Resin and turpentine are obtained from

The Vernier scale of a travelling microscope has $50$ divisions which coincide with $49$ main scale divisions. If each main scale division is $0.5 \text{ mm}$,then the least count of the microscope is

$A, B$ and $C$ are three points in a uniform electric field as shown in the figure. The electric potential is

The number of ways that $8$ beads of different colours can be strung as a necklace is

Haversian canals are found in

If a copper rod carries a direct current,the magnetic field associated with the current will be