PHOTOMETRY AND ELECTRIC LIGHTING. 



333 



general way to both direct-current arcs and alternating-ourrent 

 arcs. There are, however, differences between direct-current and 

 alternating-current arcs which are of considerable practical im- 

 portance. In a direct-current arc the positive carbon becomes 

 slightly concave and the negative carbon becomes pointed, as 

 shown in Fig. 1 84. The concave end of the positive carbon is 

 called the crater, it is excessively hot, and it emits a great amount 

 of light. The point of the negative carbon is not nearly so hot as 



(So 



the crater so that it emits very much less light, and the column 

 of hot conducting vapor, the arc itself, emits a pale violet light. 

 The result is that the light emitted by a direct-current arc is dis- 

 tributed as indicated by the curved line in Fig. 184. In direct- 

 current arc lamps the upper carbon is generally the positive 

 carbon so that the greater amount of light may be thrown down- 

 wards where it is needed. The direct-current arc lamp which is 

 used in search -lights and projecting lanterns is arranged as shown 

 in Fig. 185. 



