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OUR PHYSICAL WORLD 



passage of the current. This difficulty is overcome in several 

 ways: First by using chemicals which will not liberate hydrogen 

 as in the gravity battery described below, or secondly by the use 

 of some chemical which unites with the hydrogen. Thus in the 

 chromate battery a solution of potassium bichromate, K 2 Cr 2 7 , 

 is used. This readily gives up a part of its oxygen, and the 

 oxygen and hydrogen unite to form water. 



In general, when two substances like plates of two metals 

 are partially immersed in a chemical and chemical action occurs, 



FIG. 93. Several types of batteries: (a) gravity battery; (b) bichromate 

 battery (La Clanche); (c) Bunsen battery; (d) Daniell battery. 



the electric current passes in a wire connecting the plates from 

 the one where chemical action is less rapid to the one where it is 

 more rapid. Thus if a zinc and a copper strip were used in the 

 simple battery described above, the copper would be the positive 

 plate and the zinc the negative. 



The development of the electric current is explained in the 

 gravity battery somewhat as follows (Fig. 93). This battery 

 consists of a jar with a copper plate at its bottom and a zinc plate 

 near its top. A solution of common salt is used to fill the jar, 

 into which some copper sulphate crystals are thrown. Some of 



