STATIC ELECTRICITY 



135 



that electricity passes through a conductor ; this way of 

 speaking is in common use, but it should be noted that 

 the charge is on the surface of the body and not actually 

 within it. 



We learn, then, that the electric charge upon a con- 

 ductor moves rapidly and covers its whole surface, while 

 that upon a " nonconductor " (insulator) remains at rest 

 upon the part where it was developed. In this section 

 we shall study the case of charged insulators, where 

 the charge is at rest, and to this study we give the 

 name electrostatics. 



158. Positive and Negative Charges. When bodies 

 are electrified we find that their charges may be one 

 or the other of two sorts, which seem 

 to have certain different effects. 



Experiment 99. Charge a glass rod by rub- 

 bing with silk, and hang it by a silk thread, as 

 in Fig. 100. Using that part of the silk cloth 

 which touched the rod, bring it near to one end 

 of the latter as it hangs free to turn. Carefully 

 note what happens. Now charge another glass 

 rod in the same way, and bring its charged 

 portion near that of the suspended rod. Note 

 the result in this case. Is there anything in 

 this experiment that would seem to show a dif- 

 ference between the electrification of the body 

 that is rubbed and that of the one that does the 

 rubbing? If you conclude that the glass and 

 the silk are differently electrified, what would you say about the 

 behavior of two unlike charges toward each other? Supposing 

 the two rods to bear like charges, what do you learn about the 

 behavior of two like charges toward each other? 



FIG. 100 



