MAGNETISM AND ELECTRICITY 211 



whose electricity is, in turn, used for lighting, for propelling 

 street cars, for heating, for factory fcower, and in various 

 other ways. Electricity may be cheaply made by water 

 power and conducted great distances by wires to cities 

 and mills where it is needed (Figure 187) . 



Electric Motors. - - The appearance of the electric 

 motor is similar to that of the dynamo, but its action is 

 reversed. We revolve the armature of a dynamo in a 

 magnetic field to produce a current, but in the electric 

 motor the current is passed through coils of the armature, 

 causing it to revolve. The dynamo is a machine for 

 producing electric current, while the motor is a machine 

 for utilizing electric current and may be used to run 

 machinery wherever power is needed. 



QUESTIONS 



1. Explain how polarization stops the current. 



2. Why are gravity cells called by that name? 



3. Why is copper used in electric wiring? 



4. What do we mean by " earthing " a wire? 



5. Name four uses of the electromagnet. 



6. Draw a diagram of an electric bell, showing how it works. 



7. Why do we use soft iron in an electromagnet? 



8. Why are fuse plugs used on electric circuits? 



9. How does a current given by a cell differ from that given 

 by a Leyden jar? 



10. Learn how to make a sal ammoniac cell. 



