﻿7*18 Profs. Wellisch and Bronson on the Distribution of 



In practice it was found convenient to measure the case 

 activity at 10, 15, and 20 minutes, and the central electrode 

 activity at 25,30, and 35 minutes after the emanation had been 

 removed ; the maximum activity, i. e. the activity when in 

 equilibrium with the emanation, was then calculated by means 

 of the figures given by one of us * for the rate of decay of the 

 activity resulting from a long exposure. The consistency of 

 this calculated maximum determined the number of readings 

 necessary for the measurement of the activity, but usually 

 those just mentioned amply sufficed. 



In practically all our experiments the activities were 

 measured by observing the ionization currents to which they 

 gave rise in air under the ordinary conditions when a potential 

 of + 160 volts was applied to the vessel employed. Although 

 this potential was not sufficient to saturate the currents, sub- 

 sequent experiments showed that the ratio of the two activities 

 as thus measured was not altered by using a larger voltage. 



A typical set of readings is given later. 



3. Experiments with Small Applied Potentials. 



It is well known that for moderately high potentials the 

 activity is concentrated to a very large extent on the negative 

 electrode in an electric field ; Schmidtf has shown that when 

 small potentials are employed the cathode activity decreases 

 in a manner analogous to the diminution in electric current 

 which follows as a result of recombination of ions ; in fact, 

 he came to the conclusion that the active deposit particles 

 behave in a manner similar to the positive ions. The first 

 set of experiments tried in the present series had as object to 

 find whether the amount of activity tiiat was deposited on 

 the cathode when a small positive potential was applied to 

 the case could be altered when the ionization in the gas was 

 so increased by means of Eontgen rays that the electric 

 current through the gas was at least as great as if a large 

 potential had been applied without any extraneous source of 

 ionization. 



In this set of experiments the rods were exposed as 

 cathodes to the process of activation for a period of 10 

 minutes, both with and without the application of Rontgen 

 rays, the testing vessel containing air at 1 atmosphere ; the 

 activities were measured at various intervals after removal, and 

 were then directly compared. Preliminary trials which were 



* Bronson, Phil. Mag. ser. 6, x'i. p. 73 (1906). In order to calculate 

 the maximum activity from the rates determined at the times given 

 above, the observed rates have to be multiplied by the factors 2'02, 

 225, 2-41, 258, 2*75, and 2'96 respectively. 



t Schmidt, Phys. Zeits. ix. p. 184 (1908). 



