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ADD. VOL.

ENCYCLOPÆDIA BRITANNICA.

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E L E

E L E
Electricity E

LECTRICITY, MEDICAL. See MATERIA ME- can conveniently be made, and they are suspended by Electrome.
DICA.

exceeding fine silver wires. These wires are shaped in Electronac. 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 flat piece of ivory H, which has two boles for that pur-
of amber (electrum) which they produced. They pose. By this method of suspension, wbich is applica.
were at the mouth of the Po, according to Apollonius ble to every sort of electrometer, the friction is lessened
of Rhodes; but some historians doubt of their exis- almost to nothing, and thence the instrument is sen-

sible 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 instruments for ascertaining the presence of electri- bottom AB; they serve to convey off that electricity,
city, 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, selves, we shall give an account of them here.

this electrometer is set upon a table or other convenient Plate CC.,

Fig. 1. Plate cc. is a geometrical representation support; then it is electrified by touching the brass cap
Fig. 1. of Mr Cavallo's improved atmospherical electrometer, EF with an electrified body, wbich electricity will

of half its real size. The principal part of this in- sometimes be preserved for more than an hour. Mr
strument is a glass tube CDMN, cemented at the bota Cavallo had one of these electrometers which would re-
tom into the wooden piece AB, by which part the instru- 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 electrified 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 shew the species of that
smaller extremity, which is entirely covered with seal. body's electricity.
ing-wax, melted by heat, and not dissolved in spirits. It is necessary to remark, that to communicate any
Into this tapering part a small tube is cemented, the electricity to this electrometer, by means of an excited
lower extremity Ğ of which being also covered with seal. electric, e. g. a piece of sealing-wax, (which we sup-
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, wbich with its dily done in the usual manner, on account of the cap
lower extremity touches the flat piece of ivory H, fasten- 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 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 was so near Fig. 3.

part of the instrument from the rain, &c. In fig. 3. the cap, that one or both the corks may touch the side
à section of this brass cap is represented, in order to of the bottle CDM; after which, they will soon col-
show its internal shape, 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 sealing-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 upscrew it from its case,
their repulsion show the electricity, &c. are as small as and, holding it by the bottom AB, to present it to the
Vol. VIII. Part I.
+

open

A

ter.

cap

Fig. 4.

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

verge if there be any sufficient quantity of electricity ; A conductor raised for the purpose of making obser-
whose nature, i. e. whether positive or negative, may vation on atmospherical electricity will be found to ex-
be ascertained by bringing an excited piece of sealing. hibit signs of electricity, only when the electric fluid is
wax, or other electric, towards the brass EF. more or less condensed in the air, than in the earth.

It is perhaps unnecessary to remark, that this obser- Though the air resists the passage of the electric fluid,
vation must be made in an open place, as the roads out it is not absolutely impermeable to it; it suflers it to pass
of town, the fields, the top of a house, &c.

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

ing to discover at what height it is necessary to be ele1. The smallness of its size. Mr Cavallo made one vated, in order to find a sensible difference between the so 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 perfectly 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 sluggishness 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 heiglit is not sufficient. 4. Its great sensibility; and,

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

circumstances, some of which are known, as the de-
II. Saussure's ELECTROMETER. M. de Saussure's gree of dryness or humidity of the air, and others are
electrometer, with which he made tbe observations on unknown.
atmospherical electricity that have been related in the The degree of intensity, at a given height, 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 electri-
fer: First, the fine wires, by which the balls are sus- 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
foil 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 ball, a fourth or eighth, &c. by dividing
this tin-foil twice consecutively, and thus deprive them between two equal and similar bodies, tbe electricity
in a moment of their electricity. To prevent this de- contained in one ; he took two of bis unarmed electró.
fect, and yet give them a sufficient degree of motion, meters, which were as similar as possible, and electri-
it is necessary to use larger glasses than those that are fied one of them, so that the balls separated precisely
generally applied to Mr Cavallo's electrometer; two six lines; he then touched the top thereof by the top
or three inches in diameter will be found to answer 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 electricity which may be communicated to the in- dent by the divergence of the balls, which was four
side of the glass, and thus be confounded with that lines in each; consequently a diminution of half the
which belongs to those substances that are under exa- density bad only lessened 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-
inside 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 silver other as before ; the remaining electricity divided it.
wires, moving freely in holes nicely roanded. The self 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. fore; in the third operation they fell to pineteen ; in
tricity, by touching the bottom and top at the same 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 electricity electrometer to the other, and distribute itself uniform-
from the air, the electrometer is furnished with a point ly between them. The same experiment, repeated se-
ed wire, 15 inches or two feet long, which anscrews 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

portable ; see fig. 4. When it rains or snows, the small the positive. The following table may therefore be Fig. 4. & 5. parapluie, fig. 5. is to be screwed on the top of the considered as giving a general

