576 



PHYSIOLOGY. 



pieces of carbon and dioxide of manganese. Its electromotive force 

 is 1.5 volts. The dry cell is usually made of a zinc cup lined with 

 plaster of Paris, saturated with amomnium chloride. A carbon plate 

 is placed in the center of this and surrounded with black oxide of 

 manganese. 



Polarization of Plates. The voltaic battery consists of two 

 metals, zinc and copper, which are surrounded by an electrolyte con- 

 taining various ions. The positive ions, Cu and H, will work their 

 way towards the positive element, the copper plate, and the OH and 

 S0 4 , being negative ions, will go towards the zinc. The hydrogen 

 gas settles in minute bubbles upon the surface of the copper plate and 



K 



Fig. 214. Daniell Cell. 



at once interferes with the action of the battery. It interferes both 

 by the resistance it offers to the passage of the current, and also by 

 setting up a current in an opposite direction, which tends to weaken 

 the original current by neutralization. This action is called polari- 

 zation of the plates. Besides this, in such an element some of the 

 sulphate of zinc produced in the element is attacked by the hydrogen 

 and deposited on the copper plate, so that the copper plate begins to 

 approach the condition of the zinc plate, and, of course, the. difference 

 of potential or electromotive force is reduced. -In all these ways the 

 current is diminished and the cell is not of constant strength. 



Polarization in the Daniell cell is overcome by the solution of 

 copper sulphate, and in the Leclanche cell by the manganese dioxide. 



Resistances. There are two kinds of resistance to electric cur- 

 rents : Internal resistance or the resistance of the element, or the 





