THE ALTERNATING CURRENT GENERATOR 235 



circuits would be equal in magnitude, but the phase of 

 one would be 90 behind that of the other. The generator 

 is said to be a two-phase generator and the two circuits 

 to which it supplies two-phase power is called a two-phase 

 system. 



FIG. 139. E.M.F. Wave Forms of a Two-phase Generator. 



Sometimes the beginnings of the two coils are con- 

 nected together inside the armature winding; this con- 

 nection is brought out to one slip-ring and the two ends 

 are connected to two other rings. Such a machine is called 

 a two-phase, three-wire alternator, as contrasted to that 



FIG. 140. E.M.F. Wave Forms of a Three-phase Generator. 



shown in Fig. 139, which is a two-phase, four-wire alter- 

 nator. 



Now instead of placing two coils on the armature, 90 

 apart, we might place on it three coils 120 apart as shown, 

 in Fig. 140. The three e.m.f. waves generated by such a 

 winding would all be of the same magnitude but would 



