418 
ELECTRICITY. 
fixation altogether, we fliould make a previous courfe of 
experiments, in a variety of circuniftancesj in order to 
determineMie diminution per minute correfponding to the 
circumftances of th.e experiments that are to be made with 
further views. 
A mod valuable eleClrometcr has been invented by Mr. 
Coulomb, of the royal academy of fciences at Paris, de¬ 
fended in their Memoirs for 1785. It is conftrutted as 
reprefented in the engraving at fig. 18. ■ ABDC repre¬ 
sents a glafs cylinder, twelve inches in diameter and in 
’ height. This is covered by a glafs plate fitted to it by 
a projecting fillet on the under furface. This cover is 
pierced with two round holes of one inch three-fourths 
in diameter. One of them, is in the center, and it re¬ 
ceives thejower end of the glafs tube fk, of twenty-four 
inches-bright, yviiich is fixed in the hole with a cement 
madeof feeding wax, or other eledtric fubfiance. The 
top of this tube receives the brafs collar H, bored truly, 
cylindrical, and having a finall fhoulder, which refts or: 
the top of the tube. This collar is fattened with cement, 
and receives the hollow cylinder P, to which is joined 
the circular plate ab, divided on the edge into 360 de¬ 
grees. It is alfo pierced with a round hole in the center, 
which receives the cylindrical pin z, having, a milled 
head, and an index o, vvhofe point is bent down, fo as to 
mark the divifions on the circle ab. This pin turns ftiflly 
in the cylinder P, which turns fteadily in the collar H. 
To the lower end of the center pin is fattened a little pin- 
ccr, formed like the end of 3 port-crayon, and tightened 
by a ring, fo as to hold faft the fnfpenfion-wire h f the 
lower end of which is grafped by a fimilar pincer P. The 
lower end o, is cylindrical, and it is of fuch weight'as' to 
drain the wire perfectly ftraight, but without any rifk of 
breaking it. It may be made half of the weight that will 
jutt break if. 
This pincer is enlarged at E, and pierced with a hole, 
which receives tightly the arm gq, of the electrometer. 
This is eight inches long, and conlifts of a dry fi Ik thread, 
or flender draw of (ome grafs completely dried, and dip¬ 
ped in melted gum lac or fine fe.fling-wax, and held up¬ 
right before a clear fire, till it form a flender cylinder of 
about i-qoth of an inch in diameter. This occupies fix 
of tire eight inches, from g to q: the remaining two 
inches is a fine thread of the lac or fealing-wax, as it 
drains off in forming-the arm. A t a is a ball of pith of 
elder or fine cork, one-fourth or one-half of an inch in 
diameter, made very fmooth, and gilded. It is balanced 
by a vertical circle g, of, paper ftiftened with varnifli. 
The whole is feen in its place in the engraving, where the 
arm hang's horizontally about the middle of the height of 
the-great cylinder. In its ofcillations the ball a moves 
round in a circle, wdrofe center is in the axis of the whole 
inftrument. Its fituation is indicated by a graduated cir¬ 
cle Z OQ^drawn on a flip of paper, and adhering to the 
glafs with varnifli. The electrified, body, whole aCtion 
is to be obferved, is another (mall ball of cork t, alfo 
gilt, or a brafs ball well poliflied. This is carried by a 
(talk of gum lac mn } incloling a dry fllk thread. This 
(talk is grafped by a clamp of cleft deal, or any fimilar 
contrivance, which lies firm on the glafs cover. When 
this ball is let down through the hole m, it Hands fo as 
to touch the ball a on the arm, when that ball is oppo¬ 
site o on the graduated circle. 
To eleClriiy the ball t, we employ a flender flick of' 
fealing-wax or lac, holding a mefal wire that carries a 
fm-all poliflied metal ball. We touch with it foma elec¬ 
trified body, fucli as the prime conductor of a machine, 
the knob of a jar, &c. Introduce this electrified ball 
cailtioufly into the hole m, and touch the ball t with it. 
T he ball a is immediately repelled, and goes to a dif- 
tance, twitting the fiifpenli-on-wire, till the force of twift 
exerted by the wire balances the mutual repullion of the 
balls t and a. 
