usually discharge earth rods connected conductors electric
LIGHTNING CONDUCTOR, or LIGHTNING ROD (Paratonnerre, 131itz-ableiter), is the name usually given to apparatus designed to protect buildings or ships from the destructive effects of lightning. The title, alike in English, French, and German, is misleading ; for, when properly constructed, lightning rods serve rather to prevent the occurrence than to ward off the effects of a flash of lightning. Damping the enemy's powder would be a most efficient precaution against cannon-shot, but it would be very inappropriately termed fortification. When a conductor charged with electricity is brought near to another conductor connected with the earth, it induces on it a charge of the opposite kind of electricity. The result is an attractive force which tends to bring the conductors nearer to one another, and to augment the electric density on their opposed surfaces. When the density is sufficiently great, there is rupture of the dielectric (air) between the conductors, and the disruptive discharge takes place as an electric spark. If one of the conductors have projecting points or angles, the electric density is usually much greater at such places than over the rest of the surface. But, though the density is great at such places, the charge on them is usually small, and the discharge takes place in an almost continuous manner by a brush or glow. When, for instance, a large conductor, connected with an electric machine, is giving a rapid succession of bright sparks to a ball connected with the ground, the sparks cease as soon as a pointed wire, connected with the ground, is held in the vicinity of the conductor. No discharge is heard, but in the dark a faint glow is seen at the end of the wire, which continues as long as the machine is turned. Remove the wire and the sparks instantly recommence. This glow is known to sailors as St Elmo's (San Telmo's) fire, in old days Castor and Pollux (Plin., H. llr., ii. 37). Suppose now one of the conductors to be a thundercloud, the other the surface of the earth, the discharge will usually take place between the places of greatest surface density ; and it will in general be the more gradual as these are more pointed, and of less capacity. Hence Franklin's idea of furnishing buildings or other prominent objects with a projecting metal spike well connected with the ground, for the purpose of preventing a lightning discharge by substituting for it what is practically a continuous electric current.
To effect this object thoroughly, only three things are necessary :-(1) the points should so project from the building or ship to be protected as to prevent any great development of electric density elsewhere than on themselves ; (2) they should be effectually connected with the earth ; (3) the connecting rod ought to be so good a conductor as not to be injured even by a powerful electric discharge.
The first of these conditions is realized by making the rod branch out to all the salient portions of the building or ship, and furnishing it with points projecting beyond each of them. No general rule can be laid down as to the extent of the region protected by a single point, though it may usually be assumed with safety that the region extends throughout a vertical cone whose vertex is at the point, and whose semivertical angle is about 45°. This is probably not true if the point be very high, - on the top of a tall chimney or tower, for instance. Objects not far from the base of such a protected tower, and within the cone just described, have occasionally been damaged by lightning.
The second condition is easily fulfilled in towns by connecting the lower end of the rod with the iron gas and water-mains, which form an excellent " earth," as it is technically called. Water-pipes, being usually jointed with metallic-lead-washers, are preferable to gas-pipes, which are usually put together with white lead. This condition is also easy to secure in ships and in lighthouses, where large metal plates (in the case of a ship, the copper sheathing is precisely what is required) can easily be permanently immersed in sea-water. In country houses it is usually more difficult to obtain a proper earth. Plates and tubes of metal, of large surface