EPITOMEOFELECTRICITY.
EPITOMEOFELECTRICITY.
EPITOMEOFELECTRICITY.
EPITOME
OF
ELECTRICITY.
Electric attraction and repulsion, observed in excited amber, was, as we have already had occasion to remark, the first phenomenon which was noticed in the science of which we treat. We have also hinted that, to the present time, no explanation of this attraction and repulsion which is entirely satisfactory, has been given. Facts, however, are known in abundance; and certain principles, relative to this part of our subject, are clearly ascertained. To exhibit and illustrate these has been our object in selecting the following experiments; some of which may be considered as intended chiefly for amusement, but all of which, if thoroughly comprehended, will serve to fix the principles of the science more deeply in the mind of a learner.
EXPERIMENTS.
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.
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 anelectrical ball.
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 termvacuumwhich 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.
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 ortube 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.
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.
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.
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 luminouswordproduced.
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.
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, andwhen 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.”
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 severalcards 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.”
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.
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.
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 ratherphosphoreal 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.
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, bysimply passing the discharge of a jar through the wick and smoke.
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.
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 otherhand 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.
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, thebladder will now be rather repelled than attracted, especially if the point be very suddenly presented to it.
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.
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 ofthe 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.
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.
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.
The inflammable air with which this cannon is to be fired, may be prepared in a common porter bottle, by mixing a handful of iron filings with two wine-glassfuls of water, and an ounce of sulphuric acid, commonly calledoil of vitriol. The air when thus made should be kept in a bottle closely stopped.
To use the instrument, have ready a cork, fitted to the mouth of the cannon,—uncork the bottle containing the air, and immediately apply the cannon to the mouth of the bottle; a sufficient quantity of the gas will rise into the cannon, in the course of a few seconds, when both the cannon and bottle must be corked. Now, if the knob of the wire passing through the tube be applied to the prime-conductor, so that a spark may pass through it to the inner surface of the cannon, the gas will be inflamed with a loud report, and the cork will be forced out with considerable violence.
Professor Lichtenburg first observed some curious figures made with pulverized rosin, on a large electrophorus; but since this original discovery, a variety of other methods have been contrived, for making them upon glass, paper, resinous substances and many others. The ingenious electrician may derive considerable information from these figures; their various appearances, in many instances, showing him the direction and quality of the electric fluid.
The principal method of making these impressions is to electrify a perfect or imperfect electric, and then to throw certain powders upon the electrified substance, which will be arranged in different forms. The most convenient method of projecting these powders is to put them into a small bottle of India-rubber, and then fasten a tube of glass or metal to the neck of the bottle; the orifice of this tube must be covered with a piece of flannel when used.
As to the nature of the powders, almost every substance which can be pulverized will do.—Thus chalk, rosin, sulphur, rose-pink, dragon’s blood, gum-arabic, lake, and evaporated decoctions of colouring woods, may be used with advantage, either singly or mixed.
Take a clean pane of glass, fourteen or fifteen inches square, and after drying it thoroughly, hold it by one corner, and pass over its surface the knob of a jar, moderately charged with positive electricity—then, keeping it suspended, project upon it, by means of the bottle above described, a mixed powder of dragon’s blood and gum-arabic, in equal parts. If you examine the glass, you will find that the two powders will be separated upon it, the red powder of dragon’s blood falling on certain places, and the white powder of gum-arabic falling upon certain other places, so as to form a track upon the parts which were touched with the charged jar, consisting of two colours disposed in a thousand different ways.
If, instead of drawing the knob of the jar over the surface of the glass, you only touch it here and there with it, and then throw on the mixed powders as before, separate star-shaped figures will be formed about these places. The stars will be better defined when a single powder is used; their rays are sometimes few and strong; at others, many and slight, and frequently they do not go entirely round the parts which have been touched by the phial. These different effects depend chiefly upon the quantity of the charge in the jar.
If the jar be charged negatively, the appearances will be very different, from those occasioned by positive electricity. Very few rays will now be observed, the powders for the most part disposing themselves in roundfigures, and generally a central spot of one powder will be surrounded by another of a different colour.
Some powders adhere but slightly to the glass, so as not to bear being touched; but if a piece of paper be laid upon the painted side, without disturbing the figures, and the edge of it be fastened all round to the edge of the glass, the figure may be preserved without injury. But a better method is to lay another pane of glass over the one with the figures upon it, and then to fasten them together with sealing-wax, or a piece of paper pasted over the edges.
If the powders of such colours as are used for enamel-painting be projected upon glass or porcelain, and these substances be afterwards exposed to a proper degree of heat, as that of an enameller’s furnace, the figures will be rendered indelible.
Take a piece of common writing paper, and hold it near the fire, so as to make it quite dry and very hot—lay it upon a dry table and pass the knob of a charged jar over it—then take up the paper by one corner, and holding it suspended, throw upon it a mixed powder of dragon’s blood and gum-arabic, in the way above mentioned.—The figures in this instance will be very beautiful, and may be made in various shapes, as letters, stars, or stripes. If the paper thus painted be held near the fire for a few seconds, the powder of dragon’s blood, being a resinous substance, will be melted and fastened to the paper, after which the gum-arabic may be taken off.
