2. The mechanical action of bodies up on each other produce electrical effects. If two metals or other conductors be brought into contact, and separated, or if they be pressed or rubbed together, electric signs are produced ; and the same consequences follow, if one or both the bodies be non-conductors : but the elec tricity is more manifest where the non conducting property prevails. When non conductors are broken or torn asunder, the surfaces which were before in contact are found to be in opposite electric states; and this difference is so considerable in Muscovy talc, that bright sparks pass be tween them. From these facts, there is ground to suspect, that the opposite elec tric states prevail amongst the parts of bodies, and may perhaps be in some man ner concerned in the general attraction they exert upon each other.
3. The electricity in our common ma chines is produced by the friction of a conducting body against a non-conductor.
See MACHINE, electric.
The non-conductor may be a tube, a globe, a cylinder, or a plate of glass, and the conducting rubber is usually a cu shion, upon which a mixture of the amal gam of zinc with a little tallow has been smeared. It is found to be a condition, that atmospheric air should he present ; and if the electricity he taken off from the surface of the cylinder while it re volves, the cushion will not restore or supply the electric state, unless it be ad mitted to communicate with the earth. So that, if an insulated conductor be placed near the cylinder, it will receive electricity for a time, though the rubber be also insulated ; but the rubber itself, after assuming the negative state, will soon cease to give any more electricity to the cylinder, than the little it may obtain from the imperfect nature of its insula tion. But if a communicating branch from the positive conductor be brought within a short distance of the negative cushion, the positive sparks will fly through the interval, and supply the cu shion; and in this manner the circulation of electricity may, as far as yet has been determined by experiment, be kept up for an unlimited time. It seems, there fore, as if a chemical process requiring atmospheric air, and therefore of the na ture of combustion, were carried on at the face of the cushion, and that a pecu liar substance, on which the electric state depends, becomes deposited or disposed in a different manner from that which it possessed before ; and that the relative motion of the non-conducting body car ried it off to a situation where it tends to its firmer state, and consequently advan ces in a current towards such parts as al.
low of the restoration of that state. It seems reasonable to conclude, that the disturbances of the electric state or equi librium, and the currents by which they are restored, are in most natural opera tions performed through very short and good conductors; so- that, though in all probahiliiy they may contribute to very important results, the immediate changes elude our observation, except in a few instances, such as that of lightning and luminous meteors. And it seems from the facts to be nearly decided, that we should never have had it in our power to exhibit the plmnomenon of the electric spark, which is electricity producing ig nition by breaking through a noncon ductor, if we had not fortuitously experi mented in circumstances, where the elec tricity is first made to take the form of a charge, and afterwards brought into a state of considerable intensity, by sepa rating those bodies from each other, which produced the compensation by their opposite states. Thus in the elec trical machine, (see Nicholson, in the Philos. Trans. 1789,) little or no electric signs are produced by a cylinder ribbed by a very flat amalgamed leather, termi nating in a neat line of contact. But this rubber and cylinder will, without any al teration, afford electricity, if a flat piece of metal, or the hand, or any other flat conductor, be held over that part of the cylinder which is in the act of receding from the cushion, even though this con ductor be held at the distance of an inch or more, without touching either the cylinder or its rubber. It is proved from experiment, that the conducting body thus presented acquires the opposite state, and enables the cylinder to carry off a greater quantity of electricity in the form of a charge, the interposed air being the electric. When the cushion is thick and round ed, as is the case with the human hand, which was first used for this purpose, the rounded part opposite the receding sur face of the cylinder, performs the office of compensation ; and the best applica tion, which has yet been made for this purpose, is that of a flap of silk proceed ing from beneath the cushion, which as sumes the negative state, so as to com pensate the positive state on the cylinder, in a very considerable charge, which is conveyed by the rotation to the farther end of the silk, where it becomes uncom pensated electricity upon the naked sur face, at an intensity which could not otherwise have been produced.