Problem 5.196

A certain substance is placed in a longitudinal magnetic field of a solenoid located between two Polaroids. The length of the tube with substance is equal to $l=30 \mathrm{~cm} .$ Find the Verdet constant if at a field strength $H=56.5 \mathrm{kA} / \mathrm{m}$ the angle of rotation of polarization plane is equal to $\varphi_{1}=+5^{\circ} 10^{\prime}$ for one direction of the field and to $\varphi_{2}=-3^{\circ} 20^{\prime},$ for the opposite direction.