CHAP. XI.A description of the electrophorus, and some of its phenomena accounted for.
The electrophorus is a machine, consisting of two plates, usually of a circular form. At first the under plate was of glass covered with sealing wax; but there is little occasion to be particular, with regard, either to the substance of the lower plate, or to the electric with which it is covered; a metallic plate however is preferable to a wooden one, though the latter will answer very well. This plate must be covered with an electric: pure sulphur answers nearly as well as the dearer electrics gum lac, sealing wax &c.
The upper plate is made of brass, or a piece of paste-board covered with tin foil or silvered paper, which must be nearly of the same dimensions as the electric plate: this plate must be furnished with an electric handle, which, by means of a metallic or wooden socket is fastened to its centre.
This instrument was invented by Mr. Volta, an Italian philosopher. The manner of using it is as follows.
First, The under plate is excited, by rubbing its coated surface with a piece of new white flannel, or a fox’s tail. A hard shoe brush, having the bristles a little greased, will also excite sulphur very well. When this plate is excited as much as possible, it is placed on a table with the electric side uppermost; the metallic plate is then laid on the excited electric; then the metallic plate is touched with the finger, or with any other conducting substance, which receives a spark from it; finally the metallic plate being held by the extremityof its electric handle, is separated from the electric and after it is raised some distance, it is, on examination, found strongly electrified, with an electricity contrary to that of the electric, and will give a strong spark to a conductor brought near it. By placing the metallic plate upon the electric, touching it with the finger and separating them successively, a great number of sparks may be obtained, apparently of the same strength, and without exciting the electric again.—If these sparks be repeatedly given to the knob of a coated jar, it will become charged.
The action of these plates depends upon the principle already laid down (page22,) that an excited electric has the power of inducing a contrary electricity in a body brought within its sphere of action. The metal plate therefore, when set upon the excited electric, acquires a contrary electricity, by giving its electric fluid to the hand or other conductor which touches it, when the electric is positively electrified; or by acquiring an additional quantity from the hand &c. when the electric is negatively electrified.
More fully to explain the principle here considered let the following easy experiment be made—
Electrify any insulated conductorpositively. Then if an electrometer[15]of cork balls be held at some distance from it, the balls will diverge withnegativeelectricity. This may be proved by bringing a piece of excited glass near them, as the balls will be attracted by it. But if you present to them a piece of excited sealing wax, they will immediately avoid it—that is, supposingthe glass to be excited always positively, and the sealing wax always negatively.
Again. Insulate, in a horizontal position, a metallic rod with blunt terminations, and about two feet long. We shall designate the ends of this rod by A and B. Let a cork ball electrometer be affixed to the extremity A; then bring an excited glass tube within eight or ten inches of the other end B—the balls will immediately diverge with positive electricity. If the tube be removed the balls will immediately collapse, and no electricity will remain in them, or in the rod.—But if, while the tube is near one end B of the rod, and the balls diverge with positive electricity, the other end A be touched with a finger or other uninsulated conductor, the cork balls will immediately come together, as if the rod were in its natural state: but if, in this state of things, the excited tube be removed, the balls will again diverge, but with negative electricity, shewing that the whole rod AB is now under-charged.
This last experiment is thus explained.—When the rod is in its natural state, the electric fluid proper to it is equably distributed throughout the rod; but when the excited glass tube is brought near one of its ends as B, the fluid belonging to that end will be driven towards A; which extremity becomes over-charged, and the other extremity B under-charged; yet the rod has no more electricity now than it had before, and when the tube is removed beyond the sphere of its action, the redundant fluid of A returns to its former place B, and the equilibrium is restored. But if the extremity A be touched, while it is over-charged, by a conductor, this will carry off its superfluous fluid, and leave the extremity A in its natural state, the extremityB being at the same time negatively electrified: and when the tube is removed, part of the fluid naturally belonging to A goes towards B, and the whole rod remains under-charged.
We have already seen that it is a general law of electricity, that similar electricities repel, and that dissimilar electricities attract each other.—On this law all electrometers are constructed. In fact the cork balls, which have been mentioned are electrometers, and exhibit at once the most important phenomena for the explanation or ascertaining of which the instruments which bear this name are constructed. Still it is of use to see the application which may be made of this general principle. It is applied to ascertain the quantity of the electric fluid collected either in a prime conductor or a coated jar; and also the state of the atmosphere in regard to electricity, and the character of that electricity at any particular time and place.
