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frequently for nearly an hour; then take it off which they cast on a flat surface placed behind the fire, and continue the stirring till it is cold them. This was certainly a very simple apparaand fit for use.
tus, though, at the same time, very imperfect : 140. To the above account of the electrical it was improved by Mr. Waitz, who appended battery we shall here add Mr. Morgan's rules for small weights to the threads. its construction. They are the following.
150. The electrometer of Mr. Canton con141. Its connecting wires should be perfectly sisted of a pair of sınall pith-balls suspended by free from all points and edges.
very fine flaxen threads from a peg enclosed in a 142. They should be easily moveable, so that small box having a sliding cover fitted to it; when accident has lessened the number of jars, when he used this instrument the lid was drawn the number of wires may be reduced so as to off, the box held horizontally, so that the balls correspond with the remaining quantity of glass. might hang freely. A more complicated instru
143. The jars should not be crowded; for in ment, but founded on the same simple principles, such a case, if necessity should oblige us to em- and containing four electrometers, is represented ploy jars of different heights or sizes, the tin-foil in fig. 12. A is the basis of the stand which of the higher ones, being in contact with the un- supports these, and is made of mahogany. B coated glass of the lower ones, the insulation will is a pillar of wax, glass, or baked wood. To the thus be rendered less complete.
top of the pillar, if it be of wax or glass, a cir144. The size of the jars should not be large; cular piece of wood, C, is fixed; but if the pilfor though an increase of magnitude lessens the lar be of baked wood, that may constitute the trouble of cleaning the battery, it at the same whole. From this circular piece of wood protime increases the expense of repairing damages ceed four arms of glass, or baked wood, suspendwhich frequently occur.
ing at their ends four electrometers, two of 145. The several wires should be fixed very which, D, E, are silk threads about eight inches steadily, or in such a manner as not to admit of long, suspending each a small downy feather at any shaking.
its end. The other two electrometers, F, G, are 146. The battery should take up the least pos- made of very small balls of cork, or of the pith sible room; for as it increases in size, so is the of elder; and they are constructed in the followprobability increased of its being exposed to the ing manner :-ab is a rod of glass about six influence of surrounding conductors.
inches long, covered with sealing-wax, and 147. The strength of the charge that can be formed at top into a ring: from the lower exproduced by either a single jar or battery is tremity of this stick proceed two fine linen estimated according to the number of square feet threads, cc, about five inches long, each suspendof coated surface in each. Hence arises the ne- ing a cork or pith-ball d, about one-eighth of an cessity of forming a combination of jars, as single inch in diameter. These threads should be moisones of convenient size cannot be obtained. tened with a weak solution of salt. When this One of the largest, perhaps, ever constructed electrometer is not electrified, the threads cc hang was used by Mr. Singer in his lectures on elec- parallel to each other, and the cork balls are in tricity. This jar, he informs us, was eighteen contact; but when electrified they repel one inches in diameter, and two feet in height; its another, as represented in the figure. When it is external coating exposing a surface of about six inconvenient to use the insulating stand, A B, the square feet.
The principal experiments per- electrometers may be easily supported by a formed by the aid of the electrical battery will glass rod or tube. be explained in another part of this article. 151. Electrometers constructed of pith of
elder were employed by Mr. Cavallo in many of INSTRUMENTS FOR MEASURING ELECTRICITY.
