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Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard called unit. The result of a measurement of a physical quantity is expressed by a number (or numerical measure) accompanied by a unit.

Although the number of physical quantities appears to be very large, we need only a limited number of units for expressing all the physical quantities, since they are interrelated with one another. The units for the fundamental or base quantities are called fundamental or base units.

The units of all other physical quantities can be expressed as combinations of the base units. Such units obtained for the derived quantities are called derived units. A complete set of these units, both the base units and derived units, is known as the system of units.

The International System of Units:
In earlier time scientists of different countries were using different systems of units for measurement. Three such systems, the CGS, the FPS system and the MKS system were in use extensively till recently.

The base units for length, mass and time in these systems were as follows
  • In CGS system they were centimetre, gram and second respectively.
  • In FPS system they were foot, pound and second respectively.
  • In MKS system they were metre, kilogram and second respectively.
The system of units which is at present internationally accepted for measurement is the Système Internationale d’ Unites (French for International System of Units), abbreviated as SI.

The SI, with standard scheme of symbols, units and abbreviations, was developed and recommended by General Conference on Weights and Measures in 1971 for international usage in scientific, technical, industrial and commercial work. Because SI units use decimal system, conversions within the system are quite simple and convenient. We shall follow the SI units.

In SI, there are seven base units. Besides the seven base units, there are two more units that are defined for,
  1. plane angle 'dθ' as the ratio of length of arc 'ds' to the radius 'r' 
  2. solid angle 'dΩ' as the ratio of the intercepted area 'dA' of the spherical surface, described about the apex O as the centre, to the square of its radius 'r'.
The unit for plane angle is radian with the symbol rad and the unit for the solid angle is steradian with the symbol sr. Both of them are dimensionless quantities.

Table SI Base Quantities and Units

Basic Quantity

SI Units







The metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. (1983)




The kilogram is equal to the mass of the international prototype of the kilogram (a platinum-iridium alloy cylinder) kept at international Bureau of Weights and Measures, at Sevres, near Paris, France. (1889)




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 cesium-133 atom. (1967)

Electric current



The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2×10–7 newton per metre of length. (1948)

Thermo dynamic Temperature



The kelvin, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. (1967)

Amount of substance



The mole is the amount of substance of a system, which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon - 12. (1971)

Luminous intensity



The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. (1979) 

Note that when mole is used, the elementary entities must be specified. These entities may be atoms, molecules, ions, electrons, other particles or specified groups of such particles.

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