The instruments by which these purposes are effected we shall now shortly describe.
To ascertain the quantity of electricity in a prime conductor or jar, an electrometer the most easily constructed and of the most general use has been invented by Mr. Henley—called the quadrant electrometer.—Of this we have given a representation in the frontispiece, (letter X.)
It consists of a perpendicular stem formed at the top like a ball, and at the lower end with a screw, by whichit is fastened to the prime conductor. A graduated semicircle of ivory, horn or stiff paper, is fixed near the uppermost end of the stem. A moveable index, made of a slender piece of hickory, extends from the centre of the graduated semicircle a little distance beyond its circumference, having a small ball of cork or pith at its lower extremity.
When the conductor or jar is not electrified, the index is parallel to the stem, but when it is electrified the index recedes more or less, according to the degree of the electrization, which is marked on the graduated circle.
A simple atmospheric electrometer was constructed by Mr. Cavallo in the following manner.—
To the end of a common fishing rod, he affixed a slender glass tube covered with sealing wax, and having a cork at its end, from which two cork or pith balls were suspended by hempen strings. From the other end of the rod proceeded a flaxen or hempen twine a little longer than the whole rod and tube, with a pin attached to it, which was stuck into the cork at the extremity of the glass tube, for the purpose of taking off the insulation. The twine, to prevent its falling when the pin was pulled out of the cork, was attached to the rod, by a small string, running from it and meeting the rod at a little distance from the glass tube.
To use this instrument, let the pin be pushed into the cork. Then, holding the rod by the extremity farthest from the cork balls, project it out, from a window in the upper part of the house, into the air, raising the end of the rod to which the balls are appended, so as to make an angle of 50° or 60°, with the horizon.—Afterhaving kept it in this situation a few seconds, by pulling the twine, detach the pin from the cork.—This leaves the electrometer insulated, and electrified with an electricity contrary to that of the atmosphere. Now draw the instrument into the room and you may examine the quality of the electricity, by applying the knob of a phial positively charged to one of the balls; if the ball is attracted by the knob it is negatively electrified—if repelled, positively electrified.
The satisfaction arising from these experiments is sometimes abated, from the circumstance that the quantity of electricity obtained in this way, is so small that its quality cannot be ascertained. To remedy this inconvenience Cavallo and Nicholson, have invented machines which they denominatedoublersormultipliersof electricity. But the structure of these machines is complex and delicate, and the explanation of them is long, and not easily understood without the aid of plates. Our epitome therefore does not admit of inserting them. Those who may choose to pursue the subject we refer to the writers above mentioned.
To prevent the inconvenience arising from wind and rain in the use of the atmospheric electrometer, the following device has been used by Mr. Cavallo.—Take a glass vessel open at top and bottom—cement it at bottom to a convenient piece of wood—let the upper part be tapering like the neck of a phial, and cement into it a glass tube, extending a little above and a little below the neck of the larger vessel. Cover the tube with sealing wax, both within and without the neck of the vessel, so as to give it the appearance of one body. Into this tube cement a brass wire extending a verylittle below the bottom of the tube, and flattened at the lower end so as to be perforated with two small holes. Through these holes insert two flaxen threads, or two very fine silver wires, with small balls of cork or pith at the end of them, and touching each other:—if wires are used they should be suspended by small rings at the top, that they may act more easily. Let the top of the brass wire screw into a brass cover on the top of the whole vessel, which cover will not only secure the vessel against rain, but serve as a conductor to a very slightly electrified atmosphere—conveying the fluid, first to the wire, and by means of that to the balls, which will exhibit, within the vessel, the state of electricity collected from the atmosphere. There should be two narrow slips of tin foil stuck to the inside of the glass vessel, and communicating with the wooden bottom, which will serve to carry off that electricity which, when the corks touch the glass is communicated to it, and which, if accumulated, would disturb the free motion of the corks.
