Principle : When a current-carrying coil is placed in magnetic field, it experiences a torque.
Construction : It consists of a narrow rectangular coil PQRS consisting of a large number of turns of fine insulated copper wire wound over a frame made of light, non-magnetic metal. A soft iron cylinder known as the core is placed symmetrically within the coil and detached from it. The coil is suspended between the two cylindrical pole-pieces(N and S of a strong permanent horse-shoe magnet) by a thin flat phosphor bronze strip, the upper end of which is connected to a movable torsion head T. The lower end of the coil is connected to a hair-spring ‘s’ of phosphor bronze having only a few turns.
In order to eliminate air-disturbance, the whole arrangement is enclosed in a brass case having a glass window on the front. Leveling screws are provided at the base.The torsion head T is connected to a binding terminal T1. So, the phosphor-bronze strip acts as one ‘current lead’ to the coil. The lower end of the spring ‘s’ is connected to a binding terminal T2. A plane mirror or a concave mirror of larger radius of curvature is rigidly attached to the phosphor bronze strip. This helps to measure the deflection of the coil by lamp and scale arrangement.
