The Discharge of Electricity through Gases. 



541 



Volume Electrification near the Kathode. 



I have spent considerable time during the last few years in in- 

 vestigating the fall of potential in the different parts of the negative 

 glow with a view to deciding whether there is any considerable 

 volume electrification of the gas itself. 



De la Hue and Miiller* have already endeavoured to measure the 

 potential at different places in the dark space ; but for some reason, 

 which I cannot trace, their results are altogether anomalous. They 

 used what they call four different arrangements of leads, which, 

 however, all should give identical results ; but no regularity can be 

 traced. In many cases they found that the potential becomes more and 

 more negative away from the kathode, so that in one of their observa- 

 tions some point in the dark space was at a potential —1068 volt, 

 the negative electrode being at potential zero. I have never observed 

 anything but a regular increase of potential within the dark space. 

 In order to be able to subject the results of experiments to a simple 

 analysis, I have used two different kinds of vessel. In one the lines 

 of flow were, throughout, parallel to each other ; this I shall call the 

 axial discharge. Cylindrical tubes were used, and the kathode was 

 a closely fitting aluminium plate, care being taken to prevent a 

 discharge from the back. The other vessels used were large cylin- 

 drical receivers, the kathode being a wire forming the axis of the 

 cylinder, and the anode a cylinder made of wire gauze. The lines 

 of flow in these vessels were radii drawn from the axis of the 

 cylinder, and I shall call the discharge in this case the radial dis- 

 charge. The fall of potential between the kathode and any point 

 inside the negative glow was found to be capable of being repre- 

 sented by the formula 



V = Y (l-e— ) (1,), 



where Y is the potential at any point, V the potential in the 

 glow proper, k a constant, and x the distance from the kathode. 

 The potential of the kathode is taken as zero. The formula is not 

 meant to be more than an approximate one, but within the limits of 

 error of experiment the formula may be taken as correct through the 

 dark space and the inner parts of the glow. It may, in the first' 

 place, only be considered as an interpolation formula, and others 

 might be found representing the facts equally well, but the equation 

 gains some reality owing to the fact that k for a given pressure is 

 found to be very nearly independent of the strength of the current. 

 The complete account of my experiments will show how far this is- 

 correct. Owing to the circumstances of the case, the fall of potential 



* 'Phil. Trans.,' 1883 (vol. 174, p. 477). 



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