In most cases of friction, the nature of the rubbing and rubbed surfaces determines the kind of E. which each assumes. Thus, if glass be rubbed by a cat's fur instead of silk, its E. is instead of In the following list, each body, when rubbed by any one preceding it, is negatively electrified ; by any one succeeding it, positively: cat's fur, smooth glass, linen, feathers, wood, paper, silk, shell-lac, ground glass. When two pieces of the same material are rubbed together, the colder or smoother becomes positively excited. Metal filings rubbing against a plate of the same metal determine -- E. in themselves, and + E. in the plate. When a white silk ribbon is rubbed by a black one of the same texture, the white one becomes +. A plate of glass becomes -I- when a stream of air is directed against it from a pair of bellows. The friction caused by steam of high tension issuing from a narrow pipe develops electricities in the steam and pipe which depend on the material of the latter. This fact has been turned to advantage by Armstrong in the construction of a boiler electrical machine of immense power.
E. has the power of inducing the bodies in its neighborhood to assume a peculiar electrical condition; this is exhibited in the following simple way: A brass cylinder, rounded at both ends, is insulated on a glass pillar. Two balls, hung by cotton threads, are attached at either extremity. When a glass tube is briskly rubbed, and placed within a few inches of the end of the cylinder, the balls at each end diverge, showing that each pair is charged with similar electricities. When the glass tube is withdrawn, the balls hang down as before, so that the electrical excite ment of the cylinder is merely temporary, and dependent on the proximity of the excited tube. If, while the balls are apart, a proof plane, consisting of a small disk of gilt paper insulated at the end of a glass rod, be made to touch the end next the tube, and then transferred to an electrometer, the E. is found to be ; if the same be done at the other end, it is +. The nearer end of the cylinder is thus induced by the + E. of the glass to assume the negative electric state; and as no E. can be excited without as much + E., we find the other end positively electrified to the same extent. It appears, besides, from the positions taken up by them, that both electricities observe the same attractions and repulsions as the bodies affected by them. This action of the E. of the tube inducing in the cylinder this peculiar electrical condition, is called induction; and the cylinder in this state is said to be is, to have its poles or ends like a magnet, each having its similar, but relatively opposite force. If the hand touch the cylinder, the balls next the tube diverge further than before, and the other two cease to be affected. In this case, electrically speaking, the cylinder is a portion of the ground, for the hand and body are conductors; the ground is thus brought nearer, more E. appears, and the + E. is lost in the spark with the hand.
The E. is kept fixed in the part of the cylinder opposite the tube by the + E. of the latter; and when the -first removed, and then the tube, it causes the balls at both ends to diverge permanently. We thus that E. can be produced and insulated in conductors by the action of free E. on them. The + E. of the further half of the cyl inder is as free and insulated as if no E. existed within it. This is shown by placing a cylinder near the first, forming a continuation of it, as it were, without touching, when the second cylinder, under the induction of the + E. of the first, is thrown into the-same state as the first. This second can induce the same state in a third, and so on. As the excited tube is withdrawn, the whole series return to their natural condition without being in any way permanently affected. The moment, however, it is again brought near, there is manifested at the further termination of the last a +E., which exerts the same influence there as if a portion of the E. of the tube had been actually communicated or transferred to it..
The air intervening between the tube and the cylinder is termed the dielectric, for it is through it that the electric action is propagated. In proof of this, we have only to place a cake of shellac between the' tube and cylinder, when the polarity of the cylin -der will rise higher than before, as would be shown by the further divergence of the balls; and if this or a similar experiment be conducted with sufficient care, we find that the inductive action varies in amount for each non-conductor. Induction, there fore, we have to conclude, is not the direct action of one body on another, but an action transmitted through, or possibly residing in, the medium between them. In further proof of this, Faraday, who was the first to examine the function of the dielec tric in induction, has shown that the action takes place through air in curved as well as in straight lines, which implies the action of an intervening medium. The relative powers of different substances in facilitating induction, are termed by this philosopher their specific inductive capacities. The following table by sir W. S. Harris gives the .specific inductive capacities of the more important non-conducting substances, taking that of air as unity: Air, 1.00; resin, 1.77; pitch, 1.80; beeswax, 1.86; glass, 1.90; sul phur, 1.93; shellac, 1.95; indiarubber, 2.8. All gases, whether simple or compound, have the same inductive capacity, and this is not affected by temperature or density. If a large plate of metal be placed between the glass tube and the cylinder, the polari zation of the cylinder instantly vanishes, for the induction is diverted by it into the .ground.