The International System of Units (SI)
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The International System of Units (abbreviated to SI) was
established by the 11th General Conference on Weights and Measures (CGPM) in
1960. It is the modern metric system of measurement used throughout the World.
There are, according to NIST (2008),
seven SI base quantities:
|
Abbreviation
|
Base quantity
|
SI base unit
|
Symbol
|
|
|
length
|
metre*
|
m
|
|
|
mass
|
kilogram
|
kg
|
|
|
time
|
second
|
s
|
|
|
electric current
|
ampere
|
A
|
|
|
thermodynamic temperature
|
kelvin
|
K
|
|
|
amount of substance
|
mole
|
mol
|
|
|
luminous intensity
|
candela
|
cd
|
* In the US spelt “meter”.
Any SI derived quantity, 
,
can be expressed in terms of these SI base units by multiplication and
division, i.e.:
Its dimension is then defined to be:
where L, M, T, I, 
, N
and J are the dimensions of the corresponding SI base quantities
Definitions of each of the SI units are given in SI Unit definitions.
Examples of SI derived units include:
|
Derived quantity
|
SI derived unit
|
Symbol
|
|
area
|
square metre
|
|
|
mass density
|
kilogram per cubic metre
|
|
|
current density
|
ampere per square metre
|
|
|
luminance
|
candela per square metre
|
|
These can also be expressed using negative powers, e.g.
current density is equivalently measured in 
,
which can also be written as 
.
There are specific rules about how SI units should be written, see here.
Quantities can be expressed in terms of these base units
using ‘scientific’, i.e. ‘exponent’ notation, e.g. the mass of an electron is
approximately 
kg.
Alternatively, the SI system has a series of prefixes defining decimal
multiples and decimal divisions of the base unit, e.g. a millimetre can be
written as 0.001 m = 
m =
1 mm, see SI prefixes.
Certain SI derived units have special names and symbols, see
SI
derived units with special names and symbols.
There are some units that are outside the SI but are so
commonly used that they are widely accepted, see units
commonly accepted for use with the SI. These include units of time such as
hours and minutes and units relevant in specific disciplines such as the
electron volt. Some non-SI units are commonly used in certain fields and
according to NIST (2008) are also acceptable for use with the SI, as long as
their use is not extended beyond fields in which they are already used. Nearly
all of these are defined by reference to their value in SI units.
There are also some specialised units that are given by the
International Organisation for Standardisation (ISO) or the International
Electrotechnical Commission (IEC) which NIST also think are compatible for use
with the SI for similar reasons, e.g. the octave, phon and sone and some units
used in information technology including the baud (Bd), bit (bit), erlang (E),
hartley (Hart), and Shannon (Sh).
In some instances, physicists also use quantities expressed
in terms of fundamental
constants of nature, such as the speed of light in a vacuum. These are
sometimes used in conjunction with the SI.
There are a large number of other units that are used in
some circumstances, e.g. pints, quarts, inches, ounces etc. as per Imperial
weights and measures. Some of these are defined by reference to SI units (or in
some cases are special names for SI units or multiples or submultiples of such
units that are not accepted for use with the SI), whilst others may be
converted to SI units based on values of experimental measurements. The
Nematrian website provides users with a large number of unit conversion
functions that can be used to convert from one set of units to another.
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