VOLTA'S COUPLE. 249 



corresponds to the maximum of polarization for the conditions of 

 the experiment In this way are explained the various peculiarities 

 to which the phenomena of polarization give rise. 



When we connect, with a voltameter, a source of electromotive 

 force insufficient to produce a continuous disengagement of gas, 

 experiment shows that the electromotive force of polarization in- 

 creases with the strength of the permanent current in question, but 

 less rapidly ; that for a given value of this current, the electromotive 

 force diminishes when the surface of the electrodes is increased ; 

 and finally, that the electromotive force is constant if the current, 

 and the surface of the electrodes, increase in the same ratio. 



261. VOLTA'S COUPLE. A few words only are now needed to 

 complete the theory of the battery. Volta's couple, in the strict 

 sense of the word, consists of a plate of zinc and a plate of copper 

 placed in water, to which a small quantity of sulphuric acid, or of 

 any salt has been added, to make it conduct ; the plate of copper 

 being soldered to a plate of zinc which forms part of the next couple. 

 Thus, between two terminals of the same kind there are three 

 contacts, zinc-copper, copper-water, and water-zinc. The electro- 

 motive force may be expressed by the ordinary symbols 



E = Zn|Cu + Cu|Aq + Aq|Zn. 



Volta assumed that water only played the part of a conductor, and 

 thus we shall have 



Cu|Aq + Aq|Zn = 0, 



and therefore 



E = Zn|Cu. 



On this point of view, the electromotive force of a Voltaic couple 

 only depends on the contact zinc-copper, and these two metals joined 

 by a layer of water are at the same potential. The alteration of the 

 surface of the metal when in contact with the liquid or the gas, makes 

 it very difficult to establish Volta's hypothesis in a rigorous manner. 



However this may be, this alteration is so rapid, and produces 

 such changes in the electromotive force, that the electromotive force 

 of Volta's couple must practically be considered as depending, to a 

 considerable extent, on the medium which forms the third element. 



When the couple is closed by a conductor whose resistance is R, 

 a current is produced the strength of which is given by the ratio 



4 



