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UNITS AND MEASUREMENT 3 Table 1.2 Some units retained for general use (Though outside SI) Note that when mole is used, the elementary digits plus the first uncertain digit are known entities must be specified. These entities as significant digits or significant figures. If may be atoms, molecules, ions, electrons, we say the period of oscillation of a simple other particles or specified groups of such pendulum is 1.62 s, the digits 1 and 6 are particles. reliable and certain, while the digit 2 is We employ units for some physical uncertain. Thus, the measured value has three quantities that can be derived from the seven significant figures. The length of an object base units (Appendix A 6). Some derived reported after measurement to be 287.5 cm has units in terms of the SI base units are given four significant figures, the digits 2, 8, 7 are in (Appendix A 6.1). Some SI derived units certain while the digit 5 is uncertain. Clearly, are given special names (Appendix A 6.2 ) reporting the result of measurement that and some derived SI units make use of these includes more digits than the significant digits units with special names and the seven base is superfluous and also misleading since it units (Appendix A 6.3). These are given in would give a wrong idea about the precision of Appendix A 6.2 and A 6.3 for your ready reference. Other units retained for general measurement. The rules for determining the number of use are given in Table 1.2. significant figures can be understood from the Common SI prefixes and symbols for multiples and sub-multiples are given in Appendix A2. following examples. Significant figures General guidelinesforusingsymbolsforphysical indicate, as already mentioned, the precision quantities, chemical elements and nuclides are of measurement which depends on the least count of the measuring instrument. A choice given in Appendix A7 and those for SI units and of change of different units does not some other units are given in Appendix A8 for your guidance and ready reference. change the number of significant digits or figures in a measurement. This important 1.3 SIGNIFICANT FIGURES remark makes most of the following observations clear: Every measurement involves errors. Thus, the (1) For example, the length 2.308 cm has four result of measurement should be reported in a way that indicates the precision of significant figures. But in different units, the measurement. Normally, the reported result of same value can be written as 0.02308 m or 23.08 measurement is a number that includes all mm or 23080 mm. All these numbers have the same number of digits in the number that are known reliably plus the first digit that is uncertain. The reliablenificant figures (digits 2, 3, 0, 8), namely four. Reprint 2026-27