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OR, A

DICTIONARY

OF

ARTS, SCIENCES, AND MISCELLANEOUS

LITERATURE;

ENLARGED AND IMPROVED.

THE SIXTH EDITION.

Jllustrated with nearly sir þundred Engravings.

VOL. VIII.

INDOCTI DISCANT; AMENT MEMINISSE PERITI.

EDINBURGH:
PRINTED FOR ARCHIBALD CONSTABLE AND COMPANY;

AND HURST, ROBINSON, AND COMPANY, 90, CHEAPSIDE,

LONDON.

1823.

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ENCYCLOPÆDIA BRITANNICA.

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1823
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Ele&ricity E

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E L E

E L L E
LECTRICITY, MEDICAL. See MATERIA can conveniently be made, and they are suspended by Electrome-
Medica.

exceeding fine silver wires. These wires are shaped in Electrome.

ELECTRIDES, anciently islands in the Adriatic a ring at the top, by which they hang very loosely on the
sea, which received their name from the quantity fat piece of ivory H, which has two holes for that pur-
of amber electrum) which they produced. They pose. By this method of fufpenfion, which is applica-
were at the mouth of the Po, according to Apollonius ble to every fort of electrometer, the fri&tion is leffened
of Rhodes; but some historians doubt of their exis- almost to nothing, and thence the instrument is sensible
tence.

of a very small degree of electricity. IM, and KN,
ELECTROMETER. In various parts of the ar. are two narrow slips of tin-foil, stuck to the inside of
ticle ELECTRICITY, we have described a great variety the glass CDMN, and communicating with the wooden
of inftruments for ascertaining the presence of electrici. bottom AB ; they serve to convey off that electricity,
ty, and measuring its quantity or proportion.

which, when the corks touch the glass, is communicated
But there are several instruments of this kind that to it, and being accumulated, might disturb the free
have not been described in that article ; and as they are motion of the corks.
well deserving a place in this work, either from the in- In regard to its use, this instrument may serve to
genuity of their construction, the reputation of their in- observe the artificial, as well as the atmospherical elec-

ventors, or the intrinsic value of the instruments them- tricity. When it is to be used for artificial electricity, Plate CG. felves, we shall give an account of them here.

this electrometer is set upon a table or other convenient Fig. 1.

Fig. 1. Plate cc. is a geometrical representation support ; then it is electrified by touching the brass cap of Mr Cavallo's improved atmospberical electrometer, EF with an electrified body, which electricity will of half its real fize. The principal part of this in- sometimes be preserved for more than an hour. Mr ftrument is a glass tube CDMN, cemented at the bot- Cavallo had one of these electrometers which would retom into the wooden piece AB, by which part the inftru- main electrical for more than twelve hours, though in ment is to be held when used for the atmosphere ; and a room without a fire. If in an electrified state, any

it also serves to screw the instrument into its wooden ele&rified substance be brought near the cap EF, the Fig. 2.

case ABO, fig. 2. when it is not to be used. The corks of the electrometer, by their converging, or by in-
upper part of the tube CDMN, is shaped tapering to a creasing their divergency, will thew the species of that
smaller extremity, which is entirely covered with seal body's electricity.
ing-wax, melted by heat, and not diffolved in spirits. It is necessary to remark, that to communicate any
Into this tapering part a small tube is cemented, the electricity to this ele&trometer, by means of an excited
lower extremity G of which being also covered with seal electric, e. g. a piece of sealing-wax, (which we fup-
ing-wax, projects a short way within the tube CDMN. pose is always negatively electrified), is not very rea-
Into this smaller tube a wire is cemented, which with dily done in the usual manner, on account of the cap
its lower extremity touches the flat piece of ivory H, faften- Ef being well rounded, and free from points or sharp
ed to the tube by means of cork; the upper extremity of edges. By the approach of the wax, the electrometer
the wire projects about a quarter of an inch above the will be caused to diverge ; but as soon as the wax is re-
tube, and screws into the brass cap EF, which cap is moved, the wires immediately collapse. The best me-
open at the bottom, and serves to defend the waxed thod to electrify it, is to bring the excited wax so near
part of the instrument from the rain, &c.

