334 



ELEMENTS OF ELECTRICAL ENGINEERING. 



Fig. 185. 



The positive carbon wastes away much faster than the negative 

 carbon, usually about twice as fast. This is due largely to the 



higher temperature of the positive 

 carbon which causes it to be 

 Condensing burned more rapidly by the oxy- 

 gen of the air ; and it is partly 

 due to something akin to electro- 

 lytic action in the arc, for carbon 

 is carried continuously across the 

 arc from the positive carbon to 

 the negative carbon. 



The direct-current arc, when it 

 is operating properly, does not 

 make very much noise, although the pulsations of current due to 

 the passage of the successive segments of the commutator under 

 the dynamo brushes always produce an audible tone. 



The alternating-current arc. 

 In an alternating-current arc the 

 ends of the carbons are heated 

 nearly to the same extent, the 

 upper carbon being perhaps the 

 hotter of the two, on account of 

 the upward movement of the 

 heated air and vapor. The re- 

 sult is that light is emitted about 

 equally from the ends of both car- 

 bons, and the light is distributed 

 as indicated by the curved line 

 in Fig. 1 86. 



On account of the great amount \ \ , 50 



of light that is thrown upwards js '7<> l6 

 by an alternating-current arc a 



reflector is usually employed with an alternating-current arc 

 lamp to throw the light downwards. 



The light emitted by an alternating-current arc lamp is not 



5 



60 



130 



140 



