Mobility of the Positive Ion at Low Pressures. 171 



Curves II. and III. show the changes produced by frequencies 

 of 220 and 300 respectively. 



In previous work on the positive ion the author was not 

 able to detect any certain difference in the mobility with 

 change in the rate of alternation. This can be accounted 

 for in two ways. It was not possible, in measuring mobilities 

 below the " critical " pressure, to employ, with the dimensions 

 of the apparatus, very different frequencies. The fields also 

 were not strong enough to cause collisions sufficiently violent 

 to prevent the formation of the normal ionic aggregates. 



It has been shown* that in gases subjected to an ionizing 

 influence at ordinary pressures, positive ions exist with 

 mobilities corresponding to aggregates of from one up to 

 five or six molecules of the gas. Altberg found also that a 

 small percentage of the negative ions in a gas at ordinary 

 pressures had a mobility giving an ionic mass of less than a 

 single molecule, a result which one would expect if the 

 nucleus of the negative ion were a corpuscle existing for a 

 fraction of its life in the free state. The same experimenter 

 found that the smaller ions had a shorter life than the larger 

 aggregates. Now we should expect when the pressure is 

 diminished, and consequently the mean free path increased, 

 that the lives of all aggregates would increase in the same 

 proportion. Hence for normal mobilities to be obtained, 

 the measuring apparatus must also be increased in the same 

 proportion. 



It should be mentioned that the experimental results 

 obtained in the present paper lend some support to a theo- 

 retical investigation ou the nature of an ion by Kleemanf, 

 in which he develops the idea of a continually changing 

 aggregate to explain some properties of ions. 



If the views expressed are correct, it ought to be possible 

 to obtain abnormal mobilities at ordinary pressures by the 

 alternating field method. The plates of the alternating 

 chamber would have to be verv near together, and the 

 frequency of alternation very high. Some trouble would be 

 caused by diffusion with the plates so near together. 



This work was carried out at the Cavendish Laboratory, 

 and the author, in concluding, wishes to express his many 

 thanks to Sir J. J. Thomson for his interest in it. 



Birmingham, Nov. 15, 1912. 



* Altberg, Ann. d. Phys. p. 849 (1912). 

 t Proc. Camb. Phil. Soc. xvi. p. 285. 



