Ionizing Processes in a Point discharging in Air. 289 



N present. Xos. 9, 10, 13 and 14 agree with one another 

 to within about 0'5 per cent., and nos. 24 and 25, the only 

 others available, are practically coincident with one another, 

 but 24 and 25 are about 3 per cent, higher than the four 

 earlier ones. 



Relation betioeen the Fields for Positive and 

 Negative Discharge. 



The field at the outer surface of the ionizing Liver at a 

 positive point is the minimum in which positive ions can 

 ionize. At a negative point the field at the surface of the 

 metal is that required by positive ions to knock out corpuscles ; 

 and if from any cause they are unable to do this there is 

 still the ionizing of the gas itself open to them. It follows 

 that the ionizing field at the surface of a negative point can 

 never be quite as great as that at the surface of a positive 

 point if the positive ions produced at each are the same. 



In Curves VI. , the line B B represents the field-current 

 curve for positive discharge against a plate only, and it will 

 be seen that the majority of the negative points are well 

 above it. 



As explained in the preceding paper, the absolute values 

 of the negative fields are hot so accurately known as those of 

 the positives, but it is unlikely that this will account for so 

 large a discrepancy as the one in question. 



We are inclined to explain it as follows : — The negative 

 glow stands out a long way from the point. A considerable 

 proportion of the positive ions formed in it have consequently 

 some distance to travel before reaching the point, and will 

 have grown beyond their initial size when they arrive. We 

 shall thus probably be dealing with older positive ions on 

 the average in negative than in positive point discharge, and 

 the occurrence of the stronger fields at the negative than at 

 the positive point is thus reasonable. 



It is consistent with this that the field at a negative point 

 becomes less, relatively to that at the same point positive, as 

 ihe sharpness of the point increases * ; for at sharp points 

 the slow does not stand out so far, and as the ions thus 

 have a shorter distance to go, and also move faster in 

 approaching the point, they will be newer when they get 

 there. 



* Chattock, Phil. Mag. [5] vol. xxxii. p. 285 (1891). 



