ELECTROPHOR

, or Electrophorus, an instrument for shewing perpetual electricity; which was invented by Mr. Volta, of Como, near Milan, in Italy. The machine consists of two plates, fig. 8, one of which B is a circular plate of glass, covered on one side with some resinous electric, and the other A is a plate of brass, or a circular board, coated with tinfoil, and furnished with a glass handle I, which may be screwed into its centre by means of a socket. If the plate B be excited by rubbing it with new white flannel, and the plate A be applied to its coated side, a finger, or any other conductor, will receive a spark on touching this plate; and if the plate A be then separated, by means of the handle I, it will be found strongly electrified, with an electricity contrary to that of the plate B. By replacing the plate A, touching it with the finger, and separating it again, it will be found electrified as before, and give a spark to any conductor, attended with a snapping noise; and by this means a coated phial may be charged. The same phenomena may repeatedly be exhibited, without any renewed excitation of the electric plate B; the electric power of B having continued for several days, and even weeks, after excitation; though there is no reason to imagine that it is perpetual.

Mr. Cavallo prepares this machine by coating the glass plate with sealing wax; and Mr. Adams, philosophical instrument maker, prepares them with plates formed from a composition of two parts of shell-lac, and one of Venice turpentine, without any glass plate.

The action of this plate depends on a principle discovered and illustrated by the experiments of Franklin, Canton, Wilcke, and Æpinus, viz, that an excited electric repels the electricity of another body, brought within its sphere of action, and gives it a contrary electricity. Thus the plate A, touched by a conductor, whilst in contact with the plate B, electrified negatively, will acquire an additional quantity of the electric fluid from the conductor; but if it were in contact with a plate electrified positively, it would part with its electricity to the conductor connected with it. See an account of several curious experiments with this machine, by Mr. Henley, Mr. Cavallo, and Dr. Ingenhousz, in the Philos. Trans. vol. 66, pa. 513; vol. 67, pa. 116 and 389; and vol. 68, pa. 1027 and 1049.