40 



of the ciirreiit deiisily meuiis the tension with respect to the auxiliary 

 cledrodcs. This tension, which is also called jiolarisation tension, is 



Fig. 1. 



positive when the metal is anode, and negative, when it is used 

 as cathode. 



Further this |)Ossibility was still to be foreseen that when the 

 metal assumes internal equilibrium very slowly a distinct change ot 

 the potential difference would have to be demonstrated even after 

 the current had been interrupted. 



Now it should be noted here that when a base metal has become 

 noble during its use as anode, and the difference of potential between 

 the metal and electrolyte has risen to the tension of liberation of 

 the oxygen, at the anode two processes will begin to proceed side 

 by side; besides the going in solution of the metal we get also the 

 discharge of the OH'-ions and the possible formation of oxide skins, 

 the influence of which should be examined. 



We get something of the same kind at the cathode. When viz. 

 the difference of tension metal-electrolyte at the cathode has become 

 greater than the tension of liberation of the hydrogen, besides dis- 

 charge of metal ions, also dischar-ge of H'-ions will take })lace there. 



Method of Investiyation. 



The measurement of the polarisation tensions took place in the 

 following way (see Fig. 2j. Two electrodes of the metal that is to 

 be ijivestigated, in the shape of wire or rods, were placed in a solution 



