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frequently for nearly an hour; then take it off the fire, and continue the stirring till it is cold and fit for use.

140. To the above account of the electrical battery we shall here add Mr. Morgan's rules for its construction. They are the following. 141. Its connecting wires should be perfectly free from all points and edges.

142. They should be easily moveable, so that when accident has lessened the number of jars, the number of wires may be reduced so as to correspond with the remaining quantity of glass. 143. The jars should not be crowded; for in such a case, if necessity should oblige us to employ jars of different heights or sizes, the tin-foil of the higher ones, being in contact with the uncoated glass of the lower ones, the insulation will thus be rendered less complete.

144. The size of the jars should not be large; for though an increase of magnitude lessens the trouble of cleaning the battery, it at the same time increases the expense of repairing damages which frequently occur.

145. The several wires should be fixed very steadily, or in such a manner as not to admit of any shaking.

146. The battery should take up the least possible room; for as it increases in size, so is the probability increased of its being exposed to the influence of surrounding conductors.

147. The strength of the charge that can be produced by either a single jar or a battery is estimated according to the number of square feet of coated surface in each. Hence arises the necessity of forming a combination of jars, as single ones of convenient size cannot be obtained. One of the largest, perhaps, ever constructed was used by Mr. Singer in his lectures on electricity. This jar, he informs us, was eighteen inches in diameter, and two feet in height; its external coating exposing a surface of about six square feet. The principal experiments performed by the aid of the electrical battery will be explained in another part of this article. INSTRUMENTS FOR MEASURING ELECTRICITY. 148. The instruments used for the purpose of ascertaining the presence of electricity, and measuring its intensity, are denominated electrometers, and hold a very important place among the necessary articles of electrical apparatus. Some of these are so constructed as to be of use only in indicating the presence of electricity; others perform the two-fold office of showing its presence, and indicating the precise degree of its intensity at the same instant; while others are constructed for the purpose of exactly measuring the strength of, and giving the required direction to any accumulation of the electric matter, from the charge of a small phial to that of the most powerful battery. We shall here give a very brief description of the principal of these useful instruments.

149. The first electrometer is generally allowed to have been that of the abbé Nollet; it was composed of two silk threads, which were made to recede from each other on being approached by an electrified body. The angle of the divergency of the threads was observed by the shadow

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which they cast on a flat surface placed behind them. This was certainly a very simple apparatus, though, at the same time, very imperfect: it was improved by Mr. Waitz, who appended small weights to the threads.

150. The electrometer of Mr. Canton consisted of a pair of small pith-balls suspended by very fine flaxen threads from a peg enclosed in a small box having a sliding cover fitted to it; when he used this instrument the lid was drawn off, the box held horizontally, so that the balls might hang freely. A more complicated instrument, but founded on the same simple principles, and containing four electrometers, is represented in fig. 12. A is the basis of the stand which supports these, and is made of mahogany. B is a pillar of wax, glass, or baked wood. To the top of the pillar, if it be of wax or glass, a circular piece of wood, C, is fixed; but if the pillar be of baked wood, that may constitute the whole. From this circular piece of wood proceed four arms of glass, or baked wood, suspending at their ends four electrometers, two of which, D, E, are silk threads about eight inches long, suspending each a small downy feather at its end. The other two electrometers, F, G, are made of very small balls of cork, or of the pith of elder; and they are constructed in the following manner:-ab is a rod of glass about six inches long, covered with sealing-wax, and formed at top into a ring: from the lower extremity of this stick proceed two fine linen threads, cc, about five inches long, each suspending a cork or pith-ball d, about one-eighth of an inch in diameter. These threads should be moistened with a weak solution of salt. When this electrometer is not electrified, the threads c c hang parallel to each other, and the cork balls are in contact; but when electrified they repel one another, as represented in the figure. When it is inconvenient to use the insulating stand, A B, the electrometers may be easily supported by a glass rod or tube.

151. Electrometers constructed of pith of elder were employed by Mr. Cavallo in many of his experiments in electricity, particularly in those on the electricity of the atmosphere. One of these instruments is represented in figs. 13 and 14. The case or handle of this instrument is formed of a glass tube, about three inches in length, and three-tenths of an inch in diameter, one-half of which is coated with wax on the outside. From one extremity of this tube, viz. that without sealing-wax, a small loop of silk proceeds, which occasionally serves to hang the electrometer on a pin, &c. To the other extremity of the tube a cork is adapted, which, being cut tapering on both ends, can fit the mouth of the tube with either end. From one extremity of this cork two linen threads proceed, a little shorter than the length of the tube, suspending each a little cone of pith of elder. When this electrometer is to be used, that end of the cork which is opposite to the threads is pushed into the mouth of the tube; the tube then forms the insulated handle of the pith electrometer, as represented in fig. 13. But when the electrometer is to be carried in the pocket, the threads are put into the tube, and the cork stops it, as repre

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sented in fig. 14. The advantages of this electrometer are, its convenient small size, its great sensibility, and its continuing longer in good order than any other. Fig. 15 represents a case to carry the above described electrometer in. This case is like a common toothpick-case, except that it has a piece of amber fixed on the extremity A, which may occasionally serve to electrify the electrometer negatively; and on the other extremity a piece of ivory fastened upon a piece of amber B C. This amber serves only to insulate the ivory; which, when insulated, and rubbed against woollen cloths, acquires a positive electricity, and is therefore useful to electrify the electrometer positively.

