Alternating current is unsuitable for certain purposes, such as electroplating; and by making a small alteration in our dynamo we get a continuous or direct current, which does not reverse its direction. Fig. 20 shows the new arrangement. Instead of the two rings in Fig. 19, we have now a single ring divided into two parts, each half being connected to one end of the revolving coil. Each brush, therefore, remains on one portion of the ring for half a revolution, and then passes over on to the other portion. During one half-revolution we will suppose the current to be flowing from brush B¹ in the direction of the lamp. Then during the next half-revolution the current flows in the opposite direction; but brush B¹ has passed on to the other half of the ring, and so the current is still leaving by it. In this way the current must always flow in the same direction in the main circuit, leaving by brush B¹ and returning by brush B². This arrangement for making the alternating current into a continuous current is called a _commutator_. [Illustration: PLATE IV. _By permission of_ _Lancashire Dynamo & Motor Co. Ltd._ A TYPICAL DYNAMO AND ITS PARTS.] In actual practice a dynamo has a set of electro-magnets, and the armature consists of many coils of wire mounted on a core of iron, which has the effect of concentrating the lines of force. The armature generally revolves in small dynamos, but in large ones it is usually a fixture, while the electro-magnets revolve. Plate IV. shows a typical dynamo and its parts. As we saw in an earlier chapter, an electro-magnet has magnetic powers only while a current is being passed through its winding, and so some means of supplying current to the electro-magnets in a dynamo must be provided. It is a remarkable fact that it is almost impossible to obtain a piece of iron which has not some traces of magnetism, and so when a dynamo is first set up there is often sufficient magnetism in the iron of the electro-magnets to produce a very weak field. The rapid cutting of the feeble lines of force of this field sets up a weak current, which, acting upon the electro-magnets, gradually brings them up to full strength. Once the dynamo is generating current it keeps on feeding its magnets by sending either the whole or a part of its current through them. After it has once been set going the dynamo is always able to start again, because the magnet cores retain enough magnetism to set up a weak field. If there is not enough magnetism in the cores to start a dynamo for the first time, a current from some outside source is sent round the magnets.