INTERNATIONAL SYSTEM OF UNITS

iii) SECOND:

 Second is the base unit of time. Before 1960, the standard of time was defined in terms of the mean solar day.

1^st Definition:

It is defined as 1/86400 part of an average (mean) solar day of the year 1900 A.D. It is denoted by‘s’.

2^nd Definition:

Second is redefined as that time during which 9192631770 vibrations of cesium-133 atom take place.

iv) KELVIN:

The unit of temperature is Kelvin it can be defined as following:

Definition:

The fraction 1/273.16 of the thermodynamic temperature of the triple point of water. It is denoted by ‘K’.

Note:-

It should be noted that Triple point of a substance means the temperature at which solid, liquid and vapors phases are in equilibrium. The triple point of water is taken as 273.16 K. This definition was adopted in 1967.

v) AMPERE:

The unit of electric current is ampere. It can be defined as following:

Definition:

It is that constant current which if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section and placed a meter apart in vacuum, would produce between these conductors a force equal to 2x10^-7 newton per meter (N/m) of length. It is denoted by ‘A’. This unit of electricity was set up in 1971.

vi) CANDELA:

The unit of luminous intensity is candela. It can be defined ad following:

Definition:

The luminous in the perpendicular direction of a surface of 1/600000 square meter of a black body radiator at the solidification temperature of platinum under standard atmospheric pressure. It is denoted by ‘cd’. This definition was established in 1967.

vii) MOLE:

Mole is the unit of the amount of substance (number o particles)

Definition:

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. It is denoted by ‘mol’. This unit was adopted in 1971.