The metric system of measurement is an
internationally agreedupon set of units for expressing the amounts of
various quantities such as length, mass, time, and temperature. As of
1994, every nation in the world has adopted the metric system, with only
four exceptions: the United States, Brunei, Burma, and Yemen (which use
the English units of measurement).
Because of its convenience and consistency,
scientists have used the metric system of units for more than 200 years.
Originally, the metric system was based on only three fundamental units:
the meter for length, the kilogram for mass, and the second for time.
Today, there are more than 50 officially recognized units for various
scientific quantities.
Measuring
units in folklore and history
Nearly all early units of size were based
on the alwayshandy human body. In the Middle Ages, the inch is reputed
to have been the length of a medieval king's first thumb joint. The yard
was once defined as the distance between English king Henry I's nose and
the tip of his outstretched middle finger. The origin of the foot as a
unit of measurement is obvious.
Eventually, ancient "rules of
thumb" gave way to more carefully defined units. The metric system
was adopted in France in 1799.
The
metric units
The metric system defines seven basic
units: one each for length, mass, time, electric current, temperature,
amount of substance, and luminous intensity. (Amount of substance refers
to the number of elementary particles in a sample of matter; luminous
intensity has to do with the brightness of a light source.) But only
four of these seven basic quantities are in everyday use by
nonscientists: length, mass, time, and temperature. Their defined units
are the meter for length, the kilogram for mass, the second for time,
and the degree Celsius for temperature. (The other three basic units are
the ampere for electric current, the mole for amount of substance, and
the candela for luminous intensity.)
The meter was originally defined in terms
of Earth's size; it was supposed to be one tenmillionth of the distance
from the equator to the North Pole. Since Earth is subject to geological
movements, this distance does not remain the same. The modern meter,
therefore, is defined in terms of how far light will travel in a given
amount of time when traveling at the speed of light. The speed of light
in a vacuum—186,282 miles (299,727 kilometers) per hour—is
considered to be a fundamental constant of nature that will never
change. The standard meter is equivalent to 39.3701 inches.
The kilogram is the metric unit of mass,
not weight. Mass is the fundamental measure of the amount of matter in
an object. Unfortunately, no absolutely unchangeable standard of mass
has yet been found on which to standardize the kilogram. The kilogram is
therefore defined as the mass of a certain bar of platinumiridium alloy
that has been kept since 1889 at the International Bureau of Weights and
Measures in Sèvres, France. The kilogram is equivalent to 2.2046
pounds.
Metric
System
MASS
AND WEIGHT
Unit

Abbreviation

Mass
of Grams

U.S.
Equivalent (approximate)

metric
ton

t

1,000,000

1.102
short tons

kilogram

kg

1,000

2.2046
pounds

hectogram

hg

100

3.527
ounces

dekagram

dag

10

0.353
ounce

gram

g

1

0.035
ounce

decigram

dg

0.1

1.543
grains

centigram

cg

0.01

0.154
grain

milligram

mg

0.001

0.015
grain

microgram

μm

0.000001

0.000015 grain

LENGTH
Unit

Abbreviation

Mass
of Grams

U.S.
Equivalent (approximate)

kilometer

km

1,000

0.62
mile

hectometer

hm

100

328.08
feet

dekameter

dam

10

32.81
feet

meter

m

1

39.37
inches

decimeter

dm

0.1

3.94
inches

centimeter

cm

0.01

0.39
inch

millimeter

mm

0.001

0.039
inch

micrometer

μm

0.000001

0.000039
inch

AREA
Unit

Abbreviation

Mass
of Grams

U.S.
Equivalent (approximate)

square
kilometer

sq
km or km^{2}

1,000,000

0.3861
square miles

hectare

ha

10,000

2.47
acres

are

a

100

119.60
square yards

square
centimeter

sq
cm or cm^{2}

0.0001

0.155 square
inch

VOLUME
Unit

Abbreviation

Mass
of Grams

U.S.
Equivalent (approximate)

cubic
meter

m^{3}

1

1.307
cubic yards

cubic
decimeter

dm^{3}

0.001

61.023
cubic inches

cubic
centimeter

cu
cm or cm^{3} or cc

0.000001

0.061 cubic
inch

CAPACITY
Unit

Abbreviation

Mass
of Grams

U.S.
Equivalent (approximate)

kiloliter

kl

1,000

1.31
cubic yards

hectoliter

hl

100

3.53
cubic feet

dekaliter

dal

10

0.35
cubic foot

liter

l

1

61.02
cubic inches

cubic
decimeter

dm^{3}

1

61.02
cubic inches

deciliter

dl

0.10

6.1
cubic inches

centiliter

cl

0.01

0.61
cubic inch

milliliter

ml

0.001

0.061
cubic inch

microliter

μl

0.000001

0.000061
cubic inch

The metric unit of time is the same second
that has always been used, except that it is now defined in a very
accurate way. It no longer depends on the wobbly rotation of our planet
(1/86,400th of a day), because Earth is slowing down. Days keep getting
a little longer as Earth grows older. So the second is now defined in
terms of the vibrations of a certain kind of atom known as cesium133.
One second is defined as the amount of time it takes for a cesium133
atom to vibrate in a particular way 9,192,631,770 times. Because the
vibrations of atoms depend only on the nature of the atoms themselves,
cesium atoms will presumably continue to behave exactly like cesium
atoms forever. The exact number of cesium vibrations was chosen to come
out as close as possible to what was previously the most accurate value
of the second.
The metric unit of temperature is the
degree Celsius, which replaces the English system's degree Fahrenheit.
It is impossible to convert between Celsius and Fahrenheit simply by
multiplying or dividing by 1.8, however, because the scales start at
different places. That is, their zerodegree marks have been set at
different temperatures.
Bigger
and smaller metric units
In the metric system, there is only one
basic unit for each type of quantity. Smaller and larger units of those
quantities are all based on powers of ten (unlike the English system
that invents differentsized units with completely different names based
on different conversion factors: 3, 12, 1760, etc.). To create those
various units, the metric system simply attaches a prefix to the name of
the unit. Latin prefixes are added for smaller units, and Greek prefixes
are added for larger units. The basic prefixes are: kilo (1000), hecto
(100), deka (10), deci (0.1), centi (0.01), and milli (0.001).
Therefore, a kilometer is 1,000 meters. Similarly, a millimeter is
onethousandth of a meter.
Minutes are permitted to remain in the
metric system even though they don't conform strictly to the rules. The
minute, hour, and day, for example, are so customary that they're still
defined in the metric system as 60 seconds, 60 minutes, and 24
hours—not as multiples of ten. For volume, the most common metric unit
is not the cubic meter, which is generally too big to be useful in
commerce, but the liter, which is onethousandth of a cubic meter. For
even smaller volumes, the milliliter, onethousandth of a liter, is
commonly used. And for large masses, the metric ton is often used
instead of the kilogram. A metric ton (often spelled tonne) is 1,000
kilograms. Because a kilogram is about 2.2 pounds, a metric ton is about
2,200 pounds: 10 percent heavier than an American ton of 2,000 pounds.
Another oftenused, nonstandard metric unit is the hectare for land
area. A hectare is 10,000 square meters and is equivalent to 0.4047
acre.
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