132 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
minute. It is often preferable to define this frequency in 
terms of cycles per second. A cycle is the period from a 
given K. M. F. to a second E. M. F. in the same direc- 
tion. If Fig. 73 shows a single alternation of current, then 
Fig. 74 depicts a single cycle. ‘To reduce alternations per 
minute to cycles per second, divide by 120. Thusthe cur- 
rent delivered by the armature of Fig. 72 connected as an 
alternator would be of twenty-five cycles (~)per second. 
In designing alternating generators for high voltage it is 
desirable to have all the armature conductors in series, so 
that the armature winding is arranged with that in view. 
The usual procedure is to wind alternate armature coils in 
   
   
   
PR AT SRR 
  
T 
S 
  
  
FIGS. 73 AND 74. 
opposite directions so that as they approach or recede from 
each successive pair of poles the E. M. Fs. will be in the 
same direction. : 
Inasmuch as the frequency employed in power trans- 
mission work is quite often as high as 6o~ it is evi- 
dent that either the speed must be high or there must be a 
considerable number of poles. The result of arranging a 
generator to meet these conditions is the production of a 
highly specialized type of alternator apparently quite dis- 
tinct from ordinary continuous current dynamos. As a 
matter of fact most of the latter class can be made into 
fair alternators by the proper connection of collecting 
rings, as already shown, but very few alternators could be 
made to give continuous current successfully by the addi- 
tion of a commutator. Fig. 75 shows the general type