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| Significant Figures and Measuring Instruments |
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| Let us consider the following entries in a Students' Journal: |
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| (a) Weight of a calorimeter = 42 g |
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| (measuring instrument used is spring balance) |
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| The smallest reading the spring balance can record is 1g. |
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| There are 42 of 1 g units of measurement. Hence, there are two significant figures in the result. The figures are 4 and 2. |
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| (b) The diameter of a bob is measured by (i) the ordinary scale, (ii) the vernier callipers, and (iii) the micrometer. |
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| (i) Diameter = 2.1 cm (measured by the ordinary scale) |
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| The answer has two significant figures. |
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| (ii) Diameter = 2.13 cm (measured by vernier callipers) |
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| The answer has three significant figures. |
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| (iii) Diameter = 2.132 cm (measured by micrometer) |
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| The answer has four significant figures. |
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| The number of significant figures depends on the instrument used and accuracy required. |
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| (c) The exact inner diameter of a test - tube is 2 cm. |
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| (i) The ordinary scale will record 2.0 cm because the least count of the instrument is 0.1 cm. |
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 There are 20 of 0.1 cm units. |
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| (ii) The vernier callipers will record 2.00 cm because the least count of the instrument is 0.01 cm. |
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| There are 200 of 0.01 cm units |
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 The diameter is 2.00 and has 3 s.f. |
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| (iii) The micrometer will record 2.000 cm because the least count of the instrument is 0.001 cm. |
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| There are 2000 of 0.001 cm units. |
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 The diameter is 2.000 cm and has 4 s.f. |
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| (i) It is wrong to record diameter as 2 cm. It means the reading is approximated to the nearest 1 cm. If the exact reading is 2 cm then it should be recorded as 2.0 cm. |
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| (ii) The significant figures express number of units, rounded to the nearest such unit. |
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