DYNAMO USED AS ALTERNATOR 79 



position of the brushes of a continuous-current machine. The two 

 diametrically opposite points C and D are, on the other hand, points 

 fixed in the wind-ing, and are carried round in space, as shown in the 

 consecutive diagrams, during the rotation of the armature. We shall 

 suppose C and D to be in permanent connection with two slip-rings 

 provided with brushes as shown in Fig. 59 (a)* by means of which 

 permanent contact is maintained between the terminals of an external 

 circuit and the points C and D in the winding. In the position of 

 the armature corresponding to (a), it is clear that there will be zero 

 p.d. between C and D, since the clockwise e.in.f. induced in the 

 quadrant AC of the winding is exactly balanced by the counter- 

 clockwise e.m.f. induced in the quadrant AD; similarly, the e.m.f. 



Cf-B 



FIG. 59. To illustrate Action of Continuous-current Armature when generating 

 Single-phase Current. 



in DB balances that in CB. When the armature reaches the position 

 shown in diagram (b), the e.m.f.'s in the sections AC and DB of the 

 winding overpower those in the smaller sections AD and BC respec- 

 tively, so that there is now a p.d. acting from C to D. Since the 

 first two sections (AC and DB) steadily increase in length, while the 

 last two (AD and BC) steadily decrease, the p.d. acting from C to D 

 will increase until, as shown in diagram (c), the points C and D 

 coincide with B and A respectively when it reaches its maximum 

 value. Beyond this position, the p.d. will steadily decrease, since, 

 as is evident from diagram (d), the e.m.f.'s in the sections AC and 

 DB are now decreasing, while those in CB and AD are increasing. 



* In order to simplify the figures us much us possible, tht- slip-rings arc omitted 

 from the diagrams (6) to (e). 



