If a conductor not insulated be brought within the atmosphere (that is the sphere of action) of any electrified body, it acquires the electricity opposite to that of the electrified body, and the nearer it is brought, the stronger opposite electricity does it acquire, till the one receive a spark from the other, and then the electricity of both will be discharged.
The electric substance which separates the two conductors possessing these two opposite kinds of electricity, is said to be charged. Plates of glass are the most convenient for this purpose, and the thinner the plate the greater is the charge it is capable of holding. The conductors contiguous to each side of the glass are called their coating.
Agreeably to the above-mentioned general principle, it is necessary that one side of the charged glass have a communication with the rubber, while the other receives the electricity from the conductor, or with the conductor, while the other receives from the rubber.
It follows also, that the two sides of the plate thus charged are always possessed of the two opposite electricities; that side which communicates with the excited electric having the electricity of the electric, and that which communicates with the rubber, that of the rubber.
There is, consequently, a very eager attraction between these two electricities with which the different sides of the plate are charged, and when a proper communication is made by means of conductors, a flash of electric light, attended with a report (which is greater or less in proportion to the quantity of electricity communicated to them, and the goodness of the conductors) is perceived between them, and the electricity of both sides is thereby discharged.
The substance of the glass itself in, or upon, which these electricities exist, is impervious to electricity, and does not permit them to unite; but if they be very strong, and the plate of glass very thin, they will force a passage through the glass. This, however, always breaks the glass, and renders it incapable of another charge.
The flash of light, together with the explosion between the two opposite sides of a charged electric, is generally called the electric shock, on account of the disagreeable sensation it gives any animal whose body is made use of to form the communication been them.
The electric shock is always found to perform the circuit from one side of the charged glass to the other by the shortest passage through the best conductors. Common communicated electricity also observes the same rule in its transmission from one body to another.
It has not been found, that the electric shock takes up any sensible space of time in being transmitted to the greatest distances.
The electric shock, as also the common electric spark, displaces the air through which it passes; and if its passage from conductor to conductor be interrupted by non-conductors of a moderate thickness, it will rend and tear them in its passage, in such a manner as to exhibit the appearance of a sudden expansion of the air about the center of the shock.