Bar magnet induces current in a coil

Bar magnet induces current in a coil, shown on galvanometer. A changing magnetic field cutting across a coil of wire induces an electric current. In 'A', a cylindrical bar magnet is thrust into a tall narrow coil which is hooked to a projection galvanometer. The induced current causes the needle to swing full scale in both directions. In 'B', the narrow coil is placed inside a coil of larger diameter. A 6 volt D.C. battery and key switch are hooked to the larger coil. When the switch is pressed, the surge of ceramic magnetic separator from the larger coil cuts the narrow coil. A current surge is registered when the switch is pressed or released, smaller than in 'A'. In 'C', a soft iron cylindrical core is inserted into the narrow coil. When the switch is pressed or released, a much larger current swing is registered than in 'B'. Other cores can be inserted: a bundle of iron wires, a brass rod, or a lucite rod.