THEORY OF THE ELECTRIC ARC 163 



in a Crookes' tube and the cathode itself must be very hot. 

 In the arc the cathode drop instead of being 300 volts may 

 be as low as 5 volts (p. 96). That the cathode must be hot 

 was shown by Luggin 1 who found that a carbon disk kept 

 cool by being rotated could be used as the anode, but not 

 as the cathode of the arc. Stark and Cassuto 2 showed the 

 same thing for a metal kept cool by being rotated, and for 

 an electrolyte which can never be raised to a high tem- 

 perature. In some work by the present writer 3 it was found 

 that the anode end of an arc in a vacuum could easily be 

 made to jump to a carbon electrode placed near the arc, 

 but that the cathode end could not be made to do so, except 

 when the pencil was white hot. 



Assumption that the Electrons are from Within the 

 Cathode. As a result of this fact all agree in assuming 

 that the production of the ions at the cathode is the basis 

 of any explanation of the arc. They differ, however, as 

 to how these ions are produced. According to one view 

 the cathode is heated by the impact on it of positive ions, 

 until it becomes sufficiently hot to send out ions from 

 within the solid to the surrounding gas, as in experiments 

 performed with hot platinum wires. According to the 

 other view the electrons are produced by the impact of the 

 positive ions on the molecules at the surface of the cathode, 

 and the cause for the small cathode drop is the fact that 

 ionization occurs very much more easily when the surface 

 of the cathode is at a high temperature. Both Stark and 

 Thomson have made the former assumption the basis of 

 their explanations. They suppose the hot cathode to be 

 the origin of the negative ions and that the rise in potential 



1 Wien. Sitzungber., 96, 2 A, 767; 1887. 2 Phys. ZS., 5, 264; 1904. 



1 Phys. Rev., 19, 119; 1904. 



