The Story of Electricity | Page 4

the rubber vigorously, it will fail to attract the pithball, for there
is no trace of electricity upon it. This is because the metal differs from
the glass in another electrical property, and they must therefore be
differently treated. Brass, in fact, is a conductor of electricity and glass
is not. In other words, electricity is conducted or led away by brass, so
that, as soon as it is generated by the friction, it flows through the hand
and body of the experimenter, which are also conductors, and is lost in
the ground. Glass on the other hand, is an INSULATOR, and the
electricity remains on the surface of it. If, however, we attach a glass
handle to the rod and hold it by that whilst rubbing it, the electricity
cannot then escape to the earth, and the brass rod will attract the
pith-ball.
All bodies are conductors of electricity in some degree, but they vary so
enormously in this respect that it has been found convenient to divide
them into two extreme classes--conductors and insulators. These run
into each other through an intermediate group, which are neither good
conductors nor good insulators. The following are the chief examples
of these classes:--
CONDUCTORS.--All the metals, carbon.
INTERMEDIATE (bad conductors and bad insulators).--Water,
aqueous solutions, moist bodies; wood, cotton, hemp, and paper in any
but a dry atmosphere; liquid acids, rarefied gases.
INSULATORS.--Paraffin (solid or liquid), ozokerit, turpentine, silk,
resin, sealing-wax or shellac, india-rubber, gutta-percha, ebonite, ivory,
dry wood, dry glass or porcelain, mica, ice, air at ordinary pressures.
It is remarkable that the best conductors of electricity, that is to say, the
substances which offer least resistance to its passage, for instance the
metals, are also the best conductors of heat, and that insulators made
red hot become conductors. Air is an excellent insulator, and hence we
are able to perform our experiments on frictional electricity in it. We
can also run bare telegraph wires through it, by taking care to insulate
them with glass or porcelain from the wooden poles which support
them above the ground. Water, on the other hand, is a partial conductor,
and a great enemy to the storage or conveyance of electricity, from its
habit of soaking into porous metals, or depositing in a film of dew on
the cold surfaces of insulators such as glass, porcelain, or ebonite. The
remedy is to exclude it, or keep the insulators warm and dry, or coat

them with shellac varnish, wax, or paraffin. Submarine telegraph wires
running under the sea are usually insulated from the surrounding water
by india-rubber or gutta-percha.
The distinction between conductors and non-conductors or insulators
was first observed by Stephen Gray, a pensioner of the Charter-house.
Gray actually transmitted a charge of electricity along a pack-thread
insulated with silk, to a distance of several hundred yards, and thus
took an important step in the direction of the electric telegraph.
It has since been found that FRICTIONAL ELECTRICITY APPEARS
ONLY ON THE EXTERNAL SURFACE OF CONDUCTORS.
This is well shown by a device of Faraday resembling a small butterfly
net insulated by a glass handle (fig. 5). If the net be charged it is found
that the electrification is only outside, and if it be suddenly drawn
outside in, as shown by the dotted line, the electrification is still found
outside, proving that the charge has shifted from the inner to the outer
surface. In the same way if a hollow conductor is charged with
electricity, none is discoverable in the interior. Moreover, its
distribution on the exterior is influenced by the shape of the outer
surface. On a sphere or ball it is evenly distributed all round, but it
accumulates on sharp edges or corners, and most of all on points, from
which it is easily discharged.
A neutral body can, as we have seen (fig. 4), be charged by CONTACT
with an electrified body: but it can also be charged by INDUCTION, or
the influence of the electrified body at a distance.
Thus if we electrify a glass rod positively (+) and bring it near a neutral
or unelectrified brass ball, insulated on a glass support, as in figure 6,
we shall find the side of the ball next the rod no longer neutral but
negatively electrified (-), and the side away from the rod positively
electrified (+).
If we take away the rod again the ball will return to its neutral or
non-electric state, showing that the charge was temporarily induced by
the presence of the electrified rod. Again, if, as in figure 7, we have two
insulated balls touching each other, and bring the rod up, that nearest
the rod will become negative and that farthest from it positive. It
appears from these facts that electricity has the power of disturbing