716 Prof. E. Rutherford and Mr. S. J. Allen : Excited 



could leak across the insulator to the inner cylinder, which 

 was connected to the electrometer in the usual way. The 

 outer cylinder was connected to one pole of a battery of small 

 storage-cells, the other pole of iwhich was earthed. 



The electrometer needle showed quite a rapid moyement 

 due to the ionization current between the electrodes with a 

 P.D. of a few volts between the cylinders. 



The cylinder was made fairly air-tight and allowed to stand 

 undisturbed. Observations of the current between the 

 cylinders were made at intervals for over a month. In order 

 to avoid correction for the slight variations in sensitiveness 

 of the electrometer from day to day, the ionization current 

 between two paralleled insulated plates due to a standard 

 sample of uranium oxide was observed at the same time. 

 Previous experiments have shown that the uranium oxide is 

 a very constant source of radiation. 



The following tables show the variation of the current, due 

 to the spontaneous ionization of the air, with the P.D. between 

 the cylinders. Table I. is for air which has stood undisturbed 

 for a month inside the vessel ; Table II. for the ordinary air 

 of the room several hours after it had been introduced into 

 the apparatus. 



Table I. 



Table II. 





Current in divisions 





Current in divisions 



P.D. in volts. 



per sec. of 



P.D. in volts. 



per sec. of 



I 



electrometer. 





electrometer. 



•4 



■34" | 



.9 



•04 



•8 



•50 



105 



•22 



2-1 



•59 



21 



•32 



4-2 



•65 



6-5 



•52 



Cr5 



•67 



13 



•61 



13 



•71 



26 



•65 



26 



•72 



39 



•67 



52 



•73 



52 



•68 



The results are expressed graphically in fig. 6, curves I. 

 and II. respectively. 



The curves are very similar to those observed when the air 

 is ionized by Becquerel or Rontgen rays. The current first 

 increases approximately directly as the voltage, but soon 

 reaches a stage in which large variations of the voltage only 

 cause a slight increase in the current. On account of the 

 very small amount of ionization of the air and consequent 

 slow rate of recombination of the ions, the maximum current 

 is reached for a very small voltage. The current for 50 volts 



