ELECTRIC MACHINES.

181

the surface of the cushions of the rubbers, unless there happens to be plenty left on from a previous experiment, in which case the surface is to be cleaned by rubbing it with a piece of rough brown paper, or by scraping it with a knife. The rubber, or rubbers are to be then applied, and by means of the adjusting screws, made to press moderately against the surface of the cylinder or plate; on then turning the winch, and holding the hand towards the revolving glass near the lower surface of the silk flap, the electricity will be felt rushing between the hand and glass, like a brisk wind, attended by a crackling sound, and in the dark, by a lambent blue flame. The prime conductor is next placed, in such a manner that its points stand about one eighth of an inch from the glass : on holding the hand towards it, and turning the winch, vivid sparks, often some inches in length, appear; these are attended by a loud snapping noise, and on striking the hand, produce a pungent pricking sensation, often producing a papular eruption on the skin.

263. The development of free electricity upon the prime conductor is so intimately connected with the theory of induction already developed (248), that the remarks there made, will be sufficient to remove all obscurity as to the mode in which it is effected. On turning the glass plate or cylinders, the electricity naturally present in the rubber becomes decomposed, its positive adhering to the surface of the glass, and its negative to the rubber; the positive electric portions of the glass coming, during its revolution, opposite to the points on the conductor, act powerfully by induction upon the latter, decomposing its electricity into the component fluids, attracting the negative, which being accumulated in a state of tension (256), at the points of the conductor, dart off towards the cylinder, to meet the positive fluid, and thus reconstitute the neutral compound; the prime conductor is thus left powerfully positive, not by acquiring electricity from the revolving glass, but by having given up its own negative fluid to

the latter. The rubber is left in a proportionately negative state, and consequently, after revolving the glass for a few minutes, can develop no more free positive electricity, providing the rubber be (as in the cylindric machine) insulated; on this account, it is necessary to make it communicate with the earth, for the purpose of obtaining a sufficient supply of positive electricity to neutralize its negative state. In very dry weather, indeed, the electric machine will frequently not act, until the rubber is connected by a good conductor, not merely to the tube supporting it, but to the moist earth, or, what in large towns is more convenient and preferable, with the leaden pipes supplying the building with water.

264. Much discrepancy of opinion has existed concerning the modus agendi of the amalgam applied to the rubber; it certainly acts very powerfully in increasing the excitation of electricity; the best for this purpose consists of two parts of zinc and one of tin, melted together, and added to six parts of mercury, previously heated in a crucible: the mixture being stirred until cold, is readily reduced to a fine powder, which requires merely to be formed into a paste with lard to be ready for use. It has been, with good reason, supposed that the oxydation of the amalgam, by the friction employed, is essential to the increased excitation; for amalgams of gold, and other difficultly oxydizable metals, do not increase the development of electricity; and, in accordance with this view, Dr. Wollaston found that an electric machine, when worked in an atmosphere of carbonic acid, gave no signs of free electricity. Instead of an amalgam, the deutosulphuret of tin, or aurum musivum, may be rubbed upon the cushions of the machine, and with similar results. This latter substance acts probably like the amalgam, by undergoing oxydation, as by friction it gives rise to the formation of bisulphate of tin; in a similar manner also iron pyrites, by friction, is partly converted into sulphate of iron. The chemical influence of friction, indeed, is more energetic than is usually supposed; even siliceous minerals), as mesotype, basalt, and feldspar, become

ELECTRIC SPARK OR DISCHARGE.

183

partly decomposed, giving up a portion of their alkali in a free

state.

