ELECTRICAL APPLIANCES AND MACHINES 255 



then spreading over it a thin coat of mercury. This process 

 is called amalgamation. The mercury unites with the zinc 

 and forms a pasty amalgam. As the acid takes the 

 zinc out of the amalgam, the mercury unites with 

 other zinc beneath the surface and so keeps the same 

 amount of amalgam on the surface. The iron does not 

 dissolve in the mercury but floats to the surface of 

 the amalgam, from which a few hydrogen atoms will 

 remove it. 



176. How to Prevent Polarization. An electric cell 

 which does not give a comparatively constant current, 

 for a short time at least, is not of much value. There 

 are three possible ways of preventing polarization, but 

 only two of them are practical. 



(1) If the hydrogen bubbles are brushed from the 

 copper plate, or if the liquid of the cell is stirred suffi- 

 ciently, the bubbles will be removed and escape to the 

 surface. This is a purely mechanical method and very 

 inconvenient. Various modifications of this method 

 have been devised but without much success. 



(2) Chemical Means. If there is added to the liquid 

 of the cell a substance which will oxidize the hydrogen 

 as fast as it is liberated at the copper plate, the chemical 

 action on the zinc remains constant and so the current 

 does not decrease. Some oxidizing substances used are 

 bichromate of potash, nitric acid, and manganese dioxide. 

 The chemical means of preventing polarization is used 

 very extensively. Manganese dioxide (MnC^) is the 

 depolarizer in the common dry cell. 



(3) Electrochemical Means. In this method the copper 

 plate is in a copper sulphate solution and so pure copper 

 atoms are deposited on the copper plate instead of hydro- 

 gen. This prevents all polarization as the deposited 



