Page:Electricity (1912) Kapp.djvu/188

184 revolving part or "rotor," and may have two or any even number of poles. In Fig. 15 it is assumed that the rotor is multipolar, so that the desired frequency may be obtained with a moderate speed of revolution. The action of the machine is the same as already explained with reference to Fig. 14. In the position shown the polar faces are opposite the sides of the coils, no flux is going through the coils, but the rate of change of flux is a maximum. All the active wires on the face of the armature are being cut by lines of force, and the e.m.f. has maximum, or crest value. The term crest value is chosen to indicate the wavy or undulatory character of the current.

If the stator were made of solid iron, an e.m.f. would be induced not only in the wires, where we desire to have it, but also in the mass of the iron, where we do not desire it. We do not desire to have currents flowing in the body of the stator iron, because whenever a current flows through a conductor, be it a wire or a lump of iron, the material of the conductor gets heated, and that heat has to be paid for in the shape of some extra power which the driving engine is called on to supply. This would be pure waste, and to avoid such