528 Dr. Norman Campbell : FurtJier 



process analogous to voltaic action, for the acquirement of 

 a spontaneous positive charge by a body struck by alpha 

 rays had never been observed except when the electrodes 

 were of different materials. The first of the miscellaneous 

 experiments here described was designed to test this 

 hypothesis. 



2. There is one remarkable feature which distinguishes 

 radioactivity and its effects in ionization from almost all 

 other processes : they are independent of the temperature. 

 If the relation of the current to the potential difference were 

 determined by secondary causes unconnected with radio- 

 activity and ionization, then that relation should change 

 with the temperature ; but if it is determined by the primary 

 causes of emission of the delta rays, then it should be inde- 

 pendent of the temperature. 



The experiment was made by insulating about 1 mm. 

 apart two parallel copper plates, one of which was covered 

 with polonium, in an evacuated glass vessel. The vessel 

 was strongly heated during exhaustion to drive off air from 

 the solid contents. The relation of the current 'to the 

 potential difference between the plates was then determined 

 (1) at room temperature, (2) when the vessel was heated to 

 the softening point of the glass, (3) when the vessel was 

 immersed in liquid air. Not the smallest difference could 

 be detected between the relations for (1) and (3) ; in (2) all 

 the currents were about 3 per cent, greater than in (1), 

 a difference which could be attributed to the fact that gas 

 had not been completely eliminated from the vessel, or 

 possibly to the beginnings of thermionic emission. It may 

 be safely concluded that the emission of delta rays is inde- 

 pendent of the temperature, and that the velocity of those 

 rays is determined by processes directly connected with the 

 action of alpha rays on the atoms they strike. It is necessary 

 to imagine that the rays really start with an initial velocity 

 of some volts. 



3. Definite evidence that the form of the relation between 

 current and potential difference was influenced by the effect 

 of the field in dragging delta rays out of the electrodes had 

 been produced only in the case when one of the electrodes 

 was covered with soot. It appeared that if this action was 

 important, the relation ought to be determined by the electric 

 field between the plates rather than by the potential difference 

 between them. Experiments were accordingly made with 

 the apparatus described in the last of the three papers to 

 discover whether the relation was changed when the dis- 

 tance between the electrodes (and, therefore, the field for a 



