Page 19 - Physics - XI

zero error may be positive or negative. Negative of zero error is called zero correction which is always
added algebraically to the observed reading to obtain the corrected reading.

(i) Positive zero error: If the zero of the circular scale is below the reference line when stud A and B
just touch each other [Fig. 2.3 (a)], then zero error is positive.

Zero of
H 5 circular scale H 0
Zero of circular above the
0
5 95 Circular scale below the N reference N
scale reference line line
Reference 0 6
H line Reference line Reference
N Circular line Circular
scale scale
Fig. 2.2: Screw gauge Fig. 2.3 (a): Screw gauge with Fig. 2.3 (b): Screw gauge with
with no zero error positive zero error negative zero error
Consider Fig. 2.3 (a). When stud A just touches with B, the zero mark of the circular scale is 2 divisions
below the reference line. Hence

Zero error = + 2 × L.C.
= + 2 × 0.001 cm

= + 0.002 cm
and zero correction = – ve of zero error

= – 0.002 cm

Corrected reading = Observed reading – 0.002 cm
(ii) Negative zero error: When the zero of the circular scale is above the reference line when stud A
and B just touch each other, then zero error is negative [Fig. 2.3 (b)].
In Fig. 2.3 (b), zero of circular scale is 3 divisions above the reference line, when stud A and B just
touch each other. Hence
Zero error = – 3 × L.C.

= – 3 × 0.001 cm = –0.003 cm

Zero correction = –ve of zero error
= – [–0.003 cm] = + 0.003 cm

Corrected reading = Observed reading + 0.003 cm
(c) To determine diameter of the objects or the thickness of the sheets: Grip the object whose thickness
or diameter is to be measured between A and B gently by rotating the ratchet R.
Note the reading on linear scale. Let x mm be the reading on the pitch scale which has been just
uncovered. Also, note which of the circular scale division coincides with the reference line. Let n is
the number of circular scale division that coincides with the reference line. Then, observed thickness or
diameter is given as
Observed thickness or diameter = x mm + n × L.C.

Corrected thickness or diameter = Observed reading – (Zero error)
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