274

ELECTRICITY

the consequence of which was, that the electric effects were incom-parably greater than those produced by a single instrument.”

Several in truments have been invented for ascertaining thestate of the atmosphere with respect to the quantity of electricityit may contain, at any given time. The best of them are improve-ments upon Canton’s electrometer, which is made by inclosingthe pith-balls, or rather two slips of gold leaf, in a cylindrical glassvessel, to prevent their being affected by the wind. Bennet’selectrometer, Plate LXIY. tig. 18, is a very delicate instrument,and capable of distinguishing small quantities of electricity. Itconsists of two slips of leaf gold A, suspended in a glass B. Thefoot C may be of wood or metal; the cup D of metal. The cupis made flat on the top, that plates, books, evaporating water, orother tilings to be electrified, may be conveniently placed upon it.The cup is about an inch wider in diameter than the glass, and itsrim about three quarters of an inch broad ; which hangs parallel tothe glass, to turn off the rain, and keep it sufficiently insulated.Within this is another circular run, about half as broad as theother, which is lined with silk or velvet, and fits close upon theoutside of the glass; thus the cap fits well, and may he easilytaken off to repair any accident happening to the leaf gold. With-in this rim is a tin tube, hanging from the centre of the cap, some-what longer than the depth of the inner rim. In the tube a smallpeg is placed, and may be occasionally taken out. To the peg,which is made round at one end, and fiat at the other, the slips olleaf gold are fastened with paste, gum-water, or varnish. Theseslips suspended by the peg, and tli t in the tube fast to the centreof the cap, hung in the middle oi the glass, about three inches long,and a quarter of an inch broad. On one side of the cap there is asmall hole to place wires in. It is evident, that without the glass,the leaf gold would be so agitated by the least motion of the air,that it would be useless: and if the electricity should be commu-nicated to the surface of the glass, it would interfere with the re-pulsion of the leaf gold; therefore two long pieces, II II, of tin-foil are fastened with varnish on opposite sides of the internal sur-face of the glass, where the leaf gold may be expected to strike,and in connection with the foot. The upper end ol the glass iscovered and lined with sealing-wax as low as the outermost rim, tomake its insulation more perfect. The sensibility of this instru-ment is so great as even to astonish the most experienced electri-cians who have been witnesses of its etfeets. The brush of a fea-ther, the throwing of chalk, hair-powder, or dust, against its cap,evince strong signs of electricity. The electricity of vapour is ele-gantly shewn by pouring a tea-spoonful of water on an ignited coalplaced in a metallic cup upon the cap of this electrometer; and avery great and pleasing variety of other experiments may be madewith this excellent instrument. In performing several atmosphe-rico-electrical experiments about the year 1776, I found,” saysMr. Cavallo, “ that the use of Canton’s ..cork-ball electrometerwas much obstructed by the wind, in consequence of which I at-tempted to inclose it in a bottle, and after a variety of trials andalterations, the instrument was, in the year 1777, brought to thestate which is represented in fig. 19- The three parts of thefigure represent the instrument in its case, the same out of the case,and a section of its brass cap and neck. CDMN is an open glassvessel narrower at top than at bottom, and cemented into thewooden piece A B, by which part the instrument is held when itis to be presented to the atmosphere, or it may be rested upon atable for other experiments. This wooden pieoe also serves toscrew the instrument into its wooden case O. The upper part olCDMN is tapering like the neck of a phial; and a short glasstube is cemented into it, so as to project a little above, and a littlewithin the neck of the former: then the upper part of the instru-ment from C D to L, is covered with sealing-wax, by means olheat, which gives it the appearance of one continuate body. Theinner part G of the small glass tube is also covered with sealing-wax. into this tube a brass wire is cemented, the lower part Hof which is flattened, and is perforated with two holes; the upperpart L is formed into a screw, upon which the brass cap E F isscrewed. The office of this cap is to defend the upper part of theinstrument from the rain. The conical, or oval, or globular, corksl 5 ot this electrometer, are as small as can be made, and are sus-pended by exceedingly line silver wires the upper parts of whichare formed in rings, which pass through the holes at II, and arethereby so loosely suspended, .that they are caused to diverge

