﻿ol Mr. Norman Campbell on Delia Ray*. 



materials are due to differences in the ease with which the 

 electrons escape from the surfaces of those materials, than 

 that they are due either to a difference in the speed of the 

 electrons generated or to differences in the reflecting powers 

 of the surfaces. But the form of the curve relating current 

 to potential difference cannot be determined wholly by the 

 action of the field in aiding the electrons to escape, unless the 

 initial speed of the electrons is so low that, in the absence of 

 a field, none of them escape. Now it would simplify very 

 considerably our ideas of the mechanism of ionization by alpha 

 rays if it could be imagined that the initial speed of the 

 delta rays was practically zero ; we might then suppose that 

 the action of the alpha particle in passing through an atom 

 was merely to neutralize temporarily the electric forces in its 

 interior and so to allow to escape electrons which were 

 previously held by those forces within the atom ; the escaping 

 electrons would be similar to the " free " electrons in metals 

 and would have velocities corresponding to a potential 

 difference of a few hundredths of a volt, determined by the 

 temperature of the substance. Such an hypothesis appears 

 to me very attractive and it would explain quite adequately the 

 general form of the curves relating current between the 

 electrodes to their potential difference which have been given 

 in this paper and by other investigators, if the supposition 

 were added that the liberated electrons could not escape from 

 the surface except by the aid of an electric field, the 

 magnitude of which is determined by the nature of the 

 material and the condition of its surface. 



8. But, unfortunately, there are considerations which make 

 it impossible to accept immediately this simple and attractive 

 hypothesis. These considerations are based upon the mag- 

 nitude of the potential difference between the electrodes for 

 which the current between them becomes zero. 



If it were possible to obtain a single insulated electrode 

 emitting only delta riys surrounded by another electrode 

 which emitted no rays, the potential to which the first 

 electrode would rise automatically, which is the same as that 

 for which there is no current between the electrodes, would 

 be equal to that corresponding to the speed of the delta rays 

 emitted by it. This principle has been applied, of course, to 

 the measurement of the speed of the electrons liberated by 

 light from various substances. However, in the case of 

 delta rays it is impossible to fulfil this condition ; for, in the 

 first place, there is the charge carried by the alpha rays 

 which excite the delta rays to be taken into consideration, 

 and, in the second place, the enclosing electrode is always 



