210 EXPERIMENTAL GENERAL SCIENCE 



the strip of metal melts and breaks the circuit. Electric 

 flat-irons, toasters, and the like are heated by the resistance 

 in the small wires which they contain. The mercury vapor 

 lamps and lamps of similar structure give off light from the 

 glowing vapor of mercury, nitrogen, or other gases. 



176. Storage Batteries. Storage batteries are not strictly 

 places for storing electricity, but are devices by means of which 

 electricity may be obtained as wanted. While being charged, 

 the e lectricity changes the chemical composition of the con- 

 tents of the batteries, and when used as a source of electricity, 

 the chemicals gradually change back to the original form, giv- 

 ing up electric energy in the process. 



Practical Exercises 

 1. How could you make a weak electromagnet stronger? 



2. What would be the effect of using steel instead of soft iron for the 

 cores of electromagnets? 



3. What would be the effect of connecting the wires of an electric 

 current by means of a glass rod? Why? 



4. Why could not an alternating electric current be used for electro- 

 plating? 



5. Why cannot a storage battery continue to take up electricity 

 indefinitely? 



