184 Prof. E. Rutherford on Radioactivity 



Case of Diffusion of Radio-active Particles. 



The case where no voltage is acting is one of special 

 interest, for there the diffusion of the radio-active particles 

 is alone operative. A loose layer of paper was placed over 

 the paper envelope containing the thorium oxide. The paper 

 envelope bent into the arc of a circle covered about one 

 quarter of the circumference of the cylinder. The following 

 numbers give the rate of leak, in divisions per second, due to 

 the radioactivity on different portions of the vessel : — 



Radioactivity on inside cylinder ... 1*32 divisions per sec. 



„ on paper 2*26 „ 



„ on outside cylinder"! n^ 



and stoppers J 



Total radioactivity 6*90 „ 



The current due to the total radioactivity is thus about the 

 same as the current when 20 volts acts between cylinders. 

 The experiments on the effect of voltage extended for more 

 than a month, and some of the results showed that the thorium 

 oxide was not a constant source of radiation during the whole 

 of that time. The variations were not, however, sufficiently 

 large to obscure the general nature of the results. 



Effect of Pressure on Radioactivity. 



The diminution of the pressure of the gas from 760 to 

 20 millim. had very little action on the amount of " excited " 

 radioactivity on the — charged electrode. 



The following apparatus was employed : — 



A brass cylinder B (fig. 11a), with an ebonite stopper 0, 



Fig. 11 a. — Exciting Apparatus. 



To pv^p 



'«/>g_ 



III-- 



through which passed a brass rod A, was connected with a 

 mercury pump.. The thorium oxide inside a paper envelope 

 was placed inside the cylinder. B was connected to + pole 

 of a battery of 50 volts, and C to — pole. The apparatus was 



