432 
ELECTRICITY. 
Let a board, (baped like the gable end of a houfe, be 
fixed perpendicularly upon an horizontal board : id the 
perpendicular board let a hole be made, about an inch 
fquare and a quarter of an inch deep ; in this hole let a 
piece of wood nearly of the fame dimentions be fo in¬ 
fected as to fall eafily out of its place, and let a wire be 
faltened diagonally to this fquare piece of wood ; let ano¬ 
ther wire, terminated by a brafs ball, be fattened to the 
perpendiculafboard, with its ball above the board, and its 
lower end in contact with the diagonal wire in the fquare 
piece of wood ; let the communication be continued by 
a wire to the bottom of the perpendicular board. If the 
wires in this fiate be made part of a circuit of communi¬ 
cation, on difcharging the jar the fquare piece of wood 
will not be difplaced ; but if the communication be in¬ 
terrupted by changing the direction of the diagonal wire, 
the fquare piece of wood will, upon the dilcharge, be 
driven out of its place.—If, inftead of the upper brafs 
ball, a pointed wire be placed above the perpendicular 
board, the dilcharge will be drawn oft' without an ex- 
plofion. 
The following directions are given by earl Stanhope, 
to perfons ereCting conductors for lightning: The rods 
mult be made of Inch fub ft antes as are, in their nature, 
tlte bed: conductors of electricity ; and muft not be inter¬ 
rupted, and perfectly continuous. They mult be of fuf- 
ficient thiclcnefs, and mult be perfectly connected with 
the common ltock of electricity, that is, the earth, or 
neared water. The upper extremity of the rods mult be 
finely tapered, and as acutely pointed as poflible : they 
muft be very prominent, and l'everal feet above the chim¬ 
neys. Each rod muft be carried in the Ihorteft conve¬ 
nient direction from its upper end to the common (lock. 
There fliould be no prominent bodies of metal on the 
top of the building propofed to he fecured, but fuch as 
are connected with the conductor by fome proper metal¬ 
lic communication. There fliould be a fuificient number 
of fubftantially-ereCted high and pointed rods. See Prin¬ 
ciples of Electricity, by Charles vifcount Mahon, now 
earl Stanhope. 
To the fame work, the reader muft be referred for an ac¬ 
count of a difcovery made by his lordfliip in the fcience 
of eleCtricity, which he denominated the returning Jlroke, 
by which, he afferts, that perfons may be killed, and 
other mifehief enfue, by lightnings at a confiderable dif- 
tance from the flafh. It is proper alfo to obferve, that 
feveral refpeC'table electricians, though, willing to admit 
the faCt as difeovered by earl Stanhope, yet do not think 
that the danger attending the returning ftroke can ever he 
great or formidable. Dr. Hutton’s obfervations on tins 
phenomenon, are as follow: “To underftand properly 
the meaning of the returning Jlroke, it muft be premifed 
that, according to the noble author’s experiments, an in- 
fulated fmooth body, immerged within the eleCtrical at- 
mofphere, but beyond the' flunking diftance of another 
body, charged politively, is at the fame time in a ftate of 
threefold eleCtricity. The end next to the charged body 
acquires negative eleCtricity ; the farther end is politively 
electrified; while a certain part of the body, fomewhere 
between its two extremes, is in a natural, uneleCtrified, 
or neutral, ftate ; fo that the two contrary electricities ba¬ 
lance each other. (See Prop. XV. and tlte figure.) It 
may farther be added, that if the body be not infulated, 
but have a communication with the earth, the whole of 
it will be in a negative ftate. Suppofe then a brafs ball, 
which may be called A, to be conftantly placed at the 
ftriking diftance of a prime conductor ; fo that the con¬ 
ductor, the inftant when it becomes fully charged, ex¬ 
plodes into it. Let another large or fecond conductor be 
fufpended, in a perfectly infulated ftate, farther from the 
prime conductor than the ftriking diftance, but within its 
eleCtrical atmofphere : let a perfon ftanding on an infu¬ 
lated ftool touch this fecond conductor very lightly with 
a finger of his right hand; while, with a finger of his 
left hand, he communicates with the earth, by touching 