the cap, that one or both the corks may touch the side
a fe&tion of this brass cap is represented, in order to of the bottle CDM; after which, they will soon col-
fhow its internal fhape, and the manner in which it is lapse and appear unelectrified: if now the wax be
screwed to the wire, projecting above the tube L. The moved, they will again diverge, and remain electrified
small tube L, and the upper extremity of the large positively.
tube CDMN, appear like one continued piece, on ac- When this instrument is to be used to try the electri-
count of the fealing-wax, which covers them both. city of the fogs, the air, the clouds, &c. the observer
The conical corks P of this electrometer, which by' is to do nothing more than to unscrew it from its case,
their repulfion show the electricity, &c. are as small as and, holding it by the bottom AB, to present it to the
Vol. VIII. Part I.

А

open

Fig. 3.

In fig. 3.

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Fig. 4.

Electrome, open air, a little above his head, so that he may conve- view, the degree of intensity, and the distance from Electrome.
ter. niently see the corks P, which will immediately diverge the earth at which it first begins to be fenfible.

if there be any fufficient quantity of electricity ; whole A conductor railed for the purpose of making obser-
nature, i. e, whether positive or negative, may be af- vation on atmospherical electricity will be found to ex.
certained by bringing an excited piece of sealing-wax, bibit signs of electricity, oniy when the ele&ric fluid is
or other ele&ric, towards the brass cap EF.

сар

more or less condensed in the air, than in the earth. It is perhaps unneceffary to remark, that this obser- Though the air refifts the passage of the electric Auid, vation must be made in an open place, as the roads out it is not absolutely impermeable to it; it suffers it to pass of town, the fields, the top of a houfe, &c.

gradually, and generally with more ease in proportion The principal advantages of this electrometer, as stat- as its mass or thickness is lefu. It is therefore interested by Mr Cavallo, are as follows.

ing to discover at what height it is necessary to be ele. 1. The smallness of its fize. Mr Cavallo made one vated, in order to find a sensible difference between the fo small, that its case, which was of brass, measured electricity of the earth, and that of the air. A very only three inches and a half in length, and nine-tenths sensible difference may be generally discovered by this of an inch in diameter, and yet it acted perfeally well. instrument, at the distance of four or five feet from the

2. Its being always ready for experiments, without ground; sometimes it .may be seen if the instrument is fear of entangling the threads, or having an equivocal placed even on the ground; while at others, it must result by the fluggilhness of its motion.

be raised seven or more feet before the balls will open ; 3. Its not being disturbed by wind or rain.

sometimes, though seldom, this height is not fufficient. 4. Its great sensibility; and,

This distance is generally greatest when the electricity
So Its keeping the communicated electricity longer is strongest, though necessarily modified by a variety of
than any other electrometer.

