ELECT 
trough the fqtinre hole h. The nut n fcrews on a , and 
fcrves to adjuft the light tube T vertically. The light 
plates PP are of copper, and move freely on the wire 
w rv fomewhat like a hinge, and reft on the copper wires 
CC, ferving to make the direCt communication between 
the intide-and out of th.e battery or jar. N N are notches 
ferving to let the tube T defcend when the difcharge is 
made. Into the tube Z the glafs pillar is ground. At 
the bottom of the notch N is a piece of brafs filed with a 
Y, and fo placed as to keep the centers of the balls L a.nd 
F under each other when they come clofe together. 
When the inftrument is adjufted, which is done by 
placing the weight W at o on the line of grains, and then 
fcrewing or unfcrevving the counterbalance-ball C, till 
the tube T rifes (lowly into its horizontal pofition ; then 
fet the ball A at the diftance from the ball L that you 
choofe, and the weight W placed at the divifion or num¬ 
ber of grains that you with the repulfive power of the 
electricity to arrive at before the difcharge is made : this 
being done, conneCt the battery or jar with the ball B by 
means of the wirejy, the end of which goes into B at the 
hole X-, and (liould (land at right angles to B, the ball 
of y relting on the battery then connect the outftde of 
the,battery or jar with the hook H ; ns the battery charges, 
the electrometer Qjeontinues to rife ; and when it is fo 
highly charged that the repulfive power between the 
balls L and F is equal to the number of grains at which 
the weight W was placed, the ball L will defcend, and 
deliver the charge of the battery to the ball A. The fiib- 
ftance or thing through which the (hock is intended to 
be palled, mult form part of the communication between 
the hook H and the outfide of the battery or jar. It is evi¬ 
dent, from confidering the inftrument, that it is attended 
with thefe excellencies; that, the whole being compaCt, 
the eleCtric atmofphere mud be fo alfo ; that it has but 
one inhibition to depend upon : that the equilibrium of 
the divided arm and ball may always be. adjufted, confe- 
-quently the weight ufed known-to a certainty ; and th.e 
hook for communicating with the outfide of the battery 
being placed on the toot of the inftrument, the communi¬ 
cation may be made by wires lying on the table-; which 
are by far the belt mode of conducting ftiocks; as chains, 
by the imperfeCt connection of their links, (efpecially if 
long,) very often, and without being fufpeCted, hinder the 
fuccefs of many very well-concerted experiments.” The 
electrometers of Bennett, Brookes, M. de Luc, and fome 
others, have been fuperfeded in their ule and application 
by thefe later improvements. 
An electrometer has been recently invented, capable 
of exhibiting abfolute meafiires, and fitfceptible of com- 
parifon with others: whence it is denominated tlie com¬ 
parable ekElrometer. This inftrument is (hewn at fig. 17, 
in the annexed engraving ; where A reprefents a poll (bed 
brafs ball, one-fourth of an inch in diameter. It is fixed 
on the point of a needle three inches long, as (lender as 
can be had of that.length. The other end of the needle 
paftes through a ball of,amber or glafs, or other firm 
non-conduCting fubftance, about half or three-fourths of 
an inch in diameter ; but the end does not reach quite to 
the furface, although the ball is completely perforated. 
From this ball rifiesta glafs rod PEL, three inches long 
from F to E, where it bends at right angles, and is corn 
tinued on to L, immediately over the center of the ball 
A. At L is fixed a piece of amber C, formed into two 
parallel cheeks, between which hangs the ftalk D C B of 
the electrometer. This is formed "by dipping a ftrong 
and dry filk thread, or fine cord, in melted fealing-wax, 
and holding it perpendicular till if remain covered with 
a thin coating, and be fully penetrated by it. This ftalk 
is fattened into a fmall cube of amber, perforated on pur- 
pofe, and having fine holes drilled in two of its oppolite 
tides. The cheeks of the piece C are wide enough to al¬ 
low this cube to move freely between them, round two 
fine pms, which are thruft through the holes in the 
cheeks, and reach about half way to the ftalk. The 
Vol, VI. No. 360, 
R I C [ T Y. 417 
lower part of the ftalk is about three inches long, and 
terminates in a gilt and buruiihed coik ball, (or may be 
made of thin metal,) a quarter-of an inch in diameter. 
