﻿the Active Deposit of Radium in an Electric Field, 721 



Calculated maximum activity on case : *38. 



Ditto cathode : 2*53. 



Ratio of max. cathode to max. case activity: 6'6G : 1. 



Percentage cathode activity: 86*9. 



True percentage cathode activity as corrected for dif- 

 fusion of uncharged carriers to cathode : 86'6. 

 It is difficult to estimate with precision the experimental 

 error in the determination of the percentage cathode activity. 

 A slight error prohably arises from the fact that the cathode 

 and case activities are determined by means of ionization 

 which is differently distributed. This has been shown ex- 

 perimentally to have a negligible effect on the relative values 

 of the percentage cathode activities for different potentials, 

 the determination of which was really the object of the ex- 

 periment. As mentioned above, it was important to make 

 an accurate determination of the maximum activity on the 

 case. An error in the determination of this activity leads to 

 approximately the same percentage error in the determination 

 of the percentage cathode activity ; this error in all our 

 experiments is probably less than 2 per cent. 



The plotting of a curve such as that of fig. 2 is obviously 

 only possible if the percentage cathode activity is independent 

 of the amount of emanation used in the experiment. This 

 fact was demonstrated repeatedly during the course of our 

 investigations ; as an instance of the wide range over which 

 this holds it may be mentioned that the percentage cathode 

 activity was the same with 900 volts on the case when the 

 amounts of emanation in the vessel were such as to afford 

 equilibrium rates of 5'32 without any added capacity and 

 16*1 with the added capacity, i. e. rates which are roughly 

 in the ratio 1 : 65. When we are working in the earlier 

 part of the curve, i. e. with relatively small potentials, this 

 independence no longer holds good ; but for the higher 

 potentials the effect of the amount of emanation within a 

 large range becomes negligible. 



The curve bears a striking resemblance to the curves 

 which have been experimentally determined for ionization 

 by a particles. Many observers have drawn attention to the 

 " lack of saturation " which is a marked feature of such 

 curves ; in particular, this phenomenon has been the subject 

 of special investigation by Bragg*, Kleemant, Moulin J, and 

 Wheelock §. Reference is made to this point later (section 7), 



* Bragg, Phil. Mag. ser. 0, xi. p. 466 (1906). 

 t Kleernan, Phil. Mag. ser. 6, xii, p. 273 (1906), 

 t Moulin, Compt. Jtend. cxlviii. p, 1757 (1909); le Radium, vii. p. 350 

 (1910). 



§ Wheelock, Am. J. Sc. xxx. p. 233 (1910). 



Phil. Mag. S. G. Vol. 23. No. 137. May 1912. 3 B 