An useful alteration of this electrometer was made by Mr. Bennet. It consists of slips of gold leaf or silver leaf, instead of the corks suspended by threads or wires. These slips of leaf are to be suspended from the cover of a cylindrical vessel, and hanging within it. The slips of leaf are to be about two and an half inches long. This electrometer is the most sensible instrument of the kind, manifesting in an unequivocal manner very small quantities of electricity. But this instrument is not as portable and easily managed as the other.—If very fine threads, stiffened with glue, be used without any balls, they will be found nearly as sensible as the gold leaf.
CHAP. XIII.The identity of electricity with lightning.
The identity of the electric matter with lightning is a discovery, which has been of more use than any other in electricity.
That the effects of this fluid bore a great resemblance to those of lightning, had been several times remarked by philosophers and especially by the Abbè Nollet; but that they should be found to be effects of the same cause, and that the phenomena of electricity could be imitated by lightning, or those of lightning by electricity, was not suspected, till our countryman Dr. Franklin made the assertion in 1750, and afterwards demonstrated its truth by undeniable experiment in 1752.
This discovery is almost the only one in the whole science which has not been the result of accident.
The Doctor had for a long time observed the effects of pointed bodies in drawing off the electric matter more powerfully than could be done by others.—Improving upon this, he supposed that pointed iron rods, raised to a considerable height in the air, when the atmosphere was loaded with lightning, might “draw off the matter of the thunder-bolt, without noise or danger.” As he was waiting for the erection of a spire in Philadelphia, that he might have an opportunity of ascertaining the correctness of his hypothesis, it occurred to him, that, by means of a common kite, he could have a readier access to the higher regions of the atmosphere than in any other way. Preparing therefore a large silk handkerchief, and two cross sticks uponwhich he might easily extend it, he took the opportunity of the first approaching thunder storm to walk into a field, where there was a shed convenient for his purpose; but, dreading the ridicule which too commonly attends unsuccessful attempts in science, he communicated his design to no one but his son, who assisted him in preparing and raising the kite.
A considerable time elapsed before there was any appearance of success: one very considerable cloud had passed over the kite without any effect; when, just as he was beginning to despair, he observed some loose threads of the hempen string to stand erect, and avoid one another just as if they had been suspended from the prime conductor of an electrical machine. On this he presented his knuckle to a key which was fastened to the string, and received a spark. Others succeeded even before the string was wet; but when the rain began to fall he collected the electrical fire very copiously.
He afterwards had an insulated iron rod, to draw lightning into his house, and performed almost every experiment with real lightning, that he had before made with electricity collected by a machine. Thus a new field was opened for the philosophy of electricity.
In the structure of an electrical kite, the circumstances to be principally attended to are those near, and on the ground. Silk being a non-conductor, the end of the string which is held in the hand is to be of that substance—a silk handkerchief tied to the hempentwine of the kite will answer very well. An iron key is to be tied on the hempen string, an inch or two above its junction with the silk, and from this key, when the kite is electrified, the sparks are to be received into a Leyden phial, to be used in the same manner as if it had been charged from the electrical machine. As curiosity may prompt many to repeat the experiments made with this kite, and as no experiments with atmospheric electricity can be made without some danger,[16]we shall give the substance of Mr. Cavallo’sdirections (the best we are acquainted with) relative to the forming and using of this instrument.—He observes that the whole power of the machine lies in the string: and that in other respects a common school boy’s kite, will answer the purpose as well as any other. The string is made by twisting two threads of twine with one of brass wire or copper, such as is commonly used for trimmings. When a kite constructed in this manner was raised, the string always gave signs of electricity except once, when the weather was warm, and the wind so weak that the kite could not be kept up for a few minutes; afterwards, however, when the wind increased, he obtained as usual a considerable quantity of electricity.
Concerning the management of this kite he gives the following directions.—
In raising the kite when the weather is very cloudy and rainy, at which time there is much danger of meeting a great quantity of electricity, I usually hang upon the string a chain with one extremity touching the ground; and sometimes I use another caution besides, which is, to stand upon an insulated stool; in which situation, I think that if any quantity of electricity, suddenly discharged by the clouds, strikes the kite, it cannot much affect my person. Although I have raised my electrical kite a hundred times without any cautionwhatever, I have very seldom received a few exceedingly slight shocks in my arms. In time of a thunder storm, if the kite has not been raised before, I would not advise a person to attempt it while the stormy clouds are over head, the danger at such time being very great, even when every caution is used. At that time the electricity of the clouds may be observed by means of a cork ball electrometer, placed in an open situation.
