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SECTION VI
Improvements of the electrical doubler, with experiments
made to discover the causes and obviate the
inconvenience of its adhering or spontaneous electricity.
Some time after the construction of my doubler
of electricity described in the philos. trans.
vol.77, and in the above section, I found that on
performing the doubling process a sufficient number
of times, it always produced electricity without previous
communications, even after every method
likely to prevent it had been tried; yet this impediment
was not so great as to render the instrument
useless, for the adherent or spontaneous electricity
in the beginning of the process, when the instrument
was cautiously used, being very small, was
easily overcome by that which was communicated
in atmospheric experiments, and in case of doubt
the atmospheric electricity was distinguishable by
applying the bottle which collected it, to the two
first plates alternately as mention'd in my description
of the instrument
However to deprive the doubler entirely of its
adherent or spontaneous electricity, was thought a
desirable circumstance, as its sensibility wou'd then
be much greater, and consequently it might with
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more certainty be applied to the discovery of new
electrical facts, and which in this and the next section
I hope will be sufficiently proved.
Dr Darwin, at the desire of Lord G A Cavendish,
made the first attempt with two plates moving
two others by a lever, so as to bring them
exactly to the same position in each operation.
This contrivance he soon improved by another
instrument in which the plates stood vertically and
moved by rack work in a direction exactly parallel
to each other. With this I tried whether the plates
wou'd act without any resinous substance, and found
that the interposed air was a good substitute, and
hoped that now since it was not necessary to varnish
the plates, nor bring them into contact, the spontaneous
electricity supposed to arise from the accidental
friction of the plates wou'd not be produced.
This instrument was sent to Mr Partington, and
the improvement of placing the plates near together
without varnish was soon after mention'd to him in
a letter.
That I might accurately try whether the instrument
wou'd be improved by this omission of varnish,
I made a doubler which consisted of three
plates standing vertical and parallel, the middle plate
sliding backwards and forwards between the two
others. The connection between the two external
plates was performed by lifting up the sliding handle,
and the distance of approach was adjusted by
screws which stopped the foot of the middle plate
any where between actual contact and the space of
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half an inch. When this doubler was completed
and its insulating pillars entirely deprived of adhering
electricity by melting the surface of the sealing
wax which covered them, the doubling process was
tried, and it produced negative electricity at about ten
operations, so that this instrument tho' free from
all accidental excitement was yet possessed of spontaneous
electricity. But when I counted the doublings
and applied an insulated candle to the first and
second plates alternately, I cou'd easily collect and
distinguish atmospheric electricity in a small room,
tho' the doors and windows were shut, this I tried
upon the supposition that the air would not be
entirely deprived of its electricity by passing through
small openings.
The next improvement I had in contemplation
was a revolving doubler, and before I had finished
my contrivance, I was informed that Mr Nicholson
had made one on the same principles, and intended
to send it to me, I therefore attempted nothing more
till I received his very elegant and useful instrument,
which is much the best mode of constructing my
doubler hitherto attempted.
This doubler consists of two insulated and immoveable
plates about two inches in diameter, and
a moveable plate also insulated which revolves in a
vertical plane parallel to the two immoveable plates,
passing them alternately. See plate 2.
The plate A is constantly insulated and receives
the communicated electricity. The plate B revolves,
and when it is opposite the plate A, the
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connecting wires at the end of the cross piece D
must touch the pins of A and C at E F, and
a wire proceeding from the plate B must touch
the middle piece G, which is supported by a brass,
wooden, or other conducting pillar in connection
with the earth. In this position if electricity be
communicated to the plate A, the plate B will
acquire a contrary state, and passing forwards, the
wires also moving with it by means of the same
insulating axis, the plates are again insulated till the
plate B is opposite to C, and then the wire at
H touches the pin in C, connecting it with the
earth, and communicating the contrary state of
electricity to that of B but of the same kind with
that of A. By moving the handle still further
B is again brought opposite to A, and the connecting
wires joining A and C.[?] they both act upon B,
which is connected with the earth as before, and
nearly double its intensity, whilst the electricity of
C is absorbed into A; because of the increased capacity
of A, whilst opposed to B, capable by its
connexion with the earth of acquiring a contrary
state sufficient to balance the influential atmospheres
of both plates.
