GENERAL ACCOUNT OF COMMON PHENOMENA 3 



their appropriate rubbers. Each rubber, if very dry and held at 

 the end of a vulcanite handle, will be found to repel the disc rubbed 

 by the other, while it attracts its own disc. Or if two other discs 

 which electrify each other be rubbed together and presented to the 

 glass disc, one attracts and the other repels it. 



It must not ;be supposed that a given substance is always 

 electrified in the same way. The nature of its electrification 

 depends on its physical condition and upon the rubber used. It is 

 found that substances can be arranged approximately in order, thus 



+ Cat's fur, 



Glass, 



Wool, 



Feathers, 



Wood, 



Paper, 



Ebonite, 



Silk, 

 Shellac. 



So that if any one of these substances is rubbed by one higher in 

 the list it is negatively electrified, while if rubbed by one lower in 

 the list it is positively electrified. The order in such a list must 

 not be taken as unite fixed, since change in physical condition may 

 alter the position of a substance in the list. A polished surface, 

 for instance, appears to increase the tendency to positive electrifi- 

 cation, so that while polished glass is near the head of the list, 

 roughened glass may below down. 



Electric actions not magnetic. We may at onco dis- 

 tinguish electric from magnetic action by the fact that iron, nickel, 

 and cobalt are, with all the other metals, absent from the above list. 



Conduction and insulation. At one time it was supposed 

 that the metals and other sub.stancos not showing electric attrac- 

 tion after friction were non-electric, differing fundamentally from 

 >ub>tances such as shellac and glass, which were alone regarded as 

 being capable of electrification. But afterwards it was shown that 

 the real distinction is in the degree to which a substance will allow 

 electrification to spread over or through it. Shellac and dry glass 

 keep the electrification for a longer or shorter time on the surface 

 where it is first developed, and are therefore termed insulators. 

 Metals allow it to spread over their whole surfaces or to be com- 

 municated to other bodies which they touch, and are therefore 

 termed conductors. If a metal rod is held by an insulating handle 

 it may be electrified easily by friction with fur, but if it is touched 

 by the hand, a conductor, it instantly loses its electrification. 



There i> no doubt a fundamental difference between conductors 

 and perfect insulators. But we term many bodies insulators which 

 arc really very slow conductors, and we may have every intervening 



