Mr. Norman Campbell on Delta Rays. 



295 



16. The most obvious explanation of this difference is that 

 when no difference of potential is imposed on the electrodes, 

 there is still a difference between them owing to the Volta 

 effect. Unfortunately the effect, though of the right magni- 

 tude (about 1 volt for Al-Cu and J volt for Al-Ag) is of 

 the wrong sign ; for this explanation would make both silver 



Fig. 4. 

 A, Copper. B ; Aluminium. 



and copper positive to aluminium, whereas really the Volta 

 difference of potential makes them negative. It seemed that 

 a test of the correctness of the assumption might be made by 

 rinding the values of the total emergent and incident radia- 

 tions, and comparing them with values found when both 

 electrodes were of the same metal. If the values for the 

 intrinsic difference of potential given above are correct (and 

 no approximation nearer than £ volt is attempted) we find, 

 in the case of Ag-Al, 0*394 as the total emergence rays from 

 silver and 0'353 as the total incidence rays for aluminium. 

 Experiments made directly before and after these, when the 

 electrodes were both of the same metal, gave 0'361 for the 



