422 
ELECTRICITY. 
not fuffer this very weak eleCtricity to clear the firfl flep 
of the conduit. The marble muff be thoroughly dried, 
and even heated in an oven, and either ufed in this warm 
date, or yarniflied, fo as to prevent the re-abforption of 
moiflure. We know that marble of (lender dimenfions, 
fo as to be completely dried throughout, will not con- 
duCt till it has again become moift. A thick piece of 
marble is rendered fo, fuperficially only, and dill con¬ 
ducts internally. It is then in the bed poflible date. 
The fame may be faid of dry unbaked wood. Varnifhing 
the upper furface of a piece of marble or wood, is equi¬ 
valent to laying a thin glafs plate on it. Now this-me- 
thod, or covering the top of the marble, or of a book, or 
even the table, with a piece of clean dry fiIk, makes them 
all the mod perfeCt condenfators. This jufl view of the 
matter has great advantages. It takes away the myfte- 
rious indiflinCtnefs and obfcurity which kept the inflru- 
ment from improvement. We can now make one incom¬ 
parably better and more dmple than any propofed by 
Volta. We need only the dmple moveable plate. Let 
this be varnifhed on the under fide with a moderately 
thick coat of the pured and harded vernis dc Martin, or 
coach.painters varnilh ; and we have a complete conden- 
fator by laying it on a' table. If it be connected by a 
wire with the fubdance in which the weak and impercep¬ 
tible eleCtricity is excited, it will be raided (provided 
there be enough of it of that fmall intendty) in the pro¬ 
portion of the thicknefs of the varnidi to the fourth part 
of the diameter of the plate. This degree of condenfa- 
tion will be procured by detaching the connecting wire 
from the infulating handle of the condenfer, and then 
raiding the condenfer from the table. It will then give 
fparks, though the original eleCtricity could not fenfibly 
affeCt a flaxen fibre. 
It mud be particularly noted, that it can produce this 
condenfation only when there is eleCtric fluid to condenfe ; 
that is, only when the weak eleCtricity is diffuded over a 
greater fpace than the plate of the condenfer. lathis 
way it is a mod excellent collector of the weak atmofperic 
eleCtricity, and of all ditfuded eleCtricity. But to derive 
the fame advantage from it in many very intcrejiing cajes, 
fuch as the enquiry into the eleCtricity excited in many 
operations of nature on fmall quantities of matter, we mull 
have condenfers of various fizes, dome not larger than a 
diver penny. To condruCt thefe in perfection, we mud 
life the pured and harded varnidi, of a kind not apt to 
crack, and highly coercive. This requires experiment 
to difcover it. Spirit varnifhes are the mod coercive; 
but, by their difference of contraction by cold from that 
of metals, they foon appear frofiy, and when viewed 
through a lens, they appear fhivered ; and are then ufe- 
lefs. Oil varnifhes have die requifite toughnefs, but are 
much inferior in coercion. We have found amber var- 
nidi inferior to copal varnidi in this refpeCt, as was pre- 
vioufiy noticed by Von Haucli. On the whole, we diould 
prefer the fined coach-painters varnilh, new from the 
diop, into which a pencil has never been dipped : and 
we mud be particularly careful to clear our pencils of 
mcidure and all conducting matter, which never fails to 
taint the varnidi. We fcarcely need remark, that the 
coat of varnidi on thefe fmall condenfers diould be very 
thin, otherwife we lofe all the advantage of their fmall - 
nefs. Mr. Cavallo has ingenioully improved Volta’s 
condenfer by connecting the moveable plate, after re¬ 
moval, with a fmaller condenfer. 
It will readily occur to the reader, that this indrument 
is not indantaneuus in its operation, and that the appli¬ 
cation mud be continued for fome time, in order to col¬ 
lect the minute eleCtricity which may be excited in the 
operations of nature. He will alfo be careful that the 
experiment be fo conducted that no ufelefs accumulation 
is made any where elfe. When we expeCt eleCtricity 
from any chemical mixture, it never diould be made in a 
glafs veflel, for this will take a charge, and thus niay ab- 
iorb the whole excited eleCtricity, accumulating it in a 
neutral or infenfible date. Let themixturc be made in 
veffels of a conducting fubdance, infulated with as little 
contaCbas poflible with the infulating fupport; for here 
will alfo be fomething like a charge. Sufpend it by filk 
threads, or let it red on the tops of three glafs rods, &c. 
