Electricity.

Date:
[1829]
Catalogue details

Licence: Public Domain Mark

Credit: Electricity. Source: Wellcome Collection.

    28/150 (page 38)
    later, according as the balls are placed either nearer to, or farther from, each other. But be¬ fore we proceed to notice the discharging electro¬ meter, 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 con¬ structed another portable electrometer for atmo¬ spherical 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 in¬ strument is to be held when Used for the atmo¬ sphere ; it also serves to screw the instrument into its brass case, AC, fig. 6. The upper part of the tube, C D M N, 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 Sealing- wax, projects a short way within the tube CDMN; into this smaller tube a wire is ce¬ mented, 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 pro¬ jecting 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 vvires, 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 suspen¬ sion, 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 elec¬ tricity. 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 elec¬ tricity, 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 in¬ strument is, to bring excited wax so near the cap, that one or both of the corks may touch the side of the bottle CDMN, 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 nega¬ tive may be ascertained, by bringing an excited piece of sealing-wax, or other electric, towards the cap E F. 160. M. Saussure has made an improvement in this electrometer. The principal circum¬ stances 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 con¬ secutively, 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 communi¬ cated to the inside of the glass, and may thus be confounded with that which belongs to those sub¬ stances 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 ac¬ quired 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 elec¬ tricity of which is to be explored. By this in¬ strument 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 conduc¬ tor 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.