ELECTRICITY. CHAPTER XIII. CONSEQUENCES OF INDUCTION. Leyden Disguised Electricity, 278. Charge and Discharge of coated Dielectrics, 279-80. Charge penetrates Substance of Dielectrics, 201. Jar, 282-5. Insulated Jars cannot be charged, 286. Battery, 287. Residual Charge, 289. Velocity of Electricity, 290. Charge does not reside in the Coating, 291. Discharger, 292. Experiments, with charged Jars, 293—with Battery, 294. Leyden Vacuum, 296. Lichtenberg's Figures, 297. Condensor, 298. Returning Shock, Conversion of Non-conductors into 299. Unipolar Bodies, 300. 278. WHEN two insulated conducting bodies are differently electrified, and approached towards each other, so as to be within the influence of their mutual attraction (258, B), but not sufficiently near to permit of luminous discharge, no signs of electricity are communicated by either to a pith-ball electrometer connected with them, until the bodies are separated to a considerable distance from each other. The electric fluids are thus said to become disguised, or paralysed, by their mutual attractive action. B 47 Exp. (A). Let two plates of tinned iron, AB, a foot in dia meter, be insulated on varnished glass legs, CD, fixed into pieces of wood moving in a groove in the board E. To the backs of each of these plates is soldered a brass wire, furnished with a binding screw, grasping wires, GH, from each of which is suspended a pith ball electrometer. Now separate A and B from each other, and touch one with an excited piece of glass, the other with excited resin, the pith balls connected with each will diverge, one with negative, the other with positive electricity. Gradually approaching the plates, and as their mutual distance diminish, the pith balls will gradually collapse, until a and в are very near to each other, when they will appear totally unelectrified. EXP. (B.) The apparatus being in this state, gradually separate A and B, and, in proportion as this is done, the pith balls will diverge as before, proving that the electric states of the plates had not been destroyed by the previous experiment. 279. These phenomenadep end upon a very simple cause, the attraction of the electricity in a being sufficient to draw all that of the opposite kind in B, from the wire H, into that part of the plate opposite it; whilst the electricity in в acts in a similar manner on that in a; and thus, by their mutual attraction the two fluids are collected into those surfaces of the plates nearest each other, and being, by their attractive influence, retained there, become incapable of action on the electrometer: on separating A and B, this attractive influence decreases (258), and the electricity, being diffused over the surfaces of A and B, act upon the electrometer connected with them. The two electric fluids cannot unite by luminous discharge, until a and в are very close to each other, and then, or by making the communication with a curved wire, they unite, and mutually neutralize each other, producing a true discharge. Next, remove all free electricity from both A and B, bring then within one sixth of an inch from each other, and touch a with an excited glass tube; it thus becoming positively electrified, acts by induction on the electricity in B, attracting its ne COATED DIELECTRICS. 193 and repelling its positive, which, running up the wire H, reaches the pith-balls and causes them to diverge. Touch B with the finger, and the positive electricity thus separated by induction, will escape, leaving в negative; its electrometer cannot diverge, because its negative fluid is retained in the surface opposed to A. Separate A and B, both electrometers will indicate free electricity of an opposite kind in each; again approach them and the pith balls will as before collapse. Then connect A and B, by a curved wire, and the two fluids will rush together, producing a luminous discharge. In this experiment we have the second plate в, becoming negatively electrified through air as a dielectric, and this plate of air is said to be charged, its particles, lying between A and B, becoming polarised, and arranged as required by the theory of induction; the latter force being necessarily and solely exerted between contiguous particles. The plate of air thus becoming charged, may be discharged and reduced to its primitively unelectrified state, in two modes; first, by gradual and silent, secondly, by explosion and sudden discharge. The conditions for producing the first, are fulfilled by merely leaving the instrument (278) exposed to the air for a sufficient space of time, gradually the electricities in the two plates combine, and the separating dielectric air is necessarily discharged; for the second mode, all that is necessary is to connect the plates A and в by means of a curved wire or other conductor, the free electricities then combine, suddenly producing a luminous discharge. 280. Any other dielectric may be substituted for air in these experiments, and if a plate of glass or resin be used, the electricities accumulated in its two surfaces may be increased to a very considerable degree of tension. EXP. (A.) Place a large pane of glass, about fourteen inches square, between the two plates of the apparatus (278), and bring A and B so near to each other as to tightly grasp the pane. Connect A with the prime conductor of the electric machine, and work the latter so as to render a powerfully positive this will act by induction through the pane of glass, on the electricity naturally present in B, as before (279), repelling its positive, which, on approaching the hand to the pack of B, will produce a series of sparks, or discharges (268). After a certain time, these will cease; then remove the wire connecting a to the prime conductor, and leave it insulated; the plate a will then be charged with positive, and в with negative electricity, both in a state of high tension; connect the two plates by means of a curved wire, and discharge, arising from the union of the two electric fluids, results, attended with a vivid flash of light and a loud snap. If, instead using a curved wire, the plates be connected by the fingers of both hands, the same discharge ensues, accompanied by an exceedingly disagreeable and painful sensation, well known as the electric shock. EXP. (B.) Instead of placing a pane of glass between the two metallic plates, coat it on each side with a piece of tinfoil, leaving about one inch and a half all round uncovered; then on connecting one side with the conductor of the machine, and the other with the earth, the glass dielectric will become charged as before, that side connected with the conductor containing positive, and the other negative electricity. 281. The charge, thus communicated to the plate of glass, penetrates its substance to a certain distance, as was first pointed out by Mr. Henley. EXP. (A.) Coat two thin pieces of window-glass on one side only with a piece of tinfoil, considerably smaller than the glasses; place these together, with their uncoated sides in contact. Charge this double plate (280) as before, and then attempt to separate them, they will be found to adhere very tightly together; on pulling them asunder, the naked side of that plate which had been connected with the conductor will be found positively, and that of the other plate negatively electrified. 282. Induction, and subsequent charge, does not become materially modified by the figure of the glass, its thickness only influencing these actions, and as the plate is a very inconvenient piece of apparatus, glass jars or bottles coated with some conductor, are almost universally substituted for it. This, indeed, was the first arrangement used, forming the celebrated electric or Leyden phial, so called from the place of its discovery, by Cuneus or Muschenbroek, in 1700. Green or white glass answer almost equally well for the construction of electric jars; wide-mouthed glass jars are very convenient, but on account of their expense, common winebottles may be very conveniently substituted, provided they are free from air-bubbles, and specks of unvitrified matter. B 283. The ordinary Leyden phial, or jar, consists of a glass bottle of any size, coated internally and externally with tinfoil to about three inches from its mouth; the latter is closed by a dry and varnished cork, or wooden disc A. A stout brass wire, furnished with a ball of the same metal, passes through the cover A, and has several thin pieces of wire, or a chain fixed to its end B, so as to touch the inside coating in several places. The knob, thus corresponds to the internal coating. When narrow-mouthed jars or bottles, as the common sixteen ounce phials of white glass (which from their thinness form excellent electric jars,) are used, it is better to coat them internally with brass filings, instead of tinfoil, on account of the difficulty of applying the latter to their interior; for this purpose some thin glue should be poured into them, and the bottle turned slowly round, until its inner surface is covered to about three inches from the mouth. Brass filings are then put in, and the bottle well shaken, so that they may be diffused equally over its surface; on inverting it, those which are in excess will fall out, and t hebottle will be left coated internally, sufficiently well for its intended purposes; some jars should always be provided with hooks, instead of knobs as it is requisite frequently to suspend them to the conductor. To prevent the too rapid |