Neighbourhood of Electrodes n Point-plane Discharge. 609 



The following were the main results obtained : — 

 The field at the surface of a point discharging on to a 

 plate is independent of the distance between the electrodes 

 and, for a positive point, is independent of the current. For 

 a negative point, the field seems to decrease with increase in 

 current, but it was pointed out that it is doubtful whether the 

 pull method may be applied with so much accuracy in the case 

 of negative discharge. The value of the field at a negative 

 point was somewhat greater with small currents, and some- 

 what less with large currents, than at a positive point. 

 Negative ions supplied from an external source decreased 

 the discharging field at the surface of a positive point, while 

 positive external ions had no effect upon that at a negative 

 point, except when the source (which was another discharging 

 point) was placed close to it. Theoretical explanations of 

 these results were suggested. 



The work described below was undertaken mainly with 

 the view of obtaining support for these results, by measure- 

 ment of the potential drop in the neighbourhood of a 

 discharging point. 



Method, 



If the potential slope between a point and a plane is 

 mapped by means of an exploring wire, then, assuming that 

 the wire itself introduces no disturbance of the field and 

 that it takes up the true potential of the air at its surface, 

 the distribution of field is directly obtained. 



The experiments, which were carried out in open air, may 

 be divided into two groups, viz., Investigations of field 

 (1) near the point, (2) near the plate. 



With regard to (1), there seemed to be no doubt whatever 

 that the exploring wire would introduce considerable dis- 

 turbance of the field and that quantitive readings would be 

 unattainable. Such disturbance being present, however, 

 in both signs of discharge, it was felt that some quali- 

 tative results might be obtained, particularly with regard 

 to the comparison of the effects of positive and negative 

 discharge. It will be shown that some interesting conclu- 

 sions have been reached, which are, in the main, consistent 

 with those previously deduced from the work already 

 mentioned. 



The investigation of the field near the plate was carried 

 out for certain special ranges of current and distance 

 between electrodes, upon which some information was 

 required. 



