In an experiment with vernier callipers of least count 0.1 mm, when two jaws are joined together the zero of vernier scale lies right to the zero of the main scale and 6th division of vernier scale coincides with the main scale division. While measuring the diameter of a spherical bob, the zero of vernier scale lies in between 3.2 cm and 3.3 cm marks, and 4th division of vernier scale coincides with the main scale division. The diameter of bob is measured as

Solution

Physics sir · 3 steps

The vernier zero sits to the right, so the instrument over-reads. Take that zero error off the reading.

1. Find zero error. With the jaws closed, the vernier zero lies to the right of the main scale zero, so the zero error is positive. It equals 6 × 0.1 mm = 0.6 mm = 0.06 cm.
2. Read the bob. The main scale reading is 3.2 cm. The vernier adds 4 × 0.01 cm = 0.04 cm, so the observed reading is 3.24 cm.

   True reading = Observed reading - Zero error = 3.24 - 0.06

3. So. The diameter of the bob is 3.18 cm, option (a).

The idea to keep. Vernier zero to the right of the main zero means positive zero error, and the instrument reads too much. Always subtract the zero error, keeping its sign.

Answer from the NTA final answer key. Solution by PrepWiser.