Describe the procedure to measure the diameter of a wire with the help of a screw gauge.

In order to measure the diameter of a wire with the help of a screw gauge we follow the following steps —

1. Find the least count and the zero error of the screw gauge.

2. Turn the ratchet anticlockwise, so as to obtain a gap between the stud A and the flat end B. Place the wire in the gap between the stud A and the flat end B. Then turn the ratchet clockwise so as to hold the given wire gently between the stud A and the flat end B of the screw.

3. Note the main scale reading.

4. Note that division p of the circular scale that coincides with the base line of the main scale. This circular scale division p when multiplied by the least count, gives the circular scale reading i.e., Circular scale reading = p x L.C.

5. Add the circular scale reading to the main scale reading to obtain the total reading (i.e., the observed diameter of the wire).

6. Repeat it by keeping the wire in perpendicular direction. Take two more observations at different places of the wire and record them in a table.

Observations —

Pitch of the screw = ……… cm

Total number of divisions on the circular scale (n) = ………..

$\text {L.C. of screw gauge} = \dfrac{\text{Pitch}}{\text{Total no. of circular scale div}} = ……….\text{cm}$

Zero error = ……. cm

Mean observed length = ……… cm

From the mean observed length, subtract zero error, if any, with its proper sign to obtain the true measurement of the length of the given project.

Thus we get,

Thus,

Observed diameter = main scale reading + (circular scale division p coinciding the coinciding the base line of main scale x least count).

True diameter = observed diameter - zero error (with sign)