Page 20 - Physics - XI

Backlash error: Due to the wear and tear or due to manufacturing defect, there may be loose fi tting
between screw and nut. Due to this, the screw does not move backward or forward for a little motion
of the cap in the opposite direction. Such an error is called backlash error. This error is quite diff erent

from zero error and cannot be calculated. The backlash error can be appreciably reduced by rotating
the screw in the same direction while taking a particular set of observations.

Procedure
(i) Measurement of Diameter of the Wire

1. Find the pitch and the least count of the given screw gauge.
2. Determine the zero error, if any, with proper sign. Repeat it three times and record them.
3. Now, insert the wire between the screw and stud A. Move the screw by rotating the ratchet till the wire
is gently gripped between A and B as shown in Fig. 2.4. Stop rotating the ratchet when the ratchet slips

without moving the screw.
4. Note the number of divisions of the linear scale visible and uncovered by the edge of the cap. The
reading x is called linear scale reading.

A Wire
B
0 5
0
95
ST S

Fig. 2.4: Measurement of diameter of a wire with a screw gauge

5. Note which number of division on the circular scale (n) is coinciding with the reference line. The
product of n and L.C. gives the circular scale reading.
6. Now, release the wire gently from the gap by loosening the screw. Rotate the wire through 90° at the

same cross-section position. Repeat the steps 3 to 5 and take the observed diameter in perpendicular
direction as shown in Fig. 2.5.
A B A B

d 1 d 1 d 2

(a) (b)
Fig. 2.5: Two magnifi ed views (a) and (b) of a wire showing its perpendicular diameters d and d .
1 2
7. Repeat the above steps 3 to 6 for the three diff erent positions of the wire.

8. Take the mean of these observed diameters.
9. Now, apply the zero correction with proper sign to the mean observed diameter and fi nd the corrected
diameter of the wire.

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