Part 9
This machine was invented by Dr. Franklin. It is made of a thin round plate of window-glass, 17 inches in diameter, covered on both sides with tin-foil, except about two inches next the edge. Two small hemispheres of wood are cemented to the two sides centrally opposite, and in each of these a strong thick wire eight or ten inches long is placed; and these form the hub and axis of the wheel. It turns horizontally on a point at the lower end of its axis, which must be insulated. The upper end of the axis, passes through a hole in a thin plate of brass, cemented to a strong piece of glass or baked wood, which keeps it six or eight inches distant from any non-electric, and is furnished with a ball of wax or metal on the top, to keep in the fire. In a circle on the table which supports the wheel, are fixed twelve small pillars of glass about four inches apart, with a thimble or metallic ball on the top of each. On the edge of the wheel is a small metallic bullet, communicating by a wire with the upper coating of the wheel; and about six inches from it, is another bullet communicating, in like manner, with the lower coating. When the wheel is to be charged by the upper coating, a communication must be made from the under one to the table. When it is well charged it begins to move; the bullet nearest to a pillar is attracted by the thimble or bullet on that pillar and passing by, electrifies it, and is immediately repelled from it; the succeeding bullet, which communicates with the other coating of the glass, more strongly attracts that thimble, on account of its being previously electrified by the other bullet; and thus the wheel increases its motion, till its velocity is regulated by the resistance of the atmosphere.—The wheel will turn half an hour, and make, one minute with another, 20 turns in a minute, which is 600 turns in the whole; the bullet of the upper coating giving, in each turn, 12 sparks to the thimbles or balls, which make 7200 sparks; and the bullet of the under coating receiving as many from the thimbles; those bullets moving in the time near 2500 feet. The thimbles are well fixed, and in so exact a circle, that the bullets may pass within a very small distance of each of them. If, instead of two bullets, there be eight, four communicating with the upper, and four with the under coating, placed alternately, the motion will be considerably increased, but then it will not continue so long. These wheels may be applied to the ringing of chimes, and the moving of light-made orreries.
_The electrical Dance._
Suspend from the prime-conductor, by means of a hook, a metallic plate, six inches in diameter. About three or four inches from this, and directly under it, place another plate of the same kind, communicating with the earth. Upon the lower plate, throw small painted figures of men and women, cut in paper, or made of the pith of elder. Now, if the cylinder be turned, the figures will begin to move between the plates, leaping from one to the other, with surprising velocity, exhibiting many curious and ludicrous attitudes and motions.
_The electrified Bells._
The phenomena of attraction and repulsion may be very satisfactorily shown with the electrified bells. In order to make this experiment, provide yourself with a piece of wire, furnished with a hook equidistant from both ends, and by which it may be suspended from the prime-conductor. At each end of this wire suspend a small bell by a chain or wire; and from the middle point between these two bells, suspend a third, by a silk thread; let a clapper be hung between each of the bells, also by silk threads. From the concave or under side of the middle bell let a chain proceed, communicating with the table, and having a silk thread at its extremity. Now if the cylinder of the machine be turned, the clappers will fly from bell to bell with a very quick motion, and the bells will ring as long as the electrization continues.
The two outer bells, being suspended by chains or wires, are electrified first; hence they attract the clappers; and having communicated to them part of their electricity, repel them. The middle bell, which is in its natural state, now attracts them, and deprives them of their acquired electricity; after which they are again attracted by the outer bells, and again repelled. If, by holding the silk thread, the chain of the middle bell be raised from the table, the bells, after ringing a short time, will stop; because the middle one, being insulated, will soon become as strongly electrified as the other two; in which case, the clappers being equally attracted by both bells, must discontinue their motion towards either.
If the experiment be made in a darkened room, a spark will be seen between the clapper and bells, at every stroke.
This experiment will have a better effect, if, instead of keeping the machine in motion, a charged jar be placed in contact with the prime-conductor; and when joined with the preceding experiment, the whole will have the appearance of an _electrical ball_.
