THE ELECTRIC CURRENT. 



Fig. lOb. 



called the active materials of the cell. These active materials 

 are mechanically weak and porous and they are usually supported 

 in the interstices of massive grids of metallic lead. These lead 

 grids serve not only as mechanical supports 

 for the active material, but they serve also 

 to deliver current to or receive current from 

 the active materials which constitute the real 

 electrodes. 



Figure io shows a commercial form of 

 lead storage cell. The electrodes consist of 

 fine grids of metallic lead in the interstices 

 of which the active material is placed. The 

 positive electrode (out of which the current 

 comes during discharge) consists of three 

 grids connected together, and the negative electrode consists of 

 four grids connected together. 



Action of the cell while discharging. When the lead storage cell delivers cur- 

 rent, the electrolyte H 2 SO 4 is split up by the current into H 2 and SO 4 . The 

 hydrogen is liberated at the cathode, where it reduces the lead peroxide to PbO, and 

 this PbO combines with a portion of the H 2 SO 4 of the electrolyte forming PbSO 4 

 and water. The SO 4 which is liberated at the anode combines with the spongy 

 lead and forms PbSO 4 . During this process the active material expands, because the 

 lead sulphate is more bulky than the spongy lead and the lead peroxide ; and the 

 electrolyte grows less concentrated (and of course increases in resistance) because of 

 the absorption of SO 4 by the active material. This decrease of concentration is 

 especially great in the pores of the active material when the cell is discharged 

 rapidly. 



Action of the cell while being charged. When the lead storage cell is regenerated 

 by forcing a reversed current through it, the above-described action is reversed. The 

 lead sulphate on one electrode is converted back to lead peroxide, the lead sulphate 

 on the other electrode is reduced to spongy metallic lead, the electrolyte grows more 

 dense (especially in the pores of the active material), and the active material 

 contracts. 



The following tabular arrangement gives a clear idea of the action of the lead stor- 

 age cell while discharging and while being charged : 



DISCHARGING. 



Positive grid. PbO, + H 2 SO 4 + H a = 2H 2 O + PbSO 4 



i Direction of current through the cell 

 (negative to positive grid). 



Negative grid. Pb + S0 4 = PbSO 4 



