ELECTRIC DISTRIBUTION AND CONDENSATION. 59 



disc, two inches in diameter, to which is attached a 

 light ebonite handle, 12 inches long. A light, flat 

 spring, which lies close to the disc, its lower end free, 

 and its upper end attached to the handle, will be found 

 convenient for attaching tin-foil in some experiments. 



Fig. 8 Proof Plane. 



The proof plane is used for examining the electric 

 condition of bodies, and for transferring a small charge 

 of definite amount. Care should be used to prevent 

 the handle from becoming charged, which may happen 

 from friction against the clothing or otherwise. 



EXPERIMENTS WITH HOLLOW CONDUCTORS. Let a 

 charge of electricity be given to the insulated sphere A, 

 Fig. 9, which has an 

 opening in the top. In- 

 troduce the proof plane 

 through this opening, 

 taking care to prevent 

 contact with the edges ; 

 and touch the inside sur- 

 face and then the disc 

 of the electroscope, with 

 it. As the leaves show 

 no divergence, it proves 

 that the inside is not 

 electrified. 



Now touch the outside, and then the disc, and the 

 leaves diverge ; proving that the charge is on the out- 

 side surface. 



Apply the same tests to the insulated cylinder B, and 



