THE POLYPHASE SYSTEM. 



99 



armature are midway under the field magnet poles. This brings 

 the electromotive forces of the two armature windings into quad- 

 rature with each other. In practice a two-phase alternator is made 

 by placing all the slots shown in Fig. 90 on one armature body, 

 the two windings being however kept as distinct as if they were 

 on separate armature bodies. Thus the armature winding shown 

 in Fig. 1 1 is a two -phase winding ; the coils which are square on 

 the ends to the right constitute one armature winding which de- 

 livers current to one receiving circuit or set of receiving circuits, 

 and the coils which bend backwards and pass behind the square 



Fig. 90. 



ends constitute the other armature winding which delivers cur- 

 rent to another receiving circuit or set of receiving circuits. 



A two-phase alternator is usually provided with four collector 

 rings, two for each armature winding ; one ring may, however, 

 be made to serve as a common terminal for the two windings A 

 and B as shown in Fig. 91. In this case three transmission 

 wires are used and the two receiving circuits x and y are con- 

 nected as shown in the figure. The objections to the arrange- 

 ment shown in Fig. 9 1 are : (a) that the voltage drop in the 

 middle main may cause the voltages across x and y to become 



