134 ELECTRICITY 



on end " after being thus combed ; sparks are sometimes 

 seen to pass from fur to the hand that is rubbing it. In 

 these cases both the body that is rubbed and the one 

 that does the rubbing are said to be electrified, or charged 

 with electricity, or to have upon them a charge. 



Experiment 98. Rub a glass rod with a silk cloth; at once 

 bring the rod near small bits of paper. Quickly do the same thing 

 with the cloth. Repeat the experiment, using a stick of sealing 

 wax and a woolen cloth. Do you see evidence that any of these 

 bodies are electrified ? 



In these cases the charge appears to be on the sur- 

 face of the charged body, and only at the parts which 

 were touched when the rubbing was done. This would 

 be true when the substances used were glass, rubber, 

 dry wood, silk, paper, sealing wax, sulphur, porcelain, 

 or cloth. If certain other substances were used, such 

 as the metals, the charge would appear not only at 

 the part touched, but all over the surface of the 

 body. Moreover, if this body, while still charged, were 

 brought to touch another body which was like it in this 

 respect, its charge would at once spread all over the 

 other body as well. Thus it is clear that, in order to 

 charge such a body, we should first separate it from 

 other bodies by a substance of the sort first described, 

 cloth, glass, etc. 



Substances of the first sort, over which the charge 

 does not spread, are called insulators ( 153). As has 

 been said, dry air is one of the most important of these. 

 Substances of the second sort are called conductors ; all 

 the metals, the earth, animal bodies, and water contain- 

 ing acids or salts are conductors. We have said ( 153) 



