The magnitude of the magnetic field (in $SI$ units) at the centre of a hexagonal coil of side $10 \, cm$, having $50$ turns and carrying a current $I$ (Ampere), in units of $\frac{\mu_{0} I}{\pi}$ is: (in $\sqrt{3}$)

$A$ telephone cable at a place has four long straight horizontal wires carrying a current of $1.0 \; A$ in the same direction from east to west. The earth's magnetic field at the place is $0.39 \; G$ and the angle of dip is $35^{\circ}$. The magnetic declination is nearly zero. What are the resultant magnetic fields at points $4.0 \; cm$ below and above the cable?

Among the following,the unit of permeability is $NOT$ represented by

In a wire of radius $1 \ mm$,a steady current of $2 \ A$ uniformly distributed across the cross-section of the wire is flowing. Then the magnetic field at a point $0.25 \ mm$ from the centre of the wire is (in $\mu T$)

The magnetic effect of current was discovered by

$A$ wire carrying current $I$ and another carrying $2I$ in the same direction produce a magnetic field $B$ at the midpoint. What will be the field when the $2I$ wire is switched off?