Radioactivity and Ionization of the Atmosphere, 709 



Fig. 3 is a decay-curve for a lead wire exposed in an attic 

 190 minutes at —25000 volts. In this case the lead wire 

 was wound in the form of a flat spiral and placed inside a 

 testing vessel consisting of two parallel plates, one of which 



Fig. 3. 



60 



lo 



60 



50 



4o 



30 



To 



r 



















y 

















\ 







-Demi 



'OfFxr/fc 



4— 







\ 









Rtidioact 



V,/y- 





I 





\ 













1 



l 







































_ Time in 



A7//7S— 









Q,o 



was connected to the electrometer and the other to the 

 battery. This again falls to half value in about 45 minutes. 



A laro-e number of curves of decay have been determined 

 under very varying atmospheric conditions, but no certain 

 differences in the rate of decay have been observed, although 

 the amount of excited activity in a given time varies greatly 

 with the weather and amount of wind. The rate of decay 

 was the same for a copper as for a lead wire, and was inde- 

 pendent of the diameter of the wire. The rate of decay for 

 a brass rod charged at - 100,000 volts was about the same 

 as for a lead or copper wire exposed at —5000 volts. The rate 

 of decav for low voltages has not been investigated. 



We may thus conclude that over the range examined the 

 rate of decav is regular and independent of conditions. In 

 this respect also it resembles the excited radiations produced 

 by thorium and radium. 



" It will be seen, from these results on the rate of decay, 

 that if the intensity of the excited radiation is initially I , 

 the intensity I after a time t is given by 



