332 



Introduction to the Study of Science 



EXTERNAL 



Q* ZINC(-) 



or horn a magneto. As the so-called dry cell is Used for many 

 other purposes than ignition, it may be studied first of all in 

 some detail. 



The dry cell. The construction of a dry cell is illustrated 

 in Fig. 110. You should break apart a worn-out cell and 

 examine its parts. The material surrounding the carbon rod is 



perhaps the only difficulty to be 

 encountered. This varies in com- 

 position in different makes of cells. 

 A very common formula calls for 

 manganese dioxid, plaster of Paris, 

 flour, and sawdust, mixed with a solu- 

 tion of sal ammoniac to form a paste. 

 The zinc container may be protected 

 inside with a few layers of absorbent 

 paper, like blotting paper. When 

 the terminals of carbon and zinc are 

 connected in circuit, there is started 

 a chemical process by the zinc and 

 sal ammoniac, which in uniting gen- 

 erate an electric current. The elec- 

 tricity passes from the carbon along 

 the wire circuit and back through the 

 zinc. Breaking the circuit stops the 

 chemical action and the generation 

 of electricity. The sal ammoniac and zinc combine and form 

 a new compound. When all the available zinc or sal ammoniac 

 has been thus used up, chemical action ceases, or we say the 

 cell is worn out. Sometimes the paste may be moistened by 

 introducing a little water through a hole bored into the top 

 covering, and the cell made active for a little longer period. 

 This one fact suggests the inaccuracy of calling it a dry cell, for 

 moisture is indispensable to its activity. 



A battery. When two or more cells of any sort are connected, 

 they make up a battery. The usual way of connecting cells is 



FIG. 110. "Dry" cell. 