, thoagli noi exact idea instrument, as by this its insulation is preserved, not- of the increase in force, which corresponds to different withstanding 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 electricity is alfogs, but that also of serene weather, and enables us to ways expressed by whole numbers, as it would be rididiscover the kind of electricity which reigns in the at- culous to put a greater degree of exactness in the nummosphere ; 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

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10

17

23

36

40 44

48

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52 56

60

64

Electrome. Distance of the balls

Corresponding forces gence of the balls, he always endeavoured to obtain a Electromein fourths of a line.

of electricity permanent electricity in the foregoing manner. I

I

The following example will render the use of the 2

foregoing observations more familiar. Choose an open 3

3

situation free from trees and houses, screw the conduc4

4

tor on the top of the electrometer, lay bold of it by its 5

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

touch the ground at the same time ; then elevate it to 7

8

the height of the eye, and observe the quantity of lines, 8

10

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

12

lower it till the balls almost touch each other, and ob14

serve at what distance the top of the conductor is from II

the ground ; and this is the height from the ground at 12

20

which the electricity of the air begins to be sensible. If 13

the electricity of the air is sufficiently strong to make 14

26

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

29

of the lengths of the electrometer must be unscrewed 16

32

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

parts of the condactor should also be unscrewed, and

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

zero, or on the surface of the earth. If, on the con20

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, or 23

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

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

with the other hand ; when this band 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 elec- say the electricity is sensible at eight feet; if it is not,
trometers 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 same operation; if any electricity is found, write down
line, with the degree of electricity that makes the electricity sensible at nine feet; 'if not, mark o, or no
smaller ones diverge six lines ; these would consequent- electricity relative to this instrument, and this mode of
ly measure a 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 so 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 thander itself, to that of a piece of amber, which One advantage of this instrument is, that it will often
only attracts a bit of straw or any other light sub- exhibit signs of electricity when none can be obtained
stance.

from a conductor of 100 feet in heigbt, because it can
In order to observe the electricity of the atmosphere more easily be preserved from humidity, &c. which will
with this instrument, we must first bring the electric destroy the insulation of the large conductors.
fluid contained in the electrometer to the same 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 oiled silk, somewhat larger than the base of the instru-
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
balls are level with the eye.

explored.
The second circumstance is to render the divergence It is easy to discover also by this instrument, the elec-
of the balls, which is occasioned by the electricity of tricity of any snbstance, 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 briskly (only once) by
acquired electricity becomes contrary to that of the the ball of the electrometer ; the kind of electricity
body by which they are electrified. Let us suppose, for may be ascertained in the usual manner. It is proper,
example, that the electrometer is at five feet from the however, to observe bere, that as the top of the electro-
ground, and the balls diverging; touch the top of the meter acts in this case as au insulated rubber, the elec-
electrometer with the finger, and the balls will close ; tricity at 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 electricity of the air, III. Cadet's ELECTROM ET ER, is thus described by
ljy being brought nearer the ground, or into the house. the author, as translated in Nicholson's Journal.
N. Saussure only employed this method when the elec- Fig. 6. In a glass tube A, 18 or 20 inches long, Fig. 6.
tricity was so weak that he coald 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 bandle of turned

A 2

wood,

Electrome. wood, C, by which it is held in the hand; the other will be observed upon the ball of the jar, and another Electrome-
ter.
end is closed by a brass cap, D; the distance between at the end of the point.

ter.
the extremities of the small tube and that of the large Let us now apply this electrometer to useful observa.
one is filled with red wax, B, B; on the cap D is tions.
screwed at pleasure, either a ring E, or a brass hook F. In order to connect the idea of a determinate quanti.
The ring is used for applying the instrument to the ty of fluid to each degree of the electrometer, it is neces-
ball of a conductor, and the hook when it is lungsary to compare these degrees with the known quanti-
to a ring: on the cap D is a brass stem G, termi- ties. Suppose for instance we bave a jar, the coating
nating by a knob. This stem is bended, and the of which is six inches square; electrify it till a spon-
extremity of its knob must be directly beneath the line taneous discharge takes place, and remark, by means.
with which the graduated scale of the small tube com- of Henley's electrometer, at what degree this discharge
mences.

