7 6 



THE ELECTRIC ARC 



pressure is diminished the number of striations becomes 

 less, as is shown in Fig. 20. With still lower pressures the 

 striations disappear and the glow at the anode moves 

 about in a very erratic way. At times it appears as a 

 ring about the anode a few centimeters above the end, as 



FIG. 20. 



in Fig. 21. Occasionally two rings of this kind have been 

 noticed. 



With pressures less than 5 mm. this ring also disappears 

 and the arc consists of a bright spot on the cathode from 

 which cathode rays appear to come and a faint lumi- 

 nosity in the space through which these rays happen to 



