of Radium on the Electric Spark, 381 



the radiation has uo influence when the discharge passes from 

 the larger to the smaller sphere, are consistent with the theory 

 that a minimum discharge potential is necessary for the 



Fig. 2, 



- VE 



VE 



-VE 



+ VE 



B 



-VE 



+ VE 



-v __ ^ 



effect to be produced. In each of these cases, the mean 

 potential gradient is less than it is when the spark is in its 

 sensitive condition. 



Although the visible discharge is extinguished, it does not 

 follow that the current actually passing is diminished ; the 

 conductivity might be so largely increased that a non-luminous 

 discharge takes place. It was found possible to measure this 

 current, the numbers, although variable, being sufficiently 

 accurate for the purpose of the experiment. To do this, the 

 positive side of the spark-gap was connected directly to the 

 machine, the other side to one terminal of a dead-beat 

 D'Arsonval galvanometer, the other terminal of which was 

 earthed. The negative pole of the machine was also earthed. 

 The current thus passed from the anode acro-stothe cathode, 

 and through the Galvanometer to earth. 



It was found that the extinction o£ the spark corresponded 

 with a decrease in the current. Further, by encasing the 

 radioactive salt in a thick lead case, so that a narrow beam 



