Difference between revisions of "System of units"

Revision as of 10:04, 21 February 2012

The International System of Units(abbreviated SI from Resolution of the International Bureau of Weights and Measures establishing the International System of Unitsis the modern form of the metric system and is generally a system of units of measurement devised around seven SI base units and the convenience of the number ten. The older metric system included several groups of units. The SI was established in 1960, based on the metre-kilogram-second system, rather than the Centimetre–gram–second system of units system, which, in turn, had a few variants. The SI is declared as an evolving system, thus prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses, and as the precision of measurements improves.

SI is the world's most widely used system of measurement, which is used both in everyday commerce and in science.Definitions of the basic units can be found at the sites bottom, as well as the CODATA report listing values for special constants such as the electric constant, the magnetic constant and the speed of light, all of which have defined values as a result of the definition of the metre and ampere. In the International System of Units, the definition of the metre fixes the speed of light in vacuum c₀, the definition of the ampere fixes the magnetic constant (also called the permeability of vacuum) μ₀, and the definition of the mole fixes the molar mass of the carbon 12 atom M(¹²C) to have the exact values given in the bottom table. Since the electric constant is related to μ₀ by ε₀ = 1/μ₀c₀², it too is known exactly.

History

The desire for international cooperation on metrology led to the signing in 1875 of the Metre Convention, a treaty that established three international organizations to oversee the keeping of metric standards:

General Conference on Weights and Measures – a meeting every four to six years of delegates from all member states;
International Bureau of Weights and Measures - an international metrology centre in France; and
International Committee for Weights and Measures —an administrative committee that meets annually at the BIPM.

The history of the metric system has seen a number of variations, and has spread around the world, to replace many traditional measurement systems. At the end of World War II, a number of different systems of measurement were still in use throughout the world. Some of these systems were metric-system variations, whereas others were based on customary systems. It was recognised that additional steps were needed to promote a worldwide measurement system. As a result, the 9th General Conference on Weights and Measures, in 1948, asked the International Committee for Weights and Measures to conduct an international study of the measurement needs of the scientific, technical, and educational communities.

Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units that were recommended are the metre, kilogram, second, ampere, degree Kelvin (later renamed kelvin), and candela. In 1960, the 11th CGPM named the system the International System of Units. The seventh base unit, the mole mole, was added in 1971 by the 14th CGPM.

One of the CIPM committees, the CCU, has proposed a number of changes to the definitions of the base units used in SI.

Units and prefixes

The International System of Units consists of a set of units together with a set of SI. The units are divided into two classes—base units and derived units. There are seven SI, each representing, by convention, different kinds of physical quantities.

SI base units
Unit name	Unit symbol	Quantity name	Quantity symbol	Dimension symbol
metre	m	length	l (a lowercase L), x, r	L
kilogram	kg	Mass	m	M
Second	s	Time	t	T
Ampere	A	Electric current	I (an uppercase i)	I
Kelvin	K	Thermodynamic temperature	T	Θ
Candela	cd	Luminous intensity	I_v (an uppercase i with lowercase non-italicized v subscript)	J
mole	mol	Amount of substance	n	N
Note Reflist

In addition to the SI units, there is also a set of Non-SI units accepted for use with SI, which includes some commonly used non-coherent units such as the Litre.

