INDUCED ELECTROMOTIVE FORCE. 



127 



be excited by direct current which is generally supplied by a 

 small auxiliary direct-current generator called the exciter. 



Definition of the cycle. Frequency. The electromotive force 

 of an alternator passes through a set of positive values while a 

 group of armature wires is passing a north pole of the field 

 magnet, and through a set of negative values while the given 

 group of armature wires is passing a south pole of the field mag- 

 net. The complete 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 p poles {pJ2 north poles 

 and />/2 south poles), then the frequency of its electromotive 

 force is pnj '2, where n is the speed of the alternator armature 

 in revolutions per second. This is evident 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 p\2 cycles in one revolution. The 



standard frequencies of com- 

 mercial alternators in practice 

 are 25 cycles per second for 

 large installations for the 

 transmission of power, 60 cy- 

 cles per second for alternators 

 which supply current for both 

 lamps and motors, and 133 

 cycles per second for the older 

 styles of alternators which 

 supply current to lamps only. 



69. The direct-current dy- 

 namo is somewhat more com- 

 plicated than the alternator. 

 The following description ap- 

 plies to the direct-current dynamo having an armature of the so- 

 called ring type, and having a bipolar field magnet. An iron ring 



