In
1960 the Conference Generale des Poids et Mesures ( CGPM ), which is
the international
authority on the metric system, accepted a universal, practical system
of units and gave it
the name Le Systeme International d'Unites with the abbreviation SI.
Since then, this most modern
and simplest form of the metric system was introduced throughout the
world and by 1970's more than
20 countries, including established metric countries, passed legislation
adopting the SI system as their
only legal system with numerous countries following their example.

THE
SEVEN SI BASE UNITS

Quantity

Name

Sym

Definition
(CGPM)

length

metre

m

The
metre is the length equal to 1 650763,73 wavelengths in vacuum
of the radiation corresponding to the transition between the levels
2 p10 and 5 d5,
of the krypton-86 atom.[ 11th CGPM (1960), Resolution 6.]

mass

kilogram

kg

The
kilogram is the mass of the international prototype of the kilogram
recognised by the CGPM and in the custody of the Bureau International
des Poids et Mesures, Sevres, France.
[ 1 st CGPM (1889).]

time

second

s

The
second is the duration of 9 192631 770 periods of the radiation
corresponding to the transition between the two hyperfine levels
of the ground state of the caesium-133 atom. [13th CGPM (1967),
Resolution 1]

electric
current

ampere

A

The
ampere is that constant current which, if maintained in two straight
parallel conductors of cu rrent infinite length, of negligible
circular cross-section, and placed one metre apart in vacuum would
produce . between these conductors a force equal to 2 x 10-7 newton
per metre of length. [CIPM (1946), Resolution 2, approved by the
9th CGPM (1948).]

thermo-
dynamic temper-ature

kelvin

K

The
kelvin, unit of thermodynamic temperature, is the fraction 1/273,16
of the thermodynamic temperature of the triple point of water.
[13th CGPM (1967), Resolution 4 ]

amount
of substance

mole

mol

The
mole is the amount of substance of a system which contains as
many elementary entities as there are atoms in 0,012 kg of carbon
12. [14th CGPM (1971), Resolution 3.]

luminous
intensity

candela

cd

The
candela is the luminous intensity, in the perpendicular direction
of a surface of 1/600000 square metre of a blackbody at the temperature
of freezing platinum under a pressure of 101 325 newtons per square
metre. [13th CGPM (1967), Reso'n 5.]

NOTES:
(i) The unit kelvin and its symbol K are also used
to indicate temperature intervals or temperature differences.
Besides thermodynamic temperature (symbol T), expressed in kelvins,
Celsius temperature (symbol t) is also used. Celsius temperature
is defined by the equation: t = T - T0 where T0 = 273,15 K by
definition. Celsius temperature is in general expressed in degrees
Celsius (symbol ^{o}C). The unit
"degree Celsius" is therefore equal to the unit "kelvin" and an
interval or difference in Celsius temperature is also expressed
in degrees Celsius (^{o}C). Note
that the Celsius temperature of the triple point of water is 0,01
^{o}C, which accounts for the factor
273,16 in the definition of the kelvin.
(ii) Whenever 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.
(iii) With the object of removing the ambiguity which still occurred
in the common use of the word "weight", the 3rd CGPM (1901) declared:
"The kilogram is the unit of mass [and not of weight or of force];
it is equal to the mass of the international prototype of the
kilogram."