charged, for its outer surface has acquired an additional quantity of
the electric fluid, and the inside has lost part of its natural
quantity; but as the outside of the glass has no coating, when it is
removed from the prime-conductor and is not in contact with the hand or
other conductor, the charged part will be discharged gradually, that is,
while its outside surface is communicating its redundant quantity to the
contiguous air, the inner surface acquires the electric fluid from the
other parts of the phial or tube, and this fluid passing through the
vacuum, causes the light which is observed for so long a time. If the
phial or tube be grasped with the hand, the discharge will be
accelerated, yet it cannot be effected in this way immediately, because
the hand cannot touch every part of the glass at once.
_The Leyden Vacuum._
Take a small phial and coat it, about three inches up the outside, with
tin-foil. At the mouth of this phial cement a metallic cap, having a
hole with a valve; and from this cap let a wire proceed a few inches
within the phial, terminating in a blunt point. When this phial is
exhausted of air, a metallic ball must be screwed upon the cap, so as to
defend the valve, and prevent the air from getting into the phial. The
reason why this phial requires no inside coating, is, because the
electric fluid pervades a vacuum, so that it can pass very easily from
the wire to the surface of the exhausted glass, without the assistance
of a non-electric coating.
This phial exhibits very plainly the direction of the electric matter,
both in charging and discharging, for if it be held by its bottom, and
the ball be presented to the prime-conductor, positively electrified,
you will perceive that the pencil of rays (which always appears when the
body is positively electrified, or is giving out the electric matter)
will proceed from the wire within the phial, and when it is discharged,
the star, (which always indicates that the body is negatively
electrified, or is receiving the electric fluid) will be seen on the
point instead of the pencil, but if the phial be held by the ball, and
its bottom be presented to the prime-conductor, the contrary will take
place.
_The luminous Conductor._
This instrument, as well as the preceding, is an invention of Mr.
Henley’s and also shows the direction of the electric fluid passing
through it. The description of it is as follows. To each end of a glass
tube, about eight inches long and three or four inches in diameter, is
cemented a metallic cap, so as to be perfectly air tight. A point
projects from one of the caps, by which it is to receive the electricity
from an excited cylinder, and from the other proceeds a wire, terminated
by a ball, from which sparks may be taken. Each cap is furnished on the
inside with a knobbed wire, which extends some distance into the tube. A
stopcock or valve must be adapted to one of the caps, by which the tube
may be exhausted of air.
The supporters of the instrument are two glass pillars, fastened to a
bottom board.
When the tube is exhausted of air, and its pointed end placed near the
excited cylinder of an electrical machine, the point will appear
illuminated with a star, and a weak light will be seen pervading the
whole tube; but from the knobbed end of the wire, within the tube, a
lucid pencil will issue, and the opposite knob will be illuminated with
a star or round body of light, which, as well as the pencil of rays from
the other knob, will be discernible among the other light which occupies
the cavity of the tube. If the point, instead of being presented to the
cylinder, be connected with the rubber, the appearance will be
reversed—the reason is too obvious to mention.
If the wires within the tube be pointed, the illumination will be the
same; but it seems not so strong in this as in the other case.
_The electric Light flashing between two metallic Plates._
Let two persons (one standing upon an insulated stool communicating with
the prime-conductor, and the other upon the floor,) each hold in his
hand a polished metallic plate, in such a manner that their surfaces may
be parallel, and about two inches asunder. Upon turning the cylinder,
you will see the flashes of light between the two plates, so dense and
frequent, that you can easily perceive any thing in the room.
By this experiment the electric light is exhibited in a very copious and
beautiful manner, and bears a strong resemblance to lightning.
_The spiral Tube._
This instrument is composed of two glass tubes, one within the other,
and furnished with a metallic ball at each end. The innermost tube has a
spiral row of small round pieces of tin-foil, stuck upon its outside
surface, and lying at the distance of one thirteenth of an inch apart.
