When both jaws of vernier callipers touch each other, zero mark of the vernier scale is right to zer

Question

When both jaws of vernier callipers touch each other, zero mark of the vernier scale is right to zero mark of main scale, $4^{	ext {th }}$ mark on vernier scale coincides with certain mark on the main scale. While measuring the length of a cylinder, observer observes 15 divisions on main scale and $5^{	ext {th }}$ division of vernier scale coincides with a main scale division. Measured length of cylinder is $\_\_\_\_$ mm. (Least count of Vernier calliper $=0.1 \mathrm{~mm}$ )

Professor PC Thomas Classes · Answer

Since the zero of the vernier scale is to the right of the main scale zero, there is a positive zero error. The zero error is $e = +4 	imes 0.1 = +0.4$ mm. During measurement, the Main Scale Reading (MSR) is 15 mm and the Vernier Scale Reading (VSR) is $5 	imes 0.1 = 0.5$ mm. The observed reading is $	ext{MSR} + 	ext{VSR} = 15 + 0.5 = 15.5$ mm. The true measured length is the observed reading minus the zero error: $15.5 - 0.4 = 15.1$ mm.