184 Prof. J. Zeleny on the Ratio of the Velocities of the 



space between the plates. By blowing the air into a tube 

 containing glass-wool these are collected, and give a positive 

 deflexion on an electrometer connected to the tube. 



If the plates are moved so close together that in the 

 excursions which the positive ions make during an alternation 

 they travel a distance greater or equal to that between the 

 plates, then at every alternation of the E.M.F. all of the ions 

 in the space between, both positive and negative, areforcedto 

 the plates, and a stream of air does not now carry away with 

 it any charge. 



Tlie path traversed by a positive ion in the time of an 

 alternation is the distance between the plates when a current 

 of air just ceases to carry away a charge ; and this can be 

 made the basis for a determination of the velocity of this ion. 



§ 7. Metals and Air oppositely Charged xohen exposed to 

 Rontgen Rays. 



The results of various observers differ as to whether metals 

 and air which are exposed to Rontgen rays are left charged 

 positively, negatively, or not at all. 



Some of this confusion has already been cleared away by 

 the fact that when two different metals are dealt with at 

 the same time, the effects observed are due, at least approxi- 

 mately, to a reduction of the charges on the metals due to the 

 contact-difference of potential. 



But there still remain those cases where a piece of metal 

 is enclosed on all sides by parts made of the same metal. It 

 is evident that if the metal receives a charge from the 

 ionized, air, this itself must remain electrified with a charge 

 of the opposite sign. 



It will be shown that an electrification does take place, 

 and that a metal placed in air which is, or recently has 

 been, exposed to Rontgen radiation receives a negative 

 charge while the air itself remains positively electrified. 

 The cause of this effect will first be pointed out, and then 

 the arrangement of the apparatus for augmenting it will be 

 readily understood. 



The difference of velocity of the two ions in air leads us 

 to believe that they are not of the same size (§ 12), and it 

 follows that they do not diffuse at the same rate when in a 

 field of no force, the smaller negative ones diffusing the 

 faster. Near a boundary the initial uniform distribution 

 of both ions is disturbed by their impacts against the surface, 

 during which circumstance something takes place that is at 

 least externally equivalent to a giving up of the charge to the 

 surface. A difference in concentration of the ions is thus 



