CONDUCTORS AND NON-CONDUCTORS. 559 



any other part of the wire be electrified by the glass 

 rod, for example, by drawing it along a, or the middle 

 of the wire, or even along c, the pendulum is also re- 

 pelled. It follows that the disc becomes electric 

 whether the glass rod is brought in direct contact with 

 it or communicates its electricity only indirectly through 

 the wire. Whatever the length of the. wire, if electrified 

 at the end c, the end b also immediately manifests elec- 

 tricity; the electricity thus moves rapidly through the 

 whole length of the wire, and the wire is hence called a 

 conductor of electricity. The human body, the floor of 

 a room, the ground, are also conductors; this explains 

 why electricity disappears when we touch an electric 

 body by the hand; the electricity of the wire or the silk 

 pendulum passes through our body into the ground. 

 It follows further that it is as immaterial at what point 

 we touch a conductor in order to remove its electricity 

 as it is immaterial at what point we communicate to it 

 electricity : a conductor becomes electric instantaneously 

 over its whole extent, and loses all its electricity in- 

 stantaneously. Vegetable fibres, like cotton and linen, 

 are conductors; a cork ball suspended by thread made 

 of these substances can therefore never assume the elec- 

 trical state, since all electricity communicated to it dis- 

 appears through the thread. A ball suspended in this 

 manner can only be attracted by an electrical body, but 

 never repelled. 



Electricity does not diffuse itself over a thread of 

 silk, or a rod of glass, sealing-wax, sulphur, or ebonite, 

 as it does over a metallic wire. Such substances are 

 hence called non-conductors or ' insulators.' In order to 

 see whether a given body is a conductor or insulator, it 



