414 
ELECTRICITY. 
preferve them from the rain, with a flick and cane bow, 
like the common ones ufed by boys, will anlwer the pur- 
pofe extremely well, and are very ufeful in determining 
tlie eleCtricity of the atmofphere. 
The next article of conlideration in the eleCtrical ap¬ 
paratus, is the cUElrometer ; an inftrument that meafures 
the quantity, and determines the quality, of eleCtricity 
in any eleitrified body. Previous to the invention of in- 
flruments of this kind, Mr. Canton eftimated the quan¬ 
tity of electricity in a charged phial, by prefenting the 
phial with one hand to an infulated conductor, and giving 
It a fpark, which lie took off with the other ; proceeding 
in this manner till the phial was difcharged, when he de¬ 
termined the flrength of the charge by the number of 
lparks. The moft Ample electrometer, is a flaxen thread, 
called by Dr. Defaguliars, the thread of trial ; which, be¬ 
ing brought near an electrified body, is attracted by it: 
but this does little more than determine whether the body 
is in any degree electrified or not; without determining 
its quantity, much lefs the quality of it. The abbe 
Nollet ufed two threads, thus ingenioufly {Viewing the. 
degree of eleCfricity by the angle of their divergency, 
exhibited in their fhadow on a board placed behind them. 
But electrometers have, of late years, been greatly im¬ 
proved. 
Mr. Canton’s electrometer, reprefented in the pre¬ 
ceding engraving at fig. n, confifts of two balls of 
cork, or pith of elder, about the fize of a fmall pea,- 
fufpended by fine flaxen threads, about fix inches long, 
wetted in a weak folution of fait. If the box containing 
thefe balls be infulated, by placing it on a drinking-glafs, 
or rummer, and an excited fmooth glafs tube be brought 
rear them, they will fir ft be attracted by ir, and then be 
repelled both from the glafs, and from each other ; but 
on the approach of excited wax, they will gradually ap¬ 
proach and come together; and vice verfa. This alfo 
ferves to determine the eleCtricity of the clouds and air, 
by holding them at a fufficient distance from buildings, 
trees, &c. for if the eleCtricity of the clouds or air be 
poutive, their mutual repulfion will, increafe by. the ap¬ 
proach of excited glafs or decreafe by the approach of 
amber or fealing-wax ; on the contrary, if it be negative, 
their repulfion will be diminifhed by the former, and 
increafed by the latter. See Philof. Tranf. vol.48. If 
two balls of this kind be annexed to a prime conductor, 
they will ferve to determine both the degree and qua¬ 
lity of the eleCtricity, by their mutual repulfion and 
divergency. 
The difeharging electrometer of Mr. Lane, is repre¬ 
fented as fixed to the prime conductor of the electrical 
machine, fig. 1, in the preceding engraving.. It confifts 
of brafs work, the lower part of which is inclofed in the 
■pillar F, made of baked wood, and boiled in linfeed oil, 
and bored cylindrically about two-thirds of its length ; 
she brafs work is fixed to the pillar by the ferew PI, 
moveable in the groove I ; and through the fame is made 
to pafs a (teel ferew L, to the end of which, and.oppo¬ 
site to K, a polifhed hemifpherical piece of brafs, at¬ 
tached to the prime conductor, is fixed aball of brafs M, 
well polifhed. To this.fcrew is annexed a circular plate 
O, divided into twelve equal parts. The ufe of this 
electrometer is to difehage the jar D, or any battery con¬ 
nected with the conductor, without a difeharging rod, and 
to give fiiocks fucceltively of the fame degree of flrength; 
on which account it is very fit for medical purpofes. 
Then, if a perfon holds a wire fattened jq the ferew H 
in one band, and another wfire fixed to E, a loop of brafs 
wire palling from the frame of the machine to a tin plate, 
on which the phial D (lands, he wil 1 perceive no (hock, 
■when K and M are in contaCl; and the degree of the ex¬ 
piation, as vvell as the quantity of eleCtricity accumulated 
in the phial, will be regulated by the diitance between 
K and M. Mr. Henley has much improved Mr. Lane’s 
eleCirometer, by taking away the ferew, the double milled 
nut, and the (harp-edged graduated plate, and adding 
other contrivances in their (lead. Mr. Henley’s dif. 
charger of this kind has two tubes, one Aiding within the 
other, to lengthen and accommodate it occafionally to a. 
larger apparatus.. 
The quadrant electrometer of Mr. Henley, delineated 
in the fame engraving at fig. 12, confifts of a Item, ter¬ 
minating at its lower end with a brafs ferrule and ferew, 
for fattening it upon any occafion ; and its upper part 
ends in a ball. Near the top is fixed a graduated femi- 
cirele of ivory, on the centre of which the index, being 
a very light rod with a cork ball at its extremity, reach¬ 
ing to the brafs ferrule of the (tem,.is made to turn on a 
pin in the brafs piece, fa as to keep near the graduated 
limb of the femicircle.- When the electrometer is not 
electrified, the index hangs parallel-to the Item ; but as- 
foori as it begins to be electrified,,the index, repelled by 
the Item, will begin to move along the .graduated edge of 
the femicircle, and fo mark the degree to which the con¬ 
ductor is electrified, or the ftrength or height to which; 
the charge of any jar or battery is advanced. 
Mr. Cavallo has invented a very fenlible electrometer,, 
well adapted for the obfervation of the prefence and 
quality of natural and artificial elcCtricity. It is deline-, 
ated at fig. 13, in the EleCtricity Plate II. A glafs tube, . 
CDNM, is cemented into the piece A B. The upper 
part of the tube is ftiaped tapering, to a fmall extremity, 
which is entirely covered with fealing-wax. Into this- 
tapering part, a (mail tube of glafs is cemented, the lower 
extremity being alfo covered with fealing-wax, projects 
a fmall way within the tube CDN M. Into this (mailer 
tube, a wire is cemented, which, with its under extremi, 
ty, touches a flat piece of ivory H, faftened to the tube 
by means of a cork. The upper extremity of the wire 
projects about a quarter of an inch above the tube, and 
ferews into the brafs cap E F, which cap is open at the 
bottom, and ferves to defend the waxed part of the in¬ 
ftrument from the rain. FromH are fufpended two fine 
filver wires, h; *ing very fmall corks at the lower ends, 
which, by. their repulfion, fliew the eleCtricity. I M, 
and KN, are two flips of tin-foil ftuck to the infide of. 
the glafs, and communicating with the brafs bottom A B. 
They fe.rv.e to convey away that electricity, which, when 
the corks touch the glafs, is communicated to it, and 
might difturb their free motion. When this inftrument 
is ufed to obferve artificial eleCtricity, it is fet on a table, 
and eteCtrified by touching the brafs cap EF with an. 
electrified body ; in this ftate, if any electrified fubftance 
is brought near the cap, the corks of the eleCirometer, 
by their converging, or diverging more, will fliew the 
fpecies of electricity. When it is to be ufed to try the: 
eleCtricity of the atmofphere, fogs, &c. it mult be 
held a little above the head by the bottom AB, fo that 
the obferver may conveniently fee the. corks, which will 
immediately'diverge if there be any fufficient quantity of 
eleCtricity. in the air,, the nature of which may be afeer- 
tained by bringing an excited piece of fealing-wax to¬ 
wards the brafs cap E F. This inftrument is ufually kept 
in a brafs cafe.. 
A very ingenious-electrometer has been lately invented, 
by Mr. Cuthbertfon, for meafuring the eledricity of large 
jars and batteries. It is reprefented in the annexed en¬ 
graving, at fig. 14. G H is an oblong piece of wood, 
ferving as a (land for the inftrument, in which are fixed, 
three glafs fupporters, D, E, F, mounted with brafs balls,. 
a, c, />. Of thefe fupporters E and F are exactly of the 
fame length ; but D is four inches fliorter. Under the 
brafs ball a is a long brafs hook ; the ball c is made of 
two hemifpheres, the under one being fixed to the brafs 
mounting, and the upper turned with a groove to flint 
upon it, fo that it can be taken otfat pleafure. The ball 
b has a brafs tube fixed to it,.about three inches long, 
cemented on the top of F, and the fame ball has a hole 
at the top, of about one-half inch diameter, correfpond- 
ing with the infide of the tube. A B is a ftraight brafs 
wire, with a knife-e'dged center in the middle, placed a 
little 
