INTERNATIONAL SYSTEM OF UNITS (SI)
The International System of Units, abbreviated as SI (from
the French name
Le Système International d’Unités
), was estab-
lished in 1960 by the 11th General Conference on Weights and
Measures (CGPM) as the modern metric system of measurement.
The core of the SI is the seven base units for the physical quantities
length, mass, time, electric current, thermodynamic temperature,
amount of substance, and luminous intensity. These base units
are:
Base quantity
SI base unit
Name
Symbol
length
meter
m
mass
kilogram
kg
time
second
s
electric current
ampere
A
thermodynamic temperature
kelvin
K
amount of substance
mole
mol
luminous intensity
candela
cd
The SI base units are defined as follows:
meter:
The meter is the length of the path travelled by light in
vacuum during a time interval of 1/299 792 458 of a second.
kilogram:
The kilogram is the unit of mass; it is equal to the
mass of the international prototype of the kilogram.
second:
The second is the duration of 9 192 631 770 periods
of the radiation corresponding to the transition between
the two hyperfine levels of the ground state of the cesium
133 atom.
ampere:
The ampere is that constant current which, if main-
tained in two straight parallel conductors of infinite length,
of negligible circular cross-section, and placed 1 meter
apart in vacuum, would produce between these conduc-
tors a force equal to 2∙10
–7
newton per meter of length.
kelvin:
The kelvin, unit of thermodynamic temperature, is the
fraction 1/273.16 of the thermodynamic temperature of
the triple point of water.
mole:
The mole is the amount of substance of a system which
contains as many elementary entities as there are atoms in
0.012 kilogram of carbon 12. When the mole is used, the
elementary entities must be specified and may be atoms,
molecules, ions, electrons, other particles, or specified
groups of such particles.
candela:
The candela is the luminous intensity, in a given di-
rection, of a source that emits monochromatic radiation of
frequency 540∙10
12
hertz and that has a radiant intensity in
that direction of 1/683 watt per steradian.
SI derived units
Derived units are units which may be expressed in terms of base
units by means of the mathematical symbols of multiplication and
division (and, in the case of °C, subtraction). Certain derived units
have been given special names and symbols, and these special
names and symbols may themselves be used in combination with
those for base and other derived units to express the units of other
quantities. The next table lists some examples of derived units ex-
pressed directly in terms of base units:
SI derived unit
Physical quantity
Name
Symbol
area
square meter
m
2
volume
cubic meter
m
3
speed, velocity
meter per second
m/s
acceleration
meter per second squared
m/s
2
wave number
reciprocal meter
m
-1
density, mass density
kilogram per cubic meter
kg/m
3
specific volume
cubic meter per kilogram
m
3
/kg
current density
ampere per square meter