But Mr. Cavallo with all his caution could not avoid danger in making experiments on atmospheric electricity, as appears from the following account of his observations on the 13th of October 1773. “After having rained a great deal in the morning and the night before, the weather became a little clear in the afternoon, the clouds appearing separated and pretty well defined; the wind was west and pretty strong; the atmosphere was in a temperate degree of heat. In these circumstances, at three o’clock P. M. I raised my electrical kite, with 360 feet of string. After the end of the string was insulated, and a leather ball coated with tin foil, hung to it, I tried the power and quality of the electricity, which appeared to be positive and pretty strong; in a short time a small cloud passing over, the electricity increased a little; but the cloud being gone it returned pretty soon to its former degree.
The string of the kite was now fastened by a silk string to a post in the yard of the house; I was repeatedly charging two phials, and giving shocks with them: while I was so doing, the electricity, which was still positive, began to decrease, and in two or three minutes it became so weak, that it could hardly be perceived, with a very sensible cork ball electrometer.—Observingat the same time that a large black cloud approaching the zenith, (which no doubt caused the decrease of electricity) indicated rain, I introduced the end of the string through the window on the first floor, where I fastened it by the silk to an old chair.—The quadrant electrometer was set upon the same window, and was, by means of a wire, connected to the string of the kite. Being now three quarters of an hour after three, the electricity was actually imperceptible, however in about three minutes it returned, but now upon examination, it was found to be negative, which was evidently occasioned by the approach of the cloud, which by this time had reached the zenith of the kite; the rain also began to fall in large drops. The cloud came farther on, the rain increased and the electricity keeping pace with it, the electrometer soon arrived at 15°. Seeing now that the electricity was strong, I began again to charge the phials and to give shocks with them; but the phials had not been charged more than three or four times, before I perceived that the index of the electrometer was arrived at 35°, and was still rising. The shocks now being very smart, I desisted from charging the phials, and considering the rapid advance of the electricity, thought to take off the insulation of the string, that if it should farther increase it might be conducted silently to the earth, without occasioning any bad accident.
To effect this, as I had no proper apparatus near me, I thought to remove the silk string, and to fasten the twine itself to the chair. I disengaged the wire which connected the electrometer with the string; untied it from the silk, and fastened it to the chair: but while I was effecting this, which took up less than half a minute,I received twelve or fifteen very strong shocks, which I felt all along my arms, in my breast, and legs, shaking me in such a manner that I had hardly power to effect my purpose, or to warn the people of the room to keep their distance. As soon as I took my hands from the string, the electricity (in consequence of the chair being a bad conductor) began to snap between the string and the window shutter, which was the nearest conductor. The cloud was now just over the kite; it was black, well defined, and nearly of a circular form, its diameter appearing to be about 40°; the rain was copious but not remarkably heavy.
As the cloud was going off, I went near the string, and finding the electricity weak, but still negative, I insulated it again, thinking to keep it up some time longer; but observing that a larger and denser cloud was approaching, I resolved to pull the kite in; accordingly a gentleman, who was near me, began pulling it while I was winding up the string, he told me he had received two or three slight shocks in his arms, and if he should feel one more, he would let the string go; upon which, I pulled the kite in as fast as I could myself, without any further observation, being ten minutes after four o’clock.
N. B. There was no thunder or lightning perceived that day, nor for some days before, nor afterwards.
The general laws which Mr. Cavallo deduced from a variety of experiments made by means of electrical kites, are the following:
1st. The air appears to be electrified at all times; its electricity is always positive and much stronger in frosty than in warm weather; it is by no means less in the night than in the day time.
2d. The presence of the clouds generally lessens the electricity of the kite, sometimes it has no effect upon it, and it sometimes, though rarely, increases it a little. To this the above mentioned instance is a remarkable exception.
3d. When it rains, the electricity of the kite is generally negative, and very seldom positive.
4th. The aurora borealis seems not to affect the electricity of the kite.
5th. The electrical spark taken from the string of the kite, or from an insulated conductor connected with it, especially when it does not rain, is seldom longer than the quarter of an inch; but it is exceedingly pungent. When the index of the electrometer is not higher than 20° the person who takes the spark will feel it in his legs; it appearing more like the discharge of an electrical jar, than the spark taken from the prime conductor of an electrical machine.