152. Another form of the pith-ball electrometer is that invented by Mr. Henley; it is simple in its construction, and extremely useful in numerous experiments, as will afterwards appear. It consists (fig. 16) of a perpendicular stem formed at top like a ball, and furnished at its lower end with a brass ferrule and pin, by which it may be fixed in one of the holes of the conductor, or at the top of a Leyden jar. To the upper part of the stem, a graduated ivory semicircle is fixed, about the middle of which is a brass arm or cock, to support the axis of the index. The index consists of a very slender rod, which reaches from the centre of the graduated arch to the brass ferrule; and to its lower extremity is fastened a small pith-ball nicely turned in a lathe. When this electrometer is in a perpendicular position, and not electrified, the index hangs parallel to the pillar; but when it is electrified the index recedes more or less according to the quantity of electricity, from the stem. Fig. 17, represents this electrometer separated from its stand, and fixed upon the prime conductor. The scale in Mr. Henley's quadrant is divided into equal parts; but M. Achard has shown that, when this is the case, the angle at which the index is held suspended by the electric repulsion is not a true measure of the repulsive force; to estimate this force truly, he demonstrates that the arc of the electrometer should be divided according to a scale of arcs, the tangents of which are in arithmetical progression.

153. One of the most useful electrometers for indicating the presence of very small portions of electricity, is that invented by the Rev. Mr. Bennet. The common construction of this instrument is very good; but a highly improved form of it is described by Dr. Brewster in the appendix to his new edition of Ferguson's Essays. The chief difference between this and the original construction consists in the cap and stand being of brass instead of wood. He thus describes it ::

It consists of two stripes of gold leaf, m, n, suspended within a glass cylinder A BED. This cylinder has a brass cap A B, a little broader than itself, in the centre of which is a hole, a, in the inside of the cap, which receives a small wedge of wood. On each side of this wedge, two equal stripes of gold leaf, free of all roughness at their edges, are fixed by a little varnish; these stripes are generally about two inches long, and about a quarter of an inch broad. The inside of the cap AB, and the upper part of the glass cylinder, are coated with sealing-wax. On

the inside of the glass cylinder are pasted tw slips of tin-foil, b, c, diametrically opposite tɔ each other, and rising higher than the stripes of gold leaf. The lower ends of the tin-foil are in contact with the brass stand DEF, which supports the whole. For observing the electricity of the atmosphere, a pointed wire, C, is inserted in the brass cap A B. To use the electrometer, turn round the cap AB, till the surfaces of the gold leaf are parallel to the surfaces of the pieces of tin-foil b, c, so that the two stripes of gold leaf may hang in contact in the middle of the cylinder. Then, if a body containing a small quantity of electricity, be brought in contact with the cap A B, the gold leaves, m, n, will diverge, and their extremities will strike the slips of tin-foil b, and thus convey the electricity to the ground. See figs. 1 and 2, plate III.

154. There have been other improvements proposed on this electrometer, one of which was by Mr. Singer, and chiefly respects the mode of insulation. This instrument is represented in fig. 3: the following brief description of it will suffice to convey a correct idea of it. Like the preceding it is constructed with a glass cylinder, surmounted by a broad cap of either wood or metal. The insulation depends on a glass tube of four inches long, and one-fourth of an inch diameter, covered on both sides with sealing-wax, and having a brass wire of a sixteenth or twelfth of an inch thick, and five inches long, passing through its axis, so as to be perfectly free from contact with any part of the tube, in the middle of which it is fixed by a plug of silk, which keeps it in a concentric position with the internal diameter of the tube. Å brass cap is screwed upon the upper part of this wire; it serves to limit the atmosphere from free contact with the outside of the tube, and at the same time to defend its inside from dust. To the lower part of the wire the gold leaves are fastened. The glass tube passes through the centre of the cap of the electrometer, and is cemented there about the middle of its length. When this construction is considered, it will be evident that the insulation of the wire, and also of the gold leaves, will be preserved until the inside as well as the outside o. the glass tube become coated with moisture; but so effectually does the arrangement preclude this, that some of these electrometers have remained for seven years without being either warmed or wiped, and have still appeared to retain the same insulating power as at first. No. 2 shows this electrometer complete.

155. An electrometer of common use in the administration of medical electricity, sometimes attached to the Leyden jar, and sometimes made to fit into one of the ends of the prime conductor, is termed Lane's electrometer. Fig. 4 is a representation of this electrometer: it consists of two brass balls of equal size, one of which is connected with the inside coating of the jar, and the other insulated opposite to the first, yet so as to admit of its being placed in contact with it, or at any required distance from it. That which is insulated is connected by a wire with the outer coating of the jar, so as to serve as a course for the discharge which, it is very obvious from an inspection of the figure, will take place sooner or

later, according as the balls are placed either nearer to, or farther from, each other. But before we proceed to notice the discharging electrometer, we must describe one or two others which are both ingenious and useful.

156. We have described Mr. Cavallo's pocket electrometer at No. 154, but this gentleman constructed another portable electrometer for atmospherical purposes, which deserves particular notice. Its principal part consists of a glass tube CDMN, fig. 5, cemented at the bottom into the brass piece AB, by which part the instrument is to be held when used for the atmosphere; it also serves to screw the instrument into its brass case, AC, fig. 6. The upper part of the tube, CD MN, is tapered to a small extremity, which is entirely covered with sealing-wax; to this tapering part a small tube is cemented; the lower extremity being also covered with sealingwax, projects a short way within the tube CDMN; into this smaller tube a wire is cemented, which with its under extremity touches the flat piece of ivory H, fastened to the tube by means of a cork; the upper extremity of the wire projects about a quarter of an inch above the tube, and screws into the brass cap EF, which cap is open at the bottom, and serves to defend the waxed part of the instrument from the rain, &c.

157. A section of the brass cap is represented in fig. 7, to show its internal structure, with the manner in which it is screwed to the wire projecting above the small tube L. This small tube and the upper extremity of the large tube, CDMN, appear like one continued piece when joined, from the sealing-wax covering them both. The conical corks, P, fig. 5, which show the electricity by their repulsion, are made very small, and suspended by very fine silver wires, shaped like rings at the top, by which they hang very loosely on the flat piece of ivory H, which has two holes in it. By this method of suspension, which Mr. Cavallo says is applicable to every sort of electrometer, the friction is reduced to almost nothing, and the instrument is thus rendered sensible to a very small degree of electricity. IM and KN are two narrow slips of tin-foil, fixed on the inside of the glass CDMN, and communicating with the brass bottom A B. They serve to convey that electricity which, when the balls touch the glass, is communicated to it, and, being accumulated, might disturb the free motion of the balls.

158. To use this instrument for artificial electricity, affect the brass cap EF, by an electrified substance, and the divergence or convergence of the balls of the electrometer, at the approach of an excited electric, will show the quality of the electricity. The best manner to electrify this instrument is, to bring excited wax so near the cap, that one or both of the corks may touch the side of the bottle CDM N, after which they will soon collapse and appear unelectrified. On removing the wax, they will again diverge, and remain electrified positively.

159. To try the electricity of the fogs, air, clouds, &c., by this electrometer, the electrician must unscrew it from its case, and hold it by the bottom AB, to present it to the air a little above

his head, so that he may conveniently see the balls P, which will immediately diverge if there is any electricity; i. e. whether positive or negative may be ascertained, by bringing an excited piece of sealing-wax, or other electric, towards the cap EF.

160. M. Saussure has made an improvement in this electrometer. The principal circumstances in which his electrometer differs from Mr. Cavallo's, are: The fine wires, by which the balls are suspended, should not be so long as to reach the tin-foil which is pasted on the inside of the glass; because the electricity, when strong, will cause them to touch this tin-foil twice consecutively, and thus deprive them in a moment of this electricity. To prevent this defect, and yet give them a sufficient degree of motion, it is necessary to use larger glasses than those that are generally applied to Mr. Cavallo's electrometer; two or three inches diameter will answer the purpose very well. But, as it is necessary to carry off the electricity, which may be communicated to the inside of the glass, and may thus be confounded with that which belongs to those substances that are under examination, four pieces of tin-foil should be pasted on the inside of the glass; the balls should not be more than of an inch diameter, suspended by silver wires, moving freely in holes nicely rounded. The bottom of the electrometer should be of brass; for this renders it more easy to deprive them of any acquired electricity, by touching the bottom and top at the same time.

161. This electrometer may be used instead of the condenser of M. Volta, by only placing it on a piece of oiled silk, a little larger than the base of the instrument: but in this case the base, and not the top, of the instrument must be brought in contact with the substance, the electricity of which is to be explored. By this instrument it is easy to ascertain the degree of conducting power in any substance. If it is placed on an imperfect conductor, as dry wood or marble, and if the instrument is electrified strongly, and afterwards the top is touched, the electricity will appear to be destroyed; but, on lifting up the instrument by the top, the balls will again diverge, because the imperfect conductor formed with the base a kind of electrophorus, by which the electric fluid was condensed, and lost its tension, till the perfect conductor was separated from the imperfect one; whereas, if the conductor had been more perfect, it would have been deprived of its electricity immediately on the application of the hand. It is useful to discover also the electricity of any substance, as of clothes, hair of different animals, &c. For this purpose it must be held by the base, and the substance rubbed briskly (only once) by the ball of the electrometer; the kind of electricity may be ascertained in the usual manner. But as the top of the electrometer acts, in this case, as an insulated rubber, the electricity it acquires is always contrary to that of the rubbed body.

162. To collect a great quantity of electricity from the air, this electrometer is furnished with a pointed wire from fifteen inches to two feet long, which unscrews in three or four pieces, to render the instrument more portable, see fig. 8.

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