174: 



Prof. E. Rutherford on Radioactivity 

 Lead Radiation. 



Distance from 

 surface. 



Current. 



d (=3 rums.) 



1 



tZ+6'25 mms. 



•79 



d + 12'5 „ 



•59 



tf+187 „ 



•46 



d+25 



•35 



d+3V2 „ 



•27 



^4-37-5 „ 



•21 



The current is taken as unity when the measurements 

 began at a distance d = 3 mms. from the active lead plate. 



For the purposes of comparison, the numbers obtained in a 

 similar manner for thin layers of thorium oxide and uranium 

 oxide on a bare plate are given below. 



Thorium Radiation. 



Uranium Radiation. 



Distance. 



Current. 





Distance. 



Current. 



d (=.2"25 mms.) 



1 



d (=2-25 mms.) 



1 



("7+5 mms. 



•73 





d-\- 2 5 mms. 



•685 



<? + 10 „ 



•50 





d+ 5 



•445 



d+lo „ 



•35 





d+ 7-5 „ 



•296 



d+20 „ 



•25 





rf+io „ 



d±Vo „ 

 d+20 » 



•188 

 •088 

 •059 



The curves in fig. 6 show the results graphically. It will be 

 seen that the intensity of the radiation falls off approximately 

 in a geometrical progression as the distance increases in 

 arithmetical progression. Curves of absorption of thorium 

 radiation in air at different pressures have been obtained by 

 Owens *. 



The distances through which the three types of radiation 

 from uranium, thorium, and active lead pass through air at 

 ordinary pressures and temperatures before the intensity is 

 reduced to one-half its value, are about 4, 10, and 16*5 mms, 

 respectively. 



* Owens, PLil. Mag. Oct. 1699. 



