Doubler
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XXVII. An Account of a Doubler of Electricity, or a Machine
by which the least conceivable Quantity of positive or negative
Electricity may be continually doubled, till it becomes perceptible
by common Electrometers, or visible in Sparks. By the Rev.
Abraham Bennet, MA; communicated by the Rev. Richard
Kaye, LLD, FRS.
Read May 10, 1787
The great importance of a machine for the purpose of
detecting very minute quantities of electricity has
occurred to many of the cultivators of this science; as by such
an assistant not only many chemical combinations or solutions,
but also many yet unexplained atmospherical phaenomena, may
become intelligible.
The labours of M. VOLTA have been very successful on this
subject by the application of his condenser (as he terms it),
which, by means of a thin coated electric, is capable of
receiving a greater quantity of electrical fluid than a common
insulated conductor, and rendering it perceptible by separating
the positive and negative sides of the charged plate. On
this ingenious contrivance Mr CAVALLO made a very considerable
improvement by transferring the received quantity of
electricity from a larger to a smaller condenser, as explained in
the Phil. Trans. Vol. LXXII.
Notwithstanding the very great sensibility of this apparatus,
the electricity of the atmosphere is sometimes too weak to be
discoverable by it: for instance, in some showers, when the
negative state of the falling rain is nearly equal to the positive
state of the air. Add to this the trouble of keeping an
insulated and elevated conductor sufficiently dry, and the danger
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Mr BENNET's Description, &c
of it in a thunder-storm. I therefore contrived the following
doubler for the purpose of more easily making an
electrico-meteological diary, which I undertook at the request
of my friend Dr DARWIN, who hoped, that from thence some
lights may be thrown on the causes of the sudden changes of
aerial currents, a circumstance of so much importance to the
early growth and maturity of vegetation.
I place upon my electrometer, described in a
former Part
of the Philosophical Transactions, a circular brass plate, three
or four inches in diameter, polished and thinly varnished on
the upper surface. On this I place another brass plate, of
equal diameter, polished and varnished on both sides, with an
insulating handle attached to one edge of it. A third plate is
also provided, of equal diameter, polished and varnished on the
under side, and with a perpendicular insulating handle from the
center of the upper side, similar to those mentioned in the
Appendix to my last Paper.
The method of collecting electricity from the atmosphere,
and continually doubling it as much as required, is as follows.
If the weather be dry, I carry into the open air a lighted torch,
not liable to be easily blown out, or a small lantern with a
lighted candle in it, to the bottom of which is fixed, by means
of a socket, an insulating handle of glass covered with sealing
wax; in the other hand is carried a coated phial: then, elevating
the flame a little higher than my head, I apply to it
the knob of the phial, holding it in this situation about half a
minute. Then returning into the house (where the above
described doubler is kept dry, by being placed on a table not
far from the fire), I apply the knob of the phial to the under
side of the first plate, which lies immediately upon the electrometer,
and at the same time touching the second plate with a
finger of the other hand. Then laying aside the phial, I lift up
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Mr BENNET's Description of
the second plate by its insulating handle, and if the electricity
be not now sensible to the electrometer, I place the third plate,
by means of its insulating handle, upon the second plate, thus
elevated: then touching the third plate, by stretching a finger
over the juncture of its insulating handle, and again withdrawing
the finger. I then again separate the third plate from
the second. In this situation it will be apparent to electricians
that two of the plates are of one kind of electricity, and
nearly of equal quantity, and one only of the other. I then
apply the third plate to touch the under surface of the first
plate which remains on the electrometer, and at the same time
covering the first plate with the second, I then touch the second
plate by stretching a finger over the juncture of its insulating
handle: and first taking away the third plate, and then
withdrawing my finger from the second, and lifting it up from the
first plate, the electricity becomes doubled. If by this first
operation the quantity of electricity does not become sensible
by the electrometer, I repeat the process to ten or
twenty times, which, by doubling it every time, makes visible
the smallest conceivable quantity of electricity, since, at the
twentieth operation, it is augmented to above 500,000 times.
And though in description the above process of doubling to
twenty times may appear tedious, yet when the operator can
perform it with sufficient readiness (which is soon acquired)
it takes less time than 40 seconds. The collection of electricity
from the air, and the touching and position of the plates, are
represented in Tab.XI figures 1.2.3.4.5. and 6.
If it be required to produce sparks, the plates are to be
placed upon an insulating stand, without an electrometer, and
the process repeated as above till the sparks appear.
