Part 5

In all the above cases, the appearance of the electric matter at the point, will indicate the kind of electricity from which it proceeds. A large divergent cone indicates positive electricity; a small globular light, that which is negative. Hence it is always easy to ascertain whether an insulated conductor be electrified positively or negatively, by presenting a point to it, as the light at the point is always definitive of the contrary electricity in the conductor.

If a pointed conductor be electrified, either positively or negatively, and the face be brought near the point during the electrization, a wind will be felt blowing from the point, accompanied with a peculiar sensation, commonly called _the spider’s web_. It is remarkable that the current of air is always in the same direction, whether the point throws off or receives electricity.

The re-action of the force, by which the air is put in motion, is exerted upon the pointed body. This is shewn by a very pleasing experiment called the electric fly. This fly is composed of four small wires, fastened into a metallic cap, similar to those used in sea-compasses, so that the wires may easily move upon a point, in a horizontal direction. They should be exactly balanced, and have their ends, which must be very sharp, all bent in the same direction. Now if this fly be placed on an insulated point and electrified, its sharp ends will become luminous in the dark, and it will revolve in a direction contrary to that in which the ends are bent; or if it be placed on an uninsulated point and brought near the electrified prime conductor, the same effect will follow.

It is to be observed, that the fly will move round in the same direction, whether electrified positively or negatively. The cause of this seeming contradiction depends upon the repulsive power existing between bodies possessed of the _same_ electricity; for the air opposite to the points acquires a strong electricity, analogous to that of the points, it is therefore repelled, and replaced by other air, which is also electrified and repelled. Hence a continual stream is produced, blowing from the points, and that equally, whether the electrization be positive or negative; and as action and re-action are equal and in contrary directions, the points, repelling the air, must themselves be repelled, and in the opposite direction; which causes the fly to be always turned one way, that is, in a direction contrary to that in which the air is moved.

In vacuo no motion is produced, because there is no air on which the electric matter can act when it issues from the points.

In like manner, if air be confined in a receiver, the motion of the fly soon ceases, because the fluid cannot pass through the air and the glass. But on applying the end of a finger to the outside of the receiver, opposite one of the points of the fly, the motion will begin again, and by moving the finger occasionally round the glass, it may be continued till most of the glass is charged.

The cause of this motion is, that when the finger is applied to the outside of the receiver, the glass, loosing part of its natural quantity of electricity from that side, (i. e. when the fly is electrified positively, and vice versa if negatively) takes up the fluid from the air on its inner surface. Hence the air becomes capable of being again electrified by the point and this renews the motion.

We have already stated that if a pointed wire be presented to a conductor _positively_ charged, it will be illuminated with a star or globe; and if the conductor be negatively charged, the illumination will have the form of a pencil or divergent cone. F. Beccaria explains this in the following manner. I suppose, says he, that the star is occasioned by the difficulty with which the electric fluid is extricated from the air, which is an electric; suppose for instance that a pointed wire is presented to a body positively electrified; the electric fluid is first communicated from that body, to the air between it and the pointed wire, and then the wire must extricate it from the air.

The pencil is occasioned by the force with which the fluid, issuing from the point, passes through the contiguous air to that which is more remote, i. e. by dividing the contiguous air, and not by affixing itself to it.

Beccaria likewise remarks, that if two equally sharp pointed bodies are brought near the prime conductor, they will appear luminous at only half the distance that one of them would. They will also discharge it in half the time.

It will not be improper to remark here, that when a point not electrified is opposed to one electrified positively, both points will have small globular lights upon them; but if a positive one be opposed to one negatively electrified, they both preserve their own characteristic properties.

From the above the following conclusions may be drawn.

First, That pointed bodies attract the electric matter more or less easily, and at a greater or less distance, according to their acuteness.

Second, That pointed bodies have the power of attracting electricity as well as of repelling it, in a greater degree than conductors of any other form.

We shall treat farther of pointed conductors under the article _Thunder-house_.

