In 1954 the C G P M decided to add two further base units to the MKS A system namely Kelvin as the base unit for measurement of thermodynamic temperature and the candela as the base unit for the measurement of luminous intensity. By 1960, the International Organization for Standardization (ISO) had also agreed to the addition of these two further base units. It was further agreed that rationalized and the supplemented M K S A system should be named as the “system International” (SI). SI further remained in a continuing state of development and in 1971 addition of seventh base unit, mole for amount of substance, has been accepted.

The seven base unit of SI system are given below :

The SI system, besides seven base units, has two supplementary units given below :

The SI seven base units and two supplementary units are defined as below :

(i)    Metre: The meter is the length equal to 1,650,763.73 wave lengths in vacuum of the radiation corresponding to the transition between the levels 2 P10 and 5d5 of the krypton-86 atom.

(ii)    Kilogram: It is equal to the mass of the international proton type of the kilogram.

(iii)    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 caesium-133 atom.

(iv)    Ampere: The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section and placed on meter apart in vacuum, would produce between these conductors and force equal to 2x10^-7 nektons per meter length.

(v)    Kelvin: The Kelvin is the fraction   1   of the thermodynamic temperature of the triple point of water.                                                  273.16

(vi)    Candela: The candela is the luminous intensity, in the perpendicular direction, of a surface of 1  
                                                                                                                                          600.000
square meter of a black body at a temperature of freezing platinum under a pressure of 101,325
nektons per square metre.

(vii)    Mole: The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kg, 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.

(viii)    Radian: The radian is the plane angle between two radii of a circle which cur off on the circumference an arc equal in length to the radius.

(ix)    Streadian :The steadier is the solid angle which, having its vertex in the centre of a sphere, cuts off an area of the surface the sphere equal to that of a square with sides of length equal to the radius of the sphere.

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