This, indeed, Faraday effected, when he inserted his copper disk edgeways (Fig. 4) between the poles of a powerful magnet, and spun it, while against edge and axle were pressed spring contacts to take off the currents. The electromotive-force, acting at right angles to the motion, and to the lines of the magnetic field, produces currents which flow along the radius of the disk. If no external path is provided, the currents must find for themselves internal return paths in the metal of the disk.