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CH AP TE RO NE U NITS AND M EASUREMENT 1.1 INTRODUCTION Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard called unit. The result of a 1.1 Introduction measurement of a physical quantity is expressed by a 1.2 The international system of number (or numerical measure) accompanied by a unit. units Although the number of physical quantities appears to be 1.3 Significant figures very large, we need only a limited number of units for expressing all the physical quantities, since they are inter- 1.4 Dimensions of physical related with one another. The units for the fundamental or quantities base quantities are called fundamental or base units. The 1.5 Dimensional formulae and units of all other physical quantities can be expressed as dimensional equations combinations of the base units. Such units obtained for the 1.6 Dimensional analysis and its derived quantities are called derived units. A complete set applications of these units, both the base units and derived units, is known as the system of units. Summary Exercises 1.2 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 International d’ Unit és (French for International System of Units), abbreviated as SI. The SI, with standard scheme of symbols, units and abbreviations, developed by the Bureau International des poids et mesures (The International Bureau of Weights and Measures, BIPM) in 1971 were recently revised by the General Conference on Weights and Measures in November 2018. The scheme is now for international usage Reprint 2026-27