Two" Tons produced in (rases by Rontgen Radiation. 13)> 



to those which do not attack the materials of which the 

 apparatus was made. 



§ 5. Difference of Velocity shown by Blowing along a 

 Charged Wire, 



A simple way for showing the difference of velocity for the 

 two ions in air is to expose to the rays or to blow Ront- 

 on i zed air into an aluminium tube which is connected to 

 earth, and in the axis of which is supported an aluminium 

 wire that can be charged from a battery. 



During the conduction through the air there is an excess 

 of ions near the wire and by the wall of the tube, of the 

 kind that is moving towards these two places respectively. 



If by a proper arrangement a current of air is blown only 

 along the surface of the wire and out through an insulated 

 tube filled with glass-wool which is connected to an electro- 

 meter, then when the wire is charged negatively a greater 

 quantity of positive electrification is blown out into the glass- 

 wool than is obtained of negative when the wire is charged 

 positively to the same voltage. This shows that the positive 

 ions are moving across the tube in the electric field with a 

 smaller velocity than are the negative, since they are more 

 readily carried away by the same stream of air, blown at right 

 angles to the electric force (see § 9 and § 10). 



§ 6. Difference of Velocity shown by using an 

 Alternating E.M.F. 



Another way to show the difference of velocity of the two 

 ions produced in air is to use an alternating E.M.F. between 

 two parallel plates, or between the wire and the tube in the 

 arrangement described in § 5. 



In the case of the two parallel plates which are connected 

 to a source of alternating E.M.F., and the space between 

 which is exposed to the rays, the rapidly alternating electric 

 force causes the ions to travel back and forth between the 

 plates over paths whose lengths are determined by the period 

 of alternation, by the average value of the potential gradient 

 between the plates, and by the velocity of the ions themselves. 



Those near the plates are brought against the sides ; 

 but if the velocity of the two ions were the same there 

 would be no permanent separation of the two kinds, and so 

 no formation of a free charge anywhere between the plates. 

 But since the velocity of the negative ions is greater than 

 that of the positive, these, at each alternation, are brought 

 to the plates from a greater distance than are the positive, 

 and there is thus left an excess of positive ions in the 



