PRESSURES GREATER OR LESS THAN ONE ATMOSPHERE 75 



Carbon Arc in a Vacuum. In more recent work we 

 find more definite statements about the conditions and 

 we can be sure of the form of discharge being used. The 

 greater part of this work has been with carbon elec- 

 trodes. When the pressure about such an arc is dimin- 



FIG. 19. 



ished, the anode becomes slightly less luminous and the 

 cross section of the luminous gas becomes somewhat 

 greater, but the appearance is not greatly changed until a 

 pressure of only a few millimeters is reached. At that 

 point a very decided change occurs. Under certain con- 

 ditions striations appear, as shown in Fig. 19. As the 



