The vernier scale used for measurement has a positive zero error of 0.2 mm. If while taking a measurement it was noted that '0' on the vernier scale lies between 8.5 cm and 8.6 cm, vernier coincidence is 6, then the correct value of measurement is ___________ cm. (least count = 0.01 cm)

Solution

Physics sir · 4 steps

Take the reading first, then subtract the positive zero error after changing it to centimetres.

1. Main scale reading. The vernier zero lies between 8.5 cm and 8.6 cm, so the main scale reading is the lower mark, 8.5 cm.
2. Vernier part. The 6th vernier division coincides, so add 6 × 0.01 = 0.06 cm. The observed reading is 8.5 + 0.06 = 8.56 cm.
3. Zero error correction. A positive zero error means the instrument reads too much, so it must be subtracted. Change it to centimetres first: 0.2 mm = 0.02 cm.

   True reading = 8.56 cm - 0.02 cm = 8.54 cm

4. So. The correct measurement is 8.54 cm, option (b).

The idea to keep. True reading = observed reading minus zero error, with the sign of the error. A positive error is subtracted and a negative error is added. Always put the error in the same unit as the reading first.

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