210 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



Fig. 138. 



Fig. 139. 



The behavior of a gold leaf electroscope when a charged body is 

 brought near to the plate D is as follows : (i) When the elec- 

 troscope has no initial charge, some of the lines of force pass 

 from the charged body into the disk and then spread out from 

 the leaves to the strips ff t causing the leaves to diverge. If 

 the charged body is removed the electroscope becomes neutral 

 and the leaves fall together. If, while the charged body is near 

 D, the disk or rod is touched with the finger, the lines of force 

 passing out from the leaves cease to exist, and the leaves fall to- 

 gether. If now, the charged body is removed, the lines of force 

 passing into the disk from the charged body spread over the disk, 

 rod and leaves, and the electroscope is left charged, as indicated 

 by the divergence of the leaves. This operation, called charging 

 by influence, is explained more fully in Art. in. 



(2) When the electroscope has an initial charge, say a positive 

 charge, then a positively charged body brought near to D pushes 

 the initial charge down into the leaves, as it were, and the diver- 

 gence of the leaves is increased. If a negatively charged body 

 is brought near to D, the positive charge on the leaves is pulled 

 up into the disk, as it were, by the attraction of the negative 

 charge on the body, and the divergence of the leaves is decreased. 

 If the negatively charged body is brought nearer, the leaves will 

 come together ; and if the negatively charged body is brought 

 still nearer the leaves will again diverge. 



