Class 11 Physics practical reading To measure diameter of a given wire using a screw gauge.

Aim 

To measure diameter of a given wire using a screw gauge.

Apparatus Required

A screw gauge, a piece of wire (whose diameter is to be measured) and a magnifier.

Theory

Linear Scale Reading (L.S.R.) = N x pitch 

If the nth division of circular scale coincides with reference line, circular scale reading (C.S.R.) = n x Least Count (L.C.) in mm. 

Total Reading (T.R.) = L.S.R. + C.S.R. = (N x pitch) + (n x L.C.)

Procedure

1. To determine the least count, find the distance moved by the screw in one complete rotation. This can be done by moving the screw for five complete rotation and then finding the distance moved by screw in one rotation. This is the pitch of the screw gauge. 

2. Determine the least count of the screw gauge by dividing the pitch by the total number of divisions on the circular scale. 

3. Find the zero error as explained above. 

4. Now grip the wire in the gap between A and B of the screw gauge. Turn the screw head until the ratchet starts slipping. Note down the main (linear) scale and circular scale. 

5. Repeat above step after rotating the wire through 90° for measuring diameter in perpendicular direction. This completes one set of observations. 

6. Repeat steps (4 to 5) for two more times at different positions of wire.

Observations

Precautions

1. The screw should always be rotated by ratchet only to avoid any undue pressure and stop as soon as ratchet starts slipping. 

2. The screw should always be moved in the same direction to avoid back lash error. 

3. Reading should be taken at different places along the length of the wire. 

4. Error due to parallax should be avoided.

Sources of Error

1. The screw may have backlash error. 

2. The wire may not be of uniform area of cross-section. 

3. The screw may have friction.