160 Prof. E. Rutherford on Uranium Radiation and 



accurate owing to the slight contact-difference of potential 

 between the plates, so that there was a small initial current 

 for zero external electromotive force. 



The general results show that the gas tends to become more 

 readily saturated with diminution of pressure. The variation 

 of the current with the E.M.F. depends on two factors — the 

 velocities of the ions, and their rate of recombination. Some 

 experiments on the velocity of the carriers * in ultra-violet 

 light conduction showed that the velocity Of the ions in a 

 given electric field is inversely proportional to the pressure. 

 This is probably also true for the ions in Rontgen conduction; 

 so that under the pressure of 95 mm. the ions would move 

 eight times as fast as at atmospheric pressure. The variation 

 of the rate of recombination with pressure has not yet been 

 determined. 



The curve for hydrogen at atmospheric pressure is also 

 given in fig. 15, and shows that hydrogen is about as easily 

 saturated as air at 190 mm. pressure. 



§ 17. Separation of the Positive and Negative Ions. 



It is a simple matter to partially separate the positive and 

 negative ions in uranium conductions and produce an 

 electrified gas, The subject of the production of electrifi- 

 cation by passing a current of air over the surface of uranium 

 enclosed in a metal vessel has been examined by Beattie f, 

 who found the electrification obtained was of the same sign 

 as the charge on the uranium. His results admit of a simple 

 explanation on the theory of ionization. The gas near the 

 surface of the uranium is far more strongly ionized than that 

 some distance away on account of the rapid absorption of the 

 radiation by the air. For convenience of explanation, let us 

 suppose a piece of uranium, charged positively, placed inside 

 a metal vessel connected to earth, and a current of air passed 

 through the vessel. Under the influence of the electromotive 

 force the negative ions travel in towards the uranium, and the 

 positive ions towards the outer vessel. Since the ionization 

 is greater near the surface of the uranium, there will be an 

 excess of positive ions in the air some distance away from 

 the uranium. Part of this is blown out by the current of air, 

 and gives up its charge to a filter of cotton-wool. The total 

 number of negative ions blown out in the same time is much 

 less, as the electromotive intensity, and therefore the velocity 

 of the carrier, is greater near the uranium than near the out- 

 side cylinder. Consequently there is an excess of positive 



* Proc. Oamb. Phil. Soc. Feb. 21, 1888. 

 f Phil. Mag. July 1897, xliv. p. 102, 



