530 



Dr. Norman Campbell : Further 



mentioned in the last paragraph and many others have been 

 repeated with the substitution of the thorium deposit for 

 polonium ; in no case has any alteration due to the substi- 

 tution been detected. It may be taken as quite certain that 

 the velocity of the delta rays is perfectly independent of that 

 of the alpha rays by which they are excited. 



5. Many of the difficulties in interpreting the results of 

 the experiments described in the first two papers arose 

 from the fact that in all cases the rays proceeded from 

 both of the electrodes between which the current was mea- 

 sured. It was difficult to decide whether the effect of an 

 increase in the potential of one electrode was due to a stop- 

 ping of the rays proceeding from that electrode or to an 

 increase in the number proceeding from the other. It 

 appeared that, if only an apparatus could be obtained in 

 which the rays proceeded from one electrode only, then it 

 would be possible to determine accurately the maximum 

 speed of the rays at least, for it would be that corresponding 

 to the positive potential on that electrode which is sufficient 

 to prevent any current passing. At length the apparatus 

 shown in fig. 1 was designed and seemed to fulfil the 

 necessary conditions. 



Fig- 1. 



A -S 



1^ 



Jo 



The active material, which in this case was the active 

 ■deposit o£ thorium, is deposited on the under side of the ring 

 O. The alpha rays from it can excite delta rays at the 

 interior surface of the box A, and these rays can emerge 

 through the hole in the top of the box and reach the other 

 electrode D ; but the alpha rays cannot strike any ether 

 part of the apparatus, so that there is no other source of 

 delta ravs. Both electrodes were, of course, contained in a 

 vessel which could be exhausted. 



Preliminary experiments showed that, if the hole in the 

 top of A were covered with the thinnest possible aluminium 

 foil, no detectable current could be made to pass between 



