Part 7
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 may be 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 mathematics in 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 only _pointed_ conductors 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 of _black 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 of _charcoal_, 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 that _pointed_ conductors 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.
CHAP. XVI. _Of animal electricity._
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. Some people 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 the _electricus_, commonly called the _electric 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, especially towards 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 the _anus_ is 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 the _electrical ray_, or _cramp 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.