The upper part CD is of the fame length, and paftes 
through (with fome friction) a fmall cork ball. This 
part of the inftrument is fo proportioned, that when KE 
is perpendicular to the horizon, and DC.B hanrs ir.eeiy, 
the balls B and A juft touch each other. The ball F is 
fixed on the end of the glafs rod FI, which paftes per-- 
pendicularly through the centre of a graduated circle 
G H O, and has a knob handle of boxwood on the farther 
end at I. This ghafs rod turns ftiftiy, but fmoothly, m 
the head of the pillar HK, &c. and has an index NH, 
which turns round it. This index is fet parallel to the 
line L A, drawn through the centre of the fixed ball of 
the electrometer. The circle is divided into 360 degrees, 
and o is placed uppermoft, and go on the right hand. 
Thus the index will point out the angle which LA makes 
with the vertical. It will be convenient to have another 
index, turning ftiftiy on the fame axis, and extending a 
good way beyond the circle. 
This inftrument is ufed in the following manner : A 
connection is made with the body whofe electricity is to 
be examined, by (ticking the point of the connecting w ire 
into the hole at F, till it touch the end of the needle ; 
or, if we would merely eleftrify the balls A and B, and 
then leave them infalated, we have only to touch one 01 
them with an electrified body. Novi', take hold of the 
handle I, and turn it to the right till the index reach 90. 
In this pofition, the line LA is horizontal, and fo is C B ; 
and the moveable ball B is reding on A, and is carried 
by it. Now electrify the balls, and gently turn the han¬ 
dle backwards, bringing the index back toward o, See. 
noticing carefully the two balls. It will happen that, 
in fome particular pofition of the index, they will be ob-- 
lerved to feparate'. Bring them together again, and again 
caufe them to feparate, till the exaCt pofition at repara¬ 
tion is afeertained. This will (hew their repulfive force 
in contaCt, or at the diftance of their centers, equal to 
the fum of their radii. Having determined this point, 
turn the inftrument (till more toward the vertical pofition. 
The balls will now feparate more and more. Let an af- 
fiftant turn the long index fo as to make it paraLlel to the 
ftalk of the electrometer, by making the one hide the 
other from his view. Thus it will give abfolute mea- 
fures : for, by poifing the ftalk, by’ laying fome grains 
weight on the cork-ball D, till it becomes horizontal and 
perfectly balanced, and computing for the proportional 
lengths of BC and DC, we know exaCtly the number 
of grains with which the balls, muft repel each other 
(when the (talk is' in an horizontal pofition), in order 
merely to feparate. Then a very fimple computation, will 
ascertain the grains of repulfion when they feparate in 
any oblique pofition of the ftalk ; and another computa¬ 
tion, by the refolution of forces, will fliew 11s {he repul¬ 
fion exerted between them when A L is oblique, and B C 
makes any given angle with it. This is Tlifficiently ob- 
_yious to need no farther explanation. The reafon for 
giving the connection between A and C fuch a circuitous 
form, was to avoid all action between the fixed and the 
moveable part of the eleCtrometer, except what is ex¬ 
erted ber een the two balls A and B. This inftrument 
will alio meafure the at.traBions of the oppolitely electri¬ 
fied bodies ; for inftead of making the ball B above A, 
by inclining the inftrument to the right hand, we may 
incline.it to the left; and then, by electrifying one of the 
balls pofitively, and the other negatively, when at a great 
diftance from each other, their mutual attraction will 
caufe them to approach ; CB will deviate from the'ver- 
tical toward A ; and we can compute the force by means 
of this deviation. But this inftrument muft be fecured 
againft diftipation as much as pollible, by varnithing all 
its parts, by having all angles, points, and roughneftesj 
removed, and by choofing a dry ftate of the air, and a 
warm room ; and, becaufe it is impoflible to prevent dif- 
5 O fipatiou 
