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Substances in which the positive and negative electricities are combined, being in a neutral state, neither attract nor repel; but there is a numerous class called electrics, in which the electric equilibrium is destroyed by friction: then the positive and negative electricities are called into action or separated; the positive is impelled in one direction, and the negative in another; those of the same kind repel, whereas those of different kinds attract each other. The attractive power is exactly equal to the repulsive force at equal distances, and when not opposed, they coalesce with great rapidity and violence, producing the electric flash, explosion, and shock; then equilibrium is restored, and the electricity remains latent till again called forth by a new exciting cause. One kind of electricity cannot be evolved without the evolution of an equal quantity of the opposite kind: thus, when a glass rod is rubbed with a piece of silk, as much positive electricity is elicited in the glass as there is negative in the silk. The kind of electricity depends more upon the mechanical condition than on the nature of the surface, for when two plates of glass, one polished and the other rough, are rubbed against each other, the polished surface acquires positive, and the rough negative electricity. The manner in which the friction is per
formed also alters the kind of electricity. Equal lengths of black and white ribbon, applied longitudinally to one another, and drawn between the finger and thumb, so as to rub their surfaces together, become electric; when separated, the black ribbon is found to have acquired negative electricity, and the white positive: but if the whole length of the black ribbon be drawn across the breadth of the white, the black will be positively, and the white negatively electric when separate. Electricity may be transferred from one body to another in the same manner as heat is communicated, and, like it too, the body loses by the transmission. Although no substance is altogether impervious to the electric fluid, nor is there any that does not oppose some resistance to its passage, yet it moves with much more facility through a certain class of substances called conductors, such as metals, water, the human body, &c., than through atmospheric air, glass, silk, &c., which are therefore called non-conductors; but the conducting power is affected both by temperature and moisture.
Bodies surrounded with non-conductors are said to be insulated, because, when charged, the electricity cannot escape; but when that is not the case, the electricity is conveyed to the earth, which is formed of conducting matter; consequently it
is impossible to accumulate electricity in a conducting substance that is not insulated. There are a great many substances called non-electrics, in which electricity is not sensibly developed by friction, unless they be insulated, probably because it is carried off by their conducting power as soon as elicited. Metals, for example, which are said to be non-electrics, can be excited, but, being conductors, they cannot retain this state if in communication with the earth. It is probable that no bodies exist which are either perfect nonelectrics or perfect non-conductors; but it is evident that electrics must be non-conductors to a certain degree, otherwise they could not retain their electric state.
It has been supposed that an insulated body remains at rest, because the tension of the electricity, or its pressure on the air which restrains it, is equal on all sides; but when a body in a similar state, and charged with the same kind of electricity, approaches it, that the mutual repulsion of the particles of the electric fluid diminishes the pressure of the fluid on the air on the adjacent sides of the two bodies, and increases it on their remote ends; consequently that equilibrium will be destroyed, and the bodies, yielding to the action of the preponderating force, will recede from or repel each other. When, on the contrary, they
are charged with opposite electricities, it is alleged that the pressure upon the air on the adjacent sides will be increased by the mutual attraction of the particles of the electric fluid, and that on the further sides diminished; consequently that the force will urge the bodies towards one another, the motion in both cases corresponding to the forces producing it. An attempt has thus been made to attribute electrical attractions and repulsions to the mechanical pressure of the atmosphere; it is, however, more than doubtful whether these phenomena can be referred to that cause, but certain it is that, whatever the nature of these forces may be, they are not impeded in their action by the intervention of any substance whatever, provided it be not itself in an electric state.
A body charged with electricity, although perfectly insulated, so that all escape of electricity is precluded, tends to produce an electric state of the opposite kind in all bodies in its vicinity; positive electricity tends to produce negative electricity in a body near it, and vice versâ, the effect being greater as the distance diminishes. This power which electricity possesses of causing an opposite electrical state in its vicinity is called induction. When a body charged with either species of electricity is presented to a neutral one, its tendency, in consequence of the law of induction, is to dis
turb the electrical condition of the neutral body. The electrified body induces electricity contrary to its own in the adjacent part of the neutral one, and therefore an electrical state similar to its own in the remote part; hence the neutrality of the second body is destroyed by the action of the first, and the adjacent parts of the two, having now opposite electricities, will attract each other. The attraction between electrified and unelectrified substances is therefore merely a consequence of their altered state, resulting directly from the law of induction, and not an original law. The effects of induction depend upon the facility with which the equilibrium of the neutral state of a body can be overcome, a facility which is proportional to the conducting power of the body; consequently, the attraction exerted by an electrified substance upon another substance previously neutral will be much more energetic if the latter be a conductor than if it be a non-conductor.
The law of electrical attraction and repulsion has been determined by suspending a needle of gum lac horizontally by a silk fibre, the needle carrying at one end a piece of electrified goldleaf. A globe charged with the same, or with the opposite kind of electricity, when presented to the gold-leaf, will repel or attract it, and will therefore cause the needle to vibrate more or less rapidly