SOME WAYS OF PRODUCING ELECTRICITY 175 



in an astonishing manner when you brushed it. When 

 the air is cold and dry, a hard rubber fountain pen or a 

 rubber comb rubbed briskly with flannel will attract 

 small bits of paper or a pith ball supported by a thread. 

 If the ball or the bits of paper become electrified, as they 

 sometimes will after clinging to the rubber for a moment, 

 they will be repelled and fly away from it. The hard rub- 

 ber was charged with electricity by friction. The objects 

 which touched the charged rod were electrified by contact. 



Demonstration 4. Properties of Electrified Bodies. 



You will need these materials : two large rubber balloons, string, 

 silk cloth, fur, glass rod, and a hard rubber rod. Fasten a string 

 across the room near the ceiling. Blow up two large rubber bal- 

 loons until they are tight. Fasten them on strings looped over the 

 ceiling string. Make them at a height level with your head and 

 six inches apart. 



A. Rub one of the balloons with fur. Is there any action now 

 between the two balloons ? Between the balloon rubbed and your 

 hand held near it? 



B. Arrange the balloons so that when neither is charged, they 

 hang just touching each other. Rub both the balloons with fur. 

 After you rub one do not let it touch anything until the second one 

 is rubbed. Release the two. What happens? Compare this 

 result with the one when only 



one was rubbed. 



C. Separate the two balloons 

 so that you can use one with- 

 out interference from the other. 

 Charge the balloon by rubbing it 

 with fur. Rub the hard rubber 

 rod with fur. Bring rod near the 

 balloon. Rub the glass rod with 

 silk. Bring the glass rod near 



the balloon. What was the ac- ndt choked & rotor 



tion between the hard rubber 



rod and the balloon? Between the glass rod and the balloon? 



Conclusion and Explanation. At first the bodies were neutral. 

 After being rubbed they were charged with electricity. Does a 

 charged body attract an uncharged or neutral body? Do two 

 of the charged bodies attract each other ? Repel each other ? Do 

 all charged bodies have the same properties ? 



