THE ALTERNATOR. 15 



mission line will be excessive. In the second case comparatively 

 small and inexpensive transmission wires may be used without 

 excessive loss of power. Therefore it is necessary to use high 

 voltages in the long-distance transmission of power. 



High voltages, however, are dangerous under the conditions 

 which ordinarily exist among users of electric power, and many 

 kinds of apparatus, such, for example, as incandescent lamps, re- 

 quire low voltages for satisfactory operation. Therefore power 

 which is transmitted at high voltage must be transformed at the 

 receiving station from high-voltage-and-small-current to low- 

 voltage-and-large-current if long-distance transmission is to be 

 successful, and the advantage of alternating current over direct 

 current lies almost wholly in the cheapness of construction, cheap- 

 ness of operation, and high efficiency of the alternating current 

 apparatus that is required for this transformation. 



In the step-down transformation of direct current, a motor 

 takes a small current from the high voltage transmission lines 

 and drives a generator which delivers a large current at low volt- 

 age to the service mains. This apparatus is expensive to con- 

 struct, it requires constant attention, and its efficiency is never 

 greater than about 90 per cent. 



The step-down transformation of alternating current is accom- 

 plished by means of the alternating current transformer, which is 

 very much cheaper to construct than a motor-generator of the 

 same capacity, it requires no attention, and its efficiency under 

 full load is usually greater than 97 per cent. The alternating 

 current transformer is described in Chapter X. 



The alternating-current system has a slight advantage over 

 the direct current system because the alternator, having no com- 

 mutator, is somewhat simpler than the direct-current generator, 

 and in the alternating-current system one may use the inductor 

 alternator and the induction motor which have no sliding elec- 

 trical contacts whatever. 



Another advantage of the alternating-current system is that 

 the current passes through zero value many times per second so 



