The copper disc spinning between the poles of a magnet while an electric current was sent from the centre to the circumference, or _vice versa_, formed the simplest electro-motor. All the later motors are simply modifications of this, designed to increase the efficiency or power of the machine. Similarly, the earliest machine for the production of an electric current at the expense of mechanical power only, but through the intervention of a permanent magnet, was the rotating disc of Faraday, described on page 262. This contrivance, however, caused a waste of nearly all the energy employed, for while there was an electro-motive force from the centre to the circumference, or in the reverse direction, in that part of the disc which was passing between the poles of the magnet, the current so generated found its readiest return path through the other portions of the disc, and very little traversed the galvanometer or other external circuit. This source of waste could be, for the most part, got rid of by cutting the disc into a number of separate rays, or spokes, and filling up the spaces between them with insulating material. The current then generated in the disc would be obliged to complete its circuit through the external conductor. If we can so arrange matters as to employ at once several turns of a continuous wire in place of one arm, or ray, of the copper disc, we may multiply in a corresponding manner the electro-motive force induced by a given speed of rotation. All magneto-electric generators are simply contrivances with this object. The iron cores frequently employed within the coils of the armature tend to concentrate the lines of force of the magnet, causing a greater number to pass through the coils in certain positions than would pass through them were no iron present. The electro-motive force of such a generator depends on the strength of the magnetic field, the length of wire employed in cutting the lines of force, and the speed with which the wire moves across these lines. The point to aim at in constructing an armature is to make the resistance as small as possible consistent with the electro-motive force required. As there is a limit to the strength of the magnetic field, it follows that for strong currents, where thick wire must be employed, the generator must be made of large dimensions, or the armature must be driven at very high speed to enable a shorter length of wire to be used.