CHAP. VIII. _Of electric attraction and repulsion._

No satisfactory theory of electric attraction and repulsion has, so far as our knowledge extends, ever yet been given. The phenomena have been differently accounted for, as the writers have embraced different opinions in regard to positive and negative electricity. One mode of explanation has been adopted by those who believe, with Franklin, that positive electricity is only an accumulation of the electric fluid in a body beyond its natural state; and that negative electricity is nothing more than a deficiency of this fluid in a body. Another mode of explanation is given by those who maintain, in opposition to Franklin, that positive and negative electricity are either two distinct fluids, or else vibrations of the same fluid—the positive electricity always rushing out of a body, and the negative always rushing in. Those who maintain this hypothesis endeavour to support it by the easy solution which they affirm it gives to the phenomena of electric attraction and repulsion. But after a careful examination of this theory, we think that, so far from being satisfactory, it is scarcely intelligible. We therefore do not choose to introduce it into our epitome, as affording any solution of the difficulties that occur on this part of our subject. We are besides of opinion that the evidence in favour of a single fluid is conclusive, as we shall show when we come to discuss the theory of electricity. Yet we confess that we cannot, on this theory, offer a rationale of electric attraction and repulsion, that satisfies ourselves. It is therefore the demand of candour, and in the spirit of the Newtonian philosophy, to avow explicitly that this part of our subject is yet involved in much obscurity. In the mean time we are acquainted with certain facts, and with the clear explanation which they give of certain phenomena.

1. That bodies positively electrified, repel each other.

2. That bodies negatively electrified, also, repel each other.

3. That bodies positively electrified, attract those which are negatively electrified.

4. That bodies either positively or negatively electrified, induce a contrary electricity in bodies in their natural state, brought within the sphere of their action.

This statement is easily verified by experiment, in the following manner.—By flaxen or hempen threads, suspend, from the prime conductor, two balls made of cork or elder-pith, so that they touch each other. On charging the conductor, these balls, being both electrified positively, will immediately repel each other, and be separated to a considerable distance.—Remove one of the balls, take it in your fingers, and bring it near to the one which remains positively electrified, and the two will immediately rush together; because there are now two substances of which one is electrified positively, and the other negatively.—Again. Suspend two balls, of the kind just mentioned, from an insulated cushion of an electric machine, and let them touch each other. Put the machine in motion and the balls, which are now both electrified negatively, will repel each other and separate, as in the case first described.

In attempting to explain the first of these phenomena Dr. Franklin once supposed that there was an electric atmosphere round each of the balls positively electrified, the particles of which atmosphere, by mutually repelling each other, separated the balls. He also supposed that as bodies negatively electrified, or not having their proportional quantity of the electric fluid, are always strongly disposed to receive it, this would account for the fact that when one of these bodies was brought near to one that had more than its proportional quantity, the two would naturally rush together; the one to impart, and the other to receive the fluid. But at this time he was not acquainted with the fact, that two bodies negatively electrified would repel each other. When this was discovered he candidly acknowledged the utter deficiency of his theory, in regard to electric attraction and repulsion. Some of his friends and followers, however, have endeavoured still to maintain it. But we think that though their zeal has been greater, their success has not exceeded that of the Doctor himself: and we have already stated that other theories are equally, if not more defective, than that of Franklin. Let us then leave the explanation of electric attraction and repulsion to be made when future and fortunate discoveries shall have furnished the means of making it, and let us proceed with the application of known facts and principles.

A pleasing exhibition of the phenomena of electric attraction and repulsion, may be made in the following manner.

Take a glass tube, and after having rubbed it, let a small light feather fall from your fingers, at the distance of eight or nine inches from it.—The feather will be immediately attracted by the tube and stick very close to its surface for some seconds, after which it will be repelled, and if the tube be kept under it, the feather will continue floating in the air, at a considerable distance from the tube, without coming near it again, except it touch some conducting substance; and if you manage the tube dexterously, you may drive the feather through the air of the room at pleasure.

The cause of this phenomenon is obvious. The feather, at first, not being electrified, rushes to the excited tube. There it becomes electrified and is then repelled, and cannot approach the tube again, unless it first touch some conducting substance; because it cannot part with its electricity while floating in the air, and therefore cannot acquire a contrary electricity; consequently it must remain in a state incapable of being again attracted by the excited tube.

