The least count of the main scale of a vernier callipers is $1 \mathrm{~mm}$. Its vernier scale is divided into 10 divisions and coincide with 9 divisions of the main scale. When jaws are touching each other, the $7^{\text {th }}$ division of vernier scale coincides with a division of main scale and the zero of vernier scale is lying right side of the zero of main scale. When this vernier is used to measure length of a cylinder the zero of the vernier scale between $3.1 \mathrm{~cm}$ and $3.2 \mathrm{~cm}$ and $4^{\text {th }}$ VSD coincides with a main scale division. The length of the cylinder is : (VSD is vernier scale division)
Correct Option: , 4
Least count $=1 \mathrm{~mm}$ or $0.01 \mathrm{~cm}$
Zero error $=0+0.01 \times 7=0.07 \mathrm{~cm}$
Reading $=3.1+(0.01 \times 4)-0.07$
$=3.1+0.04-0.07$
$=3.1-0.03$
$=3.07 \mathrm{~cm}$