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.
Mr. 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 rubber, R, is seen at the farther side of the cylinder, supported by the upright lath, R', and caused to press against the glass. S' is a flap of silk. When the handle is turned sparks may be taken, or a Leyden-jar charged at the knob C. A plate machine is shown in Fig. 19. P is the plate; R and R', two rubbers which clasp the plate. A and A' are rows of points presented by the conductor, C. C C' is an insulating 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