There is a remarkable circumstance attending this experiment, which is, that if the feather be kept at a distance from the tube by the force of electric repulsion it always presents the same part towards the tube. The reason of this phenomenon is, that the equilibrium of the fluid in the different parts of the feather being once disturbed cannot easily be restored; the feather being an electric, or at least a very bad conductor. When the feather has acquired a quantity of electricity from the tube it is plain that, by the action of the excited tube, that superinduced electricity will, for the most part be forced to that side of the feather which, at first, happened to be farthest from the tube; hence that part will always afterwards be repelled the farthest.

This experiment may be agreeably varied in the following manner.—A person may hold an excited tube of glass, within a foot and a half of a stick of sealing-wax, or any other electric negatively electrified, held by another person; a feather let fall between these differently excited electrics will leap from one to the other alternately, and the two persons will seem to drive a shuttlecock by the force of electricity.

Another experiment calculated to shew the phenomena of electric attraction and repulsion is the _electric spider_.

Cut a piece of cork in the shape of a spider, and run a few short threads through it, to represent the legs; this done, suspend it by a silk thread from the ceiling of the room, or any other support, so that the spider may hang mid-way between the knob of a jar and the knob of a wire fastened to the table, or to the outside coating of the jar when not charged; let the place where the jar stands be marked; then charge and replace it. The spider will now begin to move from knob to knob, and continue this motion for a considerable time.

In this case, the knob of the jar is charged positively, and the spider, being in its natural state, is attracted by it; the knob then communicates to it some of its electricity, and the spider becoming possessed of the same electricity with the knob, is repelled by it, and immediately runs to the other knob, which communicates with the negative coating, or with the table, where it discharges its electricity and is again attracted by the knob of the jar. This attraction and repulsion continue till the jar is discharged, when the spider finishes its motion and seemingly expires.

CHAP. IX. _Of the Leyden phial._

This consists of a glass phial, jar, or bottle, coated on the outside and inside with tin-foil, rendered adhesive by paste or gum water. About two inches of the glass at the top are left without any metallic covering, to prevent a communication between the outside and inside coatings, while the electricity is collecting.—The mouth of the phial or jar is furnished with a cork which receives a wire, ending in several ramifications which touch the inside coating. The upper end of this wire, which should extend a convenient distance above the mouth of the jar, is furnished with a metallic ball.

When the phial or jar is to be charged, it may be held in the hand or placed on an uninsulated table, with the knob of the wire touching the prime conductor. The inner surface of the glass now acquires the same electricity with the prime conductor, and the external one acquires a contrary electricity by means of its uninsulated coating.

When a phial similar to the one above described is highly charged, a spontaneous discharge will usually take place over the uncoated surface, and seldom through the glass. But if the uncoated surface be left larger than from two to three inches, the phial is more apt to crack and become useless, by the charge passing through the glass. There is not however an absolute certainty that a jar which has once discharged itself over its surface will not, at another time, break by a discharge through the glass.

It was long disputed whether the discharge of the Leyden phial resided in the coating or in the electric. The following experiment clearly decides, that its residence is in the electric.

Upon an uninsulated plate of metal, lay a plate of glass considerably larger, so that there may be a rim of three or four inches projecting beyond the metal. Upon the glass lay another piece of metal, of the same size with the first, and so as precisely to cover it.

Let this instrument be charged, by connecting the upper metallic plate with the prime conductor. Then separate the metallic plates from the glass; and upon examination the glass will be found to possess the contrary electricities on its opposite sides; that side which during the electrization communicated with the prime conductor will have a like electricity with it, and the other the contrary.

Discharge the electricity of the metallic plates, and replace the whole apparatus in its former situation.—Take a discharging rod, formed by a piece of bent wire with a metallic ball at each end; touch the under plate and bring the other end of the wire near the upper plate. The consequence will be, that a strong and loud spark will pass between the upper plate and the discharging rod; the electricity of the glass will be discharged, and there will afterwards remain no signs of electricity, either in the glass, or the metallic plates.—Hence it appears that the electricity resides in the glass, and that the coatings, whether in a plane or spherical form, are of no other use than to convey the electric fluid to the glass; to keep it equably distributed over the surface; and to form a communication between the different parts of the electrified glass, so that the discharge from them may be simultaneous.

