8 STATIC ELECTRICITY 



surface. Frequently they only adhere for a moment and are then 

 repelled. This is especially noticeable with pith balls suspended 

 by insulating silk threads. They rush to the electrified surface 

 and then rush away, standing out as in Fig. 7. The explana- 

 tion of this repulsion is that on touching the electrified surface, 



they yield up their opposite electrifi- 

 cation, being slight conductors, and 

 gather from the surface some of its 

 own electrification ; being now elec- 

 trified in the same sense as the surface, 

 they are repelled from it. 



The gold-leaf electroscope may be 

 used to illustrate some of the pheno- 

 mena of induction. Eet us use the 

 form with two leaves. If a negatively 

 electrified rod, say of vulcanite, be 

 held above the table of the elect m- 

 scope as in Fig. 8, the table is electrified 

 positively and the leaves negatively. 

 The negative in the leaves induces 



positive in the side plates, their negative going off through t he- 

 conducting communication to the table or floor, or, let us my 

 generally, to the earth symbolised by the plate E. To be MIIV 

 that we get rid of this negative charge it is advisable to connect 

 the side plates to the gas- or water-pipes In wires. 



FIG. 7. 



: A : 



The gold leaves stand out from each other. We described 

 this before as due to repulsion of the like electrifications; 

 but we may now replace this provisional description by sayino- 

 that each leaf is pulled by the plate near it which is opposite! v 

 e'ectrified, and there is still an action of this kind, even if, as in 



