improved leading to measurements with greater precision. The definitions of base units are revised to keep up with this progress. Table . Some units retained for general use (Though outside SI) 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. We employ units for some physical quantities that can be derived from the seven base units (Appendix A ). Some derived units in terms of the SI base units are given in (Appendix A . ).
Some SI derived units are given special names (Appendix A . ) and some derived SI units make use of these units with special names and the seven base units (Appendix A . ). These are given in Appendix A .
and A . for your ready reference. Other units retained for general use are given in Table . .
Common SI prefixes and symbols for multiples and sub-multiples are given in Appendix A2. General guidelines for using symbols for physical quantities, chemical elements and nuclides are given in Appendix A7 and those for SI units and some other units are given in Appendix A8 for your guidance and ready reference. . SIGNIFICANT FIGURES As discussed above, every measurement involves errors.
Thus, the result of measurement should be reported in a way that indicates the precision of measurement. Normally, the reported result of measurement is a number that includes all digits in the number that are known reliably plus the first digit that is uncertain. The reliable digits plus the first uncertain digit are known as significant digits or significant figures . If we say the period of oscillation of a simple pendulum is .
s, the digits and are reliable and certain, while the digit is uncertain. Thus, the measured value has three significant figures. The length of an object reported after measurement to be . cm has four significant figures, the digits , , are certain while the digit is uncertain.
Clearly, reporting the result of measurement that includes more