592 PROCEEDINGS OF THE AMERICAN ACADEMY. 



On long standing, a storage cell may acquire a very high resistance 

 indeed as the result of complete "sulphation." This term means that 

 the active lead sulphate formed during discharge has gradually changed 

 into the crystalline inactive form and that crystals of this form have 

 completely covered the particles of lead and lead peroxide with an in- 

 sulating coating. Authentic cases of cells of considerable size, with 

 internal resistance as high as 10 ohms, are known. But under the 

 usual conditions of charge and discharge the sulphate retains its 

 " active " state, and even after standing discharged for a month or 

 more no great change in internal resistance is usually to be observed 

 [see § 15]. 



3. In its ordinary work, a storage cell is discharged only until the 

 plate potential sinks to about 1.7-1.8 volts. This means usually that 

 only about one quarter of the active material in the plates has entered 

 into reaction and that the increased resistance in the active material 

 is due rather to separation of the particles by sulphate coatings than 

 to complete transformation of the active material at any point into 

 insulating material (sulphate). On charge these sulphate coatings and 

 bridges are rapidly broken down, and the decrease in resistance is 

 therefore much more rapid than that corresponding to change in 

 electrolyte within the plate. 



After a period of discharge, with corresponding change in resistance, 

 the cell recovers its original e. m. f along a curve which is somewhat 

 like a diffusion curve [§ 13]. This curve is made steeper by the 

 equalizing effect of local action as explained in 2. It also recovers its 

 original resistance along a somewhat similar curve. These facts indi- 

 cate clearly the dpiamic nature of the whole cell activity, for evidently 

 the change in resistance as well as the change in e. m. f. is fundamentally 

 a function of acid concentration and diffusion. The particles of active 

 material cannot have been completely covered by insulating layers, for 

 on standing the plate returned to its original condition as measured by 

 e. m. f. and resistance. 



We must evidently think of the particles of lead and lead peroxide 

 as covered with a spongy or powdery layer of lead sulphate, with inter- 

 stices so small that diffusion cannot overcome the effect of even a small 

 current and its accompanying exhaustion of acid within the pores. 

 As long as no current is flowing, and when recovery has been allowed 

 to take place completely, the total active surface has not been greatly 

 reduced by the changes in the plate, nor has the " active surface " 

 been far removed from its original plane. But the passage of even a 

 small discharge current causes exhaustion of acid in the pores to such 

 an extent that nearly pure water intervenes between the electrolyte 