6th. The electricity of the kite is generally stronger or weaker, according as the string is longer or shorter; but it does not keep any exact proportion to it; the electricity, for instance, brought down from a string of an hundred yards, may raise the index of an electrometer to 20°, when with double the length of string, the index of an electrometer will not go higher than 25°.
7th. When the weather is damp, and the electricity pretty strong, the index of an electrometer, after taking a spark from the string, or being presented to the knob of a coated phial, rises surprisingly quick to its former place; but in dry and warm weather it rises exceedingly slowly.
CHAP. XV.The structure and use of lightning rods.
Since the discovery of the identity of lightning and the electric matter, long rods of iron, or other metals, have been made use of, with a view to protect buildings from the effects of lightning. This is the most practical and important part of our whole subject, and deserves to be treated with the utmost attention. Iron and copper are the metals which, on account of their conducting power, their cheapness, and the quantity required for a lightning rod, are principally used. Copper is preferable to iron. Care should be taken that the rod be not less than half an inch in diameter. It is best to have it, if possible, of one continued piece. If this be not practicable, the pieces should be screwed into each other; or at least so constructed that the rust will not separate the perfect metal of one piece from that of another; because metallic rust is almost a non-conductor of electricity. The rod should be fastened to the house by wooden cramps or staples, rather than by those of metals of any kind; because wood is neither so good a conductor of electricity, nor so likely to promote the rust of the metal which it touches. The rod should be raised above the top of the building or chimney to which it is attached, at least five or six feet. The point or points should be made very sharp, and for a few inches should taper off in the form of a pyramid, having all the corners or edges sharp. It is not of much importance whether there be, or be not, more points than one. If the means afterwards to be mentioned be not used to preserve the points from rust, it maybe of use to gild them; and the gilding should extend downwards a foot or more. It is better to paint the point of a rod, than to leave it wholly unprotected against rust. The lower end of the rod should be driven or sunk at least five or six feet into the ground, and in a direction from the building. If it can be connected with the water of a spring, a well, or a cistern, it will be so much the better. At powder-mills, arsenals, and all depots of inflammable materials, it is better to attach the rod to a post, raised for the purpose, a foot or two from the building, than to the building itself. If the building be large, there should be a rod at each end; and it is an additional security, if these rods be connected by a piece of metal, running from the one to the other, on the roof of the house. If there be but one rod, it should, in this country, be put on the western end of the house; because thunder storms oftenest arise from that quarter. If the position of the house affords but little choice in this respect, the rod should be placed either on the kitchen chimney, or as near to it as possible; because smoke and heat are conductors, and in the summer, smoke and heat seldom ascend from any other chimney than that of the kitchen. When there is a copper spout to a house, the rod, if convenient, may be connected with it as a part of the conductor. In this case however, care should be taken to make the connexion complete, both at top and bottom. Large barns and barracks, have even more need of a rod to preserve them from lightning than a dwelling house, because the vapour which ascends from them when filled with vegetable substances, imperfectly dried, is a powerful conductor.
Ships, and all vessels which have high masts, have as much need of conductors as houses on the land. Copper conductors are in every view the best for ships, as they will not contract rust from sea water. A conductor, of this metal, should be attached to the highest mast of the vessel, and extend three or four feet above its top. It should be inserted into the side of the mast, so as to leave the surface smooth, be carried across the deck and over the side of the ship to the keel; so that it may terminate where the lower extremity may be always under the water. Chains are often used as conductors to ships, but they are far inferior to a piece of metal, whose parts are not separated.
In the above directions it has been our aim to show in what manner structures may be best and most effectually protected against danger from lightning, and whenever it is practicable the best means ought certainly to be used. But it is to be remembered that where means the most effectual cannot be applied, those of an inferior kind are not to be neglected. A small rod, however pointed or fastened to a house, is unspeakably better than none, and a chain should always be used in a ship, if a rod cannot be obtained. In ninety nine cases out of a hundred, any metallic conductor, reaching from the top to the bottom of a structure, will preserve it from destruction by lightning, and save the lives or property of the inhabitants, when the whole might otherwise have been destroyed.
