In the simplest manner, we can define the least count of an instrument as the minimum value that we can measure from it. For example, the smallest distance that we can measure from a meter scale is 1mm, therefore the least count of meter scale is 1mm.
The least count is an indication of the accuracy and precision of the instrument. Lesser is the least count of an instrument higher will be its accuracy.
Least count should not be confused with accuracy, these are two completely different terms. Accuracy is the measure of how close to a certain value an instrument can measure.
Vernier caliper is an instrument introduced by PIERE VERNIER in 1631. It is used for taking a linear measurement with very high accuracy. Vernier Calipers are also used in scientific laboratories due to its high accuracy.
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Vernier Caliper Comprises of Two Scales :
Main Scale- The main scale on the vernier caliper is similar to that on a ruler, graduated in mm and cm on one side; inches on the other side.it is the fixed scale of vernier caliper.
The main scale reading is taken as the first reading on the main scale just before to the left of the zero of the vernier scale.
Vernier Scale- The vernier scale is the movable scale. It moves parallel to the main scale and enables readings to be made even of a fraction of a division on the main scale of the vernier caliper. Vernier scale reading is the mark taken on the vernier scale which exactly coincides with a mark on the main scale
The main scale and vernier scale both contribute to the linear measurement.
According to the previous discussion the main scale reading of the vernier caliper shown in the figure will be = 37 mm
Obtained reading = Main scale reading + Least count * Vernier scale reading
Uses Of A Vernier Calliper
Vernier calipers are used to measure:
(i) The length of a pipe or any object.
(ii) The diameter of a spherical ball.
(iii) The internal and external diameter of a hollow cylinder can also be measured
When the two jaws are closed if the ‘0’ mark on the main scale and ‘0’ mark on the vernier scale correspond, then there is no zero error but otherwise, it is taken into consideration while making linear measurements.
The least count of an instrument having a secondary scale is calculated as follows.
For Vernier Caliper,
Smallest reading on the main scale = 1mm
Number of divisions on man scale = 10
Least count of the main scale = 0.1 mm
The secondary scale is the vernier scale.
No. of divisions on the secondary scale = 10
Instrument least count = 0.01 mm