108 Prof. Rutherford on Excited Radioactivity and 



the course o£ three or four hours' work, the amount of excited 

 activity, obtained for a given time of exposure, diminished 

 about 20 per cent. This was partly due to the decay of the 

 radiating power of the emanation during the interval, and 

 partly to a slight escape of the emanation in removing and 

 replacing the central rod. By determining the ratio of 

 excited activity at the beginning and end of the experiments, 

 a correction was readily made for this diminution. 



Table of distribution on anode, diameter 8*3 mms. 



Voltage. Percentage. 



-300 100 



+ 300 6 



+ 150 6 



+ 50 9 



+ 20 10 



11 



It will be seen from the table that the amount on electrode 

 with + 300 volts P.D.is 6 per cent, of amount on electrode with 

 — 300 volts P. D. The percentage increases with diminution 

 of voltage, rising to 11 per cent, for zero voltage, when the 

 distribution is due to diffusion alone of the carriers to the 

 central electrode. In order to see how much of this amount 

 on the electrode was due to transmission by the electric field 

 and how much to diffusion, the experiments were repeated 

 with the central rod of '8 mm. diameter instead of 8*3 mms. 



-300 100 



+ 300 4 



+ 50 7 



Xow, with a central rod of only about 1/10 of the surface 

 area, it is obvious that the effect due to diffusion must be very 

 much reduced. We may thus conclude from these experi- 

 ments that a proportion of the excited radioactivity from 

 radium (about 5 per cent.) travels to the positive electrode 

 in an electric field, and the carrier must in consequence have 

 a negative charge. 



A special experiment was made to determine the amount 

 of radium excited activity on the anode with an apparatus 

 consisting of parallel plates. For this purpose the emanation 

 vessel of fig. 1 was used, with the plates 1*3 cm. apart. With 

 300 volts between the plates about 10 per cent, of the total 

 activity was confined to the anode. From the previous 

 experiments we have seen that about 5 per cent, reaches the 



