AE

5

+556 1823

E356 1823

v.8

Electricity !

Electrome.

ter.

Plate CC.

Fig. 1.

Fig. 2.

Fig. 3.

'E'

ELE

LECTRICITY, MEDICAL. See MATERIA
MEDICA.

ELECTRIDES, anciently islands in the Adriatic fea, which received their name from the quantity of amber (electrum) which they produced. They were at the mouth of the Po, according to Apollonius of Rhodes; but fome hiftorians doubt of their exif

tence.

ELECTROMETER. In various parts of the article ELECTRICITY, we have defcribed a great variety of inftruments for afcertaining the prefence of electricity, and measuring its quantity or proportion.

But there are several inftruments of this kind that have not been described in that article; and as they are well deferving a place in this work, either from the ingenuity of their conftruction, the reputation of their inventors, or the intrinfic value of the inftruments themfelves, we shall give an account of them here.

Fig. 1. Plate CC. is a geometrical reprefentation of Mr Cavallo's improved atmospherical electrometer, of half its real fize. The principal part of this inftrument is a glass tube CDMN, cemented at the bottom into the wooden piece AB, by which part the inftrument is to be held when used for the atmosphere; and it also serves to fcrew the inftrument into its wooden cafe ABO, fig. 2. when it is not to be used. The upper part of the tube CDMN, is fhaped tapering to a fmaller extremity, which is entirely covered with fealing-wax, melted by heat, and not diffolved in fpirits. Into this tapering part a fmall tube is cemented, the lower extremity G of which being alfo covered with fealing-wax, projects a fhort way within the tube CDMN. Into this fmaller tube a wire is cemented, which with its lower extremity touches the flat piece of ivory H, faftened to the tube by means of cork; the upper extremity of the wire projects about a quarter of an inch above the tube, and screws into the brafs cap EF, which cap is open at the bottom, and ferves to defend the waxed part of the inftrument from the rain, &c.

In fig. 3. a fection of this brafs cap is reprefented, in order to fhow its internal fhape, and the manner in which it is fcrewed to the wire, projecting above the tube L. The fmall tube L, and the upper extremity of the large tube CDMN, appear like one continued piece, on account of the fealing-wax, which covers them both. The conical corks P of this electrometer, which by their repulfion fhow the electricity, &c. are as fmall as VOL. VIII. Part I.

ELE

can conveniently be made, and they are fufpended by Electromeexceeding fine filver wires. Thefe wires are shaped in ter. a ring at the top, by which they hang very loosely on the flat piece of ivory H, which has two holes for that purpofe. By this method of fufpenfion, which is applicable to every fort of electrometer, the friction is leffened almost to nothing, and thence the inftrument is fenfible of a very fmall degree of electricity. IM, and KN, are two narrow flips of tin-foil, ftuck to the infide of the glafs CDMN, and communicating with the wooden bottom AB; they ferve to convey off that electricity, which, when the corks touch the glass, is communicated to it, and being accumulated, might difturb the free motion of the corks.

In regard to its ufe, this inftrument may serve to obferve the artificial, as well as the atmospherical electricity. When it is to be used for artificial electricity, this electrometer is fet upon a table or other convenient fupport; then it is electrified by touching the brafs cap EF with an electrified body, which electricity will fometimes be preferved for more than an hour. Mr Cavallo had one of these electrometers which would remain electrical for more than twelve hours, though in a room without a fire. If in an electrified ftate, any electrified substance be brought near the cap EF, the corks of the electrometer, by their converging, or by increafing their divergency, will fhew the fpecies of that body's electricity.

It is neceffary to remark, that to communicate any electricity to this electrometer, by means of an excited electric, e. g. a piece of fealing-wax, (which we fuppofe is always negatively electrified), is not very readily done in the ufual manner, on account of the cap EF being well rounded, and free from points or fharp edges. By the approach of the wax, the electrometer will be caused to diverge; but as foon as the wax is removed, the wires immediately collapfe. The beft method to electrify it, is to bring the excited wax fo near the cap, that one or both the corks may touch the fide of the bottle CDM; after which, they will foon collapfe and appear unelectrified: if now the wax be moved, they will again diverge, and remain electrified pofitively.

When this inftrument is to be used to try the electricity of the fogs, the air, the clouds, &c. the obferver is to do nothing more than to unfcrew it from its cafe, and, holding it by the bottom AB, to prefent it to the A

open

ter.

Electrome- open air, a little above his head, fo that he may conveniently fee the corks P, which will immediately diverge if there be any fufficient quantity of electricity; whofe nature, i. e. whether pofitive or negative, may be af certained by bringing an excited piece of fealing-wax, or other electric, towards the brafs cap EF.

Fig. 4.

It is perhaps unneceffary to remark, that this obfervation must be made in an open place, as the roads out of town, the fields, the top of a houfe, &c.

