Suppose the magnetic field is uniform but not radial. Then, when the coil comes to rest after rotation through an angle θ, NIAB cos θ = Kθ (in usual notations).
∴ I ∝ θ/cos θ as N, A, B and K are constants in a particular case.
Thus, the current is not directly proportional to the deflection. Hence, we cannot have a linear scale for measurement.
