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Low-Speed Positive H-lons in Hydrogen. 813 



cathode fall of potential are very similar to those obtained 

 for the minimum sparking potential in gases at the same 

 pressure. In hydrogen their values are between 200 and 

 300 volts. In both these cases ionization by collision of 

 positive ions becomes very important, but the corresponding 

 pressures are so high that in no case will the accelerated 

 positive ion fall unimpeded through the total P.D. Thus 

 the essential condition for the test is not fulfilled in this 

 case. 



(d) Positive Thermions from Glowing Filaments. — Stark* 

 measured the current between a slowing carbon filament as 

 anode and a metallic cathode 6 mm. apart in air at *22 mm. 

 pressure with different applied P.D.'s. After obtaining 

 saturation of the positive current for smaller values of the 

 applied P.D., he obtained an increase of the current at 

 350 volts which he ascribed to the positive ions from the 

 filament producing ionization in the gas. McClelland + 

 obtained a similar result using an incandescent anode in air 

 at '66 mm. pressure, when he found that an increase of 

 current took place at 240 volts. In both these cases the 

 pressure is so high that the P.D. between the two ends of 

 the M.F.P. is only a small fraction of a volt. 



The only experiments in which the necessary conditions 

 have been at all fulfilled are the following three cases, in 

 which, however, the positive ions were those emitted by 

 glowing coated and uncoated filaments, so that their nature 

 was not known exactly. Pawlow J measured the ionization 

 produced by positive thermions from coated filaments, when 

 accelerated through small potentials, by Lenard's method. 

 He obtained signs of ionization in hydrogen at as low as 

 10 volts, and found that the minimum potential at which 

 ionization could be detected varied with the supply of positive 

 ions, being smaller for a greater intensity of the source. 

 He also found that positive ions were much less efficient in 

 producing ionization than electrons accelerated through the 

 same voltages. The fact that the minimum potential depends 

 upon the original number of positive ions suggests ioniza- 

 tion by successive collision. Franck and Eva v. Bahr § in 

 similar experiments with air and hydrogen obtained signs 

 of ionization in a gas at potentials below the ionization 



* Stark, Annalen der Physik, 1906, p. 427, 

 t McClelland, Phil. Mag. xxix. p. 362 (1915). 

 I Pawlow, Proc. Roy. Soc, July 1914, p. 398. 



§ Franck and Eva v. Bahr, Verh. der Deuts. Phys. Gesell. Jan. 1914, 

 p. 57. 



