586 DISTRIBUTION UPON CONDUCTORS TENSION. 



by the proof-plane, but the conductor also in time loses 

 its electricity even if not touched by the proof- plane. 



Now let the interior of the conductor be touched by 

 the proof-plane, applying it within the upper opening 

 as shown at A in fig. 313, taking care not to touch the 

 conductor with the hand and to withdraw the proof- 

 plane without coming too near the open edge (see B in 

 the figure). On touching the electroscope with the proof- 

 plane no trace of electricity is indicated. The interior 

 of the conductor is not electrical. To prove that this 

 is not due to the fact that the conductor was originally 

 charged from the outside, touch the outside by the 

 proof-plane, apply the latter, which is now electrified, to 

 the interior of the opening, withdraw it again, and test 

 its state by the electroscope. No trace of electricity 

 will be discovered : the electricity of the proof- plane has 

 passed from within to the outside and has diffused itself 

 over the external surface. 



Thus in an insulated conductor the electricity resides . 

 solely upon the external surface. This is readily 

 explained by the mutual repulsion of like electricities. 

 Every small quantity of electricity which forms a 

 portion of the total quantity with which the conductor 

 is charged repels, and is repelled by, the remainder, 

 hence a general rush to the farthest points, that is to 

 the surface. This tendency of electricity to escape, 

 caused by electrical repulsion, is called electric tension. 



The electric tension on the surface of a spherical 

 conductor is uniform in every part, simply in conse- 

 quence of the symmetrical shape of the sphere. It is 

 different with bodies of irregular shape. An elongated 

 conductor may be formed by introducing a brass wire, 