The principal advantages of this electrometer, as stated by Mr Cavallo, are as follows.

1. The fmallnefs of its fize. Mr Cavallo made one fo fmall, that its cafe, which was of brafs, measured only three inches and a half in length, and nine-tenths of an inch in diameter, and yet it acted perfectly well. 2. Its being always ready for experiments, without fear of entangling the threads, or having an equivocal refult by the fluggishness of its motion.

3. Its not being disturbed by wind or rain.
4. Its great fenfibility; and,

5. Its keeping the communicated electricity longer than any other electrometer.

II. Sauffure's ELECTROMETER. M. de Sauffure's electrometer, with which he made the obfervations on atmospherical electricity that have been related in the fecond chapter of Part V. of the article ELECTRICITY, and reprefented at fig. 4. is much the fame with that of Mr Cavallo above defcribed. The following are the most material circumstances in which they differ: First, the fine wires, by which the balls are fufpended, fhould not be long enough to reach the tinfoil which is pafted on the infide of the glafs, because the electricity, when ftrong, will caufe them to touch this tin-foil twice confecutively, and thus deprive them in a moment of their electricity. To prevent this defect, and yet give them a fufficient degree of motion, it is neceffary to use larger glaffes than thofe that are generally applied to Mr Cavallo's electrometer; two or three inches in diameter will be found to answer the purpose very well. But as it is neceffary to carry off the electricity which may be communicated to the infide of the glafs, and thus be confounded with that which belongs to thofe fubftances that are under examination; four pieces of tin-foil fhould be pasted on the infide of the glafs; the balls fhould not be more than one-twentieth of an inch diameter, fufpended by filver wires, moving freely in holes nicely rounded. The bottom of the electrometer fhould be of metal; for this renders it more easy to deprive it of any acquired electricity, by touching the bottom and top at the fame time.

In order to collect a great quantity of electricity from the air, the electrometer is furnished with a pointed wire, 15 inches or two feet long, which unfcrews in three or four pieces, to render the inftrument more Fig. 4. & S. portable; fee fig. 4. When it rains or fnows, the small parapluie, fig. 5. is to be ferewed on the top of the inftrument, as by this its infulation is preserved, notwithftanding the rain.

This inftrument indicates not only the electricity of fogs, but that alfo of ferene weather, and enables us to difcover the kind of electricity which reigns in the atmofphere; and to a certain degree to form an estimate of its quantity, and that under two different points of

view, the degree of intenfity, and the diftance from Electrome the earth at which it first begins to be fenfible.

A conductor raised for the purpose of making obfervation on atmospherical electricity will be found to exhibit figns of electricity, only when the electric fluid is. more or lefs condenfed in the air, than in the earth. Though the air refifts the paffage of the electric fluid, it is not abfolutely impermeable to it; it fuffers it to pafs gradually, and generally with more eafe in proportion as its mass or thickness is lefs. It is therefore interesting to difcover at what height it is neceffary to be elevated, in order to find a fenfible difference between the electricity of the earth, and that of the air. A very fenfible difference may be generally discovered by this inftrument, at the distance of four or five feet from the ground; fometimes it may be seen if the inftrument is placed even on the ground; while at others, it must be raised seven or more feet before the balls will open; fometimes, though feldom, this height is not fufficient. This distance is generally greatest when the electricity is ftrongeft, though neceffarily modified by a variety of circumftances, fome of which are known, as the degree of dryness or humidity of the air, and others are unknown.

The degree of intenfity, at a given height, may be difcovered thus; raife the electrometer, and judge by the divifions which are placed on the edge of it, the degree of their divergence. To find the relation between this degree of divergence, and the force of the electricity, M. de Sauffure took the following method: As he could not with certainty double or triple a given quantity of electricity; yet as a given force may be reduced one half, a fourth or eighth, &c. by dividing between two equal and fimilar bodies, the electricity contained in one; he took two of his unarmed electrometers, which were as fimilar as poffible, and electrified one of them, fo that the balls feparated precifely fix lines; he then touched the top thereof by the top of that which was not electrified; in an instant the electricity was equally divided between them, as was evident by the divergence of the balls, which was four lines in each; confequently a diminution of half the denfity had only leffened the divergence one-third. One of these electrometers was then deprived of its elec tricity, and was afterwards brought in contact with the other as before; the remaining electricity divided itfelf again between them, and the balls fell from four to twenty-eight lines, nearly in the fame proportion as before; in the third operation they fell to nineteen; in the fourth to one, where he was obliged to ftop, as there was not now fufficient force in the fluid to pafs from one electrometer to the other, and diflribute itself uniformly between them. The fame experiment, repeated feveral times, gave very nearly the fame refults. Negative electricity decreafed alfo in the fame proportion as the pofitive. The following table may therefore be confidered as giving a general, though not exact idea of the increase in force, which correfponds to different. degrees of divergence in the balls; it is only calculated to every fourth of a line; the force of electricity is always expreffed by whole numbers, as it would be ridiculous to put a greater degree of exactnefs in the num bers than is to be found in the experiments which form the bafes of the calculation.

