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to protect. It is very seldom indeed that a flash of lightning proceeds in a vertical path: perhaps never.
I never yet saw, or heard of, a vertical discharge of lightning ; they are frequently very oblique indeed. The lightning which damaged Saint Michael's Church, Liverpool, last year, was an oblique discharge, and struck the bronze cross at the top of the spire, several feet from its top.
There is such a display of ignorance in the erection of tall spires, that it is almost a miracle that the whole of them are not destroyed by lightning. The copper clamps and strings of lead, the former uniformly placed at intervals from each other, and the latter wantonly poured into the crevices of the masonry, render the spire a complete chain of alternate links of metal and masonry from top to bottom : the former inviting the lightning to the edifice, and the latter offering facilities for the most destructive explosions. From this very arrangement of the materials in the steeples of Saint Michael's and Saint Martin's, at Liverpool, and in the steeple of Brixton Church, have these three steeples been shattered by lightning. If such modes of building tall spires be indispensable to protect them from the power of the wind, conductors are quite as indispensable to protect them from lightning. Three copper rods at equal distances from one another, from the top of the spire to the ground, and united at the top, and by one or two bands below, would secure each spire from lightning on which ever side it approached.
Lightning rods, however numerous about a building, should have a general metallic union ; they then form a system of conductors in which the force of the lightning would be divided, whichever branch was struck. I have a beautiful experiment to offer to your notice illustrative of this fact.
The apparatus represented by the accompanying figure consists of a series of iron-wire chains, so connected as to form a system of conductors of many branches. The chains hang vertically from a horizontal brass wire, and their lower ends rest on a sheet of tinfoil. The brass wire first receives the fluid from a discharge of the battery of jars, and the tinfoil carries it from the chains to the outside of the jar. The electric fluid, whilst traversing this circuit, illuminates every chain in the system to the same extent, showing that it is easily divided amongst them ; and had there been ten thousand such channels it would have divided itself amongst the whole of them. This experiment shows two or more interesting facts. It proves that the iron scintillates at every link by an electric discharge thr gh a chain of that metal ; and these scintillations discover to us that the fluid occupies, and passes through, every channel in the circuit; and, as I shall prove more clearly by and bye, every metallic point in the chains throws off electric fluid into the air.
It is now time that we proceed with some of those popular experiments which have been established for the purpose of illustrating the beneficial effects of conductors when struck by lightning, and an experiment with the thunder house (an odd enough name) shall be the first on the list.
This long celebrated piece of apparatus is represented by the opposite figure, and consists of a model of the gable-end of a house, to which is attached a lightning rod, which can be made continuous or interrupted at pleasure, by means of a square piece of wood, which carries a portion of the rod, being placed in certain positions in a hole which it fits in the gable-end. When in one position its wire unites the other two portions of the lightning rod, but when in another position it disunites them. The lower extremity of the discharging rod is in connection with the outside of a Leyden jar, and over the upper extremity hangs a brass ball in connection with the inside of the jar. When the lightning rod is complete, and the machine turned till the jar charges sufficiently high to overcome the resistance between the two balls, a spontaneous discharge takes place, and the conductor protects the building; but if the square piece be placed in its hole so as to make a breach in the conductor, the next discharge of the jar throws it out of its place to some distance on the table, which is considered as a representation of a displacemnet of masonry in a building struck by lightning
The electrical pyramid is another piece of apparatus for illustrating the efficacy of lightning rods, and of the danger to which such are exposed when so protected. The plinth on which the pyramid stands is fixed, and contains the important piece on which the fate of the edifice depends. The opposite figure is a representation of the apparatus in which the wire of the key-stone D is placed at right angles with the general direction of the lightning rod c c, and
A consequently there is an interruption at
c that place. The base of the pyramid is furnished with three balls, as feet, by which it rests on the plinth B F, one of
B the balls leaning on the moveable piece. Over the apex of the pyramid is sus
pended a brass ball supported by a glass pillar. When the discharge of a jar is transmitted to the pendent ball, the lightning strikes that on the top of the pyramid, the brass rod of which conducts it safely to the ball at the bottom, but finding an interruption there in the metal, it explodes to arrive at the lower portion, c F, of the conductor, and blows out the key-stone which supported one side of the pyramid, when down it comes, and being made of several loose pieces which scatter about the table, its destruction seems complete. Had the wire of the key-stone been placed in a vertical position, it would have joined the other parts, c c, of the lightning rod, and the damage would have been prevented.
We have several other striking experiments for the purpose of illustrating the effects of lightning, but in all our models we A are obliged to take advantage of good conductors, and give them such positions as may produce the intended effect. This figure represents the model of a house containing combustible materials, such as has already been shown will ignite by an electric discharge, and the result of the experiment with this model will afford a good idea of the probable consequence of a flash of lightning striking a building that contains inflammable articles.
This model is made of tin plate to prevent its entire destruction by the experiment. A glass tube passes through each of the two opposite sides of the model, which insulate two brass wires within them. These wires have each a brass ball at their inner extremities, on one of which is placed some tow moistened with oil of turpentine. The shorter wire c D, has a ring at its outer extremity, by which and a chain it is connected with the outside of a Leyden jar. The other wire, A B, is bent upwards, and the ball A at its upper extremity will receive the discharge from the inside of the jar. An explosion takes place within the building and ignites the tow and turpentine, producing all the appearance of a destructive fire.