@@ Line 1: / Line 1: @@
-The '''International System of Units'''(abbreviated '''SI''' from {{lang-fr|Système international d'unités}}<ref>[http://www.bipm.org/en/CGPM/db/11/12/ Resolution of the International Bureau of Weights and Measures establishing the International System of Units]</ref>) is the modern form of the [[metric system]] and is generally a system of [[units of measurement]] devised around seven [[SI base unit|base units]] and the convenience of the number ten. The older metric system included several groups of units. The SI was established in 1960, based on the [[Mks system of units|metre-kilogram-second]] system, rather than the [[Centimetre–gram–second system of units|centimetre-gram-second]] system, which, in turn, had a few variants. The SI is declared as an evolving system, thus prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses, and as the precision of measurements improves.
+The '''International System of Units'''(abbreviated '''SI''' from  Resolution of the International Bureau of Weights and Measures establishing the International System of Unitsis the modern form of the metric system and is generally a system of units of measurement devised around seven SI base units and the convenience of the number ten. The older metric system included several groups of units. The SI was established in 1960, based on the metre-kilogram-second system, rather than the Centimetre–gram–second system of units system, which, in turn, had a few variants. The SI is declared as an evolving system, thus prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses, and as the precision of measurements improves.
-SI is the world's most widely used [[Systems of measurement|system of measurement]], which is used both in everyday [[commerce]] and in [[science]].<ref>[http://www.bipm.org/en/si/base_units/ Official BIPM definitions]</ref><ref>[http://www.physics.nist.gov/cuu/Units/introduction.html Essentials of the SI: Introduction]</ref><!--English units are still used in some scientific applications, but note also that parsecs in astronomy, calories and mmHg in the medical sciences, and electron volts in physics are not part of the specific system of units known as SI, to just scratch the surface--><ref>An extensive presentation of the SI units is maintained on line by [http://www.physics.nist.gov/cuu/Units/units.html NIST], including a [http://www.physics.nist.gov/cuu/Units/SIdiagram.html diagram] of the interrelations between the derived units based upon the SI units. Definitions of the basic units can be found on this site, as well as the [http://physics.nist.gov/cuu/Constants/codata.pdf CODATA report] listing values for special constants such as the [[electric constant]], the [[magnetic constant]] and the [[speed of light]], all of which have defined values as a result of the definition of the metre and ampere.
+SI is the world's most widely used system of measurement, which is used both in everyday commerce and in science.Definitions of the basic units can be found at the sites bottom, as well as the [http://physics.nist.gov/cuu/Constants/codata.pdf CODATA report] listing values for special constants such as the electric constant, the magnetic constant and the speed of light, all of which have defined values as a result of the definition of the metre and ampere.
-<blockquote>In the International System of Units (SI) (BIPM, 2006), the definition of the metre fixes the speed of light in vacuum ''c''<sub>0</sub>, the definition of the ampere fixes the magnetic constant (also called the permeability of vacuum) ''μ''<sub>0</sub>, and the definition of the mole fixes the molar mass of the carbon 12 atom ''M''(<sup>12</sup>C) to have the exact values given in the table [Table 1, p.7]. Since the electric constant (also called the permittivity of vacuum) is related to ''μ''<sub>0</sub> by ''ε''<sub>0</sub> = 1/''μ''<sub>0</sub>''c''<sub>0</sub><sup>2</sup>, it too is known exactly.</blockquote>&nbsp;– CODATA report</ref> The system has been nearly [[Metrication|globally adopted]] with the United States being the only industrialized nation that does not mainly use the metric system in its commercial and standards activities.<ref>{{cite web
+In the International System of Units, the definition of the metre fixes the speed of light in vacuum ''c''<sub>0</sub>, the definition of the ampere fixes the magnetic constant (also called the permeability of vacuum) ''μ''<sub>0</sub>, and the definition of the mole fixes the molar mass of the carbon 12 atom ''M''(<sup>12</sup>C) to have the exact values given in the bottom table. Since the electric constant is related to ''μ''<sub>0</sub> by ''ε''<sub>0</sub> = 1/''μ''<sub>0</sub>''c''<sub>0</sub><sup>2</sup>, it too is known exactly.
-|url = http://en.wikipedia.org/w/index.php?title=International_System_of_Units&action=edit
-|title = Appendix G : Weights and Measures
-|work = [[The World Factbook]]
-|publisher = [[Central Intelligence Agency]]
-|accessdate =3 September 2011}}</ref> The United Kingdom has officially [[Metrication in the United Kingdom|partially adopted metrication]], with no intention of replacing [[imperial units]] entirely. [[Metrication in Canada|Canada]] has adopted it for many purposes but [[Imperial units|imperial]]/[[United States customary units|US]] units are still legally permitted and remain in common use throughout many sectors of Canadian society, particularly in the retail food, buildings trades, and railways sectors.<ref>''[http://laws-lois.justice.gc.ca/eng/acts/W-6/page-14.html/ Weights and Measures Act]''</ref><ref>''[http://laws-lois.justice.gc.ca/eng/acts/W-6/page-2.html#h-4/ Weights and Measures Act], accessed January 2012, Act current to 18 January 2012. Canadian units (5) The Canadian units of measurement are as set out and defined in Schedule II, and the symbols and abbreviations therefor are as added pursuant to subparagraph 6(1)(b)(ii).</ref>
 ==History==
