each

second. The microchip uses these

regular vibrations to make the numbers

on the display change every second, so

the watch shows the time very precisely.

WATER CLOCK

Water flows in and out

of bowls so that changing levels

of water, or a moving float,

indicate the passing time.

This Chinese water clock dates

back to the 14th century.

PENDULUM CLOCK

In the 1580s, the Italian scientist Galileo noticed

that each swing of a suspended weight, or

pendulum, takes a fixed time. He suggested that

this regular movement could be used to control

a clock. But it was another 70 years before the

first pendulum

clock was built.

Weight pulls cord,

driving main

wheel that turns

other wheels.

This

19th-century

fob watch

was worn

on the

end of a

small chain.

Main wheel

Escape

wheel moves with

each swing of the

pendulum and turns

the second hand.

Other wheels (not

shown) turn hour

and minute hands.

Swinging

pendulum

rocks anchor.

Find out more

Electronics

Rocks and minerals

Time

ASTRONOMICAL CLOCK

This beautiful clock in Prague,

the Czech Republic, not only

shows the hours and minutes

but also the signs of the zodiac

and the phases of the moon.

Anchor

Ends of anchor engage

teeth of escape wheel.

MECHANICAL WATCHES
Mechanical watches are controlled
by the oscillations of a wheel linked
to a spring. The first watch
was invented in
Germany in
about 1500.

LCD (liquid

crystal display)

Watch unit

and strap