-230 Research Staff ol the (I. E. CI, London, on the 



potential ; when V is increased to this value the glow and 

 the absorption of gas begin. On reducing V, i follows the 

 dotted line, and the glow and absorption do not cease until 

 the falling glow potential V/ is reached. At P, just before 

 V g, when there is still no measurable absorption of gas, i is 

 actually greater than after V^. Since the current after 

 Y^ includes, as a part, the saturated thermionic emission, it 

 might appear that the greater current at P could only be 

 due to greater positive ionization. If that were so, greater 

 p >sitive ionization would be accompanied by lesser absorp- 

 tion, and any ionization theory of absorption would be 

 difficult to maintain. 



2. The solution of the difficulty is simple. The argument 

 indicated is fallacious, because it assumes that the current 

 arriving at the electrodes is a measure of the rate of forma- 

 tion of ions ; it neglects the possibility of " recombination." 



Fig. 2. 



1 1— \\yxf\?Ar^ 





A very simple alteration of the apparatus enables the 

 fallacy to be exposed. The walls of the vessel are coated 

 with a conducting layer of silver which can be used as a 

 third electrode. It will be called the " grid," because its 

 function is similar to that of the grid of a triode. Its 

 potential relative to the cathode, V, determines, in conjunc- 

 tion with the potential of the anode, V, the emergence of 

 electrons from the cathode ; and if it . is .negative to the 

 cathode, the grid tends to collect the positive ions. If it 

 collects all the ions, the current flowing to it, ig, will be 

 equal to ip, the charge on the positive ions formed in unit time, 

 while i e , the current flowing to the cathode, will be equal to 

 i n , the thermionic current ; i a , the current to the anode, is 

 ig + i e . The connexions are indicated in fig. 2. 



