Potentials of Gases observed in 'Thermionic Valves. 417 



of electrons having energy equal to Ve, but also determines 

 the energy which the bombarding electrons must have in order 

 to stimulate the gas molecules to radiate ultra-violet light of 



o © 



frequency v. In other words, it' the quantum relation holds, 

 the photo-electrons could never possess more energy than the 

 thermally emit tint electrons which are responsible for the 

 radiation of the ultra-violet light, so that photo-electric 

 emission could not cause positive ions to appear until the 

 velocity of the thermions was sufficient to cause ionization of 

 the gas. 



Finally it is to be noticed that the value of the current 

 through the tube at the critical point is of the order of 

 5 milliamperes, and this in itself appears to render any 

 photo-electric explanation very improbable, unless the marked 

 increase in currents of this magnitude is taken as repre- 

 senting a photo-electric emission far larger than any hitherto 

 described. 



(3) Experimental Results. — -The method here described for 

 the determination of ionization potentials has been applied to 

 six different gases, with the following results : — 



Mercury vapour 10*8 volts. 



Argon 12'5 „ 



Hydrogen 15 „ 



Carbon monoxide 15 ,, 



Nitrogen 17 '2 ,, 



Helium 20\S „ 



The numbers given in the above table represent the dif- 

 ference of potential between the anode and the negative end 

 of the filament, and have not been corrected in any way. A 

 short discussion of the necessary corrections is given in 

 Section (4) below. Specimen curves showing the relation 

 between current and voltage for each of the six gases are 

 shown in figs. 3 and 4. 



A certain amount of advantage is obtained by plotting 

 the logarithm of the current against the logarithm of the 

 applied voltage, as the initial part of the curve, before 

 ionization sets in, is determined by the 3/2 power of the 

 voltage, so that it becomes a straight line when plotted 

 logarithmically. The line ceases to be straight when positive 

 ions begin to appear. The curves of fig. 4 are re-pletted 

 logarithmically in fig. 5. 



