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186 wires. Their safe mechanical support thus becomes a matter of first importance. We cannot stick the wires on the very surface of the armature, but we can place them into slots or tunnels close to the surface. In this way the wires are securely held and are relieved from mechanical stress, which is now taken by the iron bridges between the wires and not by the wires themselves. The slots or tunnels are lined with tubes of insulating material, and thus complete protection of the winding, both in a mechanical and electrical sense, is secured.

It will be noticed that in the alternator diagrammatically shown in Fig. 15 more than half the inner surface of the stator is left free from winding. This free space may be utilised for a second winding placed exactly midway into the free space left by the first winding. Let us call the two systems of winding A and B. If the poles are in such a position that the e.m.f. in the A winding is zero, they are exactly opposite the wires of the B winding, and generate in these wires crest value of e.m.f. Conversely if, a moment later, the e.m.f. of the B winding is zero, that of the A winding has crest value. We have thus two waves of e.m.f. and current running