Thus by continuing to revolve the plate B, the
process is performed in a very expeditious and
accurate manner.
The ball (I) is made heavier on one side than
the other, and screw'd upon the axis opposite to the
handle, to counterbalance the plate B, which may
therefore be stopped in any part of its revolution.
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Yet notwithstanding the convenience and accuracy
of this doubler it always produced spontaneous
electricity, even after all the resinous substances used
in its construction had been melted over a candle,
and after standing a long time with its plates in
connection with the earth. I therefore conjectured that
this spontaneous electricity was not owing to accidental
friction, but to the increased capacity of approximating
parallel plates which might attract and
retain their charge tho' neither of them were insulated.
To prove my hypothesis I first endeavoured
more effectually and speedily to deprive the instrument
of the electricity last communicated, and that
I might know whether this spontaneous charge supposed
to arise from the increased capacity of the
parallel plates, wou'd be always of the same kind.
To effect this deprivation I connected the plates
A and C together by a wire hooked at each end
upon two small knobs on the backs of the plates,
the middle of the same wire touching the pillar
which supports the doubler. Another wire was
hooked at one end upon the back of the plate B,
and at the other end to the brass ball which
counterbalances this plate. Thus all the plates were
connected with the earth, and by turning the handle of
the doubler, it might be discharged of electricity in
every part of its revolution.
After often trying this method of depriving the
doubler, I observed that its spontaneous charge was
almost always negative. I then touched A and C
with a positively charged bottle, and turned the
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doubler till it produced sparks for a long time
together; and after this strong positive charge I hooked
on the wires as above, and revolved the plate B
about an hundred times, which so deprived the
doubler of its positive electricity that when the
wires were taken off, it produced a negative charge
at about the same number of revolutions which it
required before.
The positively charged bottle was again applied,
and the wires being hooked upon the plates, as
before, B was revolved only fifty times, yet this
was found sufficient to deprive it of its positive
charge, and in many experiments five or six revolutions
were sufficient; but I never thought it safe
to stop at so few, and have therefore generally turned
the handle forty or fifty times between every experiment.
Lest electricity adhering to the electrometer
shou'd obstruct the above experiments, I did not
let it stand in contact with the doubler during its
revolutions, but touched the plate A with the cap
of the electrometer, after I supposed its electricity
was become sufficiently sensible: but left even this
contact shou'd communicate any electricity, I made
a cap for my electrometer of shell lac, having a
small tin tube in the center, to which the gold leaf
was suspended within the glass, and a bent wire was
fixed to the top which might easily be joined to the
plate A of the doubler, and thus the gold leaf was
more perfectly insulated, and the electricity cou'd
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not be diffused over so large a surface. The glass
which insulates the plates and cross piece of the
doubler was also cover'd with shell lac.
The doubler and electrometer being now well
insulated, I proceeded to try the following experiments
to ascertain whether the spontaneous electricity
was attracted by the approximation of uninsulated
parallel plates.
EXPERIMENT I
The doubler was deprived of electricity by
revolving the plate B forty times with brass wires
hooked to all the plates, and during this deprivation
the electrometer was connected with the plate A
by means of a brass wire. I then took off the wires
whilst the plate B stood between C and A, in the
upper part of the plane of its revolution; and
turning the handle towards the right hand, the gold leaf
open'd negatively about a quarter of an inch, with
twenty-one revolutions.
EXPERIMENT II
The doubler was deprived of electricity, as
before, and whilst the plate B stood parallel to the
plate A, the brass wires were taken off, then turning
the handle forwards, the gold leaf open'd a
quarter of an inch negatively at the sixteenth revolution.
These experiments were repeated about thirty
times, and on different days, without any considerable
difference in the results; the number of
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revolutions being always greater when the plates set
out from a single position than when the wires were
taken off whilst A and B were parallel.
