150 PHYSICS. 
known kind of electricity. This is done by bringing a body, r ( pl. 22, fig. 62), 
of known electrical condition near the top of an electroscope, and then touch- 
ing this top with the finger. The induction of the body, r, drives into the 
finger the electricity of like character with its own, the electroscope retaining 
the opposite electricity on the removal of r._ The leaves or straws of the 
electroscope will, however, still be divergent. If, for instance, the body first 
approximated had been a glass tube excited by silk, then, its electricity 
being positive, that of the electroscope would be negative. Now, if on 
approaching a second electrified body, the leaves were still to remain 
divergent, it would be an evidence that the second kind of electricity was 
of the same character with that already in the gold leaves, negative in our 
illustration. An approximation or entire collapse of the leaves would follow 
on bringing near them the opposite electricity, positive in our instance. 
Connecting two perfectly similar electroscopes (pl. 22, fig. 64) by an 
insulated conductor, and bringing an electrified body, r, near one of 
them, the balls or leaves will diverge in both. Removing first the conductor, 
and afterwards r, the leaves in both electroscopes will remain divergent, 
indicating the presence of free electricity. This, however, will be positive 
in the one and negative in the other. 
The electric machine consists of three elementary portions, a rubber, an 
idio-electric, and an insulated prime conductor. The idio-electric generall} 
consists of a circular plate of glass, or a glass cylinder, whence the distinc- 
tion into plate and cylinder machines. The rubber -is generally a cushion 
stuffed with horse-hair, having anteriorly a rubbing surface of leather 
coated with amalgam. The prime conductor is most generally one or more 
hollow cylinders of tin or brass, with hemispherical terminations, and insu- 
lated by glass feet. Pl. 20, fig. 32, represents a cylinder machine according 
to Nairne’s construction. Here a is the glass cylinder, turned about a 
horizontal axis, b, by means of a handle, and rubbed along its whole extent 
by a single cushion. The latter is connected with the conductor, 7, while 
a second conductor, v. is placed on the opposite side of the cylinder, to which 
it presents a row of fine sharp points. A flap of oiled silk attached to the 
rubber, reaches over the cylinder nearly to these points, to prevent any 
escape of electricity from the excited glass. In turning the cylinder, it and 
the conductor v are positively electrified; the rubber and conductor r 
negatively. The entire apparatus must be well insulated by legs of glass. 
To obtain positive electricity the negative conductor must communicate 
with the earth, or some large body of conducting matter, while the positive 
conductor remains insulated. To obtain a negative fluid, the conditions as 
to the insulation of the conductors must be reversed. 
Fig. 29, pl. 20, represents a plate machine. The glass disk, a, is perforated 
in the centre, and through the aperture there passes a horizontal axis, turned 
by the handle bm. The two posts, d, sustain both the plate and the rubber, 
the latter consisting of two pairs of cushions, which reach from the edge of 
the disk to over about the fourth or sixth part of the diameter. The prime 
conductor, fgf, is insulated by the glass pillars, h, and ends in two arms, 1, 
embracing the plate horizontally. Migs. 30 and 30* represent more clearly 
324 
