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International System
SI Base Units | SI Derived Units | SI Prefixes
SI
The International System of Units,
abbreviated SI (for the
French phrase Système International d'Unités), 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 its use in everyday
commerce, and the
UK
where conversion is incomplete).
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.
-
Symbols are written in
lower case except for in symbols where the unit is eponymous, or derived
from the name of a person. This means that the
symbol for the SI unit for pressure, named for
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 meters), 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.)
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', though
Americans use the international spelling more often than the American one.
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.
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