is-effected. Again, electrify the jar, till it is nearly sa-
Round the large tube is a brass ring H, half of which torated, and measuring with this electrometer, observe,
extends to the length of twelve or fifteen lines in the that the luminous point appears for instance at two de-
form of a half tube P, applied against the sides of the grees; then say, that when the electrometer, applied
tube. This gutter serves to mark the degrees, by slid- to an electrified body, marks two degrees, the body
ing along the graduated scale by means of a button contains six inches square of electricity. Repeat this
beneath 1. On the ring H is fixed one of the small experiment with a plate of glass, the coating of whicb is
electrometers invented by Saussure, K, K, which is sur- seven, eight, ten, or twelve square inches, and we may
mounted by a stem V, on which stem is fixed at pleasure form a scale of proportion, which is of the greatest uti-
either a point L, or a ball M, of the same size as that lity in aceurate experiments.
which terminates the stem G, opposite which it is " In endeavouring to ascertain some of these pro-
placed. The extremity of this point or ball must be positions, (says M. Cadet), 1 bave made an observation
placed immediately over the extremity of the balf tube which has convinced me of the utility of my electrome-
or scale P, and horizontally to the centre of the ball, ter in discovering the capacity of electric apparatus..
which terminates the stem G.

Having taken a jar from an electric battery, I electri-
At the top of Saussure's electrometer is a emall ring fied it, and measured it with a point which I passed
N, which serves to connect it with the chain 2 when along a string of silk; on observing the distance at wbich
required.

the luminous point appeared, I joined this jar to another
To explain the use of this instrument by a single ex- of the same size, and imagined that by doubling the
periment, charge a Leyden jar, till the spontaneous quantity of matter, the measure I had taken would also
overflowing announces it to be saturated. Then place be doubled ; on the contrary, however, the latter
the ring E on the knob of this bottle, and cause the measure was not more than about one-third of the for.
clectrometer of Saussure, armed with its point, to slide mer: I then added a third bottle; and still obtained
towards it. Observe the degree at which the divergence nearly the same result; whence the following proposi-
of the thread stream commences, and at that instant tion appears to be established; namely, that ihe extent
suppress the point, and adapt in its place the ball M. of the electric atmosphere is in an inverse ratio to the
Continue to advance the electrometer of Saussure till quantity of fluid accumulated. Another observation
the electric pressure of the atmosphere in the jar eauses which I have several times made, on measuring the elec-
the threads to diverge; again observe the degrees, re- tric atmosphere of a conductor, is, that the limits of this
place the point L, and close the shutters of the room; atmosphere form an elliptic figure around the body,
then continue to advance the electrometer till the lumi- nearly similar to that represented at fig. 7.
nous point appears, which again affords new degrees. “ This doubtless arises from the electrified body
Lastly, replace the ball M, and fix the chain Z to the suspended in a chamher, being nearer to the earth than
small ring N: cause it to communicate with the exte- the ceiling; but it would be a curious experiment to
rior coating of the jar, and advance the electrometer till measure it at an equal distance from every attracting
the explosion takes place. Then comparing the different body, in order to observe whether the fluid bas. not
degrees, we may ascertain the comparative difference really a tendency to descend towards the earth, rather
between the respective methods.

than in any other direction. It is my intention to re-
As soon as these relative proportions bave been once peat this experiment, as I consider it of great impor-
accurately ascertained by attentive observations, one of tance to ascertain whether electricity gravitates towards
those methods alone will be sufficient for measuring the the globe.
intensity of electricity; and, in fact, if the body intend- • From these first attempts, I conceive my

electron.e-
ed to be submitted to examination be little charged with ter would be well adapted for measuring ihe absolute
the electric fluid, the diverging of the threads, by means capacity of Leyden jars, and also their capacity with
of the point, will fix the limits of the electric atmo- regard to their size, or to the quality of the glass of
sphere: if it be more, the pressure of the atmosphere on which they are constructed; for the latter, by its
the ball M, which is substituted for the point, will in. greater or less density, absorbs a greater or less quantity
dicate this quantity. In short, if the body be loaded of fluid.”
with a considerable mass of electric natter, it will be IV. Lawson's ELECTROMETER. This is a simplified
shown by the luminous point. If a Leyden jar, instead improvement on Brooke's steelyard electrometer, and
of being positively, is negatively electrified, the point should have been described when that instrument was
indicates it at the same time that it measures the elec- mentioned, instead of Mr Adams's: but it did not oce
tric atmosphere, for instead of a luminous point, a star cur to us till after that sheet was printed..

Fig. Ti

1

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