It is used to compare currents or to determine the value of ‘H’ (Horizontal component of earth field) if current is known. It consists of a Bakelite ring dia 17cm approx. wound with 3 coils of 2,50 and 500 turns of insulated copper wire. Compass Box is mounted at center of the ring and both can rotate independent of the base. Provided with terminals to put either of the three coils in circuit at a time. Base is supported on three levelling screws.