SIGNIFICANT FIGURES AND ROUNDING OFF
The number of significant figures in any measurement indicates the degree of precision of that measurement. The degree of precision is determined by the least count of the measuring instrument. Suppose a length measured by a meter scale ( of least count = 0.1 cm ) is 1.5 cm, then it has only two significant figures, namely 1 and 5. Measured with a vernier callipers ( of least count = 0.01 cm ) the same length 1.53 cm and it then has three significant figures. Measured with a screw gauge ( of least count = 0.001 cm ) the same length may be 1.536 cm which has four significant figures. It must be clearly understood that we cannot increase the accuracy of a measurement by changing the unit . For example, suppose a measurement of mass yields a value 39.4 kg. It is understood that the measuring instrument has a least count of 0.1 kg. In this measurement, three figures 3, 9, 4 are significant. If we change 39.4 kg to 39400 g or 39400000 mg , we cannot change
