THE PHENOMENA OF ELECTROSTATICS. 



211 



The behavior of a positively charged electroscope when a 

 negatively charged body is brought near to it, is the same as its 

 behavior when it is negatively charged and a positively charged 

 body is brought near to it. 



117. Electric charge resides wholly on the surface of a charged 

 conductor. Electrical screening. The electrostatic phenomena 

 exhibited by charged conductors are precisely the same whether 

 the bodies be solid or hollow ; 

 and, if the bodies be hollow, 

 no effect of the charges can be 

 detected inside of them how- 

 ever thin their walls may be. 

 The lines offeree of the electric 

 field end at the surface of the 

 charged conductor or, in other 

 words, the electric charge re- 

 sides wholly on the surface of a 

 charged conductor. 



A conducting shell, such as 

 a metal box, screens its interior 

 completely, so that no action of any kind reaches the interior from 

 charged bodies outside.* Thus, a hollow metal ball C, Fig. 

 140, screens its interior completely. The lines of force which 

 touch the shell C end at its surface. The ending on C of the 

 lines of force from A is negative charge and the beginning on 

 C of the lines of force which reach B is positive charge. 



The fact that electrical field cannot penetrate into a substance 

 like a metal shows that such substances cannot sustain the pecul- 

 iar kind of stress which constitutes electrical field any more than 

 a fluid can sustain the kind of stress that exists in a stretched 

 steel wire. 



Mechanical analogue of electrical screening. Consider a solid 

 body B, Fig. 141, entirely separated from the surrounding solid 

 by an empty space eee. Stress and distortion of the surrounding 



* This is not strictly true when the outside conditions are changing rapidly. 



Fig. 140. 