his experiments in electricity, particularly in 148. The instruments used for the purpose of those on the electricity of the atmosphere. One ascertaining the presence of electricity, and of these instruments is represented in figs. 13 and measuring its intensity, are denominated elec- 14. The case or handle of this instrument is trometers, and bold a very important place among formed of a glass tube, about three inches in the necessary articles of electrical apparatus. length, and three-tenths of an inch in diameter, Some of these are so constructed as to be of use one-half of which is coated with wax on the outonly in indicating the presence of electricity; side. From one extremity of this tube, viz. that others perform the two-fold office of showing its without sealing-wax, a small loop of silk propresence, and indicating the precise degree of ceeds, which occasionally serves to hang the its intensity at the same instant; while others are electrometer on a pin, &c. To the other extreconstructed for the purpose of exactly measuring mity of the tube a cork is adapted, which, being the strength of, and giving the required direction cut tapering on both ends, can fit the mouth of to any accumulation of the electric matter, from the tube with either end. From one extremity the charge of a small phial to that of the most of this cork two linen threads proceed, a little powerful battery. We shall here give a very shorter than the length of the tube, suspending brief description of the principal of these useful each a little cone of pith of elder. When this instruments.
electrometer is to be used, that end of the cork 149. The first electrometer is generally allowed which is opposite to the threads is pushed into to have been that of the abbé Nollet; it was the mouth of the tube ; the tube then forms the composed of two silk threads, which were made insulated handle of the pith electrometer, as to recede from each other on being approached represented in fig. 13. But when the electromeby an electrified body. The angle of the diver- ter is to be carried in the pocket, the threads are gency of the threads was observed by the shadow put into the tube, and the cork stops it, as repre
sented in fig. 14. The advantages of this elec- the inside of the glass cylinder are pasted two trometer are, its convenient small size, its great slips of tin-foil, b, c, diametrically opposite to sensibility, and its continuing longer in good each other, and rising higher than the stripes o: order than any other. Fig. 15 represents a case gold leaf. The lower ends of the tin-foil are in to carry the above described electrometer in. contact with the brass stand D E F, which supThis case is like a common toothpick-case, ex- ports the whole. For observing the electricity of cept that it has a piece of amber fixed on the the atmosphere, a pointed wire, C, is inserted in extremity A, which may occasionally serve to the brass cap AB. "To use the electrometer, turn electrify the electrometer negatively; and on the round the cap AB, till the surfaces of the gold other extremity a piece of ivory fastened upon a leaf are parallel to the surfaces of the pieces of piece of amber BC. This amber serves only to tin-foil b, c, so that the two stripes of gold leaf insulate the ivory; which, when insulated, and may hang in contact in the middle of the cylinder. rubbed against woollen cloths, acquires a posi- Then, if a body containing a small quantity of tive electricity, and is therefore useful to electrify electricity, be brought in contact with the cap the electrometer positively.
AB, the gold leaves, m, n, will diverge, and their 152. Another form of the pith-ball electrome- extremities will strike the slips of tin-foil b, c, and ter is that invented by Mr. Henley ; it is simple thus convey the electricity to the ground. See in its construction, and extremely useful in figs. 1 and 2, plate III. numerous experiments, as will afterwards appear.
154. There have been other improvements proIt consists (fig. 16) of a perpendicular stem posed on this electrometer, one of which was by formed at top like a ball, and furnished at its Mr. Singer, and chiefly respects the mode of inlower end with a brass ferrule and pin, by which sulation. This instrument is represented in fig. it may be fixed in one of the holes of the conduc- 3: the following brief description of it will suffice tor, or at the top of a Leyden jar. To the upper to convey a correct idea of it. Like the preceding part of the stem, a graduated ivory semicircle is it is constructed with a glass cylinder, surmounted fixed, about the middle of which is a brass arm by a broad cap of either wood or metal. The inor cock, to support the axis of the index. The sulation depends on a glass tube of four inches index consists of a very slender rod, which long, and one-fourth of an inch diameter, reaches from the centre of the graduated arch to covered on both sides with sealing-wax, and the brass ferrule; and to its lower extremity is having a brass wire of a sixteenth or twelfth of fastened a small pith-ball nicely turned in a an inch thick, and five inches long, passing lathe. When this electrometer is in a perpen- through its axis, so as to be perfectly free from dicular position, and not electrified, the index contact with any part of the tube, in the middle hangs parallel to the pillar; but when it is elec- of which it is fixed by a plug of siłk, which keeps trified the index recedes more or less according it in a concentric position with the internal diameto the quantity of electricity, from the stem. Fig. ter of the tube. Å brass cap is screwed upon the 17, represents this electrometer separated from upper part of this wire; it serves to limit the atits stand, and fixed upon the prime conductor. mosphere from free contact with the outside of The scale in Mr. Henley's quadrant is divided the tube, and at the same time to defend its ininto equal parts; but M. Aehard has shown that, side from dust. To the lower part of the wire when this is the case, the angle at which the the gold leaves are fastened. The glass tube index is held suspended by the electric repul- passes through the centre of the cap of the elecsion is not a true measure of the repulsive force; trometer, and is cemented there about the middle to estimate this force truly, he demonstrates that of its length. When this construction is conthe arc of the electrometer should be divided sidered, it will be evident that the insulation of according to a scale of arcs, the tangents of which the wire, and also of the gold leaves, will be preare in arithmetical progression.