Such is the procefs for examining the law of eleCtric 
action. But when we would examine the aCtion of dif. 
ferent bodies in different ttates, another apparatus is want 
ed. This is reprefented by the piece c A d, confiding of 
a plug of fealing-wax A, which fits tight into the hole in, 
and is pierced 'by the wire cd, hooked at c, to receive a 
wire connecting it occalionally with an electrified body, 
and having below a poliflied metal ball d, which fits into 
the fame pofition as the ball t. 
The inftrument is fitted for ufe in the following 
manner: Turn the milled button c, at top, till the twift- 
index z, is on the mark o, of the twift circle. Then turn 
the whole in the collar II, till the ball a (lands oppoiite 
to the mark o of the paper circle Z O Qjmd at the fame 
time touches the ball t or d. 
The observation is made thus r the ball t or d, is elec¬ 
trified, and a is repelled, and retires, twitting the wire ; 
and, after a few ofcillations, fettles at a diftafite.corre¬ 
fponding to the repulflon. Now' turn the twift-index, 
fo as to force the ball a nearer to t or d, as occafion may 
require- We eftimate the force of this new repulflon by 
adding the motion of the twift-index to the angle at which 
the ball firtt retted. By turning the twift index dill more, 
we bring the balls (till nearer, and have a meafure of an¬ 
other repulflon. And thus we may obtain as many mea- 
fures as we pleafe. 
In this way Monf. Coulomb afeertained the relation 
between the repulflon and the distance to be the inverfe 
duplicate ratio of the diftances. He difcovered the law 
of diflipation by air in contact, and the relation which 
this bears to the primitive repullion, by obferving the 
gradual approach of a tor as the electricity diffi pates from 
both, and by Thickening the twdft-index till the ball a re¬ 
tires to it its primitive diftance. He afeertained the dif- 
fipation along imperfeCt conductors, and the length ne- 
ceflary for infulation, by completely infulating the ball t., 
and obferving the lofs by air in contaCt with it, and then 
Aiding a metal rod down the infulating (talk, till the dif- 
fipation began to exceed what took place by the air alone. 
ITe examined .the proportion of redundant fluid in com¬ 
municating bodies, by connefting.them alternately with 
the ball d ; as alfo by electrifying one ball, and obferving 
its repulfiye force, and then (haring its electricity with 
another, and obferving the diminution. An example of 
one or two of thofe trials will give a clearer conception 
of the concluflons deduced from tliefe obfervations. 
The ball t was introduced and electrified ; a was re¬ 
pelled, and fettled at 40° ; the index was twitted 140 0 ,. 
which brought a to 20 ; and the time was noted. The 
eleCtricity gradually diflipa-ted, and a came nearer to t. 
The index was untwifted 30 0 , and a retired a little be¬ 
yond 20 0 ; but, on waiting a few feconds, it flood exaClly 
at 20 0 . The time was again noted. The interval was 
exaCtlythree minutes. The conclufion from the experi¬ 
ment was as follows : When the ball was brought to 20 0 , 
the repulfion was evidently 140 -f 20, or 160. Three mi¬ 
nutes afterwards it was no + 20, or 130; and 30 0 were 
left in three minutes, or io° per minute. T he mean 
force was 145. Therefore the me'an lofs per minute was 
Obferve alfo, that the primitive foice correfpond¬ 
ing to the diftance was 40; and the force correfpondi'ng 
to 20 was 160, or inverfely, as 20' to 40 1 . But obferve, 
that the diftances were not meafured by the angles, but 
by the chord of the angles. The obliquity of aCtion. 
mull alfo be accounted for; and the real lever is lefs 
than the arm, in the proportion of radius to the cofuie of: 
half the angle. 
The wire ufed by Coulomb in his firtt experiments on 
the law of aCtion was of fuch ftrength, that of a 
French grain, applied at the point a, hCld it fall till the 
twift-index. was turned 360° ; fo that one degree corre- 
fponded to ot a grain. A foot of this wire weigh¬ 
ed ^th of agrain. Experience having (hewn that this 
was a fallibility far exceeding what was neceff’ary for the 
meafures that he had in view, and made the inftrument 
too delicate for common ufes, he fubftitiUed much (tronger 
ancl fhorter wires, and recommends much (mailer dimen- 
ftons 