The plate B is placed at the distance of 1-16th
of an inch from the other plates, and remains the
same in the following experiments. It may be necessary
to note the circumstances, for doubtless the
number of revolutions, if not other results wou'd
vary, if the plates were placed nearer or farther
asunder.
EXPERIMENT III
A copper plate thirteen inches in diameter having
its surface rather convex, was furnished with an
insulating handle of oiled glass, four inches long
and baked till the oil was well hardened. One end
of the glass was fixed into a copper socket in the
middle of the plate, and the other end into wood,
that it might not be necessary to touch the electric
part of the handle. This copper-plate was placed
upon a mahogany table, and the doubler being deprived
of its electricity, the plate B was placed
parallel to A, so that B was connected with the
earth, then the copper-plate was lifted up, by its
insulating handle, and applied to the plate A, and
lastly, the plate B being revolved only five times,
the gold leaf diverged negatively to the distance of
a quarter of an inch.
EXPERIMENT IV
Lest accidental friction against the mahogany
table shou'd be objected, I lifted up the copper-plate,
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and after touching it with the point of a needle,
I applied it to the doubler, as before, and found
that the doubler did not produce its spontaneous
electricity at less than 15 revolutions; then touching
the plate again, I lower'd it till a part of its convex
surface touched the surface of some water contained
in a large dish, and lifting up the plate, I applied
it to the doubler, which caused the gold leaf to
diverge negatively at five revolutions, as in experiment
3d.
EXPERIMENT V
The plate B of the doubler was placed a little
beyond its former position, so that its wire did not
touch the middle piece, and consequently tho' most
of its surface was parallel to the plate A yet it was
insulated. The copper plate was then applied to B
after it had touched the water as in the last experiment,
and at the same time the plate A was touched
by a brass wire, to connect it with the earth. About
five revolutions produced a very sensible positive
divergency of the electrometer, as might be expected
in consequence of the negative charge being communicated
to B instead of A.
EXPERIMENT VI
By a frequent repetition of the foregoing experiments
I was fully convinced of the attraction of
electricity by approximating parallel plates: but
hitherto the charge was negative, and suspecting that
other substances might have a positive affinity with
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the fluid, and especially if my conjectures concerning
projected powders and vapours were true; I
therefore cover'd the surface of the copper plate
with a mixture of gum water and minium, also with
gum water and wheat flour, and found that these
substances when dried upon the surface of the copper,
changed its electricity, so that when it was applied
to the plate A, as in experiment 3d, it
produced a positive charge, and when applied to B, as
in experiment 5th, it produced a negative charge,
as might be expected if the painted copper plate by
its uninsulated approximation to the surface of the
mahogany table or water absorbed a positive charge.
EXPERIMENT VII
To render the electricity of approximating plates
more conspicuously sensible, I ground a brass plate
three inches diameter with emery till it would adhere
to the surface of a piece of black marble. This
plate and marble therefore constitute a condenser in
its original state. The marble being moderately
warmed I pressed the brass plate upon its surface
with the point of a brass wire, then lifting it up by
its insulating handle I applied it to the cap of the
electrometer, which caused the gold leaf to strike
the side negatively.
I hope it will now appear evident by the precautions
and experiments mentioned in this section,
and from the known laws of electricity.
1st. That the doubler in its present state may be
deprived of accidental or communicated electricity.
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2dly. That the principal cause of its spontaneous
charge, is the attraction of electricity by the
approximation of its parallel plates.
3dly. That this charge may be positive or
negative, according as the plates, or touching wires
are composed of substances which have a greater or
less adhesive affinity with the electrical fluid.
4thly. That the causes of spontaneous electricity
are common to the condenser both in its original
and improved state, and to the doubler, and equal
in them all as far as they are equal in their
dimensions and powers.
5thly. That since the doubler may be composed
of very small plates, and yet its power be equal to
that of a very large condenser, its spontaneous
electricity will be more easily overcome by a
communicated charge than that of a condenser of equal
power, and therefore experiments performed with
it will be less liable to equivocal results; and lastly
from these considerations I have ventured to
presume that the instrument may be advantageously
used and applied to the discovery of new and
interesting facts in the science of electricity.