24 ELEMENTS OF ELECTRICAL ENGINEERING. 



of armature wires is passing a north pole of the field magnet, and 

 through a set of negative values while the given group of arma- 

 ture wires is passing a south pole of the field magnet. The com- 

 plete set of values, including positive and negative values, is 

 called a cycle, the duration of a cycle is called a period and the 

 number of cycles per second is called the frequency. If the field 

 magnet of an alternator has / poles (//2 north poles and pJ2 

 south poles), then 



/-? (0 



in which / is the frequency in cycles per second of the electro- 

 motive force which is developed by the alternator, and n is the 

 speed of the alternator in revolutions per second. This is evi- 

 dent when we consider that a complete cycle corresponds to the 

 passage of a given group of armature wires across two field poles, 

 a north pole and a south pole, so that there are// 2 cycles in one 

 revolution. 



When north poles and south poles are similar in shape and 

 size, with similar air gaps, which is usually the case in com- 

 mercial alternators, the successive sets of positive and negative 

 values of the electromotive force are alike. That is, a cycle is 

 made up of a set of positive values of electromotive force and a 

 similar set of negative values of electromotive force. 



Synchronism. Two alternating electromotive forces or currents 

 are said to be in synchronism when they have the same frequency. 

 Two alternators are said to run in synchronism when their 

 electromotive forces are in synchronism.* 



7. Electromotive force and current curves. The successive in- 

 stantaneous values of the electromotive force of an alternator may 

 be represented by the ordinates of points on a curve, the corre- 

 sponding abscissas representing elapsed time reckoned from some 



*The term "to synchronize," as used by power-station men, has a double mean- 

 ing. Thus, to synchronize an alternator preparatory to connecting it in parallel with 

 another alternator, means not only to bring it into synchronism with the other alter- 

 nator but also to bring about a certain phase relation between the two. 



