Surface of a Needle -Point discharging in Air. 273 



It appears from the foregoing arguments that the only 

 force of any importance at the surface of a positive dis- 

 charging point is the pull of the field npon the metal. 

 This field we should expect to be independent of the current 

 from the point, at any rate for a considerable range of 

 current ; and the fact that, as the following table shows, the 

 valnes of/o calculated from the pull are nearly constant thus 

 gives considerable support to those arguments. 



In the table are given the values of V#P for various 

 currents from a positive point of radius 0*018 cm. discharging 

 against a flat metal plate 2'2 cm. distant. 



^8P. 



VfcP. 



Cin 





microamperes 



7-37 



13-38 



7-38 



9-39 



7-38 



7-21 



7-41 



4-20 



7-4L 



1-64 



7-43 



0-79 



7-46 



52 



7-93 



00 



Cin 



microamperes. 

 7-46 019 



742 184 



741 3-09 



739 5-45 



7-39 6-90 



7-39 801 



7-39 9 -39 



7-38 10-90 



Negative Discharge from a Single Point. 



12 for negative discharge is usually much less than for 

 positive — several hundred times less in the case of the large 

 point ; and the glow projects into the gas to a distance 

 comparable with the diameter of the point instead of being- 

 confined to the surface of the latter, its form varying from 

 radial to trumpet-shaped. 



That fl is small means that p 2 is large ; but as the area 

 affected by p 2 will be small in the same proportion, these two 

 effects will roughly cancel, and the only important change in 

 the suction of the C ions will be due to the easier access of 

 the outside gas to the cone of discharge, which implies a 

 greater reduction of p 2 for a negative than for a positive 

 point. 



As the glow projects into the gas the C ions start, on the 

 average, further from' the point, and this also implies a 

 reduction in p 2 . 



The B ions, so far as their mechanical effect is concerned, 

 may be expected to behave much as they did for positive 

 discharge, except that their momentum will not now be given 

 up to the point quite so completely. 



For these reasons the pull on a negative discharging point 

 is probably quite as little affected by the mechanical forces of 

 the current as that on a positive point. 



