244 ELEMENTS OF ELECTRICAL ENGINEERING. 



It is the usual practice among electrical engineers to call that 

 terminal of an electric generator out of which current flows, the 

 positive terminal, and that terminal into which current flows, the 

 negative terminal. In conformity with this usage, that electrode 

 of a storage cell which is cathode during discharge is called the 

 positive grid and the other the negative grid. The positive grids 

 are of a pale salmon color and the negative grids are a neutral 

 gray. 



The following arrangement gives a clear idea of the action of 

 the lead storage cell while discharging and while being charged : 



DISCHARGING.* 



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



A Direction of current through the cell 

 & (negative to positive grid). 



Negative grid. Pb -f- SO 4 = PbSO 4 



CHARGING.* 



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



Y Direction of current t 

 y (positive to negativ 



Negative grid. PbSO 4 + H 2 = H 2 SO 4 -f Pb 



Direction of current through the cell 

 (positive to negative grid). 



Variation of voltage during charge and discharge. The elec- 

 tromotive force between lead peroxide and spongy lead immersed 

 in dilute sulphuric acid, increases with increasing concentration of 

 the acid. Therefore the voltage of a lead storage cell, on open 

 circuit, is large when the cell is completely charged, because of 

 the great concentration of the acid ; and it grows smaller and 

 smaller with decreasing concentration of the acid as the cell is 

 discharged. Furthermore, while the cell is being charged, the 

 concentration of the electrolyte in immediate contact with and 

 especially in the pores of the active material is greater than the 



*The above-described action of the lead storage cell constitutes the simple working 

 theory of the cell. These actions do take place but they are no doubt accompanied 

 by more complex actions such as the formation of per-sulphates at the anode and of 

 sub-sulphates at the cathode. See The Theory of the Lead Accumulator^ Friedrich 

 Dolezalek (English translation by C. L. von Ende, published by John Wiley & 

 Sons). 



