STORAGE BATTERIES. 241 



of voltaic cell. It is especially great in cells of the Grenet type, 

 and it may be reduced to a minimum in a given type of voltaic 

 cell by coating the zinc with a thin layer of mercury. 



The feature of the Grenet type of cell which favors local action 

 is that no provision is made in this cell to keep the oxidizing 

 agent, which in this case is potassium bichromate or chromic 

 acid, confined to the neighborhood of the carbon cathode, where 

 it is needed to oxidize the free hydrogen ; but it is allowed to 

 mix with the whole of the electrolyte thus coming into contact 

 with the zinc anode. Under these conditions the zinc dissolves 

 rapidly in the electrolyte, whether a current is flowing or not. 

 In a well cared for Grenet cell, even while it is being used to 

 give a large current, about 80 per cent, of the zinc is consumed 

 by local action, and only 20 per cent, by voltaic action. 



The useful consumption of zinc by voltaic action, while a vol- 

 taic cell is delivering a given current for a specified time, is equal 

 to the amount of zinc that would be deposited by the given cur- 

 rent during the specified time upon the cathode of an auxiliary 

 electrolytic cell containing a solution of a zinc salt, zinc sulphate, 

 for example. 



An essential feature of voltaic action is that it is reversed if a 

 current is forced backwards through a voltaic cell by an outside 

 agent, provided that no material that has played a part in the 

 previous voltaic action has been allowed to escape from the cell. 

 Thus, in the operation of the simple voltaic cell consisting of a 

 zinc anode and carbon cathode in dilute sulphuric acid, the H 2 SO 4 

 is decomposed, ZnSO 4 is formed at the anode, and hydrogen is 

 liberated at the cathode. If the current is reversed so that the 

 carbon plate becomes the anode and the zinc plate the cathode, 

 then the ZnSO 4 previously formed will be decomposed, metallic 

 zinc will be deposited upon the zinc cathode, and SO 4 will be 

 liberated at the carbon anode where it will combine with the trace 

 of hydrogen that is clinging to the carbon plate and form H 2 SO 4 . 

 In this cell the greater part of the liberated hydrogen has of 

 course escaped and the reversed chemical action, due to a 

 16 



