502 Messrs. Wood, de Long, and Compton on 



When a current flows through the gas 



'u l 



u x 4- u 2 



and 



u Y + u c 



give the proportions of the electricity carried by the positive 

 and negative ions, respectively. Furthermore, the partial 

 ionic pressures p are proportional to the ionic concentrations n. 

 Thus we may write equation (2) in the form of equation (1) 

 used by Mr. Jenkinson. The interpretation of x here sug- 

 gested, however, is typical of a diffusion-cell rather than an 

 ordinary concentration-cell such as Mr. Jenkinson considered. 

 Experimental Tests. — We tested the validity of equation (2) 

 by an apparatus whose essential features are shown in the 

 diagram. Brass plates A and B were surrounded by guard- 

 rings Gr, mounted on ebonite blocks and enclosed in the 



thick-walled lead box L L, which was lined with brass to avoid 

 disturbances due to contact difference of potential. A tube 

 of radium at R was also enclosed in a lead box provided with 

 a fan-shaped opening. This box could be set against stops S 

 so as to expose the gas near either plate to the ionizing 

 y rays. The potential divider P enabled the potential of the 

 plate A to be adjusted until the electrometer E connected 

 with the plate B showed no deflexion. The difference 



