163 



THE POPULAR SCIENCE MONTHLY, 



Sec. 16. The Electrical Machine. An electrical machine consists 

 of two principal parts: the insulator which is excited by friction, 

 and the "prime conductor." 



The sulphur sphere of Otto von Guericke was, as already stated, 

 the first electrical machine. The hand was the rubber, and indeed it 

 long continued to be so. For the sulphur sphere Hauksbee and 

 Winckler substituted globes of glass. Boze, of Wittenberg (1741), 

 added the prime conductor, which was at first a tin tube supported 

 by resin, or suspended by silk. Soon afterward Gordon substituted 

 a glass cylinder for the globe. It was sometimes mounted vertically, 

 sometimes horizontally. Gordon so intensified his discharges as to 

 be able to kill small birds with them. In 1760 Planta introduced the 

 plate machine now commonly in use. 



Mi\ Cottrell has constructed for these lessons the small cylinder 

 machine shown in Fig. 18. The glass cylinder is about seven inches 

 long and four inches in diameter ; its cost is eighteen pence. Through 

 the cylinder passes tightly, as an axis, a piece of lath, rendered secure 



Fig. 18. 



by sealing-wax where it enters and quits the cylinder. G is a glass rod 

 supporting the conductor C, which is a piece of lath coated with tin- 

 foil. Into the lath is driven the series of pin-points, P, P. The rub- 

 ber, 7?, is seen at the farther side of the cylinder, supported by the 

 upright lath, 7?', and caused to press against the glass. S' is a flap 

 of silk. When the handle is turned sparks may be taken, or a Ley- 

 den-jar charged at the knob C. A plate machine is shown in Fig. 19. 

 P is the plate ; P and 72', two rubbers which clasp the plate. A and 

 A' are rows of points presented by the conductor, C. C C is an in- 

 sulating rod of glass, intended to cut off the connection between the 

 conductor and the handle of the machine. 



The prime conductor is thus charged : when the glass plate is 



