ELECTROPHORUS. This generally consists of a tin mold filled with shell-lac, and a movable metal cover, with a glass handle. The shell-lac is poured in when melted, and it is mixed with some other substance, to make it less brittle. Five parts of shel lac, one of wax, and one of Venice turpentine, is given as a good mixture. When used, the surface of the cake of shell-lac is smartly beaten with a cat's fur or foxtail. The cover is then put on, and touched with the finger, which receives a slight spark of electricity,just before contact takes place; and after the finger is removed, the cover, when lifted by its insulating handle, gives a brisk spark of + electricity to anything presented to it. This can be repeated for several minutes without any apparent exhaus tion of the source of electricity; and in dry weather, sparks can be got in this way hours, and frequently days, after the cake has been beaten.
The action of the E. may be thus accounted for. When the surface of the cake of shell-lac is beaten, the friction excites electricity on it. This acts inductively all round, but the tin mold being the nearest conductor, and shell-lac a good dielectric, the induction becomes concentrated on it, + electricity becoming fixed on the side next the shell-lac, and electricity being sent to the ground. The electricity of the upper surface of the shell-lac is thus fixed by the + electricity of the mold. When the cover is put on the cake, the contact between the two is not sufficient to allow the latter to communicate its charge to the former. The cover is thus acted on inductively, not conductively. The electricity of the cake, then, has the choice of two channels for
its induction, either through the cake to the mold, or through a very thin film of air to the cover. The latter, from its offering so short a passage through the dielectric, has the preference, and the inductive action of the charge is diverted from the mold to the cover, and the + electricity on the other side of the cake is thus liberated and lost in the ground. The cover being strongly polarized, + electricity is induced and fixed on its lower surface, and electricity on its upper, this last being transmitted to the ground by the finger. When the finger is withdrawn, and then the cover, the + elec tricity of the latter is free to discharge itself by spark, and inductive action again takes the direction of the mold, once more attracting + electricity to it. The induced polarity of the cover is attended with no loss to the charge of the shell-lac, which can thus continue to act with the same efficiency. The loss of electricity that all charged bodies experience in air, and especially when moist, at length discharges the cake, but this takes place all the less readily, that when the electricity is not needed to act on the cover, it is kept bound by the + electricity induced by it in the mold. In order that the + electricity of the mold should have liberty, so to speak, to come and go, the E. must not be insulated; and when it is so, the action on the cover is feeble, if at all per ceptible.