Consider a $50-$turn circular loop with a radius of $1\mathrm{.}35$ cm in a $0\mathrm{.}55-$T magnetic field. This coil is going to be used in a galvanometer that reads $75$ A for a full$-$scale deflection. Such devices use spiral springs which obey an angular form of Hooke's law, where the restoring torque is

s $=-$ $.$

Here is the torque constant and is the angular displacement, in radians, of the spiral spring from equilibrium, where the magnetic field and the normal to the loop are parallel.