Ionizing Processes in a Point discharging in Air. 287 



the fact that the abscissae of A A are all too large by the 

 currents carried on the N ions and shift A A to the left the 

 discrepancy becomes less marked. 



Roughly, then, it may be said that the effect of external 

 ions upon negative discharge is to remove temporarily the 

 two signs of ageing — high field and fluctuating field at 

 the point ; in other words, to render the old point new for 

 the time being;. 



Ageing has been attributed to some change in the surface 

 of the point, which makes it difficult for positive ions to 

 knock corpuscles out of the metal. 



Considering that a point discharging negative electricity 

 produces quantities of positive ions in its neighbourhood, it 

 is not easy at first sight to see why the arrival of a relatively 

 small number of external positives should facilitate the 

 escape of the corpuscles so much. For the only obvious 

 difference between these two sets of ion c ' that those produced 

 at the point are newly formed, whil he externals are old 

 — and this ought to render the externals less able to set free 

 corpuscles instead of more. 



An explanation is perhaps to be found in the following 

 theory. 



The negative discharge starts in a very small spot upon 

 the point surface, the glow standing out in the form of a 

 luminous trumpet to a distance comparable with the diameter 

 of the point, in a manner suggestive of a rush of corpuscles 

 escaping through some weak spot in the surface of the 

 metal . 



When the point ages a very characteristic feature of the 

 discharge is observable. It is often impossible to get small 

 currents to flow steadily. With a new point the current 

 can be made to sink gradually to nothing as the wimshurst 

 is slowed down ; but with an aged point it sinks gradually 

 to some low finite value and then stops dead, just as if it had 

 been suddenly switched off. 



Jn the light of this fact let us test the following hypo- 

 thesis ; whatever the nature of the ageing change may be, 

 let its effect be such that the metal refuses to yield up 

 corpuscles under bombardment by positive ions unless the 

 number of these ions is considerable. 



The hypothesis is consistent with the switching off effect 

 just referred to. 



