When both jaws of vernier callipers touch each other, zero mark of the vernier scale is right to zero mark of main scale, $4^{\text{th}}$ 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 $5^{\text{th}}$ division of vernier scale coincides with a main scale division. Measured length of cylinder is ______ mm.
(Least count of Vernier calliper $= 0.1$ mm )
Answer: (D) 15.1
Zero error: the vernier zero is to the right of the main-scale zero, so the error is positive:
$$e = +4\times0.1 = +0.4\ \text{mm}$$
Observed reading (1 main-scale division = 1 mm):
$$15 + 5\times0.1 = 15.5\ \text{mm}$$
Corrected length $= 15.5 - 0.4 = 15.1$ mm.
Solution by Sreeraj P, M.Sc Physics