34
NEW EXPERIMENTS
SECTION III
New experiments with M Lichtenburg's large
electrophorus
The following experiments are intended as
improvements on M Lichtenburg's beautiful
configurations, first made on a resinous
electrophorus by drawing over it the knob of a charged
phial, and render'd visible by sifting powder'd rosin
over the plate, which falling so differently according
to the circumstances in which the experiment is
made, exhibits the diffusion of electricity in a very
pleasing manner.
My first electrophorus was a glass plate fifteen
inches square, cover'd on one side with a thin
resinous black coating, with tinfoil pasted on the other
side, for if the side opposite to the resinous one be
not a conductor, the electrical fluid will not be easily
diffused over it. Glass was used that the electricity
might not be so liable to pass thro' the small
holes and blistered places which cannot well be avoided
if the resinous substance be thinly spread upon
wood or metal.
As powder'd rosin projected from a brush is negatively
electrified, there appeared no doubt but that
chalk and other powders, which by the same means
are negatively electrified, would answer as well or
better; such powders were therefore tried and found
to succeed remarkably well.
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ON ELECTRICITY
EXPERIMENT I
The plate was suspended by a loop against a wall
that the grosser part of the powder might fall to the
ground, and no more adhere to the plate than was
attracted by the electricity diffused thereon. A small
phial was charged very weakly by one revolution of
the electrical machine, and after its knob had been
drawn over the resinous plate, a cloud of chalk was
projected by rubbing the lump upon a brush near
the electrified surface of the plate, this produced a
plain white line without any ramifications.
EXPERIMENT II
When the phial was charged by three revolutions
of the machine, ramifications appeared upon the
plate at a considerable distance from each other.
EXPERIMENT III
Five or six revolutions caused the electrical fluid
to spread upon the plate in ramifications very near
each other. Close to each branch a small space was
left uncover'd with powder, forming a kind of
shade to the figure. Beyond this shade the powder
lay smooth, softening off externally.
EXPERIMENT IV
With a very strong charge the ramifications were
close and broad, resembling white feathers with a
very broad shade.
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NEW EXPERIMENTS
EXPERIMENT V
A large jar was charged as full as it wou'd hold
and its knob drawn over the resinous plate, the
middle of the figure was about an inch broad, and
mostly plain and white, the shades were now more
conspicuous than the branches, and dark irregular
streaks intersected the distant white space.
EXPERIMENT VI
When a bottle with a large wooden knob in it
was highly charged, and its knob carried at the
distance of an inch from the plate in a direction across
it, the figure produced had no ramifications, but
several cloudy white streaks appeared; these streaks
vary their figure every time the experiment is tried.
EXPERIMENT VII
When a figure was made upon the plate by drawing
with a moist finger, and the moisture touched
with the knob of the charged phial, the ramifications
darted from the sides of the figure in a perpendicular
direction; whereas the ramifications
made by sliding the knob over the plate issued from
the middle of the figure, sloping like the branches
of a tree. The reason of this difference is plainly
that in the first case each ramification is made at
the same time, and in the latter the first branch
repels that which is made after it, and thus prevents
it from darting perpendicularly.
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ON ELECTRICITY
EXPERIMENT VIII
A small wire was laid upon the plate and an
electrical shock made to pass thro' the wire, which was
then thrown off without being touched; in this case,
when the chalk was projected, the ramifications
appeared very distinct and long but not shaded, and
their direction was perpendicular to the figure. A
moderate charge produced longer ramifications than
a very strong one.
EXPERIMENT IX
A circular brass plate with an insulating handle
was placed upon the resinous plate, and a spark
from the charged bottle was communicated to the
brass plate which was then taken off by its
insulating handle, and chalk projected, which produced
a very regular circle of ramifications about four
inches long, proceeding from the circumference of
the space cover'd by the brass plate, and within the
circle were a number of irregular figures somewhat
like stars. A shock made to pass thro' the same
plate generally produced more distinct ramifications,
and sometimes without any stars within the circle;
and if the brass plate was drawn along towards the
edge of the electrophorus whilst touched with the
knob of the phial a very beautiful figure was
produced.
EXPERIMENT X
The resinous plate was fastened to the top of the
pillar which supports and insulates the cushion of the
38
NEW EXPERIMENTS
electrical machine, and whilst the wheel was turning
a brass knob was drawn over the plate, which produced
very large and fine ramifications. A point
drawn over, produced a figure very much resembling
a white ostrich feather.
