98 , : ELEMENTS OF STATIC ELECTRICITY. 



structed 'on the same principles, a glass plate being 

 substituted for. the glass cylinder, and has now come 

 into general use. Fig. 29 represents one of the prevail- 

 ing styles. 



It consists of a disc of plate-glass -A, mounted on a 

 wooden base with wooden or glass pillars, and revolved 

 by a crank with an insulated handle. A pair of rub- 

 bers B, made of soft leather or felt, are pressed against 

 the glass on opposite sides by a pair of brass springs (7, 

 the pressure being adjusted by a screw. These are 

 mounted on a glass pillar, and connected above with a 

 brass ball ; and a brass chain, which may be removed, 

 connects them with the earth. 



Mounted on a glass pillar is the prime conductor _Z), 

 made of brass, and consisting of a pair of balls, from the 

 lower one of which projects a pair of combs, which 

 extend on opposite sides of the glass, and whose teeth 

 come within a quarter of an inch of it. And, from the 

 opposite side of the same ball extends a rod, terminat- 

 ing in a small ball. 



A silk cover envelops the lower part of the glass plate, 

 and the rubbers, on the surfaces in contact vvith the glass, 

 are coated with an amalgam, composed of five parts 

 zinc, three parts tin, and nine parts mercury, melted 

 together, pulverized, and made into a paste with lard. 



The machine should be dry and warm before use, as 

 moisture condenses on the surface of the glass when it 

 is colder than the atmosphere, and suspends insulation. 

 For this reason ebonite pillars have an advantage over 

 glass, being less liable to condense moisture. 



Ebonite has also been used for the plate, but is not 

 so reliable as glass ; and its liability to warp with heat, 

 when in thin plates, makes it very objectionable. 