served until the inside as well as the outside o. 153. One of the most useful electrometers for the glass tube become coated with moisture; but indicating the presence of very small portions of so effectually does the arrangement preclude electricity, is that invented by the Rev. Mr. Ben- this, that some of these electrometers have renet. The common construction of this instrument mained for seven years without being either is very good ; but a highly improved form of it warmed or wiped, and have still appeared to reis described by Dr. Brewster in the appendix totain the same insulating power as at first. No. his new edition of Ferguson's Essays. "The chief 2 shows this electrometer complete. difference between this and the original construc 155. An electrometer of common use in the tion consists in the cap and stand being of brass administration of medical electricity, sometimes instead of wood. He thus describes it : attached to the Leyden jar, and sometimes made
It consists of two stripes of gold leaf, m, n, to fit into one of the ends of the prime conductor, suspended within a glass cylinder ABED. This is termed Lane's electrometer. Fig. 4 is a reprecylinder has a brass cap AB, a little broader sentation of this electrometer: it consists of two than itself, in the centre of which is a hole, a, in brass balls of equal size, one of which is conthe inside of the cap, which receives a small nected with the inside coating of the jar, and the wedge of wood. On each side of this wedge, other insulated opposite to the first, yet so as to two equal stripes of gold leaf, free of all rough- admit of its being placed in contact with it, or at ness at their edges, are fixed by a little varnish; any required distance from it. That which is these stripes are generally about two inches long, insulated is connected by a wire with the outer and about a quarter of an inch broad. The in- coating of the jar, so as to serve as a course for side of the cap A B, and the upper part of the the discharge which, it is very obvious from an glass cylinder, are coated with sealing-wax. On inspection of the figure, will take place sooner or
later, according as the balls are placed either his head, so that he may conveniently see the nearer to, or farther from, each other. But be- balls P, which will immediately diverge if there fore we proceed to notice the discharging electro- is any electricity; i.e. whether positive or negameter, we must describe one or two others which tive may be ascertained, by bringing an excited are both ingenious and useful.
piece of sealing-wax, or other electric, towards 156. We have described Mr. Cavallo's pocket the cap
EF. electrometer at No. 154, but this gentleman con 160. M. Saussure has made an improvement structed another portable electrometer for atmo
in this electrometer. The principal circumspherical purposes, which deserves particular stances in which his electrometer differs from notice. Its principal part consists of a glass Mr. Cavallo's, are: The fine wires, by which tube CDMN, fig. 5, cemented at the bottom the balls are suspended, should not be so long as into the brass piece AB, by which part the in- to reach the tin-foil which is pasted on the inside strument is to be held when used for the atmo- of the glass ; because the electricity, when strong, sphere; it also serves to screw the instrument into will cause them to touch this tin-foil twice conits brass case, AC, fig. 6. The upper part of secutively, and thus deprive them in a moment the tube, CDMN, is tapered to a small extremity, of this electricity: To prevent this defect, and which is entirely covered with sealing-wax; to yet give them a sufficient degree of motion, it is this tapering part a small tube is cemented; the necessary to use larger glasses than those that are lower extremity being also covered with sealing- generally applied to Mr. Cavallo's electrometer; wax, projects a short way within the tube two or three inches diameter will answer the CDM'N; into this smaller tube a wire is ce- purpose very well. But, as it is necessary to mented, which with its under extremity touches carry off the electricity, which may be communithe flat piece of ivory H, fastened to the tube by cated to the inside of the glass, and may thus be means of a cork; the upper extremity of the wire confounded with that which belongs to those subprojects about a quarter of an inch above the stances that are under examination, four pieces of tube, and screws into the brass cap EF, which tin-foil should be pasted on the inside of the cap is open at the bottom, and serves to defend glass; the balls should not be more than af of an the waxed part of the instrument from the inch diameter, suspended by silver wires, moving rain, &c.