Now if the tube be held by one of its extremities, while the other is
presented to the prime-conductor, every spark that is received from the
conductor, will cause small sparks to appear between all the round
pieces of tin-foil upon the inner tube, which in the dark appears
encompassed by a spiral line of sparkling fire.
Small pieces of tin-foil are sometimes stuck upon pieces of glass, so as
to represent various fanciful figures, and upon the same principle is
the luminous _word_ produced.
_To make an electric Spark visible in Water, and to render various other
Substances luminous._
Fill a glass tube, about an inch in diameter and six inches long, with
water, and to each extremity adapt a cork to confine the water; through
the corks let two blunt wires pass, so as nearly to touch one another
within the tube: connect the outside coating of a small charged phial
with one of these wires, and touch the knob to the other, which will
cause a vivid spark to appear between their extremities within the tube.
It is necessary in this experiment that the charge of the phial should
be exceedingly slight, otherwise the tube would burst. If you place in a
common drinking glass almost full of water, two knobbed wires, so that
their knobs may be within a little distance of one another in the water,
and make the charge of a large jar pass through the wires, the explosion
will disperse the water and break the glass with surprising
violence.—This experiment is very dangerous if not made with great
caution.
Water may be made luminous thus. Connect one end of a chain with the
outside coating of a charged jar, and let the other lie on the table;
place the end of another chain at about one fourth of an inch from the
former; then set a decanter of water on these separated ends, and on
making a discharge of the jar through the chains, the water will appear
beautifully luminous.
To render ivory or box wood luminous.—Place an ivory ball on the
prime-conductor of the machine, and take a spark or send the charge of a
phial through its center, the ball will appear perfectly luminous; but
if the charge be not taken through the center, it will pass off the
surface and corrode it.
A spark taken through a ball of box wood, not only illuminates it, but
makes it appear of a beautiful crimson, or rather scarlet colour. An egg
may also be illuminated in the same way.
But the most curious experiment to shew the electric light is made with
the real, or more easily with the artificial Bolognian stone, invented
by the ingenious Mr. J. Canton. This phosphorus is a calcareous
substance (generally used in the form of powder) which has the property
of absorbing light when exposed to it, and afterwards appearing lucid in
the dark. To make the experiment, take some of this powder, and by means
of spirits of wine or ether, stick it all over the inside of a clear
glass phial, and stop it with a good cork and sealing wax. If this phial
be kept in a darkened room, (which for this experiment must be very
dark,) it will give out no light; but let two or three strong sparks be
drawn from the prime-conductor, while the phial is kept about two inches
distant from the sparks, so that it may be exposed to their light, and
the phial will afterwards appear luminous for some time. The powder may
be stuck on a board by means of the white of an egg, so as to represent
figures of planets, letters &c. at the operator’s pleasure, and these
figures may be illuminated in the dark in the same manner as the phial,
[for the method of making this phosphorus, see appendix, No. 5.]
CHAP. III.
_Experiments with Charged Electrics._
Experiments with charged electrics should always be made with caution,
for though the discharge of a small phial through the body is seldom
attended with bad consequences, yet that of a battery is always
dangerous, and sometimes mortal. The operator should therefore be
attentive, not only to the experiments he is about to perform, but also
to the persons who may happen to be with him, forbidding them to come
near any part of the apparatus.
EXPERIMENTS.
_The Magic Picture._
This experiment was contrived by Mr. Kinnersley, and is thus described
by Dr. Franklin.