When the discharge of a coated electric is made through the body of a living animal, it occasions a sudden motion, by contracting the muscles through which it passes, and gives a disagreeable sensation commonly called the _electric shock_.

CHAP. X. _The electrical battery—and experiments performed with it._

When a greater degree of electric force is required than a single jar is capable of giving, the electrical battery is made use of as part of the apparatus, which takes its name from the formidable effects it produces. This battery consists of a number of coated jars, placed in such a manner that they may all be charged at the same time, and discharged in an instant; so that the whole force of electricity accumulated in them, may at once be exerted on the substance exposed to the shock.

In discharging electrical jars, the electricity goes in the greatest quantity through the best conductors, and by the shortest passage. Thus if a chain and a wire be made to communicate at the same time with the outer coating of a jar, and be both presented to the knob of that jar, the greater part of the charge will pass by the wire, and very little by the chain, because the latter is a worse conductor than the former, on account of its discontinuation at every link. When the discharge is made by the chain only, sparks are seen at every link, which is a proof they are not in contact.

The force of an electric shock is not affected by the inflections of a conductor through which it passes, though it is sensibly weakened by its length. Hence, when the circuit or communication between the two sides of a Leyden phial is formed by one person applying his hands to the different sides, the shock is stronger than when it is formed by many persons joining hands. Yet a considerable shock was given by the Abbè Nollet, in the presence of the king of France, to one hundred and eighty men; who formed an electrical circuit.—They were all shocked in the same instant.

Doctor Watson and many other gentlemen of eminence in science, were at the pains of making experiments of the same kind. They found, by means of a wire insulated on baked wood, that the electric shock was transmitted instantaneously through the length of 12,276 feet.

Electricity transmitted in large quantities through living vegetables, destroys their vegetable life.

When transmitted, in the same form, through animals, it generally puts an end to animal life; though it is said that there are individuals who are not affected by it. Possibly the reason why some persons are not killed by very large electric shocks is, that their muscular system, or bodily organization, has something peculiar which protects them.

If an electrical circuit be made by means of imperfect conductors, as a slender piece of wood, a wet pack-thread, the discharge will be made silently.

If a small interruption of an electrical circuit be made in water, on making the discharge, a spark will be seen in the water, which never fails to agitate it and sometimes breaks the vessel in which it is contained.

A strong shock from a battery, sent through a slender piece of metal, instantly makes it red hot. Usually it is melted in whole or in part. If the fusion be perfect it is reduced into globules of different magnitudes. In this experiment it is a little remarkable that the parts of the metal at which the fluid enters and issues, are most likely to be melted.

If the metal be enclosed between pieces of glass, the shock will force the melted metal into the substance of the glass, so that it cannot afterwards be removed, without scraping off part of the glass with it. In this experiment the glasses which enclose metal are commonly broken to pieces.—It is seldom that they resist the force of a strong shock. If the glasses enclosing metal be pressed by a heavy weight, a small shock is often sufficient not only to raise the weight, but to break glasses of considerable thickness. When the pieces of glass are not broken, they are marked by the explosion with the most lively prismatic colours, which lie sometimes irregularly, and sometimes in their prismatic order.

Gun-powder may be fired by a charge from three square feet of coated glass. The powder is to be put into a quill, and then a wire is to be thrust into each end so as nearly to meet, and afterwards these wires are to be made a part of an electrical circuit.—A less charge of electricity will be sufficient if iron filings be mixed with the gun-powder.

When a shock somewhat less than is sufficient to melt a piece of metal is sent through a chain, a black dust, in the form of smoke, is seen to proceed from the chain. This dust is probably some of the metal itself, partly calcined, and by the violence of the explosion forced from it. If the chain be laid upon a piece of paper, glass, or other electric, this, after the explosion, will be found stained with some indelible marks, and often shew evident signs of having been burnt.

What is more remarkable in considering the effects of electricity on metals is, that it often, in a considerable degree, revivifies their calces or oxyds. In making experiments of this kind, the metallic calx or oxyd is to be made a part of an electrical circuit, through which a strong shock is to be sent: when the calx or oxyd will be found in a measure restored to its metallic state: the electric shock having, as it appears, taken away from the oxyd a portion of its oxygen.