when the brass cap E is exposed to a very slightly electrified at-mosphere. I M and K. N are two narrow slips ot tin-foil stuck tothe inside of the glass, and communicating witli the wooden bot-tom AB; they serve to carry off that electricity, which, whenthe corks touch the glass, is communicated to it, and if accumu-lated would disturb the free motion of the corks.” Mr. Cavallo’selectrometer for rain is represented in fig. 17. Of this he givesthe following description: “ In principle it is nothing more thanan insulated instrument to catch the rain, and by a pith ball elec-trometer, to shew the quantity and quality of its electricity. A BCl is a strong glass tube about 2£ feet long, having a tin funnelD E cemented to its extremity, which funnel defends part of thetube from the rain. The outside surface of the tube from A to Bis covered with sealing-wax ; so also is the part of it which is co-vered by the funnel. F D is a piece of cane, round which brasswires are twisted in different directions, so as to catch the raineasily, and at the same time to make no resistance to the wind.This piece of cane is fixed into the tube; and a slender wire pro-ceeding from it goes through the bore of the tube, and communi-cates with the strong wire A G, which is thrust Into a piece of corkfastened to the end, A, of the tube. The end, G, of the wireA G, is formed in a l ing, from which I suspend a more or lesssensible pith ball eleetrometei as occasion requires. This instru-ment is fastened to the side of the window-frame, where it is sup-ported by strong brass hooks at C B; which part of the tube is co-vered with a silk lace, in order to adapt it belter to the books.The part F C is out of the w indow, with the end F elevated a littleabove the horizon. The remaining part of the instrument comesthrough a hole in one of the lights of the sash within the room, andno more of it touches the side of the window than the part C B.When it rains, especially in passing showers, this instrument,standing in the situation above described, is frequently electrified ;and, by the diverging of the electrometer, the quantity and qualityot the electricity of the rain may be observed without anv dangerot mistake. With this instrument I have observed, that the rain isgenerally, though not always, electrified negatively ; and some-times so strongly, that 1 have been able to charge a small coaledphial at the wire A G. '1 his instrument should he fixed in such amanner that it may he easily taken off from the w indow, and re-placed again as occasion requires; for it will be necessary to cleanit very often, particularly when a shower of rain is approaching.”One of the best electrometers, if not the very best hifi.erln invent-ed, for measuring artificial electricity, is that of Mr. A. Brook ofNorwich. It is acknowledged by eh ctricians to he constructedon the only true principle on which machines for rnea-nring thequantity of electricity can he made. The mere attrat tion of anylight body shews indeed that the substance which attracts it iselectrified; but this property is by no means calculated to disco-ver the comparative strength of it, on account of its continual va-riation. Thus, if we hold any body within the electrified atmo-sphere of another, though it be first attracted pretty strongly, yetthat attraction will be constantly diminishing, and at la-1 changedinto a repulsive power; but the latter, alter it has once takenplace, continues invariable as long as any degree ol electric chargeremains. Fig. 20, and 21, Plate I.XIV. represent an electro-meter, or rather two, nearly similar to Mr. Brook’s. These twoinstruments are used either conjoined or separately. 'I he armsFM Jk, fig. 21, when used, are placed as much as possible outof the atmosphere of a jar, battery, prime conductor, Ac. Thearm F M, and the ball K, are made of cupper, and as liirlit as pos-sible. The divisions on the arm I 1 ’ M are each exactly a grain.1 hey are ascertained at first by placing grain weights on a brasshall which is within the ball L; (this hall is an exact counterba-lance to the arm F M, and the ball K, when the small slide on thisarm is al the first division,) and then removing the slide till it, to-gether with the ball K, counterhalances the L II L, and the weightlaid on it. A, fig. 20, is a dial-plate, divided in'o 90 eqn d parts.The index ot this plu*c is carried once round, whcnlh i;mBClias moved through 90°, or a quarter ot a circle. That motion isgiven to the index by the repulsive power of the charge actingbetween the ball D and the b..U B. (Plul. Tran-, vol. ixxxii. p*384.) The arm B V being icprlled, shews when the charge is in-creasing, and the arm F i\I shews what this repulsive power i-. I> e "tween iwo halls of this si/e, in grains, according to the mini her theweight rests at, when lilted up by the repulsive power ol the

charge: