A microscope has its main scale with 20 divisions in 1 cm and vernier scale with 25 divisions, the length of which is equal to the length of 24 divisions of main scale. The least count of microscope is :

1. 0.002 cm
2. 0.001 cm
3. 0.02 cm
4. 0.01 cm

Identify the correct use of the thimble of a screw gauge:

1. To read length correct up to 0.01 mm
2. To read length correct up to 1 mm
3. To mark main scale and base line
4. To mark circular scale.

Identify the incorrect statement from the following :

(a) A screw gauge is used to measure very small dimensions like the thickness of a wire or sheet.

(b) Vernier callipers is used to measure larger dimensions like the diameter of a sphere, internal or external diameter lengths, etc.

(c) Vernier callipers works on the principle of a sliding scale to measure fractional parts of a main scale.

(d) Vernier callipers has a higher precision with a larger least count whereas screw gauge has a lower precision with a smaller least count.

Define the least count of an instrument.