EXPERIMENT XI
The plate remaining on the pillar as in the last
experiment, a candle was brought very near the
middle and immediately again removed. When
chalk was projected, a circular space about twelve
inches diameter was cover'd with powder, having a
dark shade round it, beyond which the powder fell
more thinly.
EXPERIMENT XII
The plate was fastened to the prime conductor,
and when the candle had been presented towards the
middle as before, the chalk fell mostly on the outside
of a circle of the same dimension.
EXPERIMENT XIII
The phial was charged weakly negative and drawn
over the plate which produced a plain line, but
with chalk it was often black upon a white ground,
contrary to the positive line. With wheat flower
the line was as white as the positive one. Projected
chalk being negative, and wheat flour positive,
they wou'd always be only attracted by those
figures whose electricity is contrary, but there is
reason to believe that some of the projected powder
39
ON ELECTRICITY
is either not electrified or possesses a state contrary
to the rest, and therefore falls into the figure, but
the difference of the powders is very conspicuous.
EXPERIMENT XIV
The phial was charged strongly negative, and
drawn over the plate. The figure was not ramified
but consisted of a number of roundish spots, the
largest about the size of a pea, with smaller spots
between the large ones, the figure was shaded, and
the space beyond cover'd with powder as in the
positive experiments.
EXPERIMENT XV
A phial strongly and negatively charged was
drawn over the plate, and afterwards a pointed wire
held in the hand only, was drawn over the same
figure, then chalk was projected, which produced
a beautiful ramified figure in the middle of the
negative one.
EXPERIMENT XVI
A conical tin funnel was placed with its base on
the middle of the resinous plate, and a negative
strong charge given by connecting the discharging
rod with the under side of the plate, then a positive
charge was given in the same manner, the funnel
was thrown off and chalk projected, which produced
very beautiful ramifications both within and
on the outside of the circle.
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NEW EXPERIMENTS
EXPERIMENT XVII
A phial was charged positively, and placed with
its knob about half an inch distant from the middle
of the plate. Whilst the phial stood in this situation
chalk was projected, which adher'd smoothly
to the knob of the phial, and to the surface of the
plate for the space of about 9 inches diameter,
every where cover'd except a very distinct circular
spot about three inches distant from the knob, if
the chalk was projected at several intervals, the
successive projections, did not come up to the edge
of the first circle, but fell round a larger circumference
and not quite parallel to the first. A negative
charge produced the same kind of spot. The
cause of these spots was found to be electricity
streaming thro' the air from the cork in the neck
of the phial, for a wire placed in the cork near that
which supported the knob alter'd the shape of the
spot. The reason that it was left entirely without
powder tho' occasioned by the communication of
electricity, will appear in a subsequent experiment.
EXPERIMENT XVIII
A knob of wood about an inch in diameter was
placed upon the wire of a phial which was charged
highly positive, and the knob drawn over the plate
so as to touch the surface, this produced a beautiful
figure, the middle of which was smoothly cover'd
with chalk, and the sides finely ramified with
shades.
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ON ELECTRICITY
EXPERIMENT XIX
A brass knob about three inches in diameter was
placed upon the phial instead of the small one, and
a ramified figure drawn with it upon the middle of
the plate; then, before chalk was projected, the
knob with the remaining charge was placed opposite
to the figure at the distance of about half an inch.
In this case the powder fell upon the knob, and on
the outside of the figure, leaving its ramifications
clean and distinct, but entirely without powder.
EXPERIMENT XX
A small candle was insulated and its flame placed
about an inch distant from the middle of the resinous
plate, then the knob of a positively charged phial
was suddenly brought to the flame, and both the
flame and the phial instantly taken away again. In this
experiment when the chalk was projected a circular
space about four inches in diameter was clean and
free from powder, the rest of the plate was cover'd,
except a great number of small circular or elliptical
spots, which shews that the electrical fluid passed to
the plate in detached balls like some atmospheric
meteors, or the plate absorbed from the air a contrary
state of electricity, which produced this appearance.
EXPERIMENT XXI
If a positive figure be first drawn and then a
negative one across it or v.v. when the powder is
projected it is easy to distinguish which was first
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NEW EXPERIMENTS
drawn, the second appearing to cover the first, and
when the positive figure is made last, the ramifications
at the place of junction extend farther than the
rest and are left without powder. But if both the
strokes are positive or negative the first will appear
to cover the second.