Having a large mezzotinto with a frame and glass, (suppose of the king)
take out the print and cut out a pannel of it, near two inches distant
from the frame, all round. If the cut is through the picture it is not
the worse. With thin paste or gum water fix the border that is cut off
on the inside of the glass, pressing it smooth and close. Then fill up
the vacancy, by gilding the glass well with leaf gold or brass. Gild
likewise the inner edge of the back of the frame all round, except the
top part. Make a communication between that gilding, and the gilding
behind the glass; then put in the board, and that side is finished. Turn
up the glass and gild the foreside exactly over the back gilding, and
when it is dry paste on the pannel of the picture which has been cut
out, observing to bring the corresponding parts of the picture and
border together, by which it will appear of a piece, as at first (only
part of it is behind the glass, and part before it.) Hold the picture
horizontally by the top, and place a little moveable gilt crown upon the
king’s head. If now the picture be moderately electrified, and another
person take hold of the frame with one hand, so that the fingers may
touch the inside gilding, and with the other hand endeavour to take off
the crown, he will receive a terrible blow. If the picture were highly
charged the consequences might be as fatal as those of high treason; for
when the spark is taken through a quire of paper, and the discharge of
the picture is made through it, a fair hole will be perceived in every
sheet, (though a quire of paper, is thought a good armour against the
push of a sword, or even against a pistol bullet,) and the crack
exceedingly loud. The operator who holds the picture by the upper end,
(where the inside of the picture is not gilt,) to prevent its falling,
feels nothing of the shock, and may touch the face of the picture
without danger, which he pretends is a test of his loyalty.
If a ring of persons take the shock among them, the experiment is called
“the conspirators.”
_Colours changed by the Electric shock._
Mr. Cavallo accidentally observing that an electric spark, passing over
the surface of a card painted red, marked it with a black stroke, was
induced to try what would be the effect of sending shocks over cards
painted with different colours; accordingly he painted several cards
with different colours, and passed the discharge of a jar, containing
about one foot of coated surface over them, the result of his
experiments are the following—
Vermilion was marked with a strong black track, about one tenth of an
inch wide. The streak was generally single, but sometimes divided in the
middle.
Carmine received a faint and slender impression, of a purple colour.
Verdigris was shook off from the surface of the card, except when it was
mixed with a strong gum water, in which case it received a very faint
impression.
White lead was marked with a strong black track, but not so broad as
that on the vermilion.
On the red lead there appeared only a slight mark, much like that on the
carmine.
The other colours he tried were orpiment, gambooge, sap green, red ink,
Persian blue, and some others which were compounds of the first, but
they received no impression.
It has frequently been observed that, when a flash of lightning strikes
the mast of a ship, it passes over those parts of the mast, which are
covered with lampblack and tar, or painted with lampblack and oil,
without the least injury; when at the same time it shatters the uncoated
part so as to render the mast entirely useless.—This singular fact
induced Cavallo to carry his investigations on the subject still
farther, particularly with a view to determine something relative to the
properties of lampblack and oil. But it will not be necessary here to
enumerate all his experiments upon this subject. It is sufficient to
state that the two following propositions are the result of his
observations.
“First—That a coat of oil paint over any substance defends it from the
effects of an electric shock, that would otherwise injure it; but that
it would by no means defend it from any shock whatever.[17]
“Second—One colour does not seem preferable to another, if it is equal
in substance and equally well mixed with oil—but that a thick coating
affords a better defence than a thin one.”
_To fire Spirit of wine._
Hang to the prime-conductor a short metallic rod, having a small ball at
the end—then pour some spirit of wine, a little warmed, into a metallic
spoon. Hold the spoon by the handle, in such a manner that the knob of
the rod may be about an inch above the surface of the spirit.—In this
situation, if by turning the cylinder a spark be made to pass to the
spoon through the spirit, it will be set on fire.
It will generally be found more advantageous to fix a metallic dish,
containing the spirit, upon the prime-conductor.
This experiment may be varied different ways, so as to render it very
agreeable to a company of spectators. A person, for instance, standing
upon an insulating stool, connected with the prime-conductor, may hold
the spoon with the spirit, in his hand—another person, standing on the
floor, may fire the spirit by bringing his finger within a small
distance of it—or, instead of his finger he may use a piece of ice,
which will make the experiment still more surprising.
_To swell Clay, and break small Tubes._
Roll up a piece of soft clay in a small cylinder, and insert two wires,
so that their ends within the tube may be about one fifth of an inch
apart.—If a shock be sent through this clay, by connecting the wires
with the coatings of a pretty large jar which has previously been
charged, the clay will be inflated, by swelling in the middle.—If the
clay be not very moist, it will be broken by the explosion, and the
fragments thrown about the room.
