570 Dr. 0. A. Skinner on the Potential Gradient in 



discharge, appeared as outlined by the dotted lines in the 

 figure. The luminous column extended up near the open 

 end of the cylinder and held there tenaciously for all cur- 

 rents up to 5 and 6 m.a., not retreating as much as a centi- 

 metre at any time. A permanent horse- Fig. 5. 

 shoe magnet held so that cathode rays would 

 he deflected upwards caused the luminosity 

 to retreat slightly, while the reverse posi- 

 tion caused it to extend. The potential 

 gradient in the luminous column rose 

 slightly in passing from anode to cathode, 

 more rapidly in the constricted portion than 

 in the other. With the striated discharge, 

 while the luminosity did not extend up to 

 the cylinder-screen, but to about 3 cm. 

 above it, it required a current considerably 

 larger than under ordinary circumstances 

 to cause it to retreat perceptibly towards 

 the anode. The striations were in this case 

 of keener outline and more intense lumi- 

 nosity on their negative face. With both 

 striated and unstriated discharge, by simply 

 reversing the current, so that the cathode 

 was above, the luminous column was quickly 

 driven behind the cylinder screen, the 

 change being accompanied by a consider- 

 able increase in the current. These results 

 indicate that the Faraday dark space is 

 under the influence of a radiation emanating 

 directly from the discharge-surface of the 

 cathode, rather than from a secondary dis- 

 turbance produced by the discharge in the 

 neighbouring gas. 



The results embodied in the curves already 

 considered, appear entirely in agreement with the conclusion 

 that the Faraday dark space is swept by cathode rays, which 

 are conceived as the negative ions driven forth with a velocity 

 acquired through the excessive " drop " at the cathode, and 

 giving up their kinetic energy in ionizing the gas through 

 which they drive. All the curves (that for "8 m.a., fig. 4, 

 possibly excepted) show, in their convexity to the axis of 

 abscissas throughout the dark space, that, according to 

 Prof. J. J. Thomson *, ionization is in excess there. There 

 is a considerable region, however, lying next the luminous 



* Prof. J. J. Thomson, Phil. Mag. xlvii. p. 252 (1899). 