The explosion of a powder magazine would be still more dreadful than the firing of a house containing other kinds of inflammable materials; and as the Royal Powder Magazines at Purfleet have been
D struck by lightning even when
E several lightning rods were at- B tached to them, but fortunately without explosion of their contents, no means thought of for
their protection ought to be neglected. The model represented by the figure, will now receive a discharge from a Leyden jar, and the result will afford a pretty good idea of the effects of lightning should it enter a magazine of gunpowder. The electric fluid shall be discharged on the ball A, and conducted to the powder barrel at c, and the wire and chain D E B, will conduct it to the outside of the jar. A wet string is also in the circuit. The powder barrel explodes, blows the roof off the magazine, and levels the walls with the floor. The various parts of ihe model are joined together by hinges, and suffer but little from the explosion, so that the same model may be used several times.
Marine lightning conductors, or those employed on board of ships, are simply chains of copper, formed of links similar to those of the surveying chain, and are hoisted to the mast head when there is an appearance of lightning striking the ship. But the lightning has frequently struck ships before the chain could be got up: showing the propriety of having a permanently attached conductor, which would always be ready to receive and carry off the flash. Such fixed conductors have been proposed, and some are now on trial in the navy: but singular enough, these conductors instead of carrying the lightning overboard, would lead it into the body of the ship, and should they ever happen to be struck with a powerful stroke of lightning the consequences might be terrible indeed.
The idea of carrying a conductor through the body of the ship originated with Mr. Benjamin Cook, of Birmingham, about the year 1811, but it has been carried out by Mr. Harris, of Plymouth. Mr. H. has formed the conductors into strips of copper, which are inserted in grooves in the after side of the masts, from top to bottom, and through the keelson to the sea. In one of the smaller men of war, Mr. H. carried his mizzen conductor through the powder magazine!!! The evils attending these conductors, arise, principally, from lateral explosions and electro-magnetic influence. I have already illustrated the magnetic effects of electric discharges on a miniature scale, and from these we can form a good idea of the magnetizing influence of a flash of lightning passing through a conductor.
Imagine a chronometer to be placed near to a conductor carrying a heavy flash of lightning: the main and pendulum springs, the chain, arbours, and in fact every morsel of steel, would be rendered permanently magnetic, and consequently the machine rendered entirely useless: and the same fate would attend every chronometer and watch within the sphere of the electro-magnetic influence, which, in such cases, would be very extensive, and on every side of the conductor.
The lateral discharges are of three kinds, which I have distinguished 1st, 2nd, and 3rd, one of which I have already shown you, by the spreading abroad the grains of gunpowder, seeds, &c., and by the breaking of glass and other lard substances. These are the first kind, and take place at every interruption in the circuit.
The second kind of lateral discharge, specimens of which I shall
now offer to your notice, occurs in the most perfect conducting circuit, unless the conductor be perfectly free from asperities, sharp angles, &c. I shall endeavour to illustrate this kind of lateral discharge by a few decisive experiments.
A thick copper wire, bent at various places into angles is suspended in the room, and I transmit a discharge of the battery, from a bigh intensity, through this wire. The room being darkened, the discharge takes place; and you will have observed that at every angle in the wire a brush of electric light sprang into the air.
I will now make a new circuit for the next discharge of the battery to traverse. In this circuit I place a strip of sheel copper about a foot long, in imitation of the copper conductors in masts. On making the discharge, both edges of the copper, from top to bottom, throw out fringes of electric fluid; which is a clear proof that in every discharge of lightning on to such conductors, an immense portion would issue from their edges, from the highest point of the mast to the step in the hold of the vessel. Of the consequences of such lateral discharges in the hold of a ship I must leave others to judge.
The third kind of lateral discharge takes place even from the best polished conductors under certain circumstances, and arises from the polarizing influence of the electric fluid whilst in inotion. I cannot illustrate this kind of lateral discharge better than by two metallic rods, one of which shall receive miniature flashes of lightning from the prime conductor, and the other shall be placed near to the former. To insure the best conducting channel for the rod which receives the fluid from the prime conductor, I connect it by copper wire with the rubber of the machine, and also with the gas pipes which lead to the gas-works. The other brass rod is in connection with the table, but which has not the opportunity of carrying away the Auid as the other rod possesses.
The arrangement is represented by the accompanying figure, in which the vertical rod receives the sparks from the prime conductor, and simultaneous lateral sparks are seen between the vertical rod and ihe vicinal end of the other. If, instead of sparks from the prime conductor, I discharge a jar down the vertical rod, the lateral discharge is seen as before. If I insulate the horizontal rod, and place another near to its remote extremity, lateral discharges take place between these two simultaneously with the other. These effects are truly in miniature, but beautifully illustrative of those which happen from the discharge of lightning on a conductor situated near to other conducting bodies. But it is not requisite to watch a lightning conductor till it is struck by the primitive discharge from a cloud to convince us of the danger attending this class of lateral explosions; since an atmospheric electric wave, produced either by a distant flash of lightning, or by the transit of