Distance

ter.

Those who are defirous to carry this measure of the electric force further, may do it by having fimilar electrometers conftructed, but made upon a larger fcale, and with heavier balls, which would only feparate one line, with the degree of electricity that makes the Imaller ones diverge fix lines; thefe would confequently measure a force 1024 times greater than that which forms the unity of the preceding table; and thus by degrees we may be enabled to difcover the ratio of the ftrongeft discharge of a great battery, or perhaps even of thunder itself, to that of a piece of amber, which only attracts a bit of ftraw or any other light subftance.

In order to obferve the electricity of the atmosphere with this inftrument, we must first bring the electric fluid contained in the electrometer to the fame degree of denfity with that at the furface of the earth; this is eafily done by letting the bottom and top touch the ground at the fame time; then raife the point, keeping the bottom ftill in contact with the ground, from whence it may be lifted up in a vertical pofition till the balls are level with the eye.

The fecond circumftance is to render the divergence of the balls, which is occafioned by the electricity of the air, permanent. This is effected by touching the top of the electrometer with the finger; but here the acquired electricity becomes contrary to that of the body by which they are electrified. Let us fuppofe, for example, that the electrometer is at five feet from the ground, and the balls diverging; touch the top of the electrometer with the finger, and the balls will clofe; but they will again open, if the electrometer is withdrawn from the influence of the electricity of the air, by being brought nearer the ground, or into the house. M. Sauffure only employed this method when the eiectricity was fo weak that he could not perceive any until the electrometer was raifed confiderably above his eye: as in this cafe he could not perceive the diver

gence of the balls, he always endeavoured tò obtain a Electrome. permanent electricity in the foregoing manner.

The following example will render the ufe of the foregoing obfervations more familiar. Choose an open fituation free from trees and houses, screw the conductor on the top of the electrometer, lay hold of it by its bafe, and place it so that the bafe and conductor may touch the ground at the fame time; then elevate it to the height of the eye, and obferve the quantity of lines, or fourths of a line, that the balls have diverged; now lower it till the balls almoft touch each other, and observe at what distance the top of the conductor is from the ground; and this is the height from the ground at which the electricity of the air begins to be fenfible. If the electricity of the air is fufficiently ftrong to make the balls diverge when it ftands upon the ground, one of the lengths of the electrometer must be unfcrewed from it. If the balls, however, ftill diverge, the other parts of the conductor should also be unscrewed, and you may mark down, that the electricity is fenfible at zero, or on the furface of the earth. If, on the contrary, the electricity is fo weak, as not to cause the balls to diverge when they are even with the eye, and confequently when the conductor is two feet higher, or feven feet from the ground, you should then raise it a foot higher; while it is thus elevated, touch the top with the other hand; when this hand is taken away, lower the electrometer, and if it is electrified, you may say the electricity is fenfible at eight feet; if it is not, raise it as high as the arm can reach, and repeat the fame operation; if any electricity is found, write down electricity fenfible at nine feet; if not, mark o, or no electricity relative to this inftrument, and this mode of employing it; for figns of electricity may ftill be obtained, by throwing a metallic ball 50 or 60 feet into the air, which is at the fame time connected with the electrometer by a metallic thread.

One advantage of this inftrument is, that it will often exhibit figns of electricity when none can be obtained from a conductor of 100 feet in height, because it can more easily be preferved from humidity, &c. which will destroy the infulation of the large conductors.

This electrometer may be used instead of the condenfer of M. Volta, by only placing it on a piece of oiled filk, fomewhat larger than the bafe of the inftrument; but in this cafe, it is the base, and not the top of the inftrument, which must be brought into contact with the substance whofe electricity is to be explored.

It is eafy to discover alfo by this inftrument, the electricity of any fubftance, as of cloths, hair of different animals, &c. For this purpose, it must be held by the base, and the fubftance rubbed brifkly (only once) by the ball of the electrometer; the kind of electricity may be ascertained in the ufual manner. It is proper, however, to obferve here, that as the top of the electrometer acts in this cafe as an infulated rubber, the electricity it acquires is always contrary to that of the rubbed body.

III. Cadet's ELECTROMETER, is thus defcribed by the author, as tranflated in Nicholson's Journal.

Fig. 6. In a glafs tube A, 18 or 20 inches long, Fig. 6. is inclofed another fhorter tube X, fealed at both ends. This tube contains a graduated scale: one of the ends of these two tubes is cemented in a handle of turned A 2 wood,

ter.