ELECTRICITY. 410 
flons for tlie whole inftrument. It is far more fenfible 
than Bennet’s gold-leaf electrometer. The fame inftru-' 
ment, with a filver wire fufpenfion, and a thread of lac 
projecting from the end g , as an index to coincide more 
clofely with the fcale, is fufficiently nice for all experi¬ 
ments of meafurement. It is always proper to have the 
diameter of the cylinder double the length of the arm, 
that the aCtion of the glafs may not difturb the pofition 
of the arm. It is greatly improved by having a round 
hole in the bottom of the inftrument, in which the cy¬ 
linder EO of the lower pincer may hang freely : this pre¬ 
vents much tedious ofcillation. For ordinary experi¬ 
ments, for meafuring charges of*batteries, and the like, 
a much'lefs delicate inftrument, with a-fufpenfion-wire 
drained at both ends, is abundantly delicate, and vaftly 
more manageable. The wire fliould extend as far below 
the arm as above it, and fttould be grafped below, by a 
pincer turning by a milled head in a hole at the end of a 
tlender fpring. This enables us to adjuft the inftrument 
fpeedily. Having placed the twift-index at o, turn this 
lower button gently till the ball a points exaCtly to o on 
the paper circle. Coulomb’s electrometer has the great 
advantage of wafting very little electricity ; whereas Hen¬ 
ley’s, or Brookes’s, or de Luc’s, wafte it very fall when 
it is intenfe. 
This electrometer, as hitherto applied, meafures only 
repulfions. It is not fo eafy to meafure attractions with 
it; for even Mr. Coulomb was obliged to take a very 
circuitous method, during which a great deal of electri¬ 
city was didipated. In this refpeCt, the comparable e: lec- 
trometer above .deferibed has the advantage ; but, in 
every other refpeCt, Mr. Coulomb’s is the fineft electro¬ 
meter that has yet been publifhed, giving abfolute mea¬ 
fures, and this with great accuracy. The honourable Mr. 
Cavendifh has employed tlm conftruCtion in his molt va¬ 
luable experiments on the force of gravity, (Phil. Tranf. 
179S, part ii.) an experiment which Newton would have 
been delighted with obferving. 
The next inftrument to be noticed is the elettrophor, or 
ekElrophorus ; this confifts of three diftinCt parts, as (hewn 
in the engraving, at fig. 19. The chief part is a cake 
A BCD, made of fome eledric ; fuch as gum-lac, feal- 
iim-wax, pitch, or other refinous compofition. This is 
melted on a conducting plate, D C FE, and allowed to 
congeal; in which ftate it is found to be negatively elec- 
tricT Another conducting plate, GHBA, is laid at top, 
and may be railed up by lilk lines, or any inhabiting han¬ 
dle. Hence we filial 1 call A BCD the cake, DCFE the 
fde, and GHBA the cover. It i^ proper to make all the 
three .parts of confiderable thicknefs, and of no great 
breadth. Although this diminilhes. greatly the moll re¬ 
markable of the aCtions, it leaves them fufficiently vivid, 
and it greatly increafes the fmaller changes which are in- 
ftruCtive in the companion. The general faCts are, 
If the foie has been infulated during the congelation of 
the eleCt^ic, till all is cold and hard, the whole is found 
negatively eleCtric, and the finger draws a fpark from any 
part of it, efpecially from the foie. If allowed to remain 
in this fituation, its eleCtricity grows gradually weaker, 
and at laft difappears : but it may be excited again by 
rubbing the cake with dry warm fiannel, or, which is 
better, with dry and warm cat or hare fur. If the co¬ 
ver be now fet on the cake by its infu.lating handle, but 
without touching the cover, and again fepamed from the 
cake, no eleCtricity whatever is obferved in the cover. 
But if it be touched while on the cake, a (harp pungent 
fpark is obtained from it; and if, at the fame time, the 
foie be touched with the thumb, a very fenfible fliock is 
felt in the,finger and thumb. 
