ELECTRICITY, ol 
Instead of these two names, however, positive and negative are now more 
frequently used ; and for convenience, the algebraic signs corresponding 
to these are employed, + and —. The sum of what has been said may 
now be stated thus: Bodies charged with either positive or negative electricity 
attract bodies charged with negative and positive electricity respectively, and 
attract bodies not electrified at all; but they repel all bodies charged with 
electricity of the same kind as ther own; further, electricity can be com- 
municated by contact from one body to another, 
In the above experiment, a particular substance is rubbed with a 
particular kind of cloth, the glass with silk, and the sealing-wax with a 
woollen cloth. When electricity is produced in a substance by friction, 
electricity of the opposite kind is produced in the rubber or substance 
(a cloth generally) with which it is rubbed; and a body, which becomes 
charged with positive electricity when rubbed with one substance, may 
become charged with negative when rubbed with another. Thus, when 
glass is rubbed with silk, the electricity of the former is positive, and 
that of the latter, necative ; but if the glass be rubbed with a cat’s fur 
instead of silk, the electricity of the former is negative, and that of the 
latter, positive. In the following list, each body, when rubbed with the 
one before it in order, becomes charged with negative electricity ; when 
with the one coming after it, with positive—cat’s fur, glass, linen, feathers, 
wood, paper, silk, shell-lac, ground glass. 
It must be remembered that all bodies do not become charged with 
electricity when rubbed ; those that do are called electrics, and those that 
_do not, non-electrics. There is another distinction to be noticed in 
bodies, and that is with regard to receiving the electricity which has 
been produced in an electric; those that do are called conductors, and those 
that do not, non-conductors. Most bodies are electrics, but it makes a 
material difference in this respect whether the body be a conductor or 
not ; because, if the electric be also a conductor, the electricity produced is 
immediately carried off; while, if it be a non-conductor, the electricity 
remains on the surface, and is more apparent. Thus it happens that 
practically the most powerful electrics are non-conductors. The most 
important class of conductors are the metals. Water is also a good con- 
ductor. <A few of the principal non-conductors, and thus, in a certain 
sense, the best electrics, are shell-lac, caoutchouc, amber, resin, sulphur, 
wax, glass, gems, silk, wool, hair, dry paper, leather, camphor, chalk, 
lime. 
We can now explain why the pith-ball, made use of in the-experiment, 
was suspended by a silk thread from a glass stand. Glass and silk are 
non-conductors, and so the electricity communicated to the pith-ball could 
not be conveyed away by these substances, as it would have been if it had 
been attached to a conductor ; as, for instance, if it had been fastened by a 
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