MAGNETISM AND ELECTRICITY 



171 



FIG. 73. Electric 

 Bell. 



pressed by the finger, an electric circuit is completed. The current 

 flows around the coils of an electromagnet, which attracts a bar 

 of soft iron metal fastened to a lever, at the IJL 

 other end of which there is a hammer that 

 strikes the bell. When the soft iron metal is 

 attracted the current is broken; this causes 

 the bar to go back. This backward move- 

 ment of the bar starts the current again and 

 the operation is repeated. These operations 

 are repeated in rapid succession so long as the 

 button is pressed. 



202. Chemical Means of Generating 

 Electricity. -Electricity may be generated 

 by chemical agencies. When any two 

 different metals, such as zinc and copper, 

 are placed in an acid or solution and 



wires are attached to them and connected, a current of elec- 

 tricity flows through the wire. This arrangement of metals 

 in a liquid is called a cell. When the wires are not connected, 

 bubbles of hydrogen collect around the zinc plate, but the 

 moment the wires are connected, the hydrogen gas begins 

 to appear on the copper plate. 



Commercial zinc contains a great many impurities, such as 

 iron and carbon. Little circuits are set up between the zinc 

 and carbon impurities; hence the bubbles which appear on 

 the zinc when it is immersed in the acid. This bubbling may 

 be avoided by amalgamating the zinc, i.e., by covering it 

 with mercury so that the zinc is used up only when the cur- 

 rent is flowing. 



203. Electrolysis. The breaking up of a substance by 

 passing electricity through a solution of the substance is 

 called electrolysis and the solution in which it takes place 



