THE ALTERNATING CURRENT GENERATOR 231 



more compactly designed and have but few poles, generally 

 two or four. 



57. Frequency. Alternators have been designed for 

 many different frequencies, but the standardization oi 

 electrical apparatus has eliminated all but two, so that all 



FIG. 137. Small Alternator Having Rotating Armature, Fitted with 

 Slip Rings. This is an early type of alternator. 



modern machines are designed for either 60 cycles or 25 

 cycles per second. Earlier machines were built to give 

 133 cycles per second and there has been much talk of 

 machines for 15 cycles but the two named above are the 

 only standard frequencies used in American practice. In 

 Europe other frequencies are much used. 



