Two Ions produced in Gases by Rontgen Radiation. 137 



ments with air just before and just after those tried with 

 the C0 2 all gave the large positive deflexions already 

 mentioned. 



It is evident, since when air was used it came out of 

 the spiral charged positively, that it must have given a negative 

 charge to the metal ; but this effect has also been obtained 

 independently by means of the following arrangement : — A 

 large closed aluminium cylinder, 15 centim. in diameter, 

 connected to earth, contained in its axis a short insulated 

 aluminium tube, 3 centim. in diameter, from which an 

 aluminium strip led to the outside of the large cylinder to an 

 electrometer connexion. Both cylinders and the wire strip 

 were made from the same sheet of aluminium. A pencil of 

 rays allowed to pass through both cylinders surrounded the 

 inner one with ionized air which gradually gave to it a 

 negative charge, as was indicated by the connected elec- 

 trometer. A similar arrangement was used with all of the 

 parts made of zinc, and a negative charge was again obtained 

 upon the zinc. 



The charge does not continue to increase long, for the 

 accumulated electrifications tend to counteract further separa- 

 tion. By blowing a gentle stream of air past the central 

 cylinder, so as to remove the excess of positive electrification 

 in the neighbourhood of the metal, the charge obtained is 

 increased. 



§ 8. Air charged negatively discharges faster to a surface 

 than when charged positively. 



Rutherford has shown (Phil. Mag. April 1897, p. 246) 

 that when air, electrified by being blown away from an 

 electrode in conduction under the action of Rontgen rays, is 

 allowed to pass through tubes of various materials it loses 

 its charge more rapidly when this charge is negative, than 

 when it is positive. This result is another consequence of 

 the difference of velocity of the two ions. 



When the charged particles enter the tube in question the 

 forces that tend to bring them to the sides of the tube, in 

 addition to those due to irregularities in the current, are 

 those of diffusion and of mutual repulsion. 



Since under both of these forces the negative ions move 

 the faster, it follows that in passing through a given length 

 of the tube in the same time more of the ions will reach the 

 walls if they are negative than when they are positive. 



The equation for the motion of a charged gas due to 

 mutual repulsion is given by Townsend (Phil. Mag. Feb. 

 181)8, p. 134). 



