ELEMENTS OF ELECTRICAL ENGINEERING. 



wound is squarely under a north pole, and in the reverse direc- 

 tion when the tooth is squarely under a south pole. The induced 

 electromotive force is always opposite in direction around adjacent 

 coils, and the armature coils are all connected in series in such a 

 manner * that their electromotive forces are added, thus giving 

 an alternating electromotive force of large value between the col- 

 lecting rings. The external circuit which is to receive alternating 

 current from the machine is connected to metal brushes bb which 

 rub on the collecting rings. 



The armature body, or core, A, is built up of sheet-iron stamp- 

 ings to prevent the formation of eddy currents as explained in 

 Art. 32. 



The field magnet of the alternator is magnetized or excited by 

 a continuous or steady current, or, as it is technically called, a 



direct current. This current 

 flows through coils of wire 

 (not shown in Fig. 16) which 

 are wound on the polar pro- 

 jections N y S, N, S, etc. 

 The direct current for excit- 

 ing the field magnet of an alter- 

 nator is usually obtained from 

 a small auxiliary direct-current 

 dynamo called the exciter. 

 Alternators used in practice 

 give from 50 to 250 or more 

 reversals of current per second. 



Fig. 17. 



28. The direct current dyna- 

 mo. The following description 

 applies to the direct-current dynamo having an armature of the 

 ring type. The relation of the ring to the drum armature and a 

 full discussion of armature windings are taken up in Appendix C. 



* The coils are connected so that in following along the armature wire from one 

 collecting ring to the other, one would pass around adjacent armature teeth in opposite 

 directions. 