After this, the elefitrophorus appears quite inactive, 
and is laid to be dead ; neither foie nor cover giving any, 
ftgn of eleCtricity. But, when the coverus railed to fome 
diftance from the cake (keeping it parallel therewith), if 
it be touched while in this fituation, a fmart fpark flies, 
to fome diftance, - between it and the finger, more rentalk- 
ably from the upper fide, and ftill more from its edge, 
which will even throw ofF [parks into the air, if it be 
not rounded off. As this diminimes the defired effeCts, 
it is proper to have the edge fo rounded. This fpark is 
not fo fiiarp as-the former, and refembles that from any 
eleCtnfied conductor. The eleCtricity of the cover, while 
thus railed, is of the oppofite kind to that of the cake, 
or is pofitivc. The eleCtricity of the cover while lying 
on the cake is the fame with that of the cake, or negative. 
Thefe*appearanees may be repeated for a very long time, 
without any fenfible diminution of their vivacity. The 
inftrument has been known to retain its power undimi- 
nifhed even for months. This makes it a fort of maga¬ 
zine'of eleCtricity, and we can take off the eleCtricity of 
the cake and of the cover as charges for feparate jars, 
the cover, when raifed, charging like the prime conduc¬ 
tor of an ordinary eleCtrical machine ; and, when fet on 
the cake, charging it like the rubber. This caufed its 
inventor, Mr. Volta, to give it the name eleBrophorus. 
If the foie be infulated before putting on the cover, 
tlie fpark obtained from the cover is not fo aCtive as it 
was before : but the fame fhock will be felt if both cake 
and cover be touched together. If the cover be again 
raifed to a confiderable height, the foie will be found 
eleCtrical, and its eleCtricity is that of the cake, and op¬ 
pofite to that of the cover. After touching both cover 
and foie, if the cover be raifed and again fet down, with¬ 
out touching it while aloft, the whole is again inactive. 
If both cover and foie be made inactive when joined, 
they fttew oppofite electricities when feparated, the foie 
having the eleCtricity of the cake. If both cover and 
foie be made inactive when feparate, they both (hew the 
oppofite to the eleCtricity of the cake when joined. 
Let 11s now attend to the difpofition of the eleCtrical 
fluid in the different parts of the inftrument in thei'r va¬ 
rious fituations, and to the forces which operate mutu¬ 
ally between them. Experiments for examining this in¬ 
ftrument are belt made by fetting the three plates verti¬ 
cally, ltipported on glafs (talks, with leaden feet, to fteady 
them. A very fmall electrometer may be attached to 
the outer furfaces of the cover and foie. 
If the extent of the plates were incomparably greater 
than their thicknefs, we may infer that the redundant 
fluid and matter would be difpofed in parallel ftrata, and 
that the aCtions would be the fame'at all diftances. But 
(ince this is not the cafe, the difpofition of the fluid will 
be fomewhat different; and whatever it is, the aCtion of 
any ftratum will be diminifhed by an increafe of diftance. 
The' following defeription cannot be very dift'erent from, 
the truth : 
1. The cake grows negative by cooling; and .if. it 
were alone, it would have a negative fuperficial ftratum 
on both (ides, of greater thicknefs near the edges ; and 
theTuid would probably grow denfer by degrees to the 
middle, where it woqld have its natural denlity. But it 
cools in conjunction with the foie, and the attraction of 
the redundant matter in the cake for the moveable fluid 
in the foie difturbs its uniform diffufion in the foie, and 
cattles it to approach the cake. And becaufe this, in all 
probability, happens while the cake is ftill a conductor, . 
the difpofition of its fluid will be different from that de-. 
feribed above. But becaufe we do not know precifely 
the gradation of denfity, and aim only at general notions 
at prefent, it will be fufticient to confider the cake and 
foie as divided into two ftrata only; one redundant in 
fluid, and the other deficient, negleCting the neutral ftra¬ 
tum that is interpofed between them in each. The cake, 
then, confifts of a ftratum A Bba A containing redundant 
matter, and a ftratum ab CD containing redundant fluid ; 
and the foie has a ftratam D Cnm containing redundant 
fluid, namely, all that belongs naturally to the fpace 
D C E E, and a ftratum m n P E containing redundant mat¬ 
ter. This may be called the primitive Jlate of the cake 
and i 
