Table of Contents

System of measurement – Basic units in SI system

Systems of Measurement

A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Systems of measurement in modern use include the metric system, the imperial system, and United States customary units.

Types of measurement systems

a) FPS System: In this system, the units of length, mass, time, temperature are Foot (or Yard), Pound (or Slug), Second, Rankine (or Fahrenheit) respectively. It is common in English speaking countries and is developed by Britain.

b) Metric System: It is a decimal system of weight & measurement is based on the Metre as the unit of length. It was first used in France. Its basic unit is Metre.

CGS prescribes Centimetre, Gram, Second for length, weight & time respectively.

MKS prescribes Metre, Kilogram, Second for length, weight & time respectively.

MKSA (Giorgi) system added Ampere, the unit of electrical current to MKS system.

c) SI system: In 1960, General Conference on Weights & Measures (CGPM) formally gave the MKSA, the title ‘’Systems International d’ unites’’ with the abbreviation ‘SI’ (also called as International System of units). In SI, the main departure from the traditional metric system is the use of ‘Newton’ as the unit of Force. India by Act of Parliament No.89, 1956 switched over to SI system.

Basic units in SI system

1) For Length : Metre (m) which is equal to 1650763.73 wavelengths in vacuum of the red-orange radiation corresponding to the transition between the levels 2p10 & 5d5 of the krypton-86 atom. (Definition by wavelength standard)

By Line standard, Metre is the distance between the axes of two lines engraved on a polished surface of the Platinum – Iridium bar ‘M’ (90% platinum & 10% iridium) kept at Bureau of Weights & Measures (BIPM) at Sevres near Paris at 0C, the bar kept under normal atmospheric pressure, supported by two rollers of at least 1 cm diameter symmetrically situated in the same horizontal plane at a distance of 588.9 mm (Airy points) so as to give minimum deflection.

2) For Mass: Kilogram (kg) which is equal to the mass of International prototype of the kilogram.

3) For Time : Second (s) which is equal to the duration of 9192631770 periods of the radiation corresponding to the transition between the hyper fine levels of the ground state of the Caesium 133 atom.

4) For Current : Ampere (A) is that constant current which, if maintained in two straight parallel conductors of infinite length of negligible circular cross section & placed one metre apart in vacuum would produce between these conductors, a force equal to 2 x 10-7 Newton per unit length.

5) For Temperature: Kelvin (K) is the fraction 1/273 of thermodynamic temperature of the triple point of water.

6) For Luminous intensity: Candela (cd) is the luminous intensity in the perpendicular direction of a surface of 1/6,00,000 m2 of a black body at the temperature of freezing platinum under a pressure of 101325 N/m2.

7) For amount of substance: Mole (mol) 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.

Supplementary SI units:

1) For Plane angle: Radian (rad)
2) For Solid angle: Steradian (sr)

Derived SI units:
1) For Frequency: Hertz (1 Hz = 1 cycle per second)
2) For Force: Newton (1 N = 1 kg-m/s2)
3) For Energy: Joule (1 J = 1 N-m)
4) For Power: Watt (1 W = 1 J/s)