265. When the plate or cylinder of the machine is turned, the rubber communicating to the earth by a metallic chain, if a brass knob, or a knuckle be held towards the prime conductor, a vivid spark darts between them: this spark is usually spoken of as a positive spark, as though it consisted of positive electricity passing from the conductor towards the knob, or knuckle. This, however, is an erroneous expression; for, as the prime conductor is positively electrified, it induces (248) an oppositely electric state in any conducting substance approaching it; and when this state has amounted to one of sufficient tension, the negative electricity rushes towards the positive of the prime conductor, and constitutes the neutral combination. This neutralization, or discharge of the electric state of the conductor is attended by a sharp snapping sound, and flash of light, constituting the electric spark; consequently, whenever an electric spark is seen, it is not to be regarded as arising from the mere passage of free electricity, but of the union of the two electric fluids, and consequent discharge of the electrified body. The sparks of positive electricity said to pass from the excited tube (245), or cover of the electrophorus (254), are of the same kind. From these facts also, we deduce the necessary consequence that all cases of discharge must be preceded by induction.

266. When the prime conductor is connected with the earth, and the rubber of the machine insulated, sparks are seen on approaching the hand, or other conductor, towards it; these are termed sparks of negative electricity, but as erroneously as in the case of sparks from the prime conductor; as they arise from the discharge of the free electricity in the rubber, by its union with the induced positive electricity in the nearest conducting body.

267. If both conductor and rubber of the machine be insulated, and a pointed wire held at a short distance from each, the positive electricity will be seen leaving the wire

ELECTRICITY.

held towards the rubber, in the form of a luminous pencil or brush; whilst the point of that held towards the conductor will be illuminated by a small star of light. These different appearances probably arise from the different degrees of facility with which the two fluids traverse a badly conducting medium, the positive appearing to effect its traverse through the air, with much greater readiness than the negative.

268. If the conductor, or rubber of the electric machine, be connected with each other, or the earth, by means of a continuous conductor, as a piece of wire, the electric fluids will traverse, and discharge take place along it invisibly, unless the machine be extremely energetic, in which case the wire will appear surrounded by a lambent flame. But if the conductor be interrupted, then vivid sparks will appear at each rupture of continuity, arising from inductive action and consequent discharge taking place at every one of these spots.

EXP. (A.) Connect the prime conductor and rubber with each other, by means of a brass chain; on working the machine, vivid sparks will appear at every link.

EXP. (B.) On a plate of glass, paste some strips of tinfoil, having portions cut out, so that the spaces

represent letters; then on communicating the first piece of foil with the conductor, and the last with the ground, the letters will appear

in characters of fire, from a spark occurring at each division of the foil.

EXP. (C.) Draw, on a pane of glass, a serpentine line with varnish, and place on it, before it dries, metallic spangles, about one tenth of an inch apart; on connecting this with the machine, a serpentine line of fire will be represented.

EXP. (D.) If, in a similar manner the spangles are placed on a glass tube in a spiral direction, a fine spiral line of spark will be produced.

269. Induction, and consequent discharge, take place

B

through a greater space in an air-pump vacuum, than under ordinary atmospheric pressures, a circumstance arising from the resisting dielectric medium being removed; this led to the error of considering a vacuum as a conductor of electricity, which is not the case, discharge being effected through it readily, only if the two surfaces be sufficiently near to permit induction to take place, otherwise, electrified bodies can be as well insulated in an air-pump vacuum as in common air. EXP. (A.) A glass tube, two feet in length a, is furnished at either end with a brass ball projecting into its interior, and carefully exhausted of its air, by means of a good air-pump: on connecting its upper end в, with the prime conductor, and its lower end c with the earth, if the machine be turned, в becomes positive, and induces a contrary state on the ball at c, induction taking place with facility in consequence of the atmospheric pressure being removed, and is followed by a discharge of the two electricities in the form of a beautiful blue light, filling the whole tube, and closely resembling the aurora borealis.

C

270. In all these experiments (268-9), it is better to allow the electricity, before passing through the tinfoil, chain, or luminous conductor (269), to acquire some degree of tension; this is conveniently effected by means of an instrument called Lane's electrometer, or more properly, discharger. This con

A

B

sists of a curved arm of varnished glass, B, fixed by a brass leg into the prime conductor A, and terminating in a ball, c, through which passes a rod furnished with

two brass knobs, capable of being placed at any distance from the conductor. If any of the above-described pieces of apparatus be connected with the ball D, electricity will be set in motion through them, as soon as it has acquired a sufficient state of tension to effect a discharge between A and E.