THE ELECTROPHORUS. 577 



no sensible quantity of electricity has been communi- 

 cated by the cake to the disc. But if the disc while it 

 rests on the cake be touched but for an instant with the 

 finger, it will give strong indications of positive electri- 

 city if raised and held even at some distance from the 

 electroscope. 



The two electricities of the cover are separated by 

 induction when the cover is placed upon the negatively 

 charged cake ; the positive electricity is attracted to 

 that side of the cover which is in contact with the cake, 

 while the negative electricity is repelled to the upper 

 side of the metallic cover. That the upper surface is 

 really charged with free negative electricity may be 

 proved by touching it (before the finger is applied) with 

 the proof-plane and bringing the latter near an electro- 

 scope. When the upper surface is touched with the 

 finger, the free negative electricity passes off and the 

 cover remains charged with positive electricity, bound, 

 however, by the negative electricity of the cake ; again, 

 if the cover is raised by the insulating cords, the positive 

 charge is no longer attracted by the negative of the 

 cake, it diffuses itself over the cover, and if a conductor 

 be brought near it, a smart spark passes. 



When the cover is raised before being touched by the 

 finger, the two electricities simply combine again and 

 the cover can give no indication of electricity. 



The metallic form upon which the cake rests increases 

 the quantity of electricity and makes it more permanent. 

 The negative electricity developed on the upper surface 

 of the cake acts inductively on the lower, attracting the 

 positive electricity but driving the negative into the 

 ground. The attraction of this positive electricity binds 



p P 



