ELECTRIC GENERATORS. 93 



On a wooden base thirteen inches square, constructed 

 of layers glued together to prevent warping, is placed 

 a thin sheet of brass of the same size ; over which is 

 placed a sheet of ebonite of equal size, iV of an inch 

 thick ; and both attached to the base by screws near 

 the corners. 



On the ebonite is placed a circular plate or cover, 

 made of No. 20 sheet brass, twelve inches in diameter, 

 perfectly flat, and having a round resistance rim joined 

 to the upper surface. In its center is an ebonite handle, 

 seven inches high ; and from its rim projects a goose- 

 neck, made of No. 8 brass rod, terminating in a half- 

 inch brass ball ; near which, on the edge of the base, is 

 a brass strip, f of an inch wide, connected with the 

 lower plate. 



The base may be made of metal, if preferred, in which 

 case the lower plate and strip are unnecessary, the base 

 itself taking the place of the plate. 



In this instrument we have two conductors separated 

 by a dielectric ; the upper one insulated, and the lower 

 connected with the earth. 



The cover being removed, the dielectric is beaten 

 briskly with a piece of catskin, or other fur, by which 

 its upper surface is electrified ; and the cover is then 

 replaced. 



Suppose the charge to be negative ; electricity having 

 been removed by the fur, the same amount is attracted 

 from the earth to the under surface of the dielectric, 

 and to the upper surface of the brass plate in connection 

 with it ; which thus become positive by induction. The 

 under surface of the cover also becomes positive and its 

 upper surface negative. 



Let a connection now be made between the lower 