EXPERIMENT XXII
If powders of different colours are mixed and
projected over the figures, some of the colours will
prevail on the middle and some on the outside, and
especially if two figures whose electricity is contrary
are made on the same plate, and most of all when
both the electrical states of the figures and powders
are contrary: for example, if minium whose electricity
is strongly positive, and sulphur very strongly
negative be pounded together, and then this mixed
powder put into the bellows, and blown upon the
contrarily electrified figures, the powders separate
and the sulphur falls on the positive figure and the
minium on the negative. This produces a very
pleasing effect. I had often tried the experiment
by projecting the powder from a brush: but the
bellows do it much better, which last method was
communicated to me by the Chevalier Landriani,
who had seen it tried in Italy.
EXPERIMENT XXIII
In some of the above experiments, when the resinous
plate was positively electrified by drawing
over it the knob of a charged phial, and chalk projected,
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ON ELECTRICITY
the middle of the figure was covered with a
plain streak of powder, or (if the charge was weaker)
with beautiful white ramifications resembling
flowers; round these ramifications there appeared a
dark shade very exactly defined, whereon no powder
fell, and next to this the powder fell smooth,
diminishing in thickness towards the edge of the plate.
On the first view of these appearances, I supposed
that the positive electricity deposited upon the plate
in the middle of the figure had render'd the shaded
part of the space negative, and that therefore alternate
spaces were positive and negative: but upon
placing a wire in the cap of my electrometer, and
drawing it over the plate in various directions, and
sometimes covering different parts of the figure
whilst others were tried I could never discover any
signs of a negative state of electricity. The whole
plate appeared positive, its intensity diminishing
from the middle of the figure.
EXPERIMENT XXIV
Not perfectly convinced of the entire positive
state of the electrified plate by examining with the
electrometer, I cemented a wire to the end of a
glass tube covered with sealing wax, in such manner
that the two ends extended each way like the points
of a pair of compasses, then taking hold of the
glass tube, I placed one end of the wire in the
middle of a positive figure, and the other end upon
a clean part of the plate, and drew the wire in the
direction of the figure, keeping the first point in
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NEW EXPERIMENTS
the middle. Upon projecting chalk where the
point had touched the clean part of the plate, shaded
ramifications appeared which plainly shewed the
middle of the figure to be strongly positive, as
might be expected.
EXPERIMENT XXV
A positive figure being drawn as above, the
white part in the middle carefully wiped off
with my finger, so as to discharge its electricity,
then the wire was placed with one point in the shaded
space and drawn along the figure. Upon projecting
chalk over the clean part of the plate positive
ramifications were produced as before, but much
weaker.
EXPERIMENT XXVI
The white ramifications and shades were both
wiped off, and the wire placed near the edge of the
smooth white space, and with the other point on the
clean part of the plate, and upon projecting chalk
there was no figure produced: but if the wire was
drawn in the same manner without wiping off the
shaded part, a white line appeared with very few
points like thorns, which sufficiently proved that it
was weakly positive; and hence it appears that it
was the natural electricity of the plate disposed to
attract powder by the influence of that which was
communicated in the figure.
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ON ELECTRICITY
EXPERIMENT XXVII
The white ramifications in the middle of the figure
were wiped off and then more chalk projected upon
the shaded part which caused the ramified points to
extend farther, and by wiping off still more and
projecting chalk again, almost the whole of the
shaded part was filled, which proves that this last
space is left uncover'd, because the stronger electricity
in the middle of the figure attracts all the powder
which might otherwise fall in that space. And
in many other experiments it is found that various
parts of the same plate will be filled with powder or
left clean, where the same state of electricity, but of
different degrees of intensity, has been communicated,
particularly exp.
3,
4,
5,
17,
18 and
19.
EXPERIMENT XXVIII
Glass plates were cover'd with red or black sealing
wax, and the figures drawn upon the wax, then
powder'd sealing wax, or other powders which
wou'd not lose their colour by heating, were projected
upon a plate of the opposite colour, and by
holding the plates before a fire or over a candle till
the surface was softened, the figures were indelibly
fixed.
EXPERIMENT XXIX
I have tried to take off the figures upon paper
in various ways, and first projected lamp black,
ivory black, powder'd rotten stone, vermilion, and
many other powders, sometimes mixing them with
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NEW EXPERIMENTS
gum arabic, and then laying over the plate a paper
softened in water, with or without gum arabic dissolved
in it, and in some experiments the figures
came off very well, especially with vermilion, but
mostly failed. I then tried the dry'd extract of
dyers woods, and succeeded very well. And since
several of my friends have expressed their desire to
be informed of the process, I will describe it as
minutely as I can.
