The curved bars are the prolongations of the cores of the electro-magnets _E E E E_. They are held firmly together by screws to the sides or bottom of the cast iron frame of the machine, making it compact and strong. The coils of the electro-magnet form with the wires of the revolving armature one continuous electric circuit, and, when the armature is caused to rotate, an electric current (which at first is very feeble) is induced by the remanent magnetism in the soft iron bars and directed through the collecting brushes into the electro-magnet coils, thus strengthening the magnetism of the iron bars,[V] which again induce a still more powerful current in the revolving armature. The electric current thus becomes stronger and stronger, and the armature therefore revolves in a magnetic field of the highest intensity, the limit of which is governed by the limit of saturation of the soft iron. At each revolution the maximum magnetic effect upon each convolution of the armature is produced just after it passes through the middle of both magnetic fields, which are in a vertical plane passing through the axis of the machine (i. e. _N__{1}_S__{1} in Fig. 173). The minimum effect is produced when in a plane at right angles to it, i. e. horizontal. [Illustration: Fig. 165.] According to the law of Lenz already referred to, when a circuit starts from a neutral position on one side of an axis towards the pole of a magnet, it has a direct current induced in it, and the other part of the circuit which approaches the opposite pole of the magnet has an inverse current induced in it; these two induced currents are, however, in the same direction as regards circuit. A similar current will also be induced in all the convolutions of wire in succession as they approach the poles of the magnets. These currents, almost as soon as they are induced, are collected by terminal rollers or brushes _B_, usually the latter, placed in contact with the commutator in the position which gives the strongest current. The position giving the strongest current gives also the least spark, so that when there are no sparks at the commutator the best lighting effect is produced. Fig. 166 shows position of brushes when the armature revolves in the direction indicated by the arrow. The circumference of the revolving armature is divided into an even number of equal parts, each opposite pair being filled with convolutions of insulated wire wound parallel to the axis of the armature. The ends of these wires are brought to a commutator and connected to the segments either by screws or by soldering. The brushes collect the electric currents as they are induced, which is nearly constant and continuous. The collecting brushes are combs of copper wire placed tangentially to the cylindrical commutator, and press lightly upon it with an elastic pressure. [Illustration: Fig. 166.]