Suppose we treat the machine as a dynamo. Bring the ends of the line wire together, thus, as we say, closing the circuit. By some external force let us cause the armature to rotate and under the influence of the magnetic field it will generate an electric current, part of which will pass through the field and part through the line circuit. We may adjust the relative amount of wire in field and line so that any portion of the current we choose will pass through the field. The amount of current it will generate depends, (1) upon the strength of the field and (2) upon the speed of the armature. Its field, although never entirely without magnetism, is very feeble at first, and hence in the first instance a very small current will be generated in the moving armature. This, however, will strengthen the field slightly, and as the field is strengthened the armature will generate more current, and thus by a mutual reaction the machine gradually "builds up" to full strength. [Illustration: Fig. 21] When now we use the machine as a motor, an electric current must be sent along the line wires in the opposite direction (Fig. 21) from which it would come out of the machine when acting as a dynamo. It will then be noticed that, although the direction of the current through the field is the same, whether the machine is used as a dynamo or a motor, the direction through the armature, when used as a motor, is the reverse of that when used as a dynamo. You may perhaps be able to notice that the amount of wire on the field is considerably more than that on the armature. Now if you will trace the wires carefully you will find that there is provided a way of supplementing the wire of the armature with some more wire in what is called the rheostat, Fig. 22. This wire, or portions of it, is introduced into the armature circuit when the machine first starts. When, however, the machine has started and the armature is moving within the influence of a magnetic field, it plays the part of a dynamo at the same time that it is acting as a motor. Two conflicting and opposite electro-motive forces therefore exist in the armature at the same time. In Fig. 22 the arrow _a_ represents the direction of the electro-motive force which is impressed upon the armature, and the arrow _b_ represents the counter-electro-motive force which the moving armature develops. [Illustration: Fig. 22]