Si System of Measurement

Introduction to S.I. systems of measurement:

The eleventh general conference of weights and measures which held in oct, 1960 recommended a unified systematically constituted system of fundamental, supplementary and derived units, called the ‘International system of units’. This system is abbreviated as the SI, system international d’ units. Fundamentally, SI units is an absolute MKS system of units with addition to three more basic fundamental units, namely- ampere(A) for current, Kelvin(K) for temperature and candela (Cd) for luminous intensity. All these units are called fundamental or base units.

Definition of Fundamental Si Units

Universally accepted SI system defines some quantities as fundamental or base units that are as follows:

1. Metre (m): It is the unit of length. Metre is the length equal to 1,650,763.73 wavelength in vaccum of radiation corresponding to the transition between the energy levels 2p10 and 5d5of the krypton 86 atom.

2. Kilogram (kg): It is the unit of mass. A kilogram is equal to the mass of the international prototype which is in custody of the International bureau of weights and measures at Severs, near Paris, France. This prototype is a cylinder of the Platinum Irridium alloy.

3. Second (s): It is the unit of time. A second is defined as the duration of 9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

4. Ampere (A): It is the unit of electric current. Ampere is that constant current which if maintained in two parallel conductors of infinite length, of negligible circular cross section and placed at a distance of one metre apart in vaccum, would produce between these conductors a force equal to 2 × 107 N/m length.

5. Kelvin (K): It is the unit of temperature. Kelvin is expressed as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

6. Candela (Cd): It is the unit of luminous intensity. It is the luminous intensity, in a perpendicular direction, of a surface of 1/600,000 square metre of a blackbody at the temperature of freezing platinum under a pressure of 101,325 newton per square metre.

Advantages of Si System of Measurement:

The main advantages of SI system are

i. It is a metric system.

ii. It is more comprehensive because it defines the units of both primary fundamental as well as auxillary fundamental quantities.

iii. It is a rationalized system of units, acceptable to both magnetism and electricity.

iv. It is a non-gravitational system of units. It clearly distinguishes between the units of mass and weight which are kilogram and newton respectively.