242 ELEMENTS OF ELECTRICAL ENGINEERING. 



reversed current cannot long continue. Local action, on the 

 other hand, being independent of current is not affected by a 

 reversal of the current. 



104. The storage cell. A voltaic cell that is free from local 

 action and in which all of the materials which take part in the 

 voltaic action are conserved in the cell, may be regenerated after 

 use by sending through it a reversed current. This regeneration 

 is due to the reversed chemical action that is produced by the 

 reversed current, as explained in the previous article. A voltaic 

 cell that is adapted to be thus regenerated is called a storage cell. 

 The process of regeneration is called charging and the use of the 

 cell as an electric generator is called discharging. 



A good storage battery must be free from local action, the 

 materials which take part in the voltaic action must be kept in the 

 cell, and the electrodes must not crumble to pieces with frequent 

 charging and discharging of the cell. 



A storage cell always requires a larger voltage between its 

 electrodes to charge it than it is able to maintain while it is being 

 discharged. This is due in part to the resistance * drop Ri in the 

 cell, inasmuch as this resistance drop opposes the flow of current 

 both in charging and in discharging ; and it is due in part to 

 variations in the concentration of the electrolyte at the electrodes 

 during charging and discharging as explained later. Conse- 

 quently more work must be done in charging a storage cell than 

 can be regained by discharging it. 



105. The lead storage cell.f--The voltaic cell which, up to 

 the present time, has been found to meet most satisfactorily the 



* The resistance of the cell is here intended to include what is sometimes called 

 transition resistance at the surface of the electrodes, together with the effect at the 

 surface of the electrodes which is sometimes called irreversible polarization. 



f The voltaic cell having a cathode of lead peroxide, an anode of zinc, and an 

 electrolyte of dilute sulphuric acid has been used to some extent as a storage cell; 

 see Secondary Batteries by E. J. Wade, pages 118-126, The Electrician Company, 

 London. The Edison storage cell (spongy iron anode, nickel peroxide cathode in a 

 solution of caustic potash) is described by A. E. Kennelly in the Trans. A. I. E. ., 

 for May, 1901. 



