If the screw on a screw-gauge is given six rotations, it moves by $3\; \mathrm{mm}$ on the main scale. If there are $50$ divisions on the circular scale the least count of the screw gauge is
A$0.001\; mm$
B$0.001\; cm$
C$0.02\; mm$
D$0.01\; cm$
JEE MAIN 2020, Medium
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B$0.001\; cm$
b On six rotation, reading of main scale changes by $3 \mathrm{mm} .$
1 rotation corresponds to $\frac{1}{2} \mathrm{mm}$
Also no. of division on circular scale $=50 .$
Least count of the screw gauge will be
$\frac{0.5}{50} \mathrm{mm}=0.001 \mathrm{cm}$
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