circumltances, some of which are known, as the de-
II. Saufure's ELECTROMETER. M. de Saussure's gree of dryness or humidity of the air, and others are
electrometer, with which he made the observations on unknown.
atmospherical ele&ricity that have been related in the The degree of intensity, at a given beight, may be
second chapter of Part V. of the article ELECTRICI. discovered thus ; raise the electrometer, and judge by
TY, and represented at fig. 4. is much the same with the divisions which are placed on the edge of it, the de-
that of Mr Cavallo above described. The following gree of their divergence. To find the relation between
are the most material circumstances in which they dif- this degree of divergence, and the force of the electric
fer: First, the fine wires, by which the balls are suf- city, M. de Saussure took the following method : As
pended, should not be long enough to reach the tin- he could not with certainty double or triple a given
fuil which is pasted on the inside of the glass, because quantity of electricity; yet as a given force may be re-
the electricity, when strong, will cause them to touch duced one half, a fourth or eighth, &c. by dividing
this tin-foil twice consecutively, and thus deprive them between two equal and fimilar bodies, the electricity
in a moment of their electricity. To prevent this de- contained in one ; he took two of bis unarmed electro-
fect, and yet give them a sufficient degree of motion, meters, which were as similar as possible, and ele&tri-
it is necessary to use larger glasses than thofe that are fied one of them, so that the balls feparated precisely
generally applied to Mr Cavallo's electrometer ; tivo fix lines ; he then touched the top thereof by the top
or three incbes in diameter will be found to anfwer the of that which was not electrified ; in an instant the elec-
purpose very well. But as it is necessary to carry off tricity was equally divided between them, as was evi-
the ele&ricity which may be communicated to the in- dent by the divergence of the balls, which was four
fide of the glass, and thus be confounded with that lines in each ; confequently a diminution of half the
which belongs to those substances that are under exa- density had only leftened the divergence one-third.
mination ; four pieces of tin-foil should be pasted on the One of these electrometers was then deprived of its elec-
ingide of the glass ; the balls should not be more than tricity, and was afterwards brought in contact with the
one-twentieth of an inch diameter, suspended by filver other as before ; the remaining electricity divided it-
wires, moving freely in holes nicely rounded. The felf again between them, and the balls fell from four to
bottom of the electrometer should be of metal; for this twenty-eight lines, nearly in the same proportion as be-
renders it more easy to deprive it of any acquired elec- fure ; in the third operation they fell to nineteen ; in
tricity, by touching the bottom and top at the fame the fourth to one, where he was obliged to stop, as there
time.

was not now sufficient force in the fluid to pass from one
In order to collect a great quantity of ele&ricity ele&trometer to the other, and diftribute itself uniform-
from the air, the ele&rometer is furnished with a point- ly between them. The same experiment, repeated se-
ed wire, 15 inches or two feet long, which unscrews veral times, gave very nearly the same results. Nega-

in three or four pieces, to render the instrument more tive electricity decreased also in the same proportion as
Tig. 4. & s. portable ; see fig. 4. When it rains or snows, the small the positive. The following table may therefore be

parapluie, fig. 5. is to be ferewed on the top of the considered as giving a general, though not exact idea
inftrument, as by this its insulation is preserved, not- of the increase in force, which corresponds to different
withftanding the rain.

degrees of divergence in the balls; it is only calculated
This instrument indicates not only the electricity of to every fourth of a line; the force of eleátricity is al-
fogs, but that also of serene weather, and enables us to ways expressed by whole numbers, as it would be ridi-
discover the kind of electricity which reigns in the at- culous to put a greater degree of exactness in the num.
mosphere; and to a certain degree to form an estimate bers than is to be found in the experiments which form
of its quantity, and that under two different points of the bases of the calculation.

Distance

a

1

ter

2

2

10 II 12

14 17

32

36

20

40 44 48 $2 56

Electrome. Distance of the balls

Corresponding forces gence of the balls, he always endeavoured to obtain a Ele&rome. ter. in fourths of a line.

of electricity. permanent electricity in the foregoing manner.
I

The following example will render the use of the

foregoing observations more familiar. Choose an open 3

3

fituation free from trees and houses, screw the condudor 4

4

on the top of the electrometer, lay hold of it by its 5

base, and place it so that the base and conductor may 6

touch the ground at the same time; then elevate it to 8

the height of the eye, and observe the quantity of lines, 10

or fourths of a line, that the balls have diverged ; now 9

I 2

lower it till the balls almost touch each other, and ob.
serve at what distance the top of the conductor is from

