16 ELEMENTS OF ELECTRICAL ENGINEERING. 



that a circuit breaker operates more effectively on an alternating- 

 current circuit than it does on a direct-current circuit. Indeed, 

 it is claimed that large generating stations, such as those at 

 Niagara Falls, could not be operated at all if it were not for this 

 peculiar effectiveness of the circuit breaker on an alternating cur- 

 rent circuit. 



These advantages of the alternating current are to some extent 

 balanced by certain disadvantages which are outlined in the fol- 

 lowing paragraphs. Where power is to be transmitted over 

 short distances, the one great advantage of the alternating cur- 

 rent system vanishes, and the direct current system is distinctly 

 the better of the two, especially if a central station is to supply 

 current, not only for lighting, but also fqr driving motors and for 

 charging storage batteries. 



Alternating-current lighting. The electric lamp depends fun- 

 damentally upon the heating effect of the electric current, and 

 insofar as the mere heating effect is concerned the alternating 

 current is as satisfactory as the direct current. There is a ten- 

 dency, however, for an alternating-current lamp to give an inter- 

 mittent light because of the cooling of the luminous element 

 between the successive pulses of current. The intermittent char- 

 acter of the light is especially marked in alternating-current arc 

 lamps, ancl glow lamps give a perceptibly intermittent light at 

 low frequencies, especially if the lamp filament is very fine. 



The electric arc, too, is peculiarly unstable when it is operated 

 by alternating current, and certain types of arc lamps, some of the 

 luminous-arc lamps for example, cannot be operated by alter- 

 nating current. Furthermore the tantalum filament glow lamp 

 cannot be operated satisfactorily by alternating current. 



On the other hand the Nernst lamp operates very much better 

 with alternating current than with direct current. 



Alternating-current motors. The simple alternating current 

 is less satisfactory for motor driving than direct current. The 

 ordinary direct-current series motor can, however, be driven by 

 alternating current if it is properly designed. When so designed 



