240 



Mr. P. J. Edmunds on the 



pressures. From these results curves were drawn showing the 

 relation between Y the sparking potential, and p the pressure. 

 The values of V for a set of values of ap were taken from 

 these curves and are shown in the following table. Thus 



for each pair of values of - and ap are shown three values 



of V, obtained from different wires. It will be seen that 

 the agreement between the corresponding values of V is 

 quite good. 



Table I. 

 Sparking Potentials in kilovolts. 

 for wires with hemispherical ends in air. 

 a = radius of wire in cm. 

 d= distance from plane in cm. 

 p = pressure of air in millimetres of mercury. 



Wire charged positively. 



ap. 



2-5 

 5-0 

 10-0 

 20-0 

 300 

 37-0 





d 







e t 







d 



= 10 





- 



-=30 



a 





a 





a 



a =-025 a = '05 



a =-075 



a = -025 



a =*05 



«=-075 



a = -025 



a ='05 



T25 



1-26 



1-20 



1-38 



1-30 



1-32 



1-46 



1-40 



1-70 



1-66 



1-63 



1-85 



1-76 



1-76 



1-96 



1-89 



2-50 



2-31 



2-30 



2-76 



2-60 



2-60 



2-90 



2-84 



3-86 



3-63 



360 



4-35 



4-00 



3-95 



4-65 



4-35 





4-80 



4-74 





5-30 



5-23 





5-61 





5-60 



5-56 





616 









a=-075 



1-37 

 1-86 

 2-72 

 412 



5-45 



Wire charged negatively. 



2-5 



105 



1-05 



1-10 



1-16 



1-25 



1-25 



; 1-30 



1-30 



5-0 



1-57 



1-70 



1-63 



1-70 



1-81 



1-80 



1 1-86 



1-92 



100 



2-40 



2-52 



2-47 



2-61 



2-80 



2-80 



! 2-90 



303 



200 



3-72 



3-97 



3-90 



4-30 



4-45 



4-30 



4-65 



4-72 



30-0 





5-27 



515 





586 



5-61 





6-20 



37-0 





6-22 



5-95 





G-80 









1-27 

 1-86 

 2-90 

 4-51 

 600 



There is considerable difficulty in finding an exact solution 

 of the electrostatic problem of a charged wire opposite a 

 plane, but an approximation for small points is obtained on 

 the assumption that the wire is shaped like a paraboloid of 

 revolution. 



The potential for a paraboloid of revolution is 



Alogr(l4- cos#), 

 where (r, 6) ar the polar coordinates referred to the axis 



