Question

What is least count?how least count for vernier calliper and screw gauge can be defined?

Answer 1

The Least count of a measuring instrument is the smallest value in the measured quantity that can be resolved on the instrument's scale.

Means: The Shortest Measure that instrument can make

in a Vernier Calliper

There are two scales Main Scale and Vernier Scale, And the Least Count can be found by dividing the smallest reading of the main scale with the total number of divisions of the vernier scale. Means  that is the smallest unit which can be measured by that vernier Calliper

in a Screw Guage

There is a Circular Scale and the pitch of the screw gauge is known and then we can find the least count of a Screw Guage.

LC of Screw Guage =   $\frac{pitch of the screw guage}{total number of division of circular scale}$