Electricity through Gases by Charged Ions. 265 



plate. Hence it follows from (13) that tho electric force at 

 the negative plate is greater than that at the positive. 



A very convenient method of determining the velocities of 

 the ions, and one which can be employed in nearly every case 

 of conduction through gases, is to produce the ions in one 

 region and measure the electric intensity at two points in a 

 region where there is no production of ions, but to which ions 

 of one sign only can penetrate under the action of the 

 electric field. Thus let A, B represent two parallel plates 

 immersed in a gas, and let us suppose that in the layer 

 between A and the plane LM we produce a supply of ions, 

 whether by Rontgen rays, incandescent metals, ultra-violet 

 light, or by other means, and suppose that the gas between 

 LM and B is screened off from the action of the ionizer. 

 Then if A and B are connected to the poles of a battery a 

 current will pass through the gas, and this current in the 

 region between LM and B will be carried by ions of one 

 sign. These will be positive if A is the positive pole, nega- 

 tive if it is the negative pole. Let us find the distribution 

 of electric intensity in the region between LM and B. Let 

 us suppose A is the positive plate, then all the ions in this 

 region are positive and we have, using the same notation as 

 before, 



dX A 



k^Xe = t, 



where i is the current through unit area ; from these equations 

 we have 



^ dX _ 4ttc 



dx ~~ k l ' 



or 



h 



Hence if we measure the values of X at two points in 

 the region between LM and B, and also the value of t, 

 we can, from this equation, deduce the value of k u and 

 hence the velocity of the positive ion in a known electric 

 field. To determine the velocity of the negative ion we 

 have only to perform a similar experiment with the plate A 

 negative. 



When the ionization is confined to a layer CD between the 

 plates A and B, the distribution of electric intensity is repre- 



