GENERATION OF ELECTRICITY 393 



QUESTIONS 



1. Why not make the cores of electro-magnets of steel? 



2. Why m.ust " keepers " (bars of steel) be kept on the ends of 

 permanent steel magnets? 



3. Why does the electric car move when the motorman " turns 

 on the current." 



4. Why is it necessary to have electro-magnets in a motor? 



5. How could a motor be made to run backward (reverse)? 



GENERATION OF ELECTRICITY 



Static Electricity. Rub a hard rubber rod briskly with a piece 

 of catskin. Hold it near two pith balls suspended by a silk thread. 

 Try the rod on a bit of paper, cork, etc. Rub a rod of glass with 

 a piece of silk or flannel. Bring this near the pith balls. 



This experiment shows that there are two kinds of electricity 

 generated. That generated by the glass rod rubbed with silk is 

 called positive; that generated by the rubber rod rubbed with 

 flannel is called negative. 



If a body charged with negative electricity is brought near a body 

 charged with plus electricity, the two bodies are attracted toward 

 each other, but two bodies charged with the same kind of electricity 

 are repelled from each other. 



Rub your fountain pen on your coat and hold it near one pith ball which 

 you have charged with plus electricity. What kind of electricity is the fountain 

 pen charged with? Test a number of different materials rubbed with fur, 

 catskin or flannel. 



Potential. If you hold a charged rod rubbed with fur near the 

 finger yOu will feel a little prick and hear a sharp snap which show 

 that a spark of electricity passes from the rod to your finger. Also 

 a charge passes from your finger to the rod, neutralizing the plus 

 and minus charges. This exchange of electricity which takes place 

 between two bodies is caused by the difference in potential or pres- 

 sure of the electricity. We have already called it volts. This elec- 

 tricity contains no amperes of current; it is simply very high voltage. 

 A spark 1 inch long has a voltage of about 75,000 volts. 



