fi 



STATIC ELECTRICITY 



body near the electroscope the leaves diverge, or the single leaf 

 stands out from the strip, showing that they have undergone 

 electrification. The electrification thus produced at a distance 

 without contact is said to be induced. 



Detection of kind of charge by the gold-leaf electro- 

 scope. If the electroscope be slightly charged, say by rubbing 

 the table with flannel or fur, it will indicate the 

 kind of charge on any body brought near to the 

 table. If, for example, an excited ebonite rod 

 held over the table makes the leaf or leaves stand 

 out further, an excited glass rod makes them 

 fall nearer together, and vice versa. We shall 

 see later how these actions may be explained. 

 Here we take them as the results of direct ex- 

 periment and shall use them to test the nature 

 of induced electrification. 



Nature of induced electrification. It 

 may be shown easily that whenever an electrified 

 body is brought near an insulated conductor it 

 induces electrification of the kind opposite to its 

 own in the nearer part, and of the kind .similar 

 to its own in the further part of the insulated body, as in l-'i^. ">. 

 in which A is a body on an insulating support, and is electrified. 

 say, positively, while B is a conductor standing on an insulating 

 support. We may verify this by mean* of a /;;-oo/'y;/////<- (1'iu 

 which is merely a small metal disc at the end of an insulating rod. 



Fio. 4. 



o 



Fro. 5. 



FIG. 0. 



When the metal disc is laid on an electrified surface it virtually 

 forms, if small enough, part of the surface, and is electrified in the 

 same way as the surface round it. When lifted from the surface it 

 preserves its electrification, and we may directly test the kind by 

 means of the gold-leaf electroscope. 



Testing the electrification at each end of B (Fig. 5), we find 

 that on the end nearer A to be negative, and that on the end farther 

 from it to be positive. If we now withdraw A, B returns to its 

 ordinary state, the ends showing no electrification. Thus positive 

 and negative have come together and have neutralised each other. 



