Ionizing Processes in a Point discharging in Air. 285 



Reference wns made in the preceding paper to the fact 

 that the field in which ordinary positive point discharge 

 occurs depends Upon the curvature of the point; and that 

 in consequence of this dependence the ionizing region pro- 

 bably extends a sufficient distance from the metal to feel, as 

 it were, the divergence of the lines of force. 



It is interesting to find that the fields in which the glow 

 first appears in the presence of N ions are similarly dependent 

 upon the point. This is shown by the following Table, in 

 which are given the results of experiments upon four points 

 of different sizes. 



t. 



fv 



A ?-°' 45 . 



u 



ALU 



/ 3 - 



A/fr 



00619 



293 



84 



157 



1-9 



30? 



10 



0-0310 



410 



86 



245 



1-7 



70? 



6 



0-0105 



662 



85 



325 



20 



130? 



5 



0-0043 



975 



84 



433 



2-2 



120? 



8 



r is the radius of the point in centimetres ; f x the field in 

 which ordinary positive point discharge is on the verge of 

 slopping ; / 2 the lowest field in which glow is caused by fully 

 formed N ions ; and / 3 the field in which corpuscles give 

 rise to glow, if the views expressed above are correct. The 

 values of / 3 are queried on account of the great uncertainty 

 attending their determination. 



The third column illustrates the exactness of the empirical 

 relation between /i and r, and the rough constancy of the 

 fifth and seventh columns shows that / 2 and / 3 also depend 

 on r in a more or less similar manner. 



It must be remembered that all these fields are rapidly 

 divergent, and that their values are given at the surface of 

 the metal. We do not yet know the values of the weakest 

 fields in which the corresponding ionizing processes can 

 occur because we do not know how far the ionizing regions 

 extend from the point. 



We found that there was a certain hysteresis in the 

 appearance and disappearance of the glow, especially for 

 small values of y, the current having to be raised consider- 

 ably before the glow would start, after which it slowly 

 worked back to a minimum. At this minimum the glow 

 could be made to appear and disappear by slightly increasing 



