MAGNETISM AND ELECTRICITY. 147 
An apparatus for showing whether a body possesses electrical properties 
is called an electroscope or electrometer. The simplest of these is the 
electrical pendulum (pl. 20, fig. 22), consisting of a small ball of elder pith. 
suspended by a silk thread. If a body, when properly excited, attract this 
ball, the presence of free electricity in the former may be inferred, the 
want of such attraction being an evidence of its absence. The electric 
needie, another electroscope, constructed somewhat like the magnetic needle, 
consists of a light straw, supported on a pivot, and having pith balls at the 
extremities. This is quite sensitive in its indications, as is also Coulomb’s 
electroscope, represented in fig. 23. Here gg’ is a light rod of shellac, 
with a gilded pith ball or slip of gold leaf, e, at one extremity, and 
suspended by a vertical filament of silk, fine wire, or glass hair. The upper 
end of the latter is wound around a horizontal beam, ¢, by whose rotation 
the thread may be elevated or depressed. A cylinder of glass, vv’, inclosing 
the bar and protecting it from the air, carries a graduated circle, dd’, and 
is covered above by a top, cc’; an opening in the latter admits of the 
gradual introduction of the body to be tested. If it contain free electricity, 
then the extremity, e, will be first attracted and then repelled. 
All bodies were formerly divided into two classes, those which became 
electrical by friction, and those which did not; the former were called 
idio-electric, the latter anelectric. It was subsequently ascertained that all 
bodies exhibited electrical properties to a greater or less degree when rubbed, 
differing, however, in the readiness with which electricity was received 
and propagated. The former were called conductors, the latter non- 
conductors or insulators ; terms nearly synonymous with anelectrics and 
idio-electrics. The division into good and bad conductors would be a much 
better one, since there is no body incapable of conducting electricity to a 
certain extent. Water and liquids in general, animal bodies, and above all 
metals, are good conductors.. A conductor 'can only remain electric as 
long as it is surrounded by bad conductors or is insulated. 
The poorest conductors are silk, glass, resin, dry air, &c. To determine 
the electricity of an insulated conductor, attach to it two pith balls, by 
means of a conducting thread. These will diverge from each other 
whenever the body from which they are suspended is electrified, the 
divergence being in proportion to the amount of charge in the conductor. 
Two strips of gold leaf suspended together at one end, and with their 
surfaces in apposition, form a very delicate electroscope. They are 
generally inclosed in a cylinder of glass to protect them from aerial 
currents. An arrangement of this kind is found in the gold leaf electrometer 
of Bennett. Here the leaves are held by one extremity in the lower end 
of a rod of brass, let into the cover of the glass vessel, and carrying a screw 
at the upper end for attaching a brass ball or plate. To insulate the leaves 
completely, the rod is wrapped in two places with silk, and inclosed in a 
glass tube, which is then coated externally and internally with shellac. 
(See pl 22, fig. 65.) 
The straw electrometer of Volta (pl. 20, fig. 26), and the gold leaf 
electrometer (pl. 22, fig. 61), are exceedingly sensitive, besides serving to 
ICONOGRAPHIC ENCYCLOP&DIA.—VOL. I. 21 321 
