PRESSURES GREATER OR LESS THAN ONE ATMOSPHERE 77 



stream. When this state is reached the arc is not 

 appreciably affected by varying the distance between 

 the electrodes. 



The voltage required to maintain an arc in a vacuum is 

 much less than that required to maintain one in air. The 



FIG. 21. 



results of some observations taken by myself are given in 

 Fig. 22. l Curve i represents the anode drop at different 

 pressures, curve 2 the cathode drop, curve 3 the electric 

 force per centimeter and curve 4 the total potential dif- 

 ference between the terminals. The observations were 



1 Phys. Rev., 19, 120; 1904, and 20, 368; 1905. 