The points of rods have often been found melted by lightning, and both they and the lower extremities are often injured by rust. For an effectual method of preventing both these inconveniencies, the public are indebted to Robert Patterson Esq. professor of mathematicsin the University of Pennsylvania, and director of the Mint of the United States.—His memoir on the subject is as follows:—
“From the instances which now and then occur of houses being struck with lightning, that are furnished with metallic conductors, and the frequent instances of these conductors having their tops melted off by a stroke of lightning, it appears that this admirable contrivance for guarding houses against the dangerous effects of lightning is, in some degree, still imperfect. Some improvement seems yet to be wanting at both extremities of the rod—at the upper extremity, to secure it against the accident of being melted, which renders it afterwards unfit to answer its original intention, viz. drawing off the electricity, or lightning, from the passing cloud, in a silent imperceptible manner, for it is onlypointedconductors that possess this property—and at the lower extremity, to afford a more ready passage for the fluid into the surrounding earth.
The first of these intentions, would I am persuaded, be effectually answered by inserting in the top of the rod a piece ofblack lead, of about two inches long, taken out of a good pencil, and terminating in a fine point, projecting but a very little above its metallic socket; so that if the black lead point should happen to be broken off by any accident, of which however I think there can be but little danger, still the point of the rod would be left sharp enough to answer the purpose of a metallic conductor.
This substance is well known to be infusible, by the greatest heat, and hence its use in making crucibles; nor is it evaporable as remarked by Cronstedt, in his mineralogy, Sect. 231, except in a slow calcining heat,to which it could never be exposed at the top of a lightning rod.
At the same time its power as a conductor of electricity is perhaps equal, or but little inferior, to that of any of the metals. A line drawn on a piece of paper by a black lead pencil, will as I have often experienced, conduct an electric explosion seemingly as well as a similar line of gilding would do, and that without ever loosing its conducting power, which is not the case with gilding.
The second intention is, to facilitate the escape of the electric fluid from the lower part of the rod into the surrounding earth.
It is in many cases impracticable, from the interruption of rocks or other obstacles, to sink the rod so deep as to reach moist earth, or any other substance which is a tolerably good conductor of electricity. Nor, even if this were practicable, would it, I presume, be alone sufficient to answer the desired intention. Iron, buried in the earth, and especially in moist earth, will presently contract a coat of rust, which will continually increase till the whole is converted into rust, but rust of iron, and indeed the calx of all metals is a non-conductor, or at most but a very imperfect conductor of the electric fluid. Hence it is easy to see, that in a few years after a lightning rod has been erected, that part of it which is under ground will contribute little or nothing towards the safety of the building. Besides, the surface of this part of the rod is too small to afford an easy and copious discharge of the electric fluid into the surrounding earth, when this is but an imperfect conductor.
As a remedy for these defects I would propose, that the parts of the rod under ground be made of tin, or copper, which are far less liable to corrosion or rust, by lying under ground than iron.—Or, which perhaps would answer the purpose better, let this end of the rod, of whatever metal it be made, be coated over with a thick crust of black lead, previously formed into the consistence of paste, by being pulverised and mixed with sulphur (as in the manufactory of the ordinary kind of black lead pencils) and then applied to the rod while hot. By this means, the lower part of the rod would, I apprehend, retain its conducting power for ages, without any diminution.
In order to increase the surface of the lower part of the conductor, let a hole or pit, of sufficient extent, be dug as deep as convenient; and into this pit let there be put a quantity ofcharcoal, round the lower extremity of the rod. Charcoal possesses two properties, which, in a peculiar manner, fit it for answering the purpose here in view.—(1st.) It is a very good conductor of electricity and, (2d.) It will undergo little or no change of property by lying ever so long in the earth. Thus might the surface of that part of the conductor, in contact with the earth, be increased, with little trouble or expense to any extent at pleasure; a circumstance which every one acquainted with electrical experiments, must acknowledge to be of great importance to the end here proposed.”
The following experiments with a thunder-house, shew the utility of lightning rods, and ascertain what termination of the rod best answers the end proposed.
To shew the effect of lightning on a house not furnished with a conductor, or when the conductor is discontinued.
