Electric Wind in Hydrogen containing traces of Oxygen. 455 



This result proves that here at any rate we have a case of 

 discharge which cannot consist of the simple stream from 

 point to plate of ions of like sign to the point postulated for 

 the formula. It strongly suggests the presence of positive 

 ions travelling against the wind, ions which, even if present 

 in smaller numbers than those from the point, might still 

 reverse the pressure as they travel slower; and it is then of 

 course only reasonable to regard these ions as the cause also 

 of the lowering of dp/dz in those samples of hydrogen which 

 contained oxygen, and for which dp/dz was not negative but 

 only very small. 



The view that back discharge is the cause of the reduction 

 of the wind is further borne out by the appearance of the 

 plate when examined in the dark. In the purest hydrogen, 

 and for a negative point, the whole plate glows brightly ovei 

 the surface presented to the point ; but the glow fades 

 gradually as small quantities of oxygen are added to the gas. 

 At a current of 2 microamperes, for instance, the plate was 

 still faintly luminous with 0*1 per cent, of oxygen present, 

 but became quite dark before the percentage reached 09. 



With a positive point no such effect is observed. At 2 

 microamperes the plate was quite dark, but a very faint 

 luminosity was always obtainable with much stronger currents 

 which was apparently little if at all affected by the amount 

 of oxygen present. 



It thus appears that for negative discharge the glow is a 

 maximum when dp/dz is lowest, viz. for the purest hydrogen, 

 and that as oxygen is added the glow fades and dp/dz in- 

 creases, until at about the point where dp/dz gives' normal 

 values of V— the glow vanishes. 



If, then, we may take the glow to mean ionization at the 

 plate, we have in these facts strong support for the view that 

 the lowering of dp/dz is due to back discharge. 



It is interesting that under certain conditions the glow 

 seems to become unstable, tending to contract suddenly often 

 to a point which glows brightly, and is probably coincident 

 with some slight projection on the plate. For if at any 

 region on the plate the back discharge accidentally increased, 

 the lines of force from the point would converge towards that 

 region and concentrate the current there, thus tending to 

 increase the back discharge still further, and so to intensify 

 the concentration of the lines. We have seen this effect on 

 several occasions, and there can be little doubt that it has 

 been responsible for a certain irregularity which characterizes 

 the wind of negative discharge. And this suggests another 

 consideration. As the point is moved further from the plate 