To make red figures, take a pound of rasped Brazil
wood, put it into a kettle with as much water as
will cover it, or rather more; also put in about an
ounce of gum arabic and a lump of allum about as
big as a large nut, let it boil about two hours, or
till the water is strongly coloured; strain off the
extract into a broad dish, and set it in an iron oven,
where it is to remain till all the water be evaporated,
which with me was effected in about 12 hours; but
this depends on the heat of the oven, which shou'd
not be so hot as to endanger its burning. Sometimes
I have boiled the strained extract till it was
considerably inspissated [?] before it was placed in the
oven, that it might be sooner dry.
When it is quite dry but not burnt, scrape it
out of the dish and grind it in a mortar till it be
finely pulverized. In doing this it is proper to
cover the mortar with a cloth having a hole thro' to
prevent the powder from flying away and offending
the nose, and also to do it out of doors if the weather
be dry and calm, that the air may carry away
the powder necessarily escaping, and which otherwise
47
ON ELECTRICITY
is very disagreeable. When ground fine let it
be sifted thro' muffin or a fine hair sieve, returning
the coarser part into the mortar to be ground again.
When the grinding and sifting are finished the powder
is ready for use. The resinous plate I have
mostly used was composed of five pounds of rosin,
half a pound of bees wax, and two ounces of
lampblack, melted together and poured upon a board
sixteen inches square, with ribs upon the edges at
least half an inch high, to confine the composition
whilst fluid, thus the resinous plate was half an inch
thick, which is better than a thinner plate, the
figures being more distinct. After the composition
is cold, it will be found covered with small blisters,
which may be taken out by holding the plate before
the fire, till the surface be melted, then let it cool
again, and upon holding it a second time to the
fire, more blisters will appear; but by this repeatedly
heating and cooling the surface, it will at last
become perfectly smooth. Some plates were made
smaller and the resinous composition confined to the
form of an ellipsis, a circle or escutcheon, by a rim
of tin half an inch broad, and fixed upon a board.
The next thing to be done is to prepare the paper,
which is to be soften'd in water, either by laying
the pieces upon each other in a vessel of cold
water, or first pouring a little hot water upon the
bottom of a large dish, then laying upon it a piece
of paper, so that one edge of the paper may lie over
the edge of the dish, to remain dry, that it may
afterwards be more conveniently taken up. Then
48
NEW EXPERIMENTS
pour more hot water upon its upper surface. Upon
this place another piece in the same manner, again
pouring on more water, and thus proceed till all the
pieces are laid in. By using hot water the paper
will be more softened in a few minutes than if it
remains in cold water a whole day.
When the figures are to be made the resinous
plate must lie horizontally whilst the electricity is
communicated, if the experiment requires any thing
to be placed upon the plate: but it is convenient
afterwards to hang it up in a vertical position whilst
the powder is projected, left too much powder
should fall where it is not required.
A little of the powder may be taken between a
finger and thumb, and projected by drawing it over
a brush, or which is better, a quantity of powder
may be put into the bellows and blown towards the
place. When the figure is sufficiently covered with
powder, let the plate be again laid horizontally upon
a table, then take one of the softened papers out of
the water by its dry edge, and lay it carefully
between the leaves of a book, pressing the book
together, and let it lie in this situation about half a
minute. Then remove the paper to a dry place in
the book, and press it again about the same time,
which will generally be sufficient to take off the
superfluous moisture. Then take up the paper by
the two corners of its dry edge, and place the wet
edge a little beyond the figure on the resinous plate
lowering the rest of the piece gradually till it covers
the figure without sliding, then lay over it a piece of
49
ON ELECTRICITY
clean dry paper, and press it gently, let it remain
a short time, and then rub it closer to the plate
with a cloth, or which is better, press it down by
means of a wooden roller cover'd with cloth, taking
care that the paper be not moved from its first
position. When the paper is sufficiently pressed
let it be taken up by its dry edge, and laid upon
the surface of a vessel of water with the printed side
downwards, by this means the superfluous powder
will sink in the water, and the figure will not be so
liable afterwards to spread in the paper. After the
paper has remain'd on the water during a few
minutes, take it up and place it between the leaves
of a book, removing it frequently to a dry place.
