The magnetic field (B) inside a long solenoid having ' $n$ ' turns per unit length and carrying current ' i ' when iron core is kept in it, is ( $\mu_0=$ permeability of vacuum, $\chi=$ magnetic susceptibility)

The work done in turning a magnet of magnetic moment ' $M$ ' by an angle of $90^{\circ}$ from the meridian is ' $n$ ' times the corresponding Work done to turn it through an angle of $60^{\circ}$ where the value of ' $n$ ' is $\left(\cos 90^{\circ}=0, \cos 60^{\circ}=0.5\right)$

A bar of iron having magnetic moment $2.4 \mathrm{Am}^2$ weighs 66 g . If the density of the material of the bar is $7700 \mathrm{~kg} / \mathrm{m}^3$, the intensity of magnetisation in $\mathrm{Am}^{-1}$ is

To protect the instrument from magnetic field, it is completely surrounded by