ELECTRICITY.

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like a plate of glass. This coated plate seems to have had a great-er power than a glass plate of the same dimensions, even when theweather was not very drv: and if it is not subject to break veryeasily by a spontaneous discharge, it may be very conveniently used;for it doth not very readily attract moi-ture, and consequently mayhold a charge of electricity better and longer, than glass: besides, ifbroken, it may be repaired by ahotironjwhichisnotthecasewith glass.For discharging a jar, battery, or any coated electric without re-ceiving the shock, two instruments have been contrived, one call-ed the common Discharging Rod, (Plate I.Kill. fig. :>,) whichis nothing more than a semi-circular brass wire, furnished wit 1 obrass balls, one at the end of each wire. The other, which is o>very extensive use in electrical experiments, is called the JointedDischarging ltod, (tig. (j.) it is furnished with a glass handle:the legs are moveable, and may be set to any distance that may heconvenient. When the electrician is to use this instrument, heholds it by the handle; and touching one of the coated sides of thecharged electric with one knob, and approaching the other knobto tiie other coated side, or some conducting substance communi-cating with it, he completes the communication between the twosides, ami thus discharges the electric. Henley’s universal dis-charger consists of two wires, supported by glas> feet, with a smalltable between them to support any thing that may be the object otthe experiment. It is found to be a very useful instrument for avariety of purposes, and is composed of the following parts: A(tig. 7,) is a Hat board, about fifteen inches long, four broad, andone tliick. 1U5 are two glass pillars, cemented in two holes uponthe hoard A, and furnished at lop with brass caps, each of whichhas a turning joint, and supports a spring tube, through which thewires DI) slide. Each of tiie caps is composed of three pieces ofbrass, connected so that the wires DD, besides their slidingthrough the sockets, have a horizontal and vertical motion. Eachof the wires DD is furnished with an open ring at one end, and atthe other it has a brass ball, which, by a short spring socket, isslipt upon the po uted extremity, and may he removed. E is acircular piece of wood, having on its surface a slip of ivory inlaid,and furnished with a foot, which is fastened in the middle of thebottom A. The instruments for measuring the quantity, andascertaining the quality, of electricity, are called Electrometers ,The simplest of all is a pair of little balls of cork, or rather of thepith of the elder, which is still lighter, and suspended by silk andsometimes bv linen threads. When brought into contact with anelectrilied body, the halls will immediately diverge, and accord-ing to the degree of divergence, a judgement is formed of the de-gree in which the body in question is electrilied. A similar effectwill be produced by a light and downy feather. The pith-hallshave received the name of Canton’s electrometer, from the inge-nious electrician who first employed them for this purpose, andthey are found useful in many experiments. See fig. 28. For amore accurate admeasurement, Henley’s quadrant electrometer isin most general use. It consists of a perpendicular stem formedat top like a hall, and furnished at its lower end with a brass feruleand pin, by which it may be fixed in a hole made in the conduc-tor, or at the top of a Leyden bottle. To the upper part of thestem, a graduated ivory semicircle is fixed, about the middle of whichis a brass artn or cock, to support the axis of the index. ’I'lle indexis a very slender stick, which reaches from the centre of the gra-duated arch to the brass ferule ; and to its lower extremity is fas-tened a small pith-ball nicely turned in a lathe. When this elec-trometer is in a pcr|>endicular position, and not electrified, the in-dex hangs parallel to the pillar; hut when il is electrified, the indexrecedes more or less, according to the quantity'of electricity. Seefig. 8. Lam’s discharging electrometer is employed ehielly bv thepractitioners of medical electricity- It is represented by (Dll, infig. 9, ami is fixed to the wire that proceeds from the inside of tiiejar Z. The ball l> touches the prime conductor A, which is sup-posed to stand before an electrical machine. The electrometerconsists in a glass rod FD, furnished with brass caps I 1 ’, D : fromthe latter proceeds a strong brass wire, to which is attached an ho-rizontal spring-socket: through this, the wire CE, having the ballC at one end, and the open ring E at the other, may he slictedbackwards and forwards, so as to "place the ball C at any re-quired distance from (lie ball II. Suppose the distance be-tween B and C to be one-eighth of an inch or less, andthat by means of a chain X, a communication be formed from E

