MORSE AND SARGENT. — RESISTANCE OF LEAD ACCUMULATOR. 591 



concentration of the electrolyte in the pores of the active material and 

 in the layer in immediate contact with its surface. 



The change in B during charge will, moreover, be wholly different 

 from the change during discharge, for in both cases the changes in the 

 active vasXeriaX follow the electrolyte. During charge the active mate- 

 rial first to enter into reaction is that at the surface of the plate, pro- 

 vided the plate has not been allowed to " sulphate " so completely that 

 the conductivity through the active mass has been very greatly reduced. 

 Since the action is taking place at the surface the electrolyte does not 

 have to diffuse far through narrow channels. But as the diffusion path 

 increases and the cell becomes more fully charged, concentrated acid 

 is produced in the pores of the mass. In spite of the decrease in re- 

 sistance due to this better conducting acid it is still the plate itself 

 which does most of the conducting. It is therefore to be expected that 

 the change of resistance during the charge of a healthy storage cell will 

 not be large. 



During discharge a very different state of affairs exists. In this case 

 also action begins at the surface of the plate, where electrolyte is avail- 

 able for the reaction. But as discharge proceeds, and the area of ac- 

 tivity recedes into the interior of the active material, acid is used up 

 within the plate and the concentration of the active part of the elec- 

 trolyte decreases. To this loss is added the loss of conductivity of the 

 plate itself, for the particles of lead and lead peroxide in the outer 

 layers have now been covered with sulphate and more or less com- 

 pletely insulated from each other. The result is as if the distance be- 

 tween the plates had been increased, for the plate surface which is 

 actually carrying current is now well back in the interior of the mass 

 of active material, instead of at the actual outside surface. 



It should be kept in mind also that the equalization of concentration 

 differences in a working plate is not cared for wholly by diffusion. 

 While this is a potent factor always, it can be shown to be insufficient 

 to account for the facts. A considerable part of the equalization is 

 probably cared for by local concentration cells at various parts of the 

 plate, and these local actions depend for their efficiency on good con- 

 ductivity of the plate between the points where the concentration differ- 

 ences exist. Any change in the conducting power will affect the rate of 

 equalization of acid and will displace the area of action within the plate. 



Viewed in this way we should hardly expect any very large or very 

 rapid changes in cell resistance, nor any of the peculiar maxima of re- 

 sistance at various points in charge and discharge which appear in some 

 of the older reports on the subject. It is probable that polarization 

 was not eliminated in these measurements. 



