﻿Mr. Norman Campbell on Delta Rays. 55 



emitting delta rays which will be attracted to the insulated 

 electrode as soon as its potential rises. It is easy to see that, 

 if the alpha rays are emitted from ,the insulated electrode, 

 both these causes tend to decrease the potential to which the 

 insulated electrode will rise and accordingly the potential 

 difference for which the current is zero will give us a 

 minimum estimate for the speed of the rays. Experiments of 

 this kind have been carried out by Ewers *, Logeman f , and 

 Hauser % ; the first-named gives 1*9 volt as the highest 

 potential to which the insulated electrode was seen to rise , 

 from the curve given by the second it would appear that the 

 current was zero for about 1*0 volt, while in the experiments 

 of the third the corresponding potential difference would 

 appear to be about 2*6 volts. The arrangements in my own 

 experiments are less well suited to give information on this 

 point, but a little consideration will show that a minimum 

 estimate of the potential corresponding to the speed of the 

 rays is given by the potential difference between the electrodes 

 for which the current has the same value as it has when the 

 potential difference is zero and the magnetic field is applied. 

 This potential difference is never greater than 1 volt. It will 

 also be seen that, if the delta rays possessed no velocity of 

 their own, the current corresponding to " V = " must always 

 lie between zero and the current for "V = (magnet)'' ;' 

 this condition is by no means always fulfilled. 



It seems necessary to conclude therefore that, unless 

 current can be conveyed between the electrodes by some 

 agency other than the alpha and delta rays — and no such 

 agency can be suggested— the initial velocity of the delta 

 rays must be taken as at least that corresponding to a 

 potential difference of 3 volts. 



But if the delta rays possess so large an initial speed, the 

 form of the branch of the curve for positive potentials on B 

 must be affected to some extent by the speed of the rays 

 emitted by B ; for part of the change of the current in the 

 region between zero and the potential difference corresponding 

 to the speed of the rays must be due to a direct stopping of 

 the rays by the field. How far the effect of direct stoppage 

 of the rays from B will be masked by changes due to the 

 dragging out of electrons from A it is impossible to say. 

 But an examination of the + columns in Table II. shows no 

 sign whatever of any systematic difference when B is covered 

 by different materials ; so far as the evidence goes, it shows 



* Ewers, Fhys. Zeit. vii. p. 148 (1906). 



t Logeman, Proc. Roy. Soc. A. lxxviii, p. 262 (1906). 



| Hauser, Phi/s. Zeit. xii. p. 466 (1911). 