If it be desired that the paper shou'd be speedily
dry, let the book-leaves in which it is to be placed,
be previously warmed, and by removing it to several
places, it will be dry much sooner than by holding
it near a fire, and without drawing the paper
crooked. By the above process it is obvious that
leather, callico, or linen, as well as paper, may be
printed with these figures, and the effects of the
diffusion of electricity upon a resinous plate be
exhibited to those who have not leisure or inclination
to perform the experiments.
The atmospheric electricity with all its variations
of intensity, and changes from positive to negative
are marked on a resinous plate by an ingenious
machine of the Chevalier Landriani, which moves
by a clock, and the figures made on the plate
during his absence will become visible when powder is
50
NEW EXPERIMENTS
projected. These figures may therefore be taken
off by this process, and the state of a day's
electricity in Italy be transmitted to England.
EXPERIMENT XXX
Mr Wedgwood proposed to fix the figures by
projecting fine powder'd enamel, after the electricity
was diffused upon porcelain, and then to bake the
plate or vessel (thus ornamented) in the usual way.
This experiment was tried at his house, and the
figures were very well fixed; but the weather being
then unfavourable, they were not so beautiful as
might be expected. I have since made the figures
with more success, but had not the opportunity of
fixing them.
In this way the ramifications and shades may be
indelibly fixed and preserved, without any diminution
of their beauty.
EXPERIMENT XXXI
Another way of preserving the figures is to make
a resinous plate of a proper shape to be framed and
covered with glass. After the figure is made and
the powder projected, it is necessary to hold the
plate over a vessel of hot water, that by means of
the steam its electricity may be gradually discharged,
for if it be covered immediatly, the powder
will fly off to the glass. In one experiment I painted
a glass black on one side, then cover'd the
painted side with brass leaf, and after heating it in
the sunshine till all the moisture was expelled, I
51
ON ELECTRICITY
made the figures upon the uncoated surface, cover'd
it with another glass, and placed it in a frame.
When glass is made very dry the figures will be as
distinct and beautiful as those made upon a resinous
plate; but it is necessary to perform the experiment
in dry weather, and to take care that the glass and
knob of the bottle, with which the figures are drawn,
be sufficiently and equally heated, for if the knob
be colder than the plate, moisture will be condensed
and spoil the figure. In trying experiments with a
glass plate, I observed that it was very difficult to
deprive it of the electricity last communicated. I
rubbed the surface with wet cloths, then rubbed
water with my fingers upon the electrified parts, then
dipped the plate in water, and after that held it
over the flame of a candle, yet found that all these
methods were insufficient, the figures might still be
seen when the glass was dry, and chalk again projected.
I then rubbed the glass with tallow, and
cleaned it off with powder'd chalk, which succeeded
better.
EXPERIMENT XXXII
I placed the above described glass plate in the
sunshine where it was kept hot, and dropped a little
oil upon the middle, and upon touching the oil
with a positively charged bottle, it flowed in very
fine ramifications. I then touched another drop of
oil with a negative bottle, which also caused it to
flow in ramifications, which I cou'd not perceive to
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NEW EXPERIMENTS
differ from the positive ones, except that they did
not extend quite so far.
EXPERIMENT XXXIII
I rubbed a circular space in the middle of the
glass plate with water, and touched it with a charged
bottle, then projected chalk which cover'd the
moist space with a plain smooth coating of chalk,
I then dried it over the flame of a candle, which
also discharged its electricity. On the middle of
this white space I dropped some oil, and upon
touching it with the charged bottle it shew'd its
spreading ramifications more plainly than in the
last experiment. Also this plain white space being
touched with a positive bottle, (by chalk thrown off
the surface) shews ramifications more distinctly
filled with branches than those made upon clean
glass or resin.
EXPERIMENT XXXIV
The surface of a resinous plate was melted and
powder'd rosin sifted upon it so to make it very
rough, this caused the figures to spread less than
upon a smooth plate, so that the points of the ramifications
came very near the edge without striking
off
EXPERIMENT XXXV
A positive figure was drawn upon a resinous plate
and chalk projected considerably longer than usual,
and then examining the figure there appeared
53
ON ELECTRICITY
small ramified vegetations like those produced upon
camphire by M. Moscati. Also negative figures
with flour produced the same.
