Ionizing Processes at a Point discharging in Air. 277 



Conclusions. 



"When discharge occurs at a sharp point in air at atmo- 

 spheric pressure, the current, dimensions, and other conditions 

 being those considered in this paper, it is possible to calculate 

 the strength of the field in the ionizing region at the surface 

 of the point to within one or two per cent, for a positive and 

 less accurately for a negative point in terms of the mechanical 

 pull upon its surface ; and this conclusion holds if the point 

 be supplied with ions of opposite sign to itself from a second 

 point in its neighbourhood. 



XXII. On the Ionizing Processes at a Point discharging in 

 Air. By A. P. Chattock, Professor of Physics, and 

 A. M. Tyndall, B.Sc, Lecturer in Physics, in the University 

 of Bristol *. 



[Plate IV.] 



IN explaining the phenomena of discharge at sharp points 

 in gases under normal conditions, Sir J. J. Thomson 

 postulates an initial ionization of a few isolated molecules in 

 the gas as a preliminary to the process of discharge. 



Suppose a point to be gradually charged with positive 

 electricity in the presence of these isolated ions. The field 

 near its surface is at first unable to do more than clear them 

 away as fast as they are formed ; but as soon as it is strong- 

 enough to impart to the positives among them sufficient 

 energy to enable these to ionize fresh molecules in their 

 turn, ordinary positive discharge sets in, and a large current 

 may result, accompanied by glow at the point and wind. 



In the case of a negative point the field has also to reach 

 a high enough value to enable the initially formed positive 

 ions to form fresh ions ; but they now have the alternative 

 of doing this where they bombard the surface of the metal 

 instead of in the gas, and the field required is not necessarily 

 so high as when gaseous molecules are to be ionized. 



For both kinds of discharge the supply of positive ions is 

 pictured as kept up by ionization due to negative ions, 

 these having been produced by previously formed positive 

 ions and so on. Both signs of ion have therefore to be able 

 to ionize as each produces the other ; and since positive ions 

 require a stronger field for this than negative it is always 

 the field required by the positive ions which has to occur at 

 the point. 



* CommuniCcated bv the Authors. 



