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XC. On the Question of Valency in Gaseous Ionization. By 

 R. A. Millikan, Professor of Physics in the University of 

 Chicago, and Harvey Fletcher, Fellow in Physics in the 

 University of Chicago* . 



§ 1. Historical Survey. 



UP to the year 1908 the only experiments which threw 

 any light whatever upon the question of valency in 

 gaseous ionization were those made by Townsendf in 1900. 

 He found that the value of ne obtained from measurements 

 on ionized gases was identical with the value of wE obtained 

 from measurements on the deposition of the products of 

 electrolysis. In the foregoing expressions n denotes the 

 number of molecules in 1 c.c. of gas at 15° C. 76 cm. pressure, 

 e the mean charge carried by an ion in an ionized gas, and 

 E the charge carried by a univalent ion in electrolytic con- 

 duction. The mean value of this product came out in both 

 cases 1"23 x 10 10 electrostatic units, although in the measure- 

 ments on gases the individual experiments gave differences 

 from the mean as large as 30 per cent. The values of ne 



were obtained from the equation ne= 77 P> * n which K 



denotes the " mobility " of the gaseous ion, D its coefficient 

 of diffusion, and P normal atmospheric pressure. Townsend's 

 work consisted simply in the measurement of D, the value 

 of K having been taken from the work of previous observers. 

 In view 7 of these results it has been customary for the 

 past ten years to assume that all gaseous ions are univalent. 

 In 1908, however, Townsendf devised a method of measuring 



directly the ratio ^ ana " revised his original conclusions. 



His method consisted essentially in driving ions by means of 

 an electric field from the region between two plates A and 

 B (fig. 1), where they had been produced by the direct action 



Fig. 1. 



__ A 



D 



of X rays, through the gauze g, and observing what fraction 

 of these ions was driven by a field established between the 

 plates B and C to the central disk 1), and what fraction 

 drifted by virtue of diffusion to the guard-ring C. 



* Communicated by the Authors. 



t J. S. Townsend, Phil. Trans. A. vol. exciii. p. 129 ('1P001. 



J J. S. Townsend. Tree. Rot. Soc. A. vol. lxxx. p, 207 (1908). 