to the outside coating of the iar. When the jar is charged so highthat the fluid can leap from 15 to C, the discharge will take place,and the slroke pass through X from the inside to the outside of thejar. When the shocks are to be given through any part of thebody, as the arm, then, instead ot' the chain X, wires EL and ILare to be fastened, one to the ring E, and the other to a book at [of the stand HI which communicates with the outside of the jar.The other extremities of (lie wire, must he fastened to the directorsK, L, which consist of brass wires and halls as L fastened intoglass handles as shewn at K. The mode of operation is easily-understood from a view of the figure, for when the chain X isaway, the electric fluid must, it, passing from the inside of the jarto the outside, go through the wire EL; the brass of the director;the part ot the arm between the balls of the directors ; and throughthe wire LI. By m ving C nearer to, or farther from B, tiiestrength of the shocks will be regulated to any degree. If a plateof glass be laid upon an uninsulated plate of metal, the upper sur-face may he rendered electric, by friction, or by applying an elec-trified hotly successively to its parts. Part of this electricity may-be taken oil' by touching the upper surface with an uninsulatedmetallic plate of the same dimeiidons as that upon which the glassis placed. After this, if a plate ot metal, to which a glass handleisallixed, he laid upon the glass, this small quantity of electricitywill inlluence the metal, ami, without actually communicating theelectric matter, will cause it to exhibit a similar slate. If this hetaken off, by drawing the spark, and the metal then removed, bymeans of the glass handle, it will be found posse.sed of the con-trary slate of electricity, and another spark may be obtained. Themetallic plate may be then agicn applied to the surface of theglass, ami the process again repeated, and so on for a great numberof limes, without any sensible difference in the event. For theelectricity at the surface of the glass being almost in the naturalstate, as to condensation, does not disappear for a very long lime,and on tiie very near approach of the metal it causes the iluid in themetal to recede to one side, ami thus makes it capable of giving orreceiving the spark according as the glass is pos.tive or negative ;and hence when it is removed, it is electrilied without having af-fected the excited electric. It consists of two plates, (tig. to,)made of a circular form, from eight to eighteen inches in diameter,or upwards. The upper plate is generally made of bras.-,; hut atin plate, with a wire turned in upon its edge, will answer exceed-ingly well. At the centre of this plate a glass handle nine or teninches long is fixed. A thin hoard, covered with tin-foil, andsuspended by silken strings, w ill answer w ell, when the elcctro-phorus is wanted of a large diameter. The under plate may hemade of glass, sealing-wax, or the following composition, viz. fourparts rosin, three parts pitch, three parts shell-lac, two parts Ve-nice turpentine, melted together over a gentle fire. It mtrv hepoured and spread upon a thin linen cloth, to about one-fourth ofan inch thick. The linen rloth must be stretched upon a hoop,and made as tight as possible. If the surface is a little rough, itwill he no worse. The manner of using this machine is as fol-lows: Rub the coated side of the under plate with tine new llan-nel, or a hare’s or cat’s skin ; and when it. is excited as much aspossible, set it on a table, ami place the upper plate upon it, andput your linger on the upper plate ; then remove your linger, andtake hold of the topsrf the glass handle, and apply it to the knobof a coated jar. Repeat this operation for 30 or 40 times, and thejar will become charged. Cavallo mentions one of the abovekind made by him, with w hich he charged a coaled phial severaltimes, by once exciting, so strong as to pierce a hole through acard at every discharge. In fig. 1, l’Ulte LX11I. A represents theglass cylinder of a machine, turned by P, the handle. Its surfacerubs against C, the cushion, from which proceeds D, the silk flap,and the electricity is conveyed to E, the prime conductor. Thesupports of the cylinder, the cushion, and (indispensably) of tbeprime conductor, are made of glass or baked wood, iu order thatthe electricity may not he conveyed to the earth, unless when theoperator chooses to make the communication bv some conductingbody. A prime conductor is sometimes applied to the rubber.Fig. !i thews the plate machine, with Nicholson’s cylinder im-provements for changing the two states at pleasure, as adapted byDr. Van Martini, of Haerlem . 'flic glass plate GG, K fastenedto the axis UB, by means of a screw on the axis passing througha hole in the centre of the plate, and secured by a nut C, on the

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