Part 10

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.

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 called _oil 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.

_Curious Figures made upon Glass, Paper, and other Substances, by means of Electricity._

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 round figures, 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.

_The Electrified Capillary Syphon._

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, unless one of them was more powerfully electrified than the other.

_The Lateral Explosion._

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 the _Lateral explosion_.

_To represent the Constellations._

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 the tin-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.

_The Electrical Snake._

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.

_The luminous Shower._

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.

_The luminous Discharging-rod._

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.