The experiment which proves that the electricity is doubled
by each operation is this. If the two slips of pendulous leaf
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a Doubler of Electricity.
gold of the electrometer be made to diverge to a certain distance
by the above process, that distance will be nearly doubled by
repeating the operation. Another proof of this duplicate
accumulation is, that, when the third plate is applied to the
first, the divergency of the leaf gold is apparently undiminished,
though in this situation their electricity is diffused over
double the quantity of surface.
That flame will collect electricity better than points was
mentioned in my former Paper, and is very evident if two
phials of equal capacity are exposed to the air, the one
furnished with a sharp point, and the other having its knob applied
to an insulated flame, and their electricity afterwards examined
by the doubler.
If the weather be rainy, an insulated umbrella may be carried
in one hand, and the knob of the phial applied to the upper and
insulated part of the handle; and if it rains so slowly as not
sufficiently to communicate electricity to the umbrella, a torch
is carried under the umbrella, and used as described above.
It is obvious that some caution is necessary in managing
experiments of so much nicety, since, by the least friction of the
hand on the varnished sides of the plates or insulating handles,
or if the metallic side of one plate be accidentally rubbed
against the varnished side of the other, some degree of electricity
is produced, which, becoming sensible by the operation of
doubling, may render the experiments equivocal.
To obviate these inconveniences, I join a conducting handle,
by means of an insulating nut, to each of the plates. This
handle consists of turned unbaked mahogany, about three
inches long, into one end of which is inserted a nut of baked
wood, about half an inch long, covered with sealing-wax,
upon the other end of which nut the brass socket of the plate
is fixed; by this means it is not necessary to touch the sealing
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Mr BENNET's Description, &c
wax of the insulating nut, but occasionally to stretch a finger
over it to touch the plate, whilst the mahogany handle is held
in the same hand.
Having found, by repeated experiments, that two clean metallic
plates, or two equally varnished plates, rubbed together,
produced no electricity, I varnished the second plate on both
sides, but more thinly than when one side only was varnished,
and in some experiments used thimbles on the ends of the
touching fingers. In this way the inconveniences of accidental
friction were in some measure obviated, but much less than I
first expected; for, notwithstanding the utmost care, electricity
is produced without previous comunication: therefore, in
experiments requiring the electricity to be often doubled, its
communication may yet be ascertained by applying it to the first
and second plates alternately; so that positive electricity
communicated to the first plate appears positive by the electrometer;
but the same electricity, applied to the second plate whilst the
first is touched, produces negative in the electrometer.
I beg leave to add, that this method of doubling either positive
or negative electricity, as well as M. VOLTA's condenser, with
Mr. CAVALLO's improvement on it, as also the ingenious experiments
of Father BECCARIA with double plates of glass, which he
separated after charging, are all of them to be explained from the
same principles with the Leyden bottle, of which they may
be all said to be only different applications. I shall not therefore
trouble the Society with any further theory on this subject, but
proceed to lay before them the diary which I have hitherto
kept, and during which time I have found no difficulty in collecting
electricity from the atmosphere positive or negative, so
as to become sufficiently sensible by the above described apparatus,
though the hygrometer has sometimes shewn the greatest
degree of moisture.
[continued on page 295]
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Mr BENNET's Description, &c.
The atmospherical electricity was sometimes so strong as to
need no doubling, and mostly required only one application of
the second plate, yet I frequently found it neccesary to repeat
the process from two or three to twenty times. Perhaps the
exact comparative quantity of electricity residing in the atmosphere
might be measured by the number of operations required
to render it perceptible by the electrometer, all other
circumstances being cautiuosly attended to.
If the electricity of the atmosphere should happen to be
much weaker than I have yet found, there remains not only
the resource of doubling oftener, but the capacity of the
instruments may be much increased; as, first, by using a larger
flame; secondly, by elevating it higher; thirdly, by collecting
the electricity with a very thin glass ball, silvered within, and
coated on the outside in the common way, or gilt; fourthly,
by grinding and polishing the plates of the doubler very
exactly; fifthly, by making the experiments in an advantageous
situation. In all these particulars my apparatus was defective,
yet amply sufficient for the discovery of the atmospherical
electricity.
After considering the successful effect of flame, in collecting
atmospherical electricity, I placed an insulated lantern upon a
pole about fifteen feet high, and suspending a gold thread from
the lantern connected it with the electrometer, and was agreeably
amused with feeling the pendulous gold leaf open and shut
with every passing cloud.
On the 27th of February, 1787, when there was a considerable
mist whilst the lantern was thus elevated, the leaf
gold frequently struck the sides of the electrometer; and, in
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Mr BENNET's Description, &c.
about an hour, some drops of rain beginning to fall, the
appearance of electricity with this apparatus entirely ceased,
though I elevated the lantern several times in the course of
the day.
