246 ELEMENTS OF ELECTRICAL ENGINEERING. 



packed in the interstices of the lead grids. The active material 

 on the positive grid must always contain a large proportion of 

 lead peroxide to give it the necessary electrical conductivity, and 

 the active material on the negative grid must always contain a 

 large proportion of spongy lead for the same reason. Therefore 

 it is not permissible to discharge a cell so completely as to con- 

 vert a large portion of the lead peroxide and spongy* lead into 

 lead sulphate, (fr) The conversion of all of the lead peroxide on 

 the positive grid and of all of the spongy lead on the negative grid 

 to lead sulphate would involve an excessive expansion of the active 

 materials which would be more likely than a moderate degree of 

 expansion to detach the active materials from the grids, (c) When 

 a storage cell is discharged beyond a certain limit its electromotive 

 force falls off excessively. 



Usually a lead storage cell is not discharged beyond the point 

 at which its voltage falls to 1.8 volts while giving its full rated 

 current, and in the usual types of cells this degree of discharge 

 corresponds to the conversion, of from | to of the active material 

 into lead sulphate. 



Limit to charge. When all of the active material on the posi- 

 tive and negative grids is converted into lead peroxide and spongy 

 lead respectively, then a continuation of the charging current 

 causes hydrogen gas to be liberated at the negative grids and 

 oxygen gas to be liberated at the positive grids without produc- 

 ing any further chemical change in the active material. This evo- 

 lution of gas is not especially harmful to the cell unless it be- 

 comes so violent as to cause the disintegration of the active 

 material by the production of bubbles within the pores, or between 

 the active material and the solid metal of the grid. 



Capacity. The capacity of a storage cell is expressed in 

 ampere-hours, that is to say, in terms of the product of the cur- 

 rent in amperes times the number of hours that the cell can de- 

 liver the current when it is operated between the practical limits 

 of charge and discharge. The capacity of a storage cell varies 

 greatly with the discharge rate as explained later. 



