Comprises a circular coil wound on non-magnetic bobbin of bakelite about 160mm in diameter mounted vertically on a non-magnetic tripod metal base, with the coil width very small as compared to its diameter. A compass box (magnetometer) is located at the center of the coil to make its needle experience uniform magnetic field due to current in the coil. The coil consists of three set of windings of insulated copper wire with 2, 50 and 500 turns. A circular platform on base carries a set of four socket terminal for connection to the coils and are marked for each coil.