Provide yourself with the model of a house made of tin, four inches in breadth, six long, and about five in height. Let there be a chimney placed in the roof equidistant from both ends, and let a glass tube pass through it, the upper extremity of which must reach a little above the chimney, and the lower one come within an inch of the floor of the house.—Let a small wire pass through the bore of the glass tube, the upper end of which must extend a small distance above the orifice of the tube, having its extremity, which must be pointed, furnished with a screw, on which a metallic ball is to be fastened. The other end must likewise have a ball fixed upon it.—The instrument being thus prepared, fill the house with cotton, and sprinkle a little powdered rosin on that part of it, which is immediately between the lower knob of the wire, and the floor of the house. Then connect the lower part of the instrument with the outside coating of a pretty large jar.—From the prime conductor, in order to represent the clouds, suspend a small scale beam, having two balls of metal or wood coated with tin foil, in the place of the scale dishes, nicely balanced. The knob of the jar being connected with the prime conductor; bring the ball on the wire extending through the glass tube, under one of the balls representing a cloud.—Now charge the jar. The cloud will be attracted by the ball on the wire—the electricity of the cloud will be discharged—and if the experiment succeeds, the contents of the house will be set on fire.
The effects of lightning, when a house is furnished with a pointed conductor.
Repeat the above experiment with this variation: unscrew the ball from the upper extremity of the wire of the house, so that it may remain pointed. Place the house under the cloud as in the former experiment.—You will now find it impossible to charge the jar: or if you charge the jar before the house is placed under the cloud; the cloud, instead of being attracted by it, will be repelled, and the jar will be discharged without any explosion, and without firing the cotton.
These two experiments evince thatpointedconductors are more proper to secure houses from the effects of lightning that those terminating with a ball or knob, and that if the pointed conductors fairly act on the cloud the security is complete.
The electric power, observed by the ancients only in amber, and perhaps the tourmaline, was in process of time found to be in glass, rosin, silk, and several other substances. By degrees it was discovered, that very strong signs of electricity were exhibited by a number of animals. The experiment of producing sparks of electrical fire, by rubbing the back of a cat in frosty weather, proved that electricity might exist in a very active state in the bodies of animals, without injuring their functions. From animals of an inferior kind a transition was made to the human species. Somepeople were observed to have a remarkably bright lustre of their eyes, others were found to be so strongly electrified naturally, that a very sensible electrometer was perceptibly affected, when brought near them.—Others, it is affirmed, were found so sensible to the presence of electricity, as to be affected by a flash of lightning, though so distant that the thunder could not be heard. But what principally claims our attention in regard to this part of our subject is, that there are unquestionably certain animals which can at pleasure give an electric shock, of sufficient force to kill other small animals, and that in fact they often do it. We shall describe only three of the most remarkable of these electric animals—the Gymnotus electricus, the Torpedo, and the Silurus electricus.
The Gymnotus is a genus of fishes, belonging to the order of apodes. They have two tentacula at the upper lip; the eyes are covered with the common skin.—There are five rays in the membrane of the gills; the body is compressed, and carinated on the belly with a fin. There are five species; the most remarkable is theelectricus, commonly called theelectric eel. This species is peculiar to the Surrinam river, and they inhabit the most rocky parts of it, at a considerable distance from the sea.—The most accurate description of this fish, is in the Philosophical Transactions, for 1775, where Alexander Garden M. D. gives an account of three of them brought to Charleston in South Carolina. The largest was about three feet eight inches long, and from ten to fourteen inches in circumference, about the thickest part of the body. The head was large, broad, flat, and smooth, impressed here and there with holes, as if perforated with a blunt needle, especiallytowards the sides, where they are more regular. There are two nostrils on each side; one is large, tubular, and elevated above the surface; the other small and level with the skin. The mouth is large, but the jaws have no teeth, so that the animal lives by suction, or by swallowing its food entire.
The eyes are small, flat, and of a blueish colour, placed a little behind the nostrils. The whole body from a few inches below the head, was distinguished into four longitudinal parts, clearly divided from each other by lines. The carina begins a few inches below the head, and widening as it proceeds, reaches as far as the tail, where it is thinnest. The situation of theanusis very remarkable, being an inch more forward than the pectoral fins. Across the body, there are a number of small bands, annular divisions, or rather rugæ of the skin; by means of which the fish seems to partake of the vermicular nature, having the power of lengthening and shortening its body like a worm, and by means of which it can swim backwards as well as forwards.—For an anatomical description of this fish, see the appendix to the 2d. vol. of Mr. Cavallo’s “Complete treatise” page 303.
