OSMOTIC AND MEMBRANE EQUILIBRIA 209 



but in general 



[heY 9^ M". (106.4) 



As compared with the example of a purely chemical cell, we 

 have included in the present system one extra phase and 

 membrane in order that the two extreme phases or "terminals" 

 a and a' should have the same chemical composition. We may 

 therefore write 



WY - [heY = ZEFiv^' - y«), (107) 



and the difference of electric potential (7«' — "F") thus defined 

 is called the "electromotive force" E of the cell. Putting the 

 two phases a and a into contact is called short-circuiting the 

 cell and separating them "breaking the circuit." On closing 

 the circuit there will be an adjustment of membrane equilibria 

 with net flow of electric charge round the circuit in a direction 

 determined by the sign of E. This will cease when the con- 

 ditions 



are satisfied simultaneously, when the cell is said to be "run 

 down." 



We will now give a concrete example. We suppose the ionic 

 species A to be Cw++, B to be Zn++, and E to be electrons El- 

 and thus obtain the cell 



Cu 

 a 



Solution S containing Cw++ and Zn 



++ 



Zn 



Cu. 



a' 



We also imagine the boundaries between the phases to form 

 natural membranes, each permeable to only one ionic species. 

 In practice there would be irreversible deposition of copper 

 on the zinc, and this cell would not function unless some means 

 of preventing Cw*"^ ions from coming into contact with the 

 metal Zn were provided. We have oversimplified the descrip- 



