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Q. The pitch of the screw gauge is $1 mm$ and there are $100$ divisions on the circular scale. When nothing is put in between the jaws, the zero of the circular scale lies $ 8$ divisions below the reference line. When a wire is placed between the jaws, the first linear scale division is clearly visible while $72^{\text {nd }}$ division on circular scale coincides with the reference line. The radius of the wire is

JEE MainJEE Main 2021Physical World, Units and Measurements

Solution:

Least count $=\frac{1 \,mm }{100}=0.01 \,mm$
zero error $=+8 \times LC =+0.08 \,mm$
True reading (Diameter)
$=(1\, mm +72 \times LC )-\text { (Zero error) } $
$=(1 mm +72 \times 0.01 mm ) -0.08 \,mm $
$=1.72 \,mm -0.08 \,mm $
therefore radius $=\frac{1.64}{2}$
$=0.82\,mm$