International system of units

The eleventh General Conference on Weight and Measures held in Paris in 1960 adopted a world-wide system of measurements called International System of Units. The International System of Units is commonly referred as SI.

The system international consists of seven base units and two supplementary units that are given below:

Base Units

1. Meter 
2. Second
   
3. Kilogram
4. Candela
5. Ampere
6. Kelvin
7. Mole

Supplementary Units

• Radian
• Steradian

1. Meter 

                One meter is the unit of length and it is defined as: the distance covered by light in vacuum during a time interval of 1/299,792,458 of a second.

2. Second

                One second is defined as “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”. 

3. Kilogram 

                The kilogram is the unit of mass. It is defined as the mass of a particular international prototype (نمونہ) made of platinum-iridium and kept at the International Bureau of Weights and Measures.

4. Candela

                The candela is the SI unit of luminous intensity. The candela is used to measure the visual intensity of light sources, like light bulbs or the bulbs in torches. It is the only SI base unit based on human perception.

5. Ampere

                   The ampere is a measure of the amount of electric charge in motion per unit time.

6. Kelvin

                  One kelvin is formally defined as 1/273.16 of the thermodynamic temperature of the triple point of pure water. 

(Triple point: the condition of temperature and pressure under which the gaseous, liquid, and solid phases of a substance can exist in equilibrium)

7. Mole.    

       A mole is defined as 6.02214076×10²³ of some chemical unit, be it atoms, molecules, ions, or others. The mole is a convenient unit to use because of the great number of atoms, molecules, or others in any substance.

* Radian

                 One radian is the angle subtended at the centre of. a circle by an arc equal in length to the radius of. the circle. 

* Steradian

                 The solid angle subtended at the centre of a sphere, by that surface of the sphere, which is equal in area, to the square of radius of the sphere.