298 



Mr. Norman Campbell on Delta Rays. 



measure the radiation from the polonium disk, the total 

 saturated negative current the radiation from the other 

 material ; no simple interpretation can be put upon measure- 

 ments o£ the current lor differences of potential less than 

 that required to give saturation. The charge due to the 

 alpha rays themselves could not be determined, for a mag- 

 netic held would not stop the rays moving along the direction 

 of the field : the effect of it would be to make the negative 

 current appear rather too large and the positive rather too 

 small. 



It appeared at once that there was no very marked dif- 

 ference in the total negative current for different materials. 

 But it was found impossible to determine within about 

 10 per cent, whether the current for all materials was the 

 same. The reason for this impossibility lay in a progressive 

 change in the current with the time, an effect which had 

 been suspected before, but had not influenced measurements 

 always expressed as fractions of the total current. After 

 the vessel had been pumped out and the charcoal tube placed 

 in liquid air, the current of both signs decreased at an ap- 

 proximately regular rate for at least 24 hours : the total 

 change might amount to 10 per cent., but in no case was it 

 certain that a steady condition had been obtained. There is 

 no doubt that this change is due to the removal of films of 

 air covering the electrodes; for if the pressure in the vessel 

 were allowed to rise a few centimetres and then quickly 

 reduced again, the leak was found to have risen again ap- 

 proximately to its original value. The following table gives 

 the total positive and negative currents measured two hours 

 after the application of liquid air : each figure is the mean 

 of four observations which differ among themselves by not 

 more than 10 per cent. The ratio of the positive to the 

 negative current is much more constant, the extreme varia- 

 tion in the experiments on any one metal being 3 per cent. 



Table VII. 





Al. 



Cu. 



Zrr. 



Sn. 



Pb. 



864 

 663 

 130 



Paper. 



8-86 

 6-61 

 1-34 



-f- current 



8-S8 

 6-56 

 1-32 



8-56 

 5-85 

 1-46 



8-57 

 6-73 

 1-27 



8-29 

 6-12 

 1-35 





Ratio 4/— 



18. It will be observed that the values of the ratio of the 

 currents are the same for all metals except copper within the 



