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The '''metric system''' is the common name for the ''Système International d'Unités'' or ''International System of Units.'' In scientific work, it is abbreviated ''SI.'' It is called the "metric system" because its units of length are based on the meter.
The [[metric system]] is a system of measurement developed during the [[French Revolution]] and based on permanent natural standards rather than on royal decrees. [[Joseph 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.
{| class="wikitable" style="text-align:left"
In 1878 the [[United States]] signed the treaty called the ''Convention du Mètre.'' Since then, ''both'' the metric system and the [[US Customary system]] (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In international scientific usage, the ''Convention du Mètre'' is superseded by the conventions of the [[International System of Units]].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:
{| class="wikitable"
|-
|-
! Prefix !! Numerical Meaning
! The prefix:
! Abbreviated:
! Multiplies the unit by:
|-
|-
| micro (<math>\mu</math>) || 0.000001
| atto-
| a
| 10<sup>-18</sup>
|-
|-
| '''milli (m)''' || '''0.001'''
| femto-
| f
| 10<sup>-15</sup>
|-
|-
| '''centi (c)''' || '''0.01'''
| pico-
| p
| 10<sup>-12</sup>
|-
|-
| deci (d) || 0.1
| nano-
| n
| 10<sup>-9</sup>
|-
|-
| deca (D) || 10
| micro- ([[Greek]] ''small'')
| μ
| 10<sup>-6</sup>
|-
|-
| hecta (H) || 100
| milli- ([[Latin]] ''one thousand'')
| m
| 10<sup>-3</sup>
|-
|-
| '''kilo (k)''' || '''1,000'''
| centi- (Latin ''one hundred'')
| c
| 10<sup>-2</sup>
|-
|-
| Mega (M) || 1,000,000
| deci- (Latin ''ten'')
|}<ref>Wile, Dr. Jay L. ''Exploring Creation With Chemistry''. Apologia Educational Ministries, Inc. 1998</ref>
| d
| 10<sup>-1</sup>
|-
| deka- (Greek ''ten'')
| D or dk
| 10<sup>1</sup>
|-
| hecto- (Greek ''one hundred'')
| H
| 10<sup>2</sup>
|-
| kilo- (Greek ''one thousand'')
| k
| 10<sup>3</sup>
|-
| mega- (Greek ''large'')
| M
| 10<sup>6</sup>
|-
| giga- (Latin ''gigantic'')
| G
| 10<sup>9</sup>
|-
| tera- (Greek ''wonder'')
| T
| 10<sup>12</sup>
|}<ref>Wile, Dr. Jay L. ''Exploring Creation With Chemistry''. Apologia Educational Ministries, Inc. 1998</ref><ref>[http://lamar.colostate.edu/~hillger/common.html Common metric prefixes]</ref>
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<sup>-7</sup> 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 the ''liter''. Originally this was the volume of a cube that was one ''decimeter'' on an edge. Today the definition has changed, but only because the definition of ''length'' has changed, while the definition of ''mass'' (see below) has not. To understand what a standard liter is today (and why it is no longer exactly equal to one thousand cubic centimeters), one must turn to the definition of the unit of:
==References==
=== 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.
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 [[temperature|temperatures]].
Today, the standard [[kilogram]] is kept at the International Bureau of Standards headquarters in Sevres. It remains today the only generally accepted unit of measurement based on an artifact. But the definition of the liter has now changed, and is no longer dependent on the length of the meter. Today, one liter is defined as the volume of one kilogram of water at four degrees [[Celsius]]. One milliliter is simply one-thousandth of this, and one milliliter of water at this temperature weighs one gram. But to say that one liter is "one thousand cubic centimeters" is no longer correct, simply because the definition of length has changed, while that of mass has not.
== 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.
In addition to the International System (or in French, ''Systèm 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.
== References ==
<references/>
<references/>
== See Also ==
* [http://creationwiki.org/Metric_system Metric system] by [[CreationWiki]]
* [http://en.wikipedia.org/wiki/Metric_system Metric system] by [[Wikipedia]]
* [http://www.essex1.com/people/speer/metric.html Metric system] by Essex1.com
* [http://lamar.colostate.edu/~hillger/ The United States Metric Association]
[[Category:Science]]
[[Category:Science]]
[[Category:Unit of measure]]