Chap. I.] 



FRICTIONAL MACHINES. 



placed on the resin, which, owing to its non-con- 

 ductivity, is still able to retain its charge. It, 

 however, by induction, decomposes the neutral elec- 

 tricity of the cover, and attracts positive electricity to 

 the surface of the cover in contact with it, repelling 

 negative to the upper 

 surface. The elec- 

 tricity of the upper 

 surface can now be 

 withdrawn by touch- 

 ing it with the finger. 

 If, then, the cover be 

 lifted by its glass 

 handle, it is found 

 to have a charge of 

 -f- electricity which 

 will give a consider- 

 able spark. The 

 process can then be 

 repeated, because the 

 resin retains its 

 negative charge for a 

 long time. 



Frictional machines are constructed usually 

 of discs of glass, which are caused to revolve by a 

 handle, and in their revolution are rubbed by cushions 

 pushed against them. The friction develops + 

 electricity on the glass and electricity on the 

 rubber. A chain leads from the cushion to conduct 

 off the negative electricity into the earth. Metallic 

 points, brought near to the surface of the glass, 

 conduct off its positive charge to large metal cylinders, 

 called the prime conductors. The conductors are 

 insulated, and soon become highly charged with the 

 electricity developed by the friction. The rubbers are 

 usually coated with an amalgam. 



In Holtz' machine (Fig. />) the electricity is 



Fig. 4. Electropliorus. 



