How do you define least count for Vernier Calipers?

Answer
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Hint: The least count, as the name suggest is the least value that can be counted using an instrument and same goes for the Vernier Callipers in which the least count can be calculated based on the size of smallest division on the main scale and number of divisions on the Vernier scale.

Complete answer:
Vernier Calliper is an instrument, which is used for making linear measurements accurately. It consists of two graduated scales; one is like a simple ruler and is called the main scale and the other one is called the auxiliary scale or the Vernier scale. The Vernier Callipers have two jaws among which, one is fixed and the other one is movable and is attached with the Vernier scale. The Vernier scale enables us to take readings up to a fraction of divisions made on the main scale.
Least count is defined as the smallest value a measuring device can measure and is also known as Vernier constant. In case of a Vernier caliper, the Vernier scale has divisions of size a bit less than that of the smallest division on the main scale.
Least count of a Vernier scale is calculated using the following formula,
Least count = $\dfrac{\text{Smallest reading on main scale}}{\text{Number of divisions on Vernier scale}}=\dfrac{1mm}{10}$ = This is the least count for Vernier Callipers.
Hence, the least count for Vernier Callipers is 0.1mm.

Note:
Least count can also be calculated by taking the difference of one main scale division and one Vernier scale division. The information that we collect from the measurement of least count is generally an error that can be positive or negative, based on the position of zero on the Vernier scale and main scale.