The Gymnotus has the astonishing property of giving the electric shock to any person or number of persons, either by the immediate touch of the hand or by the mediation of any metallic conductor. The shock is interrupted by the intervention of a non-conducting substance. If the animal be touched only with one hand, a kind of tremor is felt in that hand only. The power of giving shocks depends entirely on the will of the animal.
As nature is ever provident for her creatures, both with regard to their preservation and support, she has endowed the Gymnotus with a peculiar instinctive faculty, so that if it be pursued by an enemy, it never fails to communicate a shock, in consequence of which it eventually makes its escape. In obtaining food it likewise makes use of its electrical property by which it kills small fish, and afterwards devours them.
But the most remarkable instinct of this fish is, that when any substance approaches it, it is sensible whether it be a conductor or non-conductor. In order to exhibit this wonderful phenomenon, a variety of methods were contrived, the easiest and most satisfactory one was the following. The extremities of two wires were dipped into the water of the vessel, in which the animal was kept, after which they were extended to a considerable distance, where they terminated in two separate glasses full of water. These wires being supported by silk at some distance from each other, the circuit was, of course, incomplete. In these circumstances if a person completed the circuit, by placing one hand in one of the glasses and the other in the other, the fish which never went purposely towards the wires, while the circuit was interrupted, would now go immediately towards them and give the shock, and this though the completion of the circuit was made out of his sight.
The next electrical fish we are to mention is the Torpedo; a genus of fishes belonging to the order of Chondropterygia; the species of this genus are remarkable and numerous; but we must content ourselves with the sixth species, called theelectrical ray, orcramp fish, or Torpedo. The head and body, which are indistinct,are nearly round, the ventral fins form on each side the quarter of a circle, the two dorsal fins are placed on a trunk of the tail, which is round, the caudal fin is broad and abrupt. The eyes are small, and placed near each other; behind each is a round spiracle with six small cutaneous rags on their inner circumference.—The mouth is small, and the teeth are minute and spicular.
These fish have been taken in Torbay, off Pembroke, near Waterford in Ireland, and many other parts of Europe, with a trawl, and sometimes with a bait; they commonly lie about forty fathoms deep. The food of the Torpedo is fish.—For an anatomical description we refer the curious reader to one given by Mr. Hunter, in the Philosophical Transactions, vol. 63.
The electrical properties of this fish are remarkable; for a long time they were considered as fabulous; but the fact having been ascertained beyond the possibility of doubt, it was endeavoured to be accounted for, by a variety of ingenious though unsatisfactory arguments. But when the phenomena of electricity began to be better understood, considerable light was thrown upon the subject; and Mr. Walsh at last, not only explained the phenomena which generally attend it, on the known principles of electricity, but actually contrived an artificial fish, by which a shock very similar to that of the natural one can be given.
The electrical power of the Torpedo is conducted by the same substances as conduct common electric matter, and is interrupted also by the same non-conductors: but its shock will not pass over the least interception of the circuit, not even if a chain be used.This singular fact was also imitated by Mr. Walsh with his artificial Torpedo.
It has not been in our power to obtain a particular account of this artificial Torpedo of Mr. Walsh.—But we know that one may be formed in the following manner.
Let a number of small thin laminæ of talc, commonly called isinglass, or thin sash glass, coated in the usual way, be joined together in the same manner as in the battery. Let these be placed in the body of an artificial fish resembling the Torpedo.—Let them then be charged, and on being touched, the same phenomena which accompany the real Torpedo will ensue; except that the shock of this will not be impeded by a small interruption in the circuit. Similar effects may also be produced, by means of a large battery weakly charged and furnished with Lane’s electrometer.
The third and last fish that we shall mention, is the Silurus or Silurus electricus, a genus in Ichthyology belonging to the order of Pisces Abdominales.—The body of this is long, smooth, and without scales, being rather large and flattened towards the lower part. The eyes are of the middle size and covered by the skin which envelopes all the head. Each of the jaws is furnished with a great number of small teeth. About the mouth it has six filamentous appendices, two from the upper, and four from the under lip. The colour of the body is greyish, with a few dark spots towards the tail.
With regard to its electrical properties very little is known, enough however to entitle it to the name of electricus.