PRACTICAL PHYSIOLOGY 



FIG. 2. DIAGRAM OF A VOLTAIC COUPLE. Z, ZINC ; C, COPPER. 



gas to form on the copper, and this not only intro- 

 duces a resistance to the flow of current through 

 the cell, but the hydrogen being electropositive 

 tends to set up a current (polarisation current) in 

 the opposite direction in the cell and circuit ; from 

 both these causes the original E.M.F. of the cell 

 becomes rapidly weakened. 



To obviate this effect Daniell placed the copper 



Copper pot. 



Zinc. 



Porous pot containing 

 dilute sulphuric acid. 



-Sulphate of copper 

 (saturated solution). 



FIG. 3. DANIELL CELL. 



