Page:Electricity (1912) Kapp.djvu/224

220 of turns. Now imagine a machine as shown in Fig. 15 having two independent windings as pointed out on p. 186. Let one winding supply current to the coil A and the other to the coil B. The phases of the two currents are displaced in point of time by a quarter period. If the current in A has crest value that in B is zero and vice versa. The magnetic field produced by these coils and passing through the copper cylinder will at these two moments have the direction at right angles to the plane of coil A and to that of coil B respectively. At intermediate times, when there is some current in both coils, the magnetic field will be due to the combined effect of both currents, and _ its direction will be intermediate between the two positions mentioned. We thus get a "revolving magnetic field," the number of complete revolutions performed in a second being equal to the frequency. It is as though a physical magnet were whirled round the copper cylinder. The lines of force of this revolving field cut through the metal of the cylinder and thus create induced currents, which in combination with the field produced by the two coils exert a drag on the surface of the cylinder and cause it to rotate in the