To make this experiment with a little variation, take a piece of the
stem of a tobacco pipe, or a glass tube (which will answer equally
well,) and fill the bore with moist clay; then insert wires as in the
preceding experiment, and send the shock through it. This tube will not
fail to be broken, and the pieces thrown to a considerable distance.
_To pierce Cards &c. with the electric Explosion._
Hold a card or the cover of a book, close to the outside coating of a
jar, then by applying one end of the discharging rod to the card,
discharge the jar; the electricity rushing through the circuit from the
positive to the negative coating, will pierce a hole through the card,
or book-cover. This hole will be larger or smaller as the card is more
or less moist. The card, upon examination, will be found to have a
sulphureous or rather phosphoreal smell. It is remarkable in this
experiment that there is a burr raised on both sides of the card.
Insects may be killed in this manner. If they are quite small the shock
of a common phial will be found sufficient to deprive them of life: but
if they are large, they will, upon receiving the shock, appear dead, but
after a short time recover.—This however depends upon the quantity of
the charge sent through them.
The shock of a jar, sent through a lump of white sugar, if strong enough
to break it, will illuminate every part of the sugar, and this
illumination will continue a short time after making the experiment.
_To light a Candle by the discharge of a Jar._
Take a wire about the size of a common knitting needle, and by means of
a small flexible chain, let one end communicate with the outside coating
of a jar, containing at least ten inches of coated surface. To the other
end of the wire some cotton must be twisted very loosely, so as to cover
the extremity of the wire completely. The cotton must be rolled or
sprinkled with powdered rosin. Now let the jar be charged and bring the
cotton to its knob pretty quickly, so that the discharge may pass
through the rosin on it; the cotton will instantly inflame, and will
last long enough to light a candle.
Paper, dipped in a solution of nitre and water, and previously dried,
may be fired in the same manner, and by this a brimstone match may be
lighted. The same effect will follow, if you grease the cotton with a
little sweet-oil, or moisten it with turpentine.—Flame may be again
excited in a candle recently blown out, by simply passing the discharge
of a jar through the wick and smoke.
CHAP. IV.
_Experiments relating to the influence of pointed Bodies on
Electricity._
These experiments, though not the most entertaining are certainly among
the most important in electricity. By the knowledge of them, mankind
have received the greatest practical advantage. But as we have already
treated of this subject, we shall, in this chapter, describe only two
experiments which may serve to set it in a clearer light, and which may,
in a more particular manner, demonstrate the utility of affixing pointed
conductors to buildings, in order to preserve them from the dreadful
effects of lightning.
EXPERIMENTS.
_The electrified Cotton._
Take a small lock of cotton, extended in every direction as much as can
conveniently be done, and by a linen thread about five or six inches
long, fasten it to the prime-conductor; then let the cylinder of the
machine be turned—the lock of cotton, by the repellency of its
filaments, will immediately swell and stretch itself towards the nearest
uninsulated conductor. In this situation, if you present your knuckle or
a knobbed wire towards the cotton, it will immediately move towards it,
and endeavour to touch it; now with the other hand present a pointed
wire to it:—the cotton will immediately shrink up, and fly towards the
prime-conductor. Remove the point, and the cotton will again approach
the knuckle or knobbed wire—present the point, and it will again recede.
This experiment shows that a point is the proper termination for a
lightning rod. For the cotton will represent the cloud, and the two
wires, the lightning rods with different terminations.
The cotton is attracted by the knuckle or knobbed wire, in order to part
with its electricity, this however cannot be effected unless they come
so near as to touch one another, and then the discharge is effected at
once. But the point is capable of drawing off the electricity when at a
distance, and it does this gradually; at the same time that it causes a
current of air which repels the cotton; the cotton being deprived of its
electricity is again attracted by the prime-conductor.
_The electrified Bladder._
Coat a bladder that is well blown, with gold, silver, or brass leaf,
which may be fastened on with gum water.—Suspend this bladder at the end
of a silk thread, six or seven feet long, from the ceiling of the room.
