34 



ELEMENTARY LESSONS ON [CHAP. i. 



upon the surface. An important proposition concerning 

 the absence of electric force within a closed conductor is 

 proved in Lesson XX. ; meanwhile it must be noted that 

 the proofs, so far, are directed to demonstrate the 

 absence of a free charge of electricity in the interior 

 of hollow conductors. Many other experiments have 

 been devised in proof. Thus, Ferguson showed that 

 a pair of gold leaves hung inside a wire cage could 

 not be made to diverge when the cage was elec- 

 trified. Faraday constructed a conical bag of linen- 



Fig. 20 



gauze, supported, as in Fig. 20, upon an insulating 

 stand, and to which silk strings were attached, by which 

 it could be turned inside out. It was charged, and 

 the charge was shown by the proof-plane and electro- 

 scope to be on the outside of the bag. On turning it 

 inside out the electricity was once more found outside. 

 Faraday's most striking experiment was made with a 

 hollow cube, measuring 12 feet each way, built of wood, 

 covered with tinfoil, insulated, and charged with a 

 powerful machine, so that large sparks and brushes 



