504 Diffusion Cells in Ionized Gases. 



We tested equation (2) in another manner, by substituting 

 dry carbon dioxide gas for air between the plates. In this 

 case, the difference between the ionic velocities u x and u 2 is 

 much less than in air, from which we should expect a smaller 

 electromotive force. In order to avoid large changes in the 

 strength of the ionizing source during the substitution of 

 one gas for the other, we used the radium for the ionizing 

 source in this test and increased the electrometer sensitiveness 

 to about 15,000 divisions per volt. The averages of a number 

 of consistent measurements gave E = 0*00090 volt for air 

 and E = 0*00035 volt for carbon dioxide. This difference is 

 in the right direction and of the general size predicted by 

 equation (2), though an exact prediction cannot be made 

 because of the difference in the degrees of ionization and 

 recombination in the two gases. 



Finally we tried the effect of filling the apparatus with 

 moist carbon dioxide, in which case the sign of the electro- 

 motive force should be reversed, owing to the fact that the 

 positive ions diffuse more rapidly than the negative ions in 

 this gas. This test, however, was unsuccessful because the in- 

 sulation broke down before the measurements could be made. 



In conclusion we might mention an attempt to test 

 equation (2) without the necessity for measuring the ionic 

 concentrations at the plates by the aid of the equation 



which follows from Thomson's equations given earlier in the 

 paper if we put A = w 2 X/D 2 and B= — a/D 2 . This equation 

 is of the type of the equation of motion of a particle moving, 

 subject to friction, under the action of a force proportional 

 to the square of the distance from a fixed point, and has not 

 been solved. Approximate solutions indicated that the ratio 

 njn is not independent of n , as we might have anticipated. 

 Thus even with an exact solution of this differential equation 

 it would still be necessary to make some sort of measurement 

 of the ionic concentration, so that this method was not 

 pursued further. 



We are glad to acknowledge our indebtedness to Professor 

 E. P. Adams for suggesting approximate solutions of the 

 differential equation mentioned above, and to Professor 

 H. L. Cooke for suggesting the method which we employed 

 for measuring the relative values of the ionic concentrations 

 at the plates. 



Palmer Physical Laboratory, 

 Princeton, N. J., U.S.A. 