Electrify the bladder by giving it a few sparks from a charged jar, and
hold towards it, at some distance, a knobbed wire; you will perceive
that the bladder approaches the knob, and when it comes within striking
distance, gives it the electricity it received from the charged jar, and
thus becomes discharged. Touch it again with the charged phial, and
instead of the knobbed wire, present the point of a needle towards it,
the bladder will now be rather repelled than attracted, especially if
the point be very suddenly presented to it.
CHAP. V.
_Promiscuous Experiments._
We shall in this chapter, describe a variety of experiments, which are
easily made, and which may serve to illustrate the principles of
electricity in general.
EXPERIMENTS.
_The electrical Jack._
This is an invention of Dr. Franklin, and turns with considerable force,
so that it may sometimes be used for the purposes of a common jack. The
construction of it is as follows.—A slender shaft of wood passes, at
right angles, through the centre of a thin, round board, about twelve
inches in diameter, and turns upon a sharp point of iron, fixed in the
lower end; while a strong wire in the upper end passes through a hole in
a brass plate, which keeps the shaft truly vertical. About thirty radii,
of equal length, made of sash glass, cut into narrow slips, issue
horizontally from the circumference of the round board, the ends
farthest from the centre, being about four inches apart, and each
furnished with a metallic ball or thimble.
If the wire of a jar, electrified in the common way, be brought near the
circumference of the wheel, it will attract the nearest ball or thimble,
and put the wheel in motion. That ball or thimble, passing by the knob
of the jar, receives a spark from it, and being thereby electrified, is
repelled, and driven forward; while the second, being attracted,
approaches the knob, receives a spark from it, and is driven after the
first. This process is repeated till the wheel has made one revolution;
when the thimbles, before electrified, approaching the wire, instead of
being attracted are repelled, and the motion presently ceases.—But if
another jar, charged through the coating, or otherwise electrified
negatively, be placed near the same wheel, its wire will attract the
thimble or ball, repelled by the first jar, and thereby double the force
which carries round the wheel.
_The self-charging Tube._
Take a glass tube, about eighteen inches long, and an inch, or an inch
and a half, in diameter; coat the inside with tin-foil, from one
extremity of it as far as the middle; then fix a cork to the aperture of
the coated end, and let a knobbed wire pass through it, and come in
contact with the coating.
The instrument being thus prepared, hold it in one hand by the uncoated
part, and with the hand clean and dry, or with a piece of buckskin,
which has had some amalgam spread upon it, rub the outside of the coated
part; after every two or three strokes, you must remove the rubbing
hand, and by applying it to the knobbed wire, you will receive sparks
from it. By this means the coated end will gradually acquire a charge,
which may be increased to a considerable degree. Now, if you grasp the
outside of the coated end with one hand, and touch the knobbed wire with
the other, you will receive a shock.
In this experiment, the coated part of the tube answers the double
purpose of the electrical machine and Leyden phial; the uncoated part
serving as a handle, to hold the instrument by. The friction on the
outside accumulates a quantity of positive electricity upon it, and this
electricity, in virtue of its sphere of action, forces out a quantity
from the inside. Then, by taking the sparks from the knobbed wire, this
inside electricity is removed, and it consequently remains
under-charged, or negatively electrified; and it also follows, that the
positive electricity of the outside, comes closer to the surface of the
glass, and begins to form the charge.
A small phial may be charged by giving the sparks from the knobbed wire
of the tube to that of the phial; but the phial will be charged
negatively, whereas the tube is charged positively.
_To fire the electrical Cannon by inflammable Air._
This instrument consists of a metallic barrel, made in the shape of a
common cannon,—a glass tube is cemented into the top of the barrel, in
the place of a touch-hole, and through this tube a wire passes, which is
bent so as to come within an eighth of an inch of the inner surface of
the cannon,—on the outer end of this wire, a ball is fixed, which serves
to receive a spark from a charged jar, or from the prime-conductor.