208 FRICTIONAL ELECTRICITY. 



(a.) If the resin were a good conductor like the metal cover, its 

 molecules would all receive + electricity from the cover and give 

 electricity to it. But as the resin is a poor conductor, only the 

 very few molecules that come into actual contact with the cover at 

 each charging have their electrical equilibrium restored. The + of 

 the cover cannot pass through them to their electrified neighbors. 

 Hence it requires a great many placings of the cover upon the plate 

 to discharge the plate by reconveying to it the + electricity removed 

 at its electrification. When the cover is charged, it gives up part 

 of its electricity ; when it is discharged, it receives this elec- 

 tricity back again from the body which discharges it. As this 

 giving and taking is neither to nor from the resin, it may be con- 

 tinued indefinitely. A Leyden jar ( 353) may be charged with an 

 electrophorus. 



344, Effect of Pointed Conductors. -^Before 

 proceeding to study the electrical machine we need to 

 understand something of the action of pointed conduc- 

 tors. The reason of this action we shall see more clearly 

 as we proceed ; but the action itself, viz., that a strong 

 charge of electricity will easily and quietly escape from 

 a pointed conductor, is clearly shown by the following 

 experiments: Place a carrot horizontally upon an insu- 

 lating support. Into one end of the carrot stick a sewing- 

 needle. Bring the electrified glass rod near the point of 

 the needle without touching it. The electricity of the 

 carrot escapes from the point to the rod and the carrot 

 is positively charged. And now for another experiment, 

 not so easily made, but still certain to succeed if you are 

 careful. Excite the glass rod, turn the needle away 

 from it, and bring the rod near the other end of the 

 carrot. The positive electricity is now repelled to the 

 point from which it will stream into the air. Remove 

 the rod and test the carrot ; it is negatively electrified. 



345. The Plate Electric Machine. This instrument is 

 represented in Fig. 146. It consists of an insulator (or electric), a 



