A Vernier calipers has $1 \mathrm{~mm}$ marks on the main scale. It has $20$ equal divisions on the Vernier scale which match with $16$ main scale divisions. For this Vernier calipers, the least count is
$0.02 \mathrm{~mm}$
$0.05 \mathrm{~mm}$
$0.1 \mathrm{~mm}$
$0.2 \mathrm{~mm}$
The smallest division on the main scale of a Vernier calipers is $0.1 cm$. Ten divisions of the Vernier scale correspond to nine divisions of the main scale. The figure below on the left shows the reading of this calipers with no gap between its two jaws. The figure on the right shows the reading with a solid sphere held between the jaws. The correct diameter of the sphere is
Area of the cross-section of a wire is measured using a screw gauge. The pitch of the main scale is $0.5 mm$. The circular scale has 100 divisions and for one full rotation of the circular scale, the main scale shifts by two divisions. The measured readings are listed below.
Measurement condition | Main scale reading | Circular scale reading |
Two arms of gauge touching each other without wire | $0$ division | $4$ division |
Attempt-$1$: With wire | $4$ division | $20$ division |
Attempt-$2$: With wire | $4$ division | $16$ division |
What are the diameter and cross-sectional area of the wire measured using the screw gauge?
Which of the following is the most precise device for measuring length:
$(a)$ a vernier callipers with $20$ divisions on the sliding scale
$(b)$ a screw gauge of pitch $1\; mm$ and $100$ divisions on the circular scale
$(c)$ an optical instrument that can measure length to within a wavelength of light?