380. =Electrical Machine.=--You are now prepared to see how the common electrical machine operates. There are two kinds--the plate and the cylindrical. The plate machine, Fig. 261, has at _p_ a large plate of glass, and at _r_ a rubber which consists of two brass plates lined with leather which is stuffed, the pressure of which upon the glass is regulated by a screw. Above this rubber is a brass ball, _d_, and a brass chain connects the rubber and the ball with the floor, or, in other words, with the earth. At _c_ is what is called the prime conductor--a hollow brass cylinder with rounded ends, having attached to it a rod with points, as seen at _a_. There is a similar rod attached to it on the other side of the glass plate. The different parts of the instrument are supported on glass pillars, _g g g_, standing on a wooden platform. The lower part of the plate is covered with a case of silk, which, being a non-conductor, prevents the electricity on the glass from being lost in the air, and also serves to keep the plate free from dust. The rubber is covered with an amalgam of tin, zinc, and mercury, this being found very effectual in exciting electricity. The operation of the machine is this: As the plate revolves positive electricity is collected upon the glass, and negative electricity upon the rubber. The former, as it comes to the points at _a_, goes to them and passes on by the rods to the prime conductor, while the latter passes from the rubber by the chain to the earth. The points at _a_ are of great service in collecting the electricity, because the fluid is always much more ready to go to points than to conductors of a blunt shape. [Illustration: Fig. 262.] The cylinder machine is represented in Fig. 262, _a a_ being a glass cylinder, which can be turned rapidly by the multiplying wheel, _b b_. At _c_ is a piece of silk, and on the rear part of the cylinder is the rubber. At _d_ is the prime conductor. 381. =Experiments.=--Many experiments may be tried with the electrical machine. I will cite a few of them: If pith balls be attached to the prime conductor, as seen in Fig. 261, they will stand out from each other as soon as the machine is worked, because they are both charged with the same kind of electric fluid. Let a small figure with its head covered with hair be placed upon the prime conductor. As soon as the conductor becomes charged with electricity the hair stands out, as represented in Fig. 263, for the same reason that the pith balls diverged in the previous experiment. [Illustration: Fig. 263. Fig. 264.] So, also, if you place on the conductor a figure having attached to it strips of tissue-paper, they will diverge in the manner shown in Fig, 264. [Illustration: Fig. 265.]