the ground; and this is the height from the ground at
20

which the electricity of the air begins to be senfible. 13

23

If the electricity of the air is sufficiently strong to make 14

26

the balls diverge when it stands upon the ground, one 15

29

of the lengths of the electrometer must be unscrewed 16

from it. If the balls, however, still diverge, the other 17

parts of the conductor should also be unscrewed, and 18

you may mark down, that the electricity is sensible at 19

zero, or on the surface of the earth. If, on the con

trary, the electricity is so weak, as not to cause the 21

balls to diverge when they are even with the eye, and 22

consequently when the conductor is two feet higher, of 23

60

seven feet from the ground, you should then raise it a 24

64

foot higher ; while it is thus elevated, touch the top

with the other hand; when this hand is taken away,
Those who are desirous to carry this measure of the

lower the electrometer, and if it is electrified, you may
electric force further, may do it by having similar e. say the electricity is sengble at eight feet; if it is not,
lectrometers constructed, but made upon a larger scale, raise it as high as the arm can reach, and repeat the
and with heavier balls, which would only separate one fame operation ; if any electricity is found, write down
line, with the degree of electricity that makes the ele&ricity sensible at nine feet; if not, mark o, or no
Imaller ones diverge fix lines; these would consequently electricity relative to this inftrument, and this mode of
measure à force 1024 times greater than that which employing it; for signs of electricity may still be ob-
forms the unity of the preceding table ; and thus by tained, by throwing a metallic ball go or 60 feet into
degrees we may be enabled to discover the ratio of the the air, which is at the same time connected with the
strongest discharge of a great battery, or perhaps even electrometer by a metallic thread.
of thunder itself, to that of a piece of amber, which One advantage of this inftrument is, that it will often
only attracts a bit of Araw or any other light sub- exhibit figns of electricity when none can be obtained
Itance.

from a conductor of 100 feet in height, because it can
In order to observe the electricity of the atmosphere more easily be preserved from humidity, &c. which will
with this inftrument, we must first bring the electric destroy the insulation of the large conductors.
fluid contained in the electrometer to the fame degree This electrometer may be used instead of the con-
of density with that at the surface of the earth ; this is denser of M. Volta, by only placing it on a piece of
easily done by letting the bottom and top touch the viled Gilk, somewhat larger than the base of the inftru-
ground at the same time ; then raise the point, keep- ment; but in this case, it is the base, and not the
ing the bottom still in contact with the ground, from top of the instrument, which must be brought into
whence it may be lifted up in a vertical position till the contact with the substance whose electricity is to be ex-
balls are level with the eye.

plored.
The second circumstance is to render the divergence It is easy to discover also by this inftrument, the elec-
of the balls, which is occasioned by the ele&tricity of tricity of any substance, as of cloths, hair of different
the air, permanent. This is effected by touching the animals, &c. For this purpose, it must be held by the
top of the electrometer with the finger ; but here the base, and the substance rubbed brikly (only once) by
acquired electricity becomes contrary to that of the bo- the ball of the electrometer; the kind of electricity
dy by which they are electrified. Let us suppose, for may be ascertained in the usual manner.
example, that the electrometer is at five feet from the however, to observe here, that as the top of the electro-
ground, and the balls diverging; touch the top of the meter acts in this case as an insulated rubber, the elec-
electrometer with the finger, and the balls will close ; tricity it acquires is always contrary to that of the rub.
but they will again open, if the electrometer is with- bed body.
drawn from the influence of the ele&ricity of the air, III. Cadet's ELECTROMETER, is thus described by
by being brought nearer the ground, or into the house. the author, as translated in Nicholson's Journal.
M. Sauffure only employed this method when the eiec. Fig. 6. In a glass tube A, 18 or 20 inches long, Fig. 6.
tricity was so weak that he could not perceive any un- is inclosed another shorter tube X, sealed at both ends.
til the electrometer was raised considerably above his This tube contains a graduated scale : one of the ends

:
eye: as in this case he could not perceive the diver. of these two tubes is cemented in a handle of turned

wood,

It is proper,

A2

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