PHOTOMETRY AND ELECTRIC LIGHTING. 



331 



tional area, and no doubt this increase in sectional area has to do 

 with the decreased voltage required to maintain the arc with in- 

 creased current. 



When the current becomes very large the arc alters greatly in 

 character, emits a loud hissing noise, and constitutes what is 

 called a hissing arc. The transition from the steady arc to the 

 hissing arc does not take place at a sharply defined value of the 

 current ; in fact the arc is very unstable in the region of transi- 



Fig. 183. Volt-ampere characteristics of an open direct -current arc between solid carbons one 

 half inch in diameter. 



tion, and when the change to the hissing variety takes place, the 

 current has to be greatly reduced in value to change the arc 

 back to the steady type. This instability of the arc in the region 

 of transition from the steady arc to the hissing arc is indicated in 

 Fig. 183 by the broken portions of the curves, and it must not 

 be confused with the kind of instability that is discussed in the 

 following paragraph. 



