# Metric system

The metric system is a system of measurement developed during the French Revolution's anti-religious Cult of Reason[1] and based on permanent natural standards rather than on royal decrees. Joseph-Louis Lagrange headed the Revolutionary weights-and-measures committee that developed it and set its initial definitions.

Traditional systems of measurement all used standard lengths of parts of the human body, or standard distances that a human might walk. The metric system, by contrast, initially used the Earth itself as the ultimate standard.

In 1878 the United States signed the treaty called the Convention du Mètre. Since then, both the metric system and the United States customary units (miles, inches, pounds, etc.) have had legal status in the US. US Customary units are in fact defined by reference to metric units; that is, legally, an inch is defined to be exactly 25.4 millimeters.

In international scientific usage, the Convention du Mètre is superseded by the conventions of the International System of Units. However, many of the conventions of the metric system survive, including definitions of units that are not, strictly speaking, SI.

## Definitions

All measurement systems must start with one basic unit. For the metric system, this unit is the meter. Every other named unit of the metric system had a definition that depended upon it.

### Prefixes

All base units of the metric system have prefixes that multiply, or divide, the unit by various factors of ten (or alternatively, a thousand). They are:[2][3]

The prefix: Abbreviated: Multiplies the unit by:
yocto- y 10−24
zepto- z 10−21
atto- a 10−18
femto- f 10−15
pico- p 10−12
nano- n 10−9
micro- (Greek small) μ 10−6
milli- (Latin one thousand) m 10−3
centi- (Latin one hundred) c 10−2
deci- (Latin ten) d 10−1
deka- (Greek ten) da 101
hecto- (Greek one hundred) H 102
kilo- (Greek one thousand) k 103
mega- (Greek large) M 106
giga- (Latin gigantic) G 109
tera- (Greek wonder) T 1012
peta- P 1015
exa- E 1018
zetta- Z 1021
yetta- Y 1024

Other, related systems (see below) use the same prefixes to multiply their units.

### Length

The base unit of length, and originally the unit from which all other definitions flowed, is the meter. The original definition was "one ten-millionth the distance from the equator to the north pole." Lagrange's surveyors computed this distance (probably using a variation on Eratosthenes' tropical-shadow experiment) and inscribed it on a metal bar that they kept at the weights-and-measures headquarters building in Sèvres, France.

But this artifact, being made of metal, tended to expand and contract. Centuries later, when the scientific community realized what had happened, the task of re-standardizing all meter sticks to the equator-to-pole-times-10−7 definition was judged too onerous. Therefore, the meter was redefined. At first it was defined as 1,650,763.73 wavelengths of the orange-red line in the excitation spectrum of krypton-86. Today it is defined as 1⁄299,792,458th of a light-second.

The meter in all its multiples replaces all other units of measurement, long and short. Long distance is measured in the meter multiplied by a thousand—the kilometer. Short distances are measured in centimeters—or millimeters, or micrometers, or however small one needs to measure them.

### Volume

The base unit of volume is derived from the meter: it's the cubic meter [m3]. Traditionally, the liter[4] is used. 1 liter is defined as 10−3 m3 = 1 dm3. Capacity, traditionally measured in liters, refers to the volume occupied by a liquid. Today, one liter is defined as the volume of one kilogram of water at four degrees Celsius.

### Mass

The base unit of mass is the gram. Originally a gram was the mass of one cubic centimeter (see above), then called a milliliter, of water. The Lagrange committee fashioned an artifact exactly one thousand times as heavy as this—the volume, in short, of one liter of water—and stored this permanently at Sèvres.

Lagrange's committee also proposed a unit of mass called the grave, to be equal to one thousand grams. But the Revolutionary government rejected this name as sounding too much like the German word graf, translated as comte or count in French. Therefore, the artifact at Sèvres would be called a kilogram instead—and that artifact has that name to this day.

Mass never changes. But in addition to the prototype meter changing its length over time, scientists later realized that water has different densities at different temperatures.

Today, the standard kilogram is kept at the International Bureau of Standards headquarters in Sèvres. It remains today the only generally accepted unit of measurement based on an artifact.

## Relation to other systems of measurement

The International System of Units borrows some definitions and concepts from the metric system—most notably the meter and the kilogram—and adds other definitions of other quantities, most notably time, force, amount of substance, plane angle, solid angle, and luminous intensity. The liter is not the unit of volume in that system, however, but liter is accepted for use with the SI.

In addition to the International System (or in French, Système International, or SI for short), scientists have invented other systems of measurement that include units of time, force, energy, and electric charge. The most popular are the meter-kilogram-second and centimeter-gram-second systems.

Various governing authorities have established conversion ratios to assist people in converting between the metric system and other, more traditional units of measurement. The definitions of US Customary units in the United States is an example of this.

The metric system is more efficient and easier to learn, as units are based on powers of 10 and are better related to each other than with the imperial system.

## Usage worldwide

According to CIA's World Factbook,[5] the SI system has been officially adopted in all countries of the world, with an exception of Burma, Liberia and the United States of America. The inertia of the United States, unfortunate though it may be, is rooted in an enormous amount of existing industrial equipment and usage. Companies that do any international business have to do a lot of conversion work in order to make their products compatible with the rest of the world. This is slowly leading to more widespread adoption of the SI system in the United States, but progress is very slow.

Some space missions have failed because of unit incompatibility in guidance software between US and SI measurements.

## References

1. However, according to researcher Pat Norton, the system was described a century earlier by Englishman John Wilkins. Australian Broadcasting Corporation, 15 July 2007 (retrieved 15 July 2007.)
2. Wile, Dr. Jay L. Exploring Creation With Chemistry. Apologia Educational Ministries, Inc. 1998
3. Common metric prefixes
4. http://www.bipm.org/en/si/si_brochure/chapter4/table6.html
5. CIA World Factbook, Appendix G