Powders of different colours may be projected upon the paper after the same manner, but unless they be of a resinous nature, so as to be easily melted by heat, it is very difficult to fasten them to the paper.
A little experience will enable the operator to make them in a neat and handsome manner. It will however be necessary to observe a few precautions.—The charge of the jar should not be too great or too small; for in the former case the figures will be confused and irregular; and in the latter they will be too faint.—These experiments should be performed as quickly as possible, for if the paper be suffered to cool too much, or the communicated electricity be dissipated, the desired effect will not be produced.
Let a small bucket of metal be suspended from the prime-conductor, and put into it a syphon of glass or metal, so narrow at the outer extremity that the water may just drop from it.—Now, if the cylinder be turned, the water, which when not electrified came over only in drops, will run in a stream, or even be subdivided into a number of smaller ones.—If the experiment be made in the dark, the streams appear luminous.
The same phenomenon may be exhibited by a small bucket, with a jet pipe fixed in the bottom. This must be hung on the prime-conductor, as in the last experiment: or the experiment may be agreeably varied, by hanging one bucket from a positively, and another from a negatively electrified conductor: so that the two jets may be about three inches from each other.—The stream issuing from the one will be attracted by that issuing from the other, and both will unite into one: but, though both are luminous in the dark, before meeting, after this has taken place they will not be so, unlessone of them was more powerfully electrified than the other.
If a jar be discharged with a rod which has no electric handle, the hand which holds the rod, on making the discharge, frequently feels something similar to a shock, especially when the charge is considerable.—This shock, or lateral explosion, as it has been called, may be rendered visible in the following manner.—Connect a chain with the outside coating of a charged jar—then discharge the jar through another circuit; for instance, a discharging rod—The chain which is connected with the outside coating, but which forms no part of the circuit, will appear lucid in the dark; that is, sparks will be seen at every link. This chain will also appear lucid, if it be only put close to the jar, without touching it; and on making the discharge a spark will be seen between the coating and the end of the chain. This luminous appearance is what has been denominated theLateral explosion.
Provide yourself with a piece of paste-board, of the size you intend the figure of the constellation, (four or five inches square will be found convenient) and cover one side with tin-foil or silvered paper. Let needles, or any other small metallic points, project from the other side of the paste-board, from the places where you intend stars to appear, taking care to form a communication between each of the points, or needles, and thetin-foil on the other side. If the instrument thus prepared be fixed upon the prime-conductor, negatively electrified, all the points will be illuminated at once.—The experiment may be performed with the prime-conductor positively electrified; but in this case, the light at the points, being in the shape of a divergent cone, does not appear so proper to represent stars, as the round globular lights, which are characteristic of points negatively electrified.—It is scarcely necessary to remark that this experiment should be performed in a darkened room.
Cut a circular piece of silvered paper into a spiral form. The outer end must be shaped like a serpent’s head, with the mouth open and the tongue protruded. Then provide an upright shaft of wood or metal, terminating upward in a point, and having the lower extremity fastened in a foot or bottom-board. The snake, being put spirally round the shaft, with its tail on the point, and then placed under a metallic point suspended from the prime-conductor, will turn round, and in a darkened room will appear to spit fire.
Electrify a common tumbler, by passing a chain, communicating with the prime-conductor, over its inner surface. Place a small heap of steel or brass-filings on an uninsulated conductor, and invert the electrified tumbler over it: the filings will be attracted up the sides of the tumbler, and then thrown off. This, at night,forms a very beautiful experiment, as the filings become luminous, and appear like a shower of fire.
If a tumbler, electrified in this way, be inverted over pith balls, instead of brass-filings, the balls will leap with surprising velocity up the sides of it.
Provide a glass tube in the shape of a common discharging-rod, about ten inches in length, and let the bore of the tube be nearly the eighth of an inch in diameter; upon one end fasten with cement, or otherwise, a brass knob, so as to be perfectly air tight. Now expel the air from the tube, by heat or the air pump, and then fix another knob upon the open end, in a way similar to the former.
If the instrument be used as the common discharging-rod, it will be found to answer its purposes equally well; while at the same time all the inner surface of the tube, during the discharge of a jar with it, is beautifully luminous.
Mr. Nairne also contrived a luminous discharging-rod. It consisted of an arched glass tube, with a metallic ball at each end, and a communication from one ball to the other was made by a brass chain, which passed through the bore of the tube.—In the discharge of a jar with it, small sparks are seen between the links of the chain within the tube.—Both these dischargers should have handles fastened to them.
Let a glass tube, having a small bore, be filled with water; then close each end of the tube with a pieceof cork, and let two wires pass through the corks, so that their extremities may come pretty near each other within the tube.
If sparks of electricity be made to pass between the ends of the wires, within the tube, the water will be converted into oxygen and hydrogen gases.
If this process be continued till the extremities of the wires become immersed in the two gases, they will explode and again form water.
Insulate a small fountain made of metal, (one on the construction ofHiero’swill be found most convenient,) and connect it with the prime-conductor—put it in operation—the jet will be undivided, except at the top—now turn the cylinder, and you will immediately perceive the jet divided much lower than at first; the drops, which before fell nearly perpendicularly, will now be thrown off in elliptical lines, and attracted by any conductor brought near them. A small Leyden phial may be charged at the top of the jet, which will present the curious spectacle of fire coming out of water.