very lightly a fecond brafs ball fixed at the top of a me¬ 
tallic (land, on the floor, which may be called B. Now 
while the prime conductor is receiving its eleCtricity, 
fparks pafs (at leaft if the diftance between the two con¬ 
ductors is not too great) from the fecond conductor to 
the right hand of the infulated perfon ; while fimilar and 
fimultaneous fparks pafs out from the finger of his left 
hand into the fecond metallic ball B, communicating 
with the earth. At length, however, the prime con¬ 
ductor, having acquired its full charge, fuddenly (trikes 
into the ball A, of the firft metallic ftand, placed for that 
purpofe at the ftriking diftance. The explolion being 
made, and the prime conductor fuddenly robbed of its 
elaftic atmofphere, its'prefl'tire or aCti >n on the fecond 
conductor, and on the infulated perfon, as fuddenly 
ceafes; and the latter inftuntly feels a finart returning 
ftroke, though he has no direCt or vifible communication 
(except by the floor) with either of the two bodies, and 
is placed at the diftance of five or fix feet from both of 
them. This returning ftroke is evidently occalioned by 
the hidden re,entrance of the eleCtric fluid naturally be¬ 
longing to his body and to the fecond conductor, which 
had before been expelled from them by the action of the 
charged prime conductor upon them ; and which returns 
to its former place in the inftant when that aCtion or elaftic 
prelTure ceafes. When the fecond conductor and the in¬ 
fulated perfon are placed in the denfeft part of the elec¬ 
trical atmofphere of the prime conductor, or juft beyond 
the ftriking diftance, the effeCts are (till more confider- 
able ; the returning ftroke being extremely fevere and 
pungent, and appearing confiderably (harper than even the 
main ftroke itfelf, received direCtly from the prime con¬ 
ductor. Lord Stanhope-obferves, that perfons and ani¬ 
mals may be deftroyed, and particular parts of buildings 
may be much damaged, by an eleCtrical returning ftroke, 
occafioned even by fome very diftant explolion from a 
thunder cloud ; pofiibly at the diftance of a mile or more. 
It is certainly not difficult to conceive that a charged ex- 
tenfive thunder cloud muft be productive of effeCts fimi¬ 
lar to thofe produced by the prime conductor ; but per¬ 
haps the effeCts are not fo great, nor the danger fo terri¬ 
ble, as have been apprehended. If the quantity of elec¬ 
tric fluid naturally contained, for example, in the body 
of a man, were immenfe or indefinite, then the eftimate 
between the effeCts producible by a cloud, and thofe 
caufed by a prime conductor, might be admitted ; but 
furely no eleCtrical cloud can expel from a body more 
than the natural quantity of eleCtricity which it contains. 
On the hidden removal therefore of the preffure by which 
this natural quantity had been expelled, in confequence 
of the explofion of the cloud into the earth, no more, at 
the utmoft, than his whole natural flock of eleCtricity 
can re-enter his body. And perhaps we have no reafon 
to fuppofe that this quantity is fo great, as that its hid¬ 
den re-entrance into his body (hould deftroy or injure 
him. Allowing therefore the exiftence of the returning 
ftroke, as fufticiently afeertained, and’well illuftrated, in 
a variety of circumftances, by the noble author.’s experi¬ 
ments, the magnitude and danger of it do not feem to be 
fo alarming as he apprehends.” See Hutton’s Diet, 
vol. ii. p. 370. 
XXV. The eleCtric fluid pafles eafily through a va¬ 
cuum. —The air being a r.on-conduCtor, in proportion as 
it is removed, the effort cf the eleCtric fluid on the fur- 
face of the body politively eleCtrified to pafs to the next 
conductor, meets with lefs refiftance, and therefore is dif- 
fufed over a greater fpace.— To exemplify this, let a jar 
be charged in vacuo. Let a luminous conductor be placed in 
the circuit, and obferve the fluid pafling through it. 
Let a vacuum be made a part of the circuit in difcharging 
a phial.—Or, make a vacuum in a double barometer, and 
let the fluid pafs from one leg to the other by connecting 
one cf the veffels of mercury with the conductor.—The 
eleCtric fluid may be made to pafs through a large tube 
three feet in length, and four or five inches in diameter. 
