646 



Dr. S. R. Milner on the 



made up of two parts : the polarizations at the anode and at 

 the cathode respectively. Experimentally, either of these 

 may be measured separately, by observing, not the total back- 

 electromotive force of the cell, but the electromotive force 



Fiff. 1. 



between the corresponding electrode and a " free " silver 

 electrode B suspended in the solution. In a cell of this type, 

 the anodic and the cathodic polarizations are practically 

 independent of each other, and either may be destroyed 

 without affecting the other, which then becomes the whole 

 polarization of the cell. Thus in the cell of fig. 1, the cathode 

 may be rendered practically unpolarizable by adding strong 

 silver nitrate to the solution in the right-hand beaker. It 

 greatly simplifies matters to deal with the two polarizations 

 separately ; and in this paper the anodic polarization of the 

 cell only is considered. 



When a current is passed through the cell in the direction 

 from A to C, the passage of each unit of charge is associated 

 with the solution of an equivalent amount of silver at the 

 anode A ; the concentration of the silver ions at the surface 

 of A is thereby increased above its initial value, while that 

 outside the tube in the neighbourhood of the free electrode B 

 remains unchanged. The actual increase produced at A does 

 not, however, depend solely on the charge passed through the 

 cell, for the excess of concentration tends to be removed by the 

 motion of the silver ions up the tube away from the surface — a 

 process which is partly an ordinary diffusion of the ions due to 

 the variation in their concentration which has resulted, and 

 partly a motion due to the electrical forces acting on them. 

 The concentration c x=0 (x representing distance up the tube 

 from A) of silver ions which is ultimately established at the 

 surface of A will thus depend on the current, the time, and 



