i6 



PHYSIOLOGICAL PHYSICS. 



[Chap. II. 



of electricity in the voltaic pile was due to the unequal 

 action of a chemical agent on two dissimilar metals 

 gave rise to the voltaic element. 



Voltaic couple, element, or cell. (Fig. 8.) 

 A plate of zinc z and one of copper c are plunged 

 into a vessel, three-fourths filled with diluted sul- 

 phuric acid L (1 acid to 7 water). The metals must 

 not touch one another. The sulphuric acid acts upon 

 the zinc, producing sulphate of zinc and liberating 

 hydrogen, as expressed in the formula 



Zn + H. 2 iSo 4 = ZnSo 4 + H 2 . 



The hydrogen collects in bubbles on the copper plate. 

 With such a combination it is found that the zinc 



plate is at a higher potential than 

 the copper; but, so long as the 

 metals remain unconnected, no cur- 

 rent is present. On connecting the 

 parts of the plates projecting above 

 the liquid, which are called the POLES, 

 by means of a copper wire E a 

 current flows from the plate of 

 higher potential through the liquid 

 to that of lower potential. In the 

 liquid, therefore, the current passes 

 from zinc to copper, outside of the 

 liquid from copper to zinc. Thus, 

 though the copper is the plate least 

 acted on, its pole is called positive, 

 because the direction of the current is from it to the 

 zinc pole. Properly speaking, the difference of poten- 

 tial is not limited to that between zinc and copper. 

 There is a difference between the zinc plate and the 

 liquid in contact with it, a difference between the 

 liquid in contact with the copper plate and the copper 

 plate ; and, again, a difference between the copper 

 wire in connection with the copper plate and the zinc 



Fig. 8. Voltaic 

 Couple. 



