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.

Define the term 'Vernier constant'.

When is a vernier callipers said to be free from zero error?