THE ALTERNATOR. 25 



chosen instant. Such a curve is called the electromotive force 

 curve of the alternator. In a similar manner the successive 

 instantaneous values of an alternating current may be represented 

 by ordinates, and elapsed times by abscissas, giving a current 

 curve. These curves are determined by means of the oscillograph, 

 or by means of the contact maker as explained in Art. 10. 



The shape of the electromotive force curve of an alternator 

 depends upon the shape and distance apart of the pole pieces of 

 the field magnet, and upon the distribution of the armature 

 winding. This is exemplified in Figs. 23, 24 and 25. Figures 

 23 and 24 are given in duplicate to show the effect of the fringe 

 of magnetic flux beyond the pole tips. 



Figure 23 represents a single armature wire a cutting the 

 uniformly distributed flux under the pole pieces, and the resulting 

 electromotive force curve is shown in the lower part of the figure. 

 Figure 230 represents the magnetic flux as stopping short at the 

 pole tips, and Fig. 23^ shows the fringe of magnetic field which 

 always exists beyond the pole tips. 



Figure 24 represents a band of armature wires ab cutting 

 the uniformly distributed flux under the pole pieces, and the 

 resulting electromotive force curve is shown in the lower part of 

 the figure. Figure 24*2 represents the magnetic flux as stopping 

 short at the pole tips, and Fig. 24^ shows the fringe of magnetic 

 field which always exists beyond the pole tips. 



It is important to note that a certain armature wire must be 

 chosen as an index point in Figs. 23, 24, and 25, and that the 

 ordinate of the electromotive force curve which is directly beneath 

 this chosen wire at a given instant as the wire moves to the right 

 represents the value of the electromotive force at that instant. 

 Thus the wire a is chosen as the index point in Fig. 24, and 

 wherever this wire may be at a given instant, that ordinate 

 of the electromotive force curve which is immediately beneath 

 a represents the electromotive force at the given instant. 



Fig. 25 is a more or less fanciful drawing which is intended to 

 show how the shape and width of the pole pieces affect the shape 



