Metric  Base Units
 
Metric Base Units
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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 oC). 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 (oC). Note that the Celsius temperature of the triple point of water is 0,01 oC, 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."

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