618 Dr. H. Geiger and Prof. E. Rutherford on 



^ = 35 , 5mm.) and the calculated fields, for + and — dis- 

 charge from P, are shown in the following table: — 



Table II. 



c. 



/+■ 



/- 



(microamps.) 



(volls/cm.) 



(volts/cm.) 



0-4 



670 



710 



0-8 



930 



890 



1-6 



1270 



1120 



23 



1480 



1300 



31 



1640 



1500 



Summary. 



The potential drop in the immediate vicinity of a point 

 discharging on to a plate is independent of (a) the distance 

 between the electrodes, (b) the current between the electrodes. 

 The potential drop required to start discharge is slightly 

 greater at a negative point than at a positive point. 



The effects on the potential drop of supplying to the point 

 ions from an external source have been investigated. 



These results are discussed in the light of previous deter- 

 minations of the field at a discharging point obtained from 

 measurements of the mechanical pull on its surface. 



The potential gradient in the vicinity of the plate has also 

 been measured over certain ranges of current and distance. 



These experiments were carried out in the Physics Labo- 

 ratories of the University of Bristol. 



I wish to acknowledge my indebtedness to Dr. Tyndall 

 for the constant ndvice and suggestion he has offered, and 

 to Mr. J. D. Fry for help in the construction of the apparatus. 



LV1I. Photographic Registration of ex. Particles. By Dr. H. 

 Geiger and Prof. E. Rutherford, F.R.S., University of 

 Manchester *. 



IN a previous paper t the authors have described an 

 electrical method of counting the a particles from radio- 

 active substances. For this purpose, the a particles passed 

 through a small opening into a cylindrical vessel which 



* Communicated bv the Authors. 



t Rutherford k Geiger, Proc. Roy. Soc. A. lxxxi. p. 141 (1908). 



