796 



Prof. Townsend and Mr. Edmunds on the 



The points were made of platinum or silver wire with ends 

 approximately hemispherical, and the planes to which the 

 discharges took place were platinized brass and were fixed at 

 right angles to the axis of the wire. The points and planes 

 were contained under bell-jars fitted with tubes for adjusting 

 the pressure of the air, and measurements of the sparking 

 potentials were made as with the cylinders. 



The results showed the same characteristics as those 

 obtained with the cylinders, as may be seen from the 

 following figures : — 







Positive 



Negative 





Pressure 



sparking 



sparking 





in mm. 



potential 



potential 



Diameter of wire '15 cm 





in volts. 



in volts. 



555 



6200 



7000 



Distance from plane T5 cm. ... 



397 



5080 



5500 





220 



3450 



3800 





103 



2200 



2450 





35 



1320 



1280 





13 



975 



825 



Diameter of wire '10 cm 



763 



6400 



6800 



Distance from plane 1 cm. ... 



213 



2950 



3130 





81 



1700 



1700 



Diameter of wire 0'5 cm 



760 



5300 



5300 



Distance from plane 1"5 cm. ..„ 



265 



2900 



2700 



8. It is interesting to see whether this result can be ex- 

 plained on the hypothesis that the effect is due to ionization 

 by collision. 



The relation * which must hold between the values of a 

 and ft along a radius r between two cylinders, radii a and 

 A, before a continuous discharge can take place is 



-j>-JV 



■P)dr d 



when the inner cylinder is negatively charged. 



The corresponding condition for the case in which the 

 inner cylinder is positively charged is obtained by inter- 

 changing cl and /3 in the above integral, and it does not 

 follow that a system of values of a and /3 which satisfies the 

 first also satisfies the second. Hence the system of forces 

 required for a positive discharge is in general different from 

 the system required to maintain a negative discharge. 



* ' Ionization of Gases by Collision/ p. 66. 