EXPERIMENT XXXVI
The powders made from the decoction of woods
described in exp.29 are very troublesome when
projected in the open air, and a considerable quantity
is left in every experiment. I therefore contrived
to project the powder by means of a circular
brush inclosed in a box. Its lid A plate 3d fig 1
is sixteen inches square, with edges about an inch
and a half deep. The resinous composition is
poured within these edges of the lid to the depth of
half an inch. The sides of the box as far as the
perpendicular part at B are six inches deep; the
sloping boards at C ten inches. The circular box
D in which the circular brush in inclosed is six
inches diameter, and three inches thick. A wire or
small glass tube is put thro' the circular box at E
that the hairs of the brush when turned round by
the handle F may flirt against it and project the
powder thro' an opening at C and diffuse it into
the air contained within the box, and thus the
figures made on the resinous plate within the lid
will attract the finest part of the powder, or that
which is most strongly electrified with the contrary
state to that of the figure, and the rest will fall down
again. If the box contains too little powder it
will be necessary to lean it on one side, and if there
be a large quantity, the brush must be turned slowly
lest it shou'd throw up too much. Smaller resinous
54
NEW EXPERIMENTS
plates may be supported within the box by bars of
wood placed across, and cover'd over with the lid
whilst the powder is projected.
EXPERIMENT XXXVII
That I might more conveniently draw a variety
of figures as well as try the effects of a very small
Leyden bottle, I made an electrical pen, by silvering
the inside and gilding the outside of large thermometer
tubes, from three to eight inches long,
with and without bulbs, covering the ends with
sealing wax. In one end was fixed a blunt needle
connected with the internal metallic surface. Some
were coated on the outside with tin foil, and a wire
served for the internal coating. Whilst drawing
with them, a charged bottle stands ready to supply
electricity by touching its knob with the point of
the needle. Small or large knobs, camel hair
pencils, thistle down, and other substances occasionally
fixed on the point, produce different effects, and
with the smallest pen lines may be drawn as fine as
can be made with ink.
EXPERIMENT XXXVIII
For the purpose of making circles, spiral lines,
volutes, &c an iron pin is fixed to the conducting
side of the lid of the projecting box at
G plate 3 fig.1;
and when electricity is to be communicated
to the resinous surface, the pin is placed in a hole
made in the table, and the plate whirled round,
whilst it is touched with the knob of a charged
55
ON ELECTRICITY
bottle, or pen described above, so as to produce
the required figure.
EXPERIMENT XXXIX
The figure mentioned at the end of exp. 9th,
made by drawing the brass plate towards the edge
of the electrophorus, by its insulating handle whilst
the plate was touched by the knob of a charged
bottle, somewhat resembles the picture of a comet;
but in the tail I frequently observed that a circular
or elliptical space was left without powder, which
may reasonably be attributed to the absorption of a
contrary state of electricity from the air, which
weakens the intensity of the charge diffused in that
space; see plate 3,fig.6. An electrified cloud or
stratum of air emits or absorbs balls of electrical
fire called meteors or falling stars, to which this
experiment may bear some resemblance.
EXPERIMENT XL
A circular plate of wood 12 inches in diameter
was covered with tin-foil, and furnished with an
insulating handle, this plate had also three feet of
glass about an inch long, cover'd with sealing wax.
A resinous plate half an inch thick and of less diameter
than the wooden plate, was laid upon a table,
and the wooden plate placed over it, its feet standing
upon the table on the outside of the resinous
plate, so that the surface of the wooden plate stood
about half an inch higher than the surface of the
resinous plate. The knob of a charged bottle was
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NEW EXPERIMENTS
applied to the wooden plate which was then removed.
Upon projecting powder the surface of the
resinous plate was covered with circular elliptical
and irregular spots and rings. If the charge was
weak there appeared only small round spots, but a
strong charge produced rings broader or narrower
according to the state of the air and strength of the
charge. After touching the wooden plate with a
positive bottle, I sometimes removed the plate a
little from its first position, and then touched it
with a negative bottle, and instead of a single powder,
the mixture of minium and sulphur were blown
upon the plate, as in exp.22, which distinguished
the positive and negative rings by the yellow and
red colours.
Upon examining these rings by the method described
in exp.24, it appeared that the whole space
included within the rings was covered with electricity;
and yet the powder was only attracted by the
edges of the space, probably because the intensity
of the middle is diminished by the equally surrounding
repulsion of the sides.
The circles of green grass called fairy rings are
with great probability attributed to the effects of
lightening. See Dr. Priestley's Hist. of Electricity,
vol.2, page 274,of which this experiment may be
esteemed a tolerable artificial imitation.
