THEORY OF THE ELECTRIC ARC 165 



cate that at 3140 C., which is the temperature of the 

 cathode according to Reich, 1 the rate of discharge from the 

 cathode should be 2 X io 8 amperes per sq.cm., whereas 

 the measurements which Reich made on the size of the 

 cathode spot indicate that the current is but 318 amperes 

 per sq. cm. 



Second Explanation of lonization at Cathode. Accord- 

 ing to a second explanation the ionization at the cathode is 

 produced by the impact of the positive ions on its surface. 

 In the discharge from a cold cathode such ionization is 

 supposed to occur and the only objection to assuming that 

 it occurs at the cathode of the arc is that the cathode drop 

 is here much smaller. However, it has been shown by 

 Hittorf 2 and Cunningham 3 that the cathode drop becomes 

 smaller as the temperature of the cathode is raised. The 

 same occurrence has been found when using hot calcium 

 oxide for the cathode. 4 It would in fact be surprising if 

 it were not much easier to ionize molecules which are at the 

 temperature of the arc, since even without impact they 

 emit some electrons. 



Almost conclusive evidence in favor of this view is given 

 by the similarity between the glow discharge in a partial 

 vacuum, the luminous discharge from hot calcium oxide, 

 and the arc, since with the first two of these we certainly 

 have ionization at the cathode produced by impact of 

 positive ions. .The similarity between these is shown by 

 the fact that it is quite possible to pass by gradual changes 

 from the ordinary form of discharge in a vacuum, where 

 the cathode drop must be several hundred volts, through 

 the discharge from a heated cathode, where Cunningham 



1 Phys. ZS., 7, 73; 1906. 3 Phil. Mag., (6), 9, 202; 1905. 



* Wied. Ann., 21, 133; 1884. * Phys. Rev., 32, 507; 1911. 



