Description
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IV. Description of a new Electrometer. In a letter from the
Rev. Abraham Bennet, M.A. to the Rev. Joseph Priestley, LLD, FRS.
Read December 7, 1786
Wirksworth, Sept 14, 1786
Rev. Sir,
I send you a description of my electrometer, which, having
the honour of your approbation, may be communicated
to the Royal Society. (See Tab II. fig.1. and 2.
which represent two sections of the Instrument).
It consists of two slips of leaf gold, aa, suspended in a
glass b. The foot c may be made of wood or metal; the cap
d of metal. The cap is made flat on the top, that plates,
books, evaporating water, or things to be electrified,
may be conveniently placed upon it. The cap is about an inch
wider in diameter than the glass, and its rim about three-quarters
of an inch broad, which hangs parallel to the glass, to turn off
the rain and keep it sufficiently insulated. Within this is another
circular rim, about half as broad as the other, which is
lined with silk or velvet, and fits close upon the outside of the
glass; thus the cap fits well, and may be easily taken off to
repair any accident happening to the leaf gold. Within this
rim is a tin tube e, hanging from the center of the cap, somewhat
longer than the depth of the inner rim. In the tube a small
peg f is placed, and may be occasionally taken out. To the
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Mr BENNET's Description, &c
peg, which is made round at one end and flat at the other, two
slips of leaf gold are fastened with paste, gum-water, or varnish.
These slips, suspended by the peg, and that in the tube fast to
the center of the cap, hang in the middle of the glass, about
three inches long, and a quarter of an inch broad. In one
side of the cap there is a small tube g, to place wires in. It is
evident, that without the glass the leaf gold would be so agitated
by the least motion of the air, that it would be useless;
and if the electricity should be communicated to the surface
of the glass, it would interfere with the repulsion of the leaf
gold; therefore two long pieces bb of tin-foil are fastened with
varnich on opposite sides of the internal surface of the glass,
where the leaf gold may be expected to strike, and in connexion
with the foot. The upper end of the glass is covered
and lined with sealing-wax as low as the outermost rim, to
make its insulation more perfect. Tab.III fig.1.
represents the instrument joined together, and ready for use.
The following experiments will shew the sensibility of this
instrument. See Tab. IV.
1st, Powdered chalk was put into a pair of bellows, and
blown upon the cap, which electrified it positively when the
cap was about the distance of six inches from the nozzle of the
bellows; but the same stream of powdered chalk electrified it
negatively at the distance of three feet, as representd in fig.2. and
3. In this experiment there is a change of electricity from
positive to negative, by the dispersion or wider diffusion of the
powder in the air. It is also changed by placing a bunch of
fine wire, silk, or feathers, in the nozzle of the bellows, and is
wholly negative when blown from a pair of bellows without
their iron pipe, so as to come out in a larger stream; this last
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Mr BENNET's Description of
The positive electricity of the chalk, thus blown, is communicated
because part of the powder sticks to the cap; but the
negative is not communicated, the leaf gold collapsing as soon
as the cloud of chalk is dispersed.
2ndly, A piece of chalk drawn over a brush, or powdered
chalk put into the brush, and projected upon the cap, electrifies
ot negatively; but its electricity is not communicated. Fig.4.
3dly, Powdered chalk blown with the mouth or bellows
from a metal plate placed upon the cap, electrifies it
permanently positive. Fig.5. Or if the chalk is blown from the
plate, either insulated or not, so that the powder may pass
over the cap, if not too far off, it is also positive. Or if a
brush is placed upon the cap, and a piece of chalk drawn over
it, when the hand is withdrawn the leaf gold gradually opens
with positive electricity as the cloud of chalk disperses.
4thly, Powdered chalk falling, from one plate, to another
placed upon the instrument, electrifies it negatively. Fig.6.
Other methods of producing electricity with chalk and
other powders have been tried; as projecting chalk from a
goose wing, chalking the edges of books and clapping the
book suddenly together, also sifting the powder upon the cap;
all which electrified it negatively: but the instrument being
placed in a dusty road, and the dust struck up with a stick near
it, electrified it positively. Breaking the glass tear upon a book
electrified it negatively, probably by friction in the act of
shivering, for when broken in water it did not electrify it.
