the Electrical Conduction produced by it. 151 



equal to — ±irp, where p is the volume-density of electrifica- 

 tion at any point. In order to produce the disturbance of 

 the electric field shown in fig. 11, there must be an excess 

 of ions of one kind distributed between the plates. Such a 

 result follows at once from what has been said in regard to 

 the inequality of the ionization between the plates due to the 

 absorption of the radiation. 



It was found that the potential gradient approached more 

 and more its undisturbed value with increase of the electro- 

 motive force between the plates. The displacement of the 

 point of zero-potential from the uranium surface increased 

 with diminution of electromotive force. For example, for 

 two plates 51 mm. apart, charged to equal and opposite 

 potentials, the points of zero potential were 28, 30, 33 mm. 

 from the uranium when the differences of potential between 

 the plates were 16, 8, and 4 volts respectively. 



When the uranium was charged positively, the point of 

 zero potential was more displaced than when it was charged 

 negatively. This is due to the slower velocity of the positive 

 ion. 



The slope of potential very close to the surface of the 

 uranium has not been investigated. The deviation from the 

 normal potential slope between the plates depends very 

 largely on the intensity of the ionization produced in the gas. 

 With very weak ionization the normal potential gradient is 

 only slightly affected. 



Child * and Zeleny f have shown that the potential gradient 

 between two parallel plates exposed to Rontgen rays is not 

 uniform. In their cases the ionization was uniform between 

 the plates, and the disturbance in the field manifested itself in 

 a sudden drop at both electrodes. In the case considered 

 for uranium radiation, the ionization is too small for this 

 effect to be appreciable. The disturbance of the field is due 

 chiefly to the inequality of the ionization, and does not only 

 take place at the electrodes. 



§ 16. Relation between Current and Electromotive Force. 



The variation with electromotive force of the current 

 through a gas exposed to uranium radiation has been investi- 

 gated by Becquerel {, and later by de Smolan and Beattie§. 



* Wied. Annal. April 1898, p. 152, 

 t Phil. Mag., July 1898. 

 \ Comptes Rendus, pp. 438, 800 (1897). 

 § Phil. Mag. vol. xliii. p. 418 (1897). 