_The inflammable air Balloons._
The following experiment may serve to illustrate some of the phenomena observed in thunder storms.
Provide two balloons, made of the allantoides of a calf, containing about two cubic feet, and fill them with inflammable air. To each of these attach, by a silk thread about eight feet long, a weight sufficient to prevent their rising higher than the above distance in the air. Then connect one of them with the positive, and the other with the negative conductor, or insulated rubber of the machine, by very thin wires, thirty feet long: keep them a considerable distance asunder, and as far from the machine as the wires will admit. On being electrified, the balloons will rise as high in the air as the silk thread will allow them, then attract each other, and uniting as it were in one cloud, will gradually descend.
The rising of these balloons is attributed to the expansion of the air contained in them, in consequence of the repulsive power communicated to its particles by the action of the electric matter upon them.—When in contact, their opposite electrical powers destroy one another, and they descend in consequence of the condensation of the internal air.
_Dr. Franklin’s Experiment for illustrating his Theory of Thunder Storms._
Take two round pieces of paste-board, two inches in diameter; from the centre and circumference of each of them suspend, by fine silk threads eighteen inches long, seven small balls of wood, or seven peas, equal in size, so that the balls appended to each paste-board will form equilateral triangles, one ball being in the centre and six at equal distances from that, and from each other, around the circumference. Thus they represent particles of air. Dip both setts in water; and some of it adhering to each ball, they will represent air loaded with moisture. Electrify one sett, and its balls will repel each other to a greater distance, enlarging the triangles. Could the water, supported by the seven balls, come in contact, it would form a drop or drops, so heavy as to break the cohesion it had with the other balls, and so fall. Let the two setts then represent two clouds, the one a sea-cloud electrified, and the other a land-cloud. Bring them within the sphere of attraction, and they will instantly draw towards each other. Now you will see the separated clouds close thus—the first electrified ball that comes near an unelectrified one, by attraction joins it, and gives it fire; instantly they separate, and each flies to another of its own party, one to give and the other to receive fire; and so they proceed through both setts, but so quick as to be in a manner instantaneous. In the collision they shake off and drop their water, which represents rain.
CHAP. II. _Experiments with electric Light._
These experiments should be made in a darkened room, for though the electric light is visible frequently in day light, yet the appearance of it is very often confused, so that a distinct idea of it cannot be formed.
Before we proceed to describe the experiments under this head, it will be necessary to inform the reader that by the term _vacuum_ which he will frequently meet with in this chapter, we mean such an one as is formed by the action of an air pump, which is a good conductor of electricity.
EXPERIMENTS.
_The Aurora Borealis._
Take a phial nearly of the shape and size of a Florence flask; fix a stopcock or valve to its neck, and exhaust it of air.—If this phial be rubbed in the usual way to excite electrics, it will appear luminous within, being full of a flashing light, very much resembling the northern lights or aurora borealis. This phial may also be made luminous by presenting one end of it to the prime-conductor, while the other is held in the hand. In this case, the whole cavity of the glass will instantly appear full of a flashing light, which remains in it for some time after the glass has been removed from the prime-conductor.
A glass tube exhausted of air in the same manner, and hermetically sealed, may be used instead of this phial, and perhaps with more advantage.
The most remarkable circumstance attending this experiment is, that after the phial or tube has been removed from the prime-conductor, and even several hours after the flashing light has ceased, strong flashes will be again visible upon applying the hand.
The causes of this phenomenon are two; first, the conducting nature of the vacuum; and second, the charging of the glass; for when one side of the phial is touched with the prime-conductor, the electric fluid communicated to that part on the outside, occasions the natural fluid of the inside surface, to leave its place and pass to the opposite side of the phial, which does not communicate with the electrified conductor; this passing of the fluid through the vacuum occasions the light within, which is more or less subdivided as the vacuum is more or less perfect.
That part of the phial which has touched the prime-conductor is actually 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.