ELECTRICITY AND MAGNETISM 137 



of motion in a piston; this motion, transmitted by the parts 

 of an engine to a dynamo, produces electrical energy. 



The waterfall and the steam boilers may be many miles 

 away from the motor and the machines. The electricity 

 can be conveniently carried by wires, properly insulated, 

 underground or overhead. Current is thus supplied to 

 towns which have no water power or railroad facilities for 

 bringing coal by which they could produce electricity. 

 Streets and houses in rural districts are now better lighted 

 than were those of a large city fifty years ago. Cars run by 

 electricity furnish cheap and rapid conveyance which brings 

 the city and the country nearer together, to the advantage 

 of both. 



EXERCISES 



1. What kind of electricity is developed by stroking a cat's fur? 



2. Explain the spark that passes from the hand to the cat's body 

 after the stroking. 



3. Which is a better conductor, dry or moist air? 



4. Why does a body retain a charge better on a dry day? 



6. How far away is an electric flash if one can count twenty between 

 the moment of seeing the flash and hearing the thunder? 



6. Name all the parts of the circuit of a simple cell whose metal 

 plates are connected by a copper wire. 



7. Why is it necessary to cover telephone and electric light wires? 



8. Why are glass or porcelain coverings used where telegraph wires 

 are supported on poles? 



9. What is meant by a " live wire "? 



10. Name changes in the form of energy between a waterfall which 

 is used in producing electricity and a trolley car run by electricity. 



