Childrens Illustrated Encyclopedia

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560

How far away is the moon? How deep are the oceans? How tall are you?

How hot is it on Mars? It is possible to measure all of these things and many

more. Every day we need to make measurements. In cooking, for example,

a recipe requires the correct weight of each ingredient, and

once mixed, the ingredients have to be cooked at a certain

temperature. we make measurements using measuring

instruments. for example, a thermometer measures

temperature, a ruler measures distance, and a clock

measures time. all measurements are based on a

system of units. Time, for example, is measured

in units of minutes and seconds; length

is measured in feet or meters.

Precise measurements are

very important in science

and medicine. Scientists have

extremely accurate measuring

instruments to determine

everything from the tiny

distance between atoms in

a piece of metal to the

temperature of a distant

planet, such as Neptune.

Body measurements

The earliest systems of units

were based on parts of the

human body, such as the

hands or feet. Both the

ancient Egyptians (about

3000 bce) and the romans

(from about 800 bce) used

units of this kind. However,

body measurements present

a problem. They always give

different answers because

they depend on the size of

the person making the

measurement.

UNITS of MEaSUrEMENT

when you measure something, such as height,

you compare the quantity you are measuring

to a fixed unit, such as a foot or a meter. Scientists

have set these units with great precision, so that if

you measure your height with two different rulers,

you will get the same answer. The meter, for

example, is defined (set) by the distance traveled

by light in a specific time. This gives a very precise

measure of length.

LENgTH aNd arEa

Tape measures and rulers indicate length.

They can also be used to calculate area, which

indicates, for example, the amount of land a

football field takes up or the amount of material

needed to make a coat.

TIME

Time is measured in hours,

minutes, and seconds.

a digital stopwatch can

measure the time of a race to

the nearest hundredth

of a second.

wEIgHT

weighing scales measure

how heavy things are.

They compare the

weight of an object in

one pan with a

known weight that

sits in the other pan.

METrIc SySTEM

a system of measurement defines fixed

units for quantities, such as weight and

time. Most countries use the metric

system, which was developed in france

about 200 years ago. Then the meter

was fixed as the 10-millionth part

of the distance between the North

Pole and the equator. The meter is

now fixed using the speed of light.

We can also measure things that we cannot

see. This digital meter measures the strength

of an electric current in amperes (A).

The

hand was divided

into four fingers.

The foot originated

in ancient Rome.

IMPErIaL SySTEM

Units of the imperial system include

inches and feet for length, pints and

gallons for volume, and pounds and

tons for weight. The imperial system

is used mainly in the US.

One cubit

The cubit and the

hand were

ancient

Egyptian

units.

Find out more

clocks and watches

Egypt, ancient

Mathematics

roman empire

Many imperial units were first used in

ancient Rome. The mile was 1,000 paces,

each pace being two steps. The word mile

comes from the word for 1,000 in Latin.

Weights and measures

Scale pan

carries

fixed weights

in units of

ounces or

grams.

VoLUME

Volume measures the

amount of space that

an object or liquid takes

up. a measuring jug

measures the volume

of a liquid. By reading the

level of the liquid against

a scale of units, you can

find the volume of the

liquid in the jug.

Thermometers

measure

temperature.

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