106 Prof. Rutherford on Excited Radioactivity and 



and the alternating E.M.F. applied. By means of side 

 tubes, the apparatus was put into connexion with the emana- 

 tion vessel and a small fraction of the emanation introduced 

 into it by sending a slow current of air into the cylinder. 

 The tubes leading into the apparatus were then closed, and 

 the alternating E.M.F. continued for an interval of 15 to 30 

 minutes. Before stopping the commutator, the emanation 

 was blown out of the apparatus by a slow current of air. The 

 plates were then removed, and the amount of activity on them 

 was compared by means of the electrometer. On account of 

 the initial rapid decay of the radiations, a difficulty arose in 

 comparing the amount of radiation on the two plates. As 

 shown in a previous paper *, the excited radiation from radium, 

 for short exposures, decreases rapidly for the first 5 minutes 

 after removal, but about 15 minutes after removal reaches a 

 value which is maintained fairly constant for an interval of 

 about 10 minutes. It then decays to zero, falling to half 

 value in about 30 minutes. The comparison of the activity 

 on the two plates was made during this constant interval. 



When experiments were made under the same conditions 

 as those for thorium, somewhat higher values of the velocity 

 of the carriers were obtained, and the numbers, for different 

 frequencies and voltages, differed considerably among them- 

 selves. These discrepancies were found to be due to the 

 fact that even in a strong electric field from 5 to 10 per 

 cent, of the total excited activity was distributed on the 

 anode. In this respect the activity excited by radium differs 

 from that of thorium. Consequently, the value of p would 

 be greater for the radium than for the thorium experiment, 

 under the same conditions, even if the carrier of excited 

 radioactivity travelled at the same rate in both cases. 



§ 7. Distribution of Excited Activity on the Anode. 



In order to throw more lio-ht on the cause of this distribu- 

 tion of excited activity on the anode, some experiments were 

 made with the apparatus shown in fig. 2 a. 



The emanation vessel A consisted of a brass cylinder 25*5 

 cms. long and 8'30 cms. diameter. A long central brass rod 

 BCDE, diameter *518 cm., passed through an ebonite cork 

 at one end of the tube. The outside cylinder was connected 

 to one pole of a large battery, the other pole of which was 

 earthed. The central rod was connected to earth. The 

 emanation was introduced into the vessel by sending a slow 

 current of air through a radium chloride solution contained 



* E. Rutherford arid Miss Brooks, Phil. Mag. July 1902. 