Whoever confiders this apparatus, will perceive that 
its office is not to manifed a fmall quantity of eleCtricity, 
but to condenfe an expanded quantity of eleCtricity into 
a fmall fpace : hence, if by means of this apparatus one 
expeCted to render more manifed than it generally is, 
when communicated immediately to an electrometer, the 
eleCtricity of a fmall tourmalin, or of a hair when rub¬ 
bed, he would dnd himfelf midaken. It is Mr. Bennet’s 
dcubler that was intended to anfwer that end ; viz. to 
multiply, by repeated doubling, a fmall, and otherwife 
unperceivable, quantity of eleCtricity, till it became fuf- 
ficient t© affeCt an eleCtrometer, to give fparks, See. The 
merit of this invention is certainly confiderable ; but the 
ufe of it is far from precife and certain ; and for this 
principal reafon, viz. becaufe it multiplies not only the 
eleCtricity which is communicated to it from the fub¬ 
dance in queltion ; but it multiplies alfo that eleCtricity 
which in the courfe of the operation is alrnod unavoid¬ 
ably produced by accidental friCtion ; or that quantity of 
eleCtricity, however fmall it may be, which adheres to 
the plates in fpite of every care and precaution. Mr, 
Cavallo has, however, with great indudry, fearched out 
all its imperfections, and has endeavoured to remove 
them by feveral ingenious condruCtions. But Mr. Ben- 
net’s original indrument, or doubler of eleblricity, has re¬ 
ceived its greated improvement from the hand of Mr. 
Nicholfon, whereby the machine becomes lefs liable to 
the objeCfjons above-mentioned. In its improved date, 
it confids of two infulated and immoveable plates about 
two inches in diameter, and a moveable plate alfo infu¬ 
lated, which revolves in a vertical plane parallel to the 
two immoveable plates, palling them alternately. A cor¬ 
rect view of the indrument is given at fig. 20, in the 
EleCtricity Plate III. of which the following is an ex¬ 
planation : The plate marked A, is condantly infulated, 
and receives the communicated eleCtricity. The plate B 
revolves; and, when it is oppofite the plate A, the con¬ 
necting wires at the end of the crofs piece D, touch the 
pins of A and C at E F, and a wire proceeding from the 
plate B, alfo touch the middle piece G, which is fup- 
ported by a brafs, wooden, or other conducting pillar in 
connection with the earth. In this podtion, if eleCtricity 
be communicated to the plate A, the plate B will acquire 
a contrary date ; and, pafling forwards, the' wires alfo 
moving with it by means of the fame infulating axis, the 
plates are again infulated till the plate B is oppolite to 
C, and then the wire at H touches the pin in C, connect¬ 
ing it with the earth, and communicating the contrary date ‘ 
of eleCtricity to that of B, but of the fame kind with that 
of A. By moving the handle dill farther, B is again 
brought oppofite to A ; and, the connecting'wires join¬ 
ing A and C, they both aCt upon B, which is connected 
with the earth as before, and nearly double its intenfity, 
whjfft the eleCtricity of C is abforbed into A ; becaufe 
of the increafed capacity of A, whild oppofed to B, ca¬ 
pable by its connection with the earth of acquiring a con¬ 
trary date, fufticient to balance the influence of both the 
other plates. Thus by continuing to revolve the plate 
B, the procefs is performed in a very expeditious man¬ 
ner. The ball I is made heavier on one fide than the 
ether, and ferewed upon the axis oppofite to the handle, 
to counterbalance the plate B, which may therefore be 
flopped in any part of its revolution. 
Mr. Nicholfon’s beautiful mechanifm for thus expe¬ 
diting the multiplication, has, however, the inconveni¬ 
ence of bringing the plates towards each other edgezvife, 
which will bring on a fpark or communication Tooner 
than may be defired : but this is no inconvenience what¬ 
ever in any philofophical refearch ; becaufe, before this 
happens, the eleCtricity has become very didinguifliable 
.as 
