ELEMENTARY LESSONS ON [CHAP. i. 



researches on the action of points. About 1760 De 

 la Fond, Planta, Ramsden, and Cuthbertson, constructed 

 machines having glass plates instead of cylinders. The 

 only important modifications introduced since their time 

 are the substitution of ebonite for glass, and the inven- 

 tion of machines depending on the principles of induc- 

 tion and convection. 



39. The Cylinder Electrical Machine. The 

 Cylinder Electrical Machine, as usually constructed, 

 consists of a glass cylinder mounted on a horizontal axis 

 capable of being turned by a handle. Against it is 

 pressed from behind a cushion of leather stuffed with 

 horsehair, the surface of which is covered with a 

 powdered amalgam of zinc or tin. A flap of silk attached 

 to the cushion passes over the cylinder, covering its 



Fig. 23. 



upper half. In front of the cylinder stands the " prime 

 conductor," which is made of metal, and usually of the 

 form of an elongated cylinder with hemispherical ends, 

 mounted upon a glass stand. At the end of the prime 

 conductor nearest the cylinder is fixed a rod bearing a 

 row of fine metallic spikes, resembling in form a rake ; 

 the other end usually carries a rod terminated in a brass 



