784 Dr. Norman Campbell on Delta Rays 



proved by Lenard * that under the action of /3 rays, with a 

 velocity greater than 11 volts, all metals emitted a "secondary 

 cathode radiation," the velocity of which was nearly the same 

 for all substances and of the order of 10 volts. His results 

 have been confirmed by many subsequent investigators, and 

 especially by Gehrts t, who finds that the quality of this 

 secondary radiation is the same from whatever metal it is 

 emitted and whatever is the velocity of the primary rays, so 

 long as it is greater than 80 volts. These conclusions are all 

 based on the measurement of the relation between the current 

 from an electrode emitting the radiation and the potential 

 difference. An examination of the curves given by Gehrts 

 (loc. cit. pp. 1014, 1015) shows that they are very similar to 

 those obtained by myself (Phil. Mag/ Aug. 1911, p. 291) 

 when measuring in a similar manner the same relation for a 

 current carried by £ rays excited by a. rays. It appears 

 probable that the $ rays of the same quality are emitted 

 under the action of both a. and j3 rays. 



3. It has been shown in the previous papers that the § rays 

 are not homogeneous in velocity and that, for this and other 

 reasons, the relation between the current carried by them 

 and the acting potential difference will vary somewhat with 

 the distribution of the rays and the arrangement of the elec- 

 trodes. Accordingly, in order to produce conclusive proof of 

 the identity of the rays produced by different agents, it was 

 necessary to ensure that they were investigated in the same 

 conditions. It was easy to attain these conditions if the /3 

 rays which were to be compared with a rays were those 

 excited by the action of Rontgen rays. 



The apparatus used was that described in a previous 

 paperj; it consists of two parallel electrodes, one of which 

 is pierced by a hole, covered with thin foil, through which 

 the rays enter the space between the electrodes, in both of 

 which they excite £ rays. Two series of experiments were 

 conducted with the electrodes in exactly the same condition ; 

 in the first the rays were a rays proceeding from polonium 

 placed above the hole ; in the second the rays were Rontgen 

 rays passing through the same hole. In each case the current 

 was measured for different potential differences between the 

 electrodes ; in the former case the measurements were made 

 by the method described already, in the latter by the usual 

 compensation method designed to render the measurements 



* Lenard, Ann. d. Phys. viii. p. 149 (1902) ; xii. p. 449 (1903) ; 

 xv. p. 485 (1904). 



t Gehrts, Ann. d. Phys. xxxvi. p. 995 (1911). 

 X Phil. Mag. April 1912, p. 464. 



