SI

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Alternate uses: see SI (disambiguation)

The International System of Units, abbreviated SI (for the French phrase Systeme International d'Unites), is the most widely used system of units. Along with the older cgs (centimetre, gram, second) system, SI is sometimes referred to as the metric system (especially in the United States, which has not widely adopted it, and the UK, where conversion is incomplete).

Table of contents

1 Origin 2 SI writing style 3 Base Units 3.1 Semi-Base Units 3.2 Derived Units of Special Importance 4 Notes 5 Related topics 6 External links 7 Further reading

Origin

The units of the SI system are decided by international conferences organised by the Bureau International des Poids et Mesures (International Office of Weights and Measures). The SI system was first given its name in 1960, and last added to in 1971.

SI is built on seven SI base units, such as the kilogram and metre. These are used to define various SI derived units. SI also defines a number of SI prefixes to be used with the units: these combine with any unit name to give subdivisions and multiples. For example, the prefix kilo denotes a multiple of a thousand, so the kilometre is 1,000 metres, the kilogram 1,000 grams, and so on.

SI writing style

• Symbols are written in lower case except for in symbols where the unit is the same as the name of a person, or derived from the name of a person. This means that the symbol for the SI unit for pressure, named after Blaise Pascal, is Pa, whereas the unit itself is written pascal. The official SI brochure lists the symbol for the litre as an allowed exception to the capitalization rules: either capital or lowercase L is acceptable.

• Symbols are written in singular e.g 25 kg (not "25 kgs")

• It is preferable to keep the symbol in upright roman type (for example, kg for kilograms, m for metres), so as to differentiate from (mathematical and physical) variables (for example, m for mass, l for length).

• A space between the numbers and the symbols: 2.21 kg, 7.3·10² m²

• SI uses spaces to separate decimal digits in sets of three. e.g. 1 000 000 or 342 142 (in contrast to the commas or dots used in other systems e.g. 1,000,000 or 1.000.000).

• SI used only a comma as the separator for decimal fractions until 1997. The number "twenty four and fifty one hundredths" would be written as " 24,51 ". In 1997 the CIPM decided that the British full stop (the "dot on the line", or period) would be the decimal separator in text whose main language is English (" 24.51 "). No allowances were made for alternate decimal separators in other languages; except in English, the comma remains the official standard.

The system can legally be used in every country in the world, and in many countries its use is obligatory. Those countries that still give official recognition to non-SI units (e.g. US, UK) define them in terms of SI units. It was adopted by the 11th General Conference on Weights and Measures (CGPM) in 1960. (See weights and measures for a history of the development of units of measurement.)

Base Units

The following are the fundamental units from which all others are derived, they are dimensionally independent. The definitions stated below are widely accepted.

Name	Unit Symbol	Measure Of	Definition
metre	m	Length	The unit of length is equal to the length of the path travelled by light in a vacuum during the time interval of 1/299 792 458 of a second
kilogram	kg	Mass	The unit of mass is equal to the mass of the international prototype kilogram(a platinum-iridium cylinder) kept at the Bureau International des Poids et Mesures (BIPM), Sevres, Paris.
second	s	Time	The unit of time is the duration of exactly 9 192 931 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of caesium-133 atom.
ampere	A	Electrical Current	The unit of electical current is the constant current which, if maintained in two straight parallel conductors, of infinite length and negligible cross-section, placed 1 metre apart in a vacuum, would produce a force between these conductors equal to 2 x 10 -7 newton per metre of length.
kelvin	K	Absolute Temperature	The unit of thermodynamic temperature (or absolute temperature) is the fraction 1/273.16 (exactly) of the thermodynamic temperature at the triple point of water.
mole	mol	Amount of Substance	The unit of amount of substance is the amount of substance which contains as many elementary entities as there are atoms in 0.012 kilogram of pure carbon-12. [elementary entities may be atoms, molecules, ions, electrons, or particles].
candela	cd	Luminous Intensity	The unit of luminous intensity is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Semi-Base Units

The following units are debated as to whether they are base, or derived. Most consider them a "class of dimensionless derived units" (as suggested by the International Committee on Weights and Measures (CIPM)).

Name	Unit Symbol	Measure Of	Definition
radian	rad	Angle	The unit of angle is the angle subtended at the centre of a circle by an arc of the circumference equal in length to the radius of the circle
steradian	sr	Solid Angle	The unit of solid angle is the solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere having an area r2.

Derived Units of Special Importance

Base units can be compounded to derive units of measurement for other quantities. As any of the base units can be put together to create such quantities not all have names. The following are of particular import, and commonly used.

Measure of	Name	Unit Symbol	True Symbol
Frequency	hertz	Hz	s-1
Force	newton	N	kg m/s 2
Energy	joule	J	N m = kg m2/s2
Power	watt	W	J/s = kg m2/s3
Pressure	pascal	Pa	N/m2 = kg/m s2
Luminous flux	lumen	lm	cd sr
Illuminance	lux	lx	cd sr/m2
Electric Charge	coulomb	C	A s
Electric Potential Difference	volt	V	J/C = kg m2/A s3
Electric resistance	ohm	Ω	V/A = kg m2/s3 A2
Electric capacitance	farad	F	A2 s4/kg m2
Magnetic flux	weber	Wb	kg m2/s2 A
Magnetic flux density	tesla	T	Wb/m2 = kg/s2 A
Inductance	henry (inductance)>henry	H	kg m2/s2 A2
Electric conductance	Siemens_(unit)>siemens	S	Ω -1 = kg-1 m-2 s3 A2
Activity (of radioactive source; nuclear transformations per unit time)	becquerel	Bq	s-1
Absorbed dose (of ionising radiation)	gray (unit)>gray	Gy	J/kg
Dose equivalent (of ionising radiation)	sievert	Sv	J/kg

Notes

Americans frequently spell 'metre' as 'meter', and 'litre' as 'liter'; however 'metre' and 'litre' are the official BIPM names for these units, although the American usage has been approved by the US government. The official US spelling for 'deca' is 'deka'.

The unit 'gram' is also sometimes spelled 'gramme' in English speaking countries, though that is an older spelling. Several other languages use the American spelling. In written practice only the abbreviated (prefixed) symbols are used, avoiding the spelling issue.

Related topics

• Other measurement systems: imperial units, U.S. customary units, Metre-tonne-second system of units
• Metrication
• Metric system in the United States
• Metrology
• UTC (Coordinated Universal Time)
• Binary Prefixes - used to quantify large amounts of computer data
• Orders of magnitude

External links

Official

• SI maintenance agency (home page)
• BIPM reference (SI reference)

Information

• US NIST reference on SI
• SI -- Its History and Use in Science and Industry
• Nelson, Robert A., "The International System of Units Its History and Use in Science and Industry". Via Satellite, February 2000.

Further reading

• I. Mills, Tomislav Cvitas, Klaus Homann, Nikola Kallay, IUPAC: Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell Science Inc 1993, ISBN 0632035838.