Wheat flour, and red lead, are strongly negative in all
cases where the chalk is positive. The following powders were
like chalk: red ochre and yellow, rosin, coal ashes, powdered
crocus metallorum, aurum mosaicum, black-lead, lampblack
(which was only sensible in the two first methods), powdered
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a new Electrometer
quick-lime, umber, lapis calamnaris, Spanish brown, powdered
sulphur, flowers of sulphur, iron filings, rust of iron, sand.
Rosin and chalk, separately alike, were changed by mixture;
this was often tried in dry weather, but did not succeed in
damp: white lead also sometimes produced positive, and sometime
negative, when blown from a plate.
If a metal cup be placed upon the cap, with a red-hot coal
in it, a spoonful of water thrown in electrifies the cup
negatively; and if a bent wire be placed in the cap, with a piece of
paper fastened to it to increase its surface, the posotive
electricity of the ascending vapour may be tried by introducing the
paper into it. Perhaps the electrification of fogs and rain is
well illustrated by pouring water through an insulated cullender,
containing hot coals, where the ascending vapour is positive, and
falling drops negative. Fig.7.
The sensibility of this electrometer may be considerably increased
by placing a candle upon the cap. By this means a
cloud of chalk, which only just opens the leaf gold, will cause
it to strike the sides for a long time together; and the electricity,
which was not before comunicated, now passes into the
electrometer, causing the leaf gold to repel, after it is carried
away. Even sealing-wax by this means communicates its fire
at the distance of twelve inches at least, which it would
scarcely otherwise do by rubbing upon the cap.
A cloud of chalk or wheat flour may be in one room,
and the electrometer, with its candle, be afterwards leisurely
brought from another room, and the cloud will electrify it
before it comes very near. The air of a room, adjoining to
that wherein the electrical machine was used, was very sensibly
electrified, which was perceived by carrying the instrument
through it with its candle. Fig. 8.
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Mr BENNET's Description of
In very clear weather, when no clouds were visible, the
electrometer has been often applied to the insulated string
of kites without metal, and their positive electricity caused the
leaf gold to strike the sides; but when a kite was raised in
cloudy weather, with a wire in the string, and when it gave
sparks about a quarter of an inch long, the electricity was
sensible by the electrometer at the distance of ten yards or
more from the string; but, when placed at the distance of six
feet, the leaf gold continued to strike the sides of the electrometer,
for more than an hour together, with a velocity increasing
and decreasing with the density or distance of the
unequal clouds which passed over.
Sometimes the electricity of an approaching cloud has been
sensible without a kite, though in a very unfavourable situation
for it, being in a town surrounded with hills, and where
buildings encompassed the wall on which the electrometer was
placed. A thunder cloud passing over, caused the leaf gold to
strike the sides of the glass very quick at each flash of lightening.
No sensible electricity is produced by blowing pure air,
projecting water, by smoke, flame, or explosions of gunpowder.
A book was placed upon the cap, and struck with silk, linen,
woollen, cotton, parchment, and paper, all which produced
negative repulsion; but when the other side of the book was
struck with silk, it became positive; this side, struck at right
angles with the former, was again negative; and, by continuing
the strokes which produced positive, it changed to negative,
for a little while; and, by stopping again, became
positive. No other book would do the same, though the sides
were scraped unchalked, upon a supposition that altering the
surface would produce it. At last, one side of a book was
moistened, which changed it; so I concluded, that one edge
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a new Electrometer.
of the book had lain in a damp place; which conjecture was
further confirmed by all the books becoming positive in damp
weaher, and one of them being dried at the fire again became
negative.
When the cap is approached with excited sealing-wax, the
leaf gold may be made to strike the sides of the glass more than
twelve times; and as the sealing wax recedes, it strikes nearly
as often; but, if it approaches much quicker than it recedes,
the second number will sometimes be greater.
The quantity of electricity necessary to cause a repulsion of
the leaf gold is so small, that the sharpest point or edges do
not draw it off without touching; hence it is unecessary to
avoid points or edges in the construction of this instrument.
I am, &c
ABRAHAM BENNET.
