200 Dr. B. Hodgson on Heating Effects at 



say 3-4 mm. in oxygen, and the current small enough to 

 give a normal cathode fall. Although ionization is necessary 

 at the cathode surface to make the current continuous *, the 

 author f has shown that only a small fraction of the 

 bombarding ions produce fresh ions at the cathode surface. 

 The relatively few corpuscles produced soon acquire energy 

 in the cathode dark space sufficient to ionize many times. 

 This energy is expended in the process of ionization in the 

 cathode dark space and the negative glow. The corpuscles 

 produced in the dark space soon produce fresh ions there^ 

 and so a progressive ionization occurs as the negative glow 

 is approached. The increase in the number of ions per cm. 

 reduces the gradient, and in the negative glow the " resist- 

 ance " of the gas there is so small, that the gradient is 

 negligible compared with that in the dark space ; and when 

 the electrons or negative ions have expended the energy 

 acquired in the cathode dark space, no further ionization 

 occurs. The formation of clusters begins, and these negative 

 carriers drift in the electric field towards the anode. The 

 hazy glow at the anode may be the result of ionization, but 

 this must be due to only a small fraction of the bombarding 

 ions. Otherwise the shape of the curve in fig. 4 is difficult 

 to account for. As the pressure is reduced the phenomena 

 occur as already described. 



The curves in fig. 2 are explained in a similar way to 

 those in fig. 3, as they represent the facts of Tables I. and II. 

 is a different form, namely, with constant pressure instead o£ 

 constant current. 



§5. Effect of Distance between the Electrodes on the Energy 

 given to the Anode and Cathode. 



A tube was arranged so that one electrode was movable. 

 The distance between the electrodes could be varied between 

 and 30 cm., and this process could be performed by using 

 a winding arrangement without impairing the vacuum. 



Though it was not possible to decide the relative values of 

 the two suggestions in §3, the results are not in discord with 

 either view. During the experiment the current and gas 

 pressure were kept constant, and the distance between the 

 electrodes and the energy given up to cathode or anode were 

 measured. In the case of the cathode, the energy given up 

 was independent of the length of the positive column except 



* J. J. Thomson, ' Conduction through Gases,' ii. ed. p. 585. 

 t Hodgson, Phil. Mag. xxvi. p. 460 (1913). 