freely in holes nicely rounded. The bottom of 157. A section of the brass cap 'is represented the electrometer should be of brass; for this in fig. 7, to show its internal structure, with the renders it more easy to deprive them of any acmanner in which it is screwed to the wire pro- quired electricity, by touching the bottom and jecting above the small tube L. This small tube top at the same time. and the upper extremity of the large tube, 161. This electrometer may be used instead of CDMN, appear like one continued piece when the condenser of M. Volta, by only placing it joined, from the sealing-wax covering them both. on a piece of oiled silk, a little larger than the The conical corks, P, fig. 5, which show the base of the instrument: but in this case the base, electricity by their repulsion, are made very and not the top, of the instrument must be small, and suspended by very fine silver wires, brought in contact with the substance, the elecshaped like rings at the top, by which they hang tricity of which is to be explored. By this invery loosely on the flat piece of ivory H, which strument it is easy to ascertain the degree of has two holes in it. Bý this method of suspen- conducting power in any substance. If it is sion, which Mr. Cavallo says is applicable to placed on an imperfect conductor, as dry wood every sort of electrometer, the friction is reduced or marble, and if the instrument is electrified to almost nothing, and the instrument is thus strongly, and afterwards the top is touched, the rendered sensible to a very small degree of elec- electricity will appear to be destroyed; but, on tricity. IM and KN are two narrow slips of lifting up the instrument by the top, the balls tin-foil, fixed on the inside of the glass CDMN, will again diverge, because the imperfect conducand communicating with the brass bottom AB. tor formed with the base a kind of electrophorus, They serve to convey that electricity which, when by which the electric fluid was condensed, and the balls touch the glass, is communicated to it, lost its tension, till the perfect conductor was and, being accumulated, might disturb the free separated from the imperfect one; whereas, if motion of the balls.
the conductor had been more perfect, it would 158. To use this instrument for artificial elec- have been deprived of its electricity immediately tricity, affect the brass cap E F, by an electrified on the application of the hand. It is useful to substance, and the divergence or convergence of discover also the electricity of any substance, as the balls of the electrometer, at the approach of of clothes, hair of different animals, &c. For an excited electric, will show the quality of the this purpose it must be held by the base, and the electricity. The best manner to electrify this in- substance rubbed briskly (only once) by the ball strument is, to bring excited wax so near the cap, of the electrometer; the kind of electricity may that one or both of the corks may touch the side be ascertained in the usual manner. But as the of the bottle CDMN, after which they will soon top of the electrometer acts, in this case, as an collapse and appear unelectrified. On removing insulated rubber, the electricity it acquires is the wax, they will again diverge, and remain always contrary to that of the rubbed body. electrified positively.
162. To collect a great quantity of electricity 159. To try the electricity of the fogs, air, from the air, this electrometer is furnished with clouds, &c., by this electrometer, the electrician a pointed wire from fifteen inches to two feet must unscrew it from its case, and hold it by the long, which unscrews in three or four pieces, to bottom AB, to present it to the air a little above render the instrument more portable, see fig. 8.