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In a galvanometer, the coil experiences a deflecting torque due to the magnetic field and a restoring torque from the spring. At equilibrium, these two torques are equal.The deflecting torque (τdeflect) on a coil in a magnetic field is given by τdeflect = BIANsinα, where B is the magnetic field, I is the current, A is the area of the coil, N is the number of turns, and α is the angle between the normal to the coil and the magnetic field. For a radial magnetic field in a galvanometer, sinα = 1, so τdeflect = BIAN.The restoring torque (τrestore) exerted by the spring is proportional to the deflection angle (θ): τrestore = kθ, where k is the spring constant.At equilibrium, τdeflect = τrestore.Therefore, BIAN = kθ or Kθ = BIAN.