l6 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



this zinc oxide combines with the caustic potash in the solution 

 forming potassium zincate (K 2 ZnO 2 ). 



The Clark standard cell is arranged as shown in Fig. 9. One 

 electrode is pure mercury and the other electrode is pure zinc. 

 When this cell delivers current, the ZnSO 4 in solution is decom- 

 posed, SO 4 is liberated at the surface of the zinc where it com- 

 bines with the zinc forming ZnSCX, and at the same time Zn is 

 liberated at the surface of the mercury electrode where it is acted 

 upon by the mercurous sulphate Hg 2 SO 4 according to the 

 equation 



Zn + Hg 2 S0 4 = Hg 2 + ZnS0 4 



This cell is remarkable for the constancy of its electromotive force 

 and it is used as a standard of electromotive force, as explained 

 in Chapter X. 



8. Voltaic action and local action. Two kinds of chemical 

 action are to be distinguished in a voltaic cell, namely, (a) the 

 chemical action which depends upon the flow of current and does 

 not exist when there is no current and ($) the chemical action 

 which is independent of the flow of current and which takes 

 place whether the current is flowing or not. 



The chemical action which depends on the current is propor- 

 tional to the current, it is essential to the operation of the voltaic 

 cell as a generator of current, its energy is available for the mainte- 

 nance of the current, and it is called voltaic action. 



The chemical action in a voltaic cell which is independent of 

 the flow of current does not help in any way to maintain the cur- 

 rent, it represents absolute waste of materials, and it is called local 

 action. Local action takes place more or less in every type of 

 voltaic cell and it is especially marked in the chromic acid cell 

 above described. It may be reduced to a minimum in a given 

 type of voltaic cell by coating the zinc with a thin layer of metallic 

 mercury (amalgamation). 



The term, local action, originated in the following considerations : When a strip of 

 clean zinc is immersed in sulphuric acid, no perceptible chemical action takes place. 



