THE CONTINUOUS CURRENT GENERATOR 97 



connected as shown in Fig. 58. As the armature revolves 

 it is evident that every coil will generate a wave of e.m.f. 

 of the same shape and magnitude as that of any other coil 

 but that these waves will be behind one another, a time 

 equal to that required for one-quarter of a revolution 

 of the armature. Moreover the e.m.f. of the machine 

 is obtained at any time by adding together the e.m.fs. 

 of the two coils that are in series with each other at that 

 time in the path considered. 



E.M.F. Form of a Four-coil Armature. The e.m.fs. 

 of the different coils are shown in Fig. 56 (a) the curves 



FIG. 58. Four-coil Armature and Four-part Commutator. 



being numbered to correspond with the coils. Thus at 

 time = coil No. 1 is just moving under the N pole and 

 beginning to generate an e.m.f. While it is still generating 

 voltage, coil No. 2 moves under the same N pole and also 

 generates an e.m.f. in the same direction as that of coil 

 No. 1. Then between the brushes these two coils are 

 acting in series and the line e.m.f. is the sum of these two. 

 The e.m.f. generated by the other half of the armature 

 winding is obtained by considering coils No. 3 and No. 4. 

 It will be found that they give, between the brushes, an 

 e.m.f. exactly equal to that given by coils No. 1 and No. 2 