-The [[metric system]] was conceived by a group of scientists (among them, [[Antoine Lavoisier|Antoine-Laurent Lavoisier]], who is known as the "father of modern chemistry") who had been commissioned by the ''[[Assemblee Nationale|Assemblée nationale]]'' and [[Louis XVI]] of France to create a unified and rational system of measures.On 1 August 1793, the National Convention adopted the new decimal ''[[metre]]'' with a provisional length as well as the other decimal units with preliminary definitions and terms. On 7 April 1795 (''Loi du 18 germinal, an III'') the terms ''[[gram]]me'' and ''[[kilogram]]me'' replaced the former terms ''gravet'' (correctly ''milligrave'') and ''[[grave (unit)|grave]]'' and on 22 June 1799, after [[Pierre Méchain]] and [[Jean-Baptiste Delambre]] completed their survey, the definitive standard metre was deposited in the French National Archives. On 10 December 1799 (a month after [[coup of 18 Brumaire|Napoleon's coup d'état]]), the metric system was definitively adopted in France.
+The desire for international cooperation on metrology led to the signing in 1875 of the Metre Convention, a treaty that established three international organizations to oversee the keeping of metric standards:
+* General Conference on Weights and Measures – a meeting every four to six years of delegates from all member states;
+* International Bureau of Weights and Measures - an international metrology centre in France; and
+* International Committee for Weights and Measures —an administrative committee that meets annually at the BIPM.
-The desire for international cooperation on [[metrology]] led to the signing in 1875 of the [[Metre Convention]], a treaty that established three international organizations to oversee the keeping of metric standards:
+The history of the metric system has seen a number of variations, and has spread around the world, to replace many traditional measurement systems. At the end of World War II, a number of different systems of measurement were still in use throughout the world. Some of these systems were metric-system variations, whereas others were based on customary systems. It was recognised that additional steps were needed to promote a worldwide measurement system. As a result, the 9th General Conference on Weights and Measures, in 1948, asked the International Committee for Weights and Measures to conduct an international study of the measurement needs of the scientific, technical, and educational communities.
-* [[General Conference on Weights and Measures]] (''Conférence générale des poids et mesures'' or CGPM) – a meeting every four to six years of delegates from all member states;
-* [[International Bureau of Weights and Measures]] (''Bureau international des poids et mesures'' or BIPM) – an international metrology centre at [[Sèvres]] in France; and
-* [[International Committee for Weights and Measures]] (''Comité international des poids et mesures'' or CIPM)—an administrative committee that meets annually at the BIPM.
-The [[Metric system#History|history of the metric system]] has seen a number of variations, and has spread around the world, to replace many traditional [[Systems of measurement|measurement systems]]. At the end of World War II, a number of different systems of measurement were still in use throughout the world. Some of these systems were metric-system variations, whereas others were based on customary systems. It was recognised that additional steps were needed to [[metrication|promote a worldwide measurement system]]. As a result, the 9th General Conference on Weights and Measures ([[CGPM]]), in 1948, asked the International Committee for Weights and Measures ([[CIPM]]) to conduct an international study of the measurement needs of the scientific, technical, and educational communities.
+Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units that were <!--"that were" expresses the preterit, and the following predicate "are" connotes the non-changing entity no matter where in time of the complements "metre," "kilogram," "second," "ampere," "kelvin," "candela"-->recommended are the metre, kilogram, second, ampere, degree Kelvin (later renamed kelvin), and candela. In 1960, the 11th CGPM named the system the ''International System of Units. The seventh base unit, the mole mole, was added in 1971 by the 14th CGPM.
-Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and [[SI electromagnetism units|electromagnetic]] quantities. The six base units that were <!--"that were" expresses the preterit, and the following predicate "are" connotes the non-changing entity no matter where in time of the complements "metre," "kilogram," "second," "ampere," "kelvin," "candela"-->recommended are the [[metre]], [[kilogram]], [[second]], [[ampere]], degree Kelvin (later renamed [[kelvin]]), and [[candela]]. In 1960, the 11th CGPM named the system the ''International System of Units'', abbreviated SI from the French name, {{lang|fr|''Le Système international d'unités''}}. The seventh base unit, the [[mole (unit)|mole]], was added in 1971 by the 14th CGPM.
+One of the CIPM committees, the CCU, has proposed a number of changes to the definitions of the base units used in SI.
-One of the CIPM committees, the CCU, has proposed a number of [[new SI definitions|changes]] to the definitions of the base units used in SI.
 ==Units and prefixes==
-The International System of Units consists of a set of units together with a set of SI. The units are divided into two classes&mdash;'''base units''' and '''derived units'''. There are seven SI, each representing, by convention, [[Dimensional analysis|different kinds of physical quantities]].
+The International System of Units consists of a set of units together with a set of SI. The units are divided into two classes&mdash;'''base units''' and '''derived units'''. There are seven SI, each representing, by convention, different kinds of physical quantities.
 {| class="wikitable" style="margin:1em auto 1em auto"
@@ Line 33: / Line 26: @@
 !Quantity name
 !Quantity symbol
-![[Dimensional analysis|Dimension symbol]]
+!Dimension symbol
 |-
-![[metre]]
+!metre
 |m
-|[[length]]
+|length
 |''l'' (a lowercase L), ''x'', ''r''
 |L
 |-
-![[kilogram]]
+!kilogram
 |kg
 |[[mass]]