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XLII. A Determination of the Charge on the Ions produced 

 in Air by Rbntgen Rays. By Harold A. Wilson, Fellow 

 of Trinity College, Cambridge*. 



THE experiments described in this paper were undertaken 

 with the object of making a fresh determination of the 

 charge on one ion. This charge will throughout this paper 

 be denoted by e. 



Prof. Townsend (Phil. Mag. Feb. 1898), in a paper on 

 the " Electrical Properties of Newly Prepared Gases," has 

 described a determination of the average charge on the 

 droplets composing the cloud formed when newly prepared 

 oxygen is bubbled through water. This charge was found 

 to be about 3 x 10~ 10 electrostatic units of electricity. There 

 are some reasons for supposing that each droplet contains 

 one ion, and consequently Townsend's result may be regarded 

 as a determination of the charge on one ion. The result 

 which I have obtained is in very good agreement with his. 



Prof. J. J. Thomson (Phil. Mag. Dec. 1898 and 1899) has 

 given two estimates of <?, the first depending on a determina- 

 tion of the average charge on the droplets of a cloud formed 

 by condensation of water-vapour on the ions produced in air 

 by Pontgen rays, and the second on a similar determination 

 for the ions given off by a zinc plate under the action of 

 ultra-violet light. The mean result of the first research was 

 g = 6-5 x 10- 10 and of the second e = 6'S X 10" 10 t. 



Since from the value of e the number of molecules in a 

 cubic centimetre of a gas can be immediately deduced, and 

 also since the absolute value of e is of considerable interest 

 in itself, a fresh determination by a different method appeared 

 to be worth making. 



The method 1 have used depends, like Prof. Thomson's, on 

 the fact discovered by C. T. R. Wilson J, that the ions pro- 

 duced in air by Pontgen rays act as nuclei for the cloudy 

 condensation of water-vapour when super saturation exceeding 

 a definite amount is produced by a sudden expansion. 



* Communicated by Prof. J. J. Thomson. 



f Since this paper was written Prof. Thomson has informed me that 

 he has lately made a fresh determination of e by his original method, 

 but with an improved apparatus, and he has very kindly consented to my 

 mentioning the result he has obtained, here. It is e = 3-Sx 10- 10 . and so 

 agrees very well with the mean result of my experiments, viz. 31 X 10- 10 . 

 It appears that in his earlier experiments the cloud was formed mainly 

 on the negative ions and not on both positive and negative ions as was 

 supposed at the time, consequently the result obtained was nearly twice 

 too big. 



| Phil. Trans. A. 1897, p. 265, and A. 1S99. p. 403. 



