STORAGE BATTERIES. 243 



requirements of a storage cell, as above specified, is a voltaic cell 

 having a cathode of lead peroxide (PbO 2 ), an anode of spongy 

 metallic lead, and an electrolyte of dilute sulphuric acid. The 

 lead peroxide and the spongy metallic lead are both converted 

 into insoluble lead sulphate (PbSOJ when this cell delivers 

 current, and this lead sulphate is converted back into lead per- 

 oxide and spongy lead respectively when a reversed current is 

 forced through the cell. The lead peroxide and the spongy lead 

 are called the active materials of the cell. These active materials 

 are mechanically weak and porous, and they are usually sup- 

 ported in the interstices of massive grids of metallic lead. 

 Furthermore, the active material being a rather poor electrical 

 conductor, the lead grids serve not only as mechanical supports, 

 but also to deliver current to or receive current from the active 

 material which constitutes the real electrodes. 



Action of the cell while discharging. When the lead storage 

 cell delivers current, 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 especi- 

 ally great in the pores of the active material. 



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. 



