CHAP, i.] "ELECTRICITY AND MAGNETISM. 31 



great that the discharge is practically instantaneous. 

 Further information on this question is given in Lesson 

 XXIII. 



28. Distribution of Electricity on Bodies. If 

 electricity is produced at one part of a non-conducting 

 body, it remains at that point and does not flow over 

 the surface, or at most flows over it excessively slowly. 

 Thus if a glass tube is rubbed at one end, only that one 

 end is electrified. If a warm cake of resin be rubbed at 

 one part with a piece of cloth, only the portion rubbed 

 will attract light bodies. The case is, however, wholly 

 different when a charge of electricity is imparted to any 

 part of a conducting body placed on an insulating 

 support, for it instantly distributes itself all over the 

 surface, though not always uniformly over all points 

 of the surface. 



29. The Charge resides on the surface. A 

 charge of electricity resides only on the surface of 

 conducting bodies. This is proved by the fact that it 

 is found to be immaterial to the distribution what the 

 interior of a conductor *s made of; it may be solid metal, 

 or hollow, or even consist of wood covered with tinfoil 

 or gilt, but, if the shape be the same, the charge will 

 distribute itself precisely in the same manner over the 

 surface. There are also several ways of proving by 

 direct experiment this very important fact. Let a hollow 

 metal ball, having an aperture at the top, be taken (as in 

 Fig. 1 8), and set upon an insulating stem, and charged 

 by sending into it a few sparks from an electrophorus. 

 The absence of any charge in the interior may be shown 

 as follows : In order to observe the nature of the 

 electricity of a charged body, it is convenient to have 

 some means of removing a small quantity of the charge 

 as a sample for examination. To obtain such a sample, 

 a little instrument known as a proof -plane is employed. 

 It consists of a little disc of sheet copper or of gilt paper 

 fixed at the end of a small glass rod. If this disc is laid 



