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# Units and Measurements ## Introduction 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) acco...

# Units and Measurements ## Introduction 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 (or British) 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(SI)' Unites* (French for International System of Units), abbreviated as SI with standard scheme of symbols, units and abbreviations, was developed and General Conference on Weights and Measures in 1971 for international technical, industrial and commercial work. Because SI units used decimal system, conversions within the system are quite simple and convenient. We shall follow the SI units.