ELECTRICITY.
Most of the phenomena which Electricity pre sents have been described at large in the ENCYCLO P.uDIA; and the experimental part of the science remains nearly in the same state as at the publi cation of that work. This is not the case, however, with the theoretical. The theory was then found ed on suppositions more or less doubtful ; on ingeni ous but contracted views of the subject; and rather on empirical relations among the phenomena, than on calculations rigorously mathematical. It is to supply this defect, that we must especially devote ourselves in this SUPPLEMENT, and happily the pro gress of science affords ample materials.
To proceed methodically in the concentration of the electrical phenomena, let us recal successively all the general laws in which they are recorded, and which are established by experiment. The first con sists in the mutual attraction or repulsion which electrified bodies exert, or seem to exert, towards each other. These properties are exhibited when we elec trify a tube of glass or of resin, by rubbing it on a woollen cloth, and then touch with this tube small and light balls of wood, or the pith of the elder, sus pended in a dry state of the air by threads of silk equally well dried. In this case, the silk prevents the escape of the electricity, and the air also pos sesses the same faculty. The little balls being only in contact with insulating bodies, are thus protected from any waste of their power. They are what we term insulated. But the moment the electrical pro perty is communicated to them, they mutually fly asunder, and, contrary to the tendency of gravity, re cede from the vertical; precisely as if the electricity which attaches itself to their surfaces had determined them to repel each other.
The result is alike, whether the little balls have been touched with the tube of glass, or with that of resin, provided both are touched with the same. But, if one of them be touched with the tube of resin, and the other with the tube of glass, these two tubes having been both rubbed on a rubber of the same nature, then the two balls approach each other, contrary to the tendency of gravity, as if by a mutual attraction. This result being opposite to
the first, obliges us to distinguish two modifications of electricity different from each other, at least in the apparent effects which they produce. These are what have been termed positive and negative electricity. We shall not employ these terms on account of their al ready presenting to the mind the ideas of addition and subtraction, which are really hypothetical, since they go beyond the facts. To express the distihction be tween the two kinds of electricity, we shall name them according to the method which serves to de velope their influence. We shall call that vitreous electricity which a tube of glass exhibits when rub bed on a rubber of wool, and that resinous electricity which is obtained on rubbing upon the same rubber a tube of resin; though either of these electrici ties could be yet produced by some other proceed ing different from what we have indicated in this de finition.
We must now seek, by experiment, for the law according to which these attractions and repulsions operate, that is to say, what their relative intensity may be at different distances. The separation of the little balls furnishes an index of this; but it is not sufficiently delicate, and, as we shall soon see, too complicated, to measure it with exactness. The same may be said of all the contrivances in which weights so small as they must necessarily be, are made to balance the attraction or the repulsion. The torsion of metallic wires is the only force suf ficiently minute, sufficiently constant, and whose effect can be estimated with sufficient ease, to be em- • ployed with advantage in measurements of this kind. Such is the object of the electrical balance of Cou lomb, described in the Encyclopaedia under the • word Electrometer, and represented in the annexed figure. (1.) In what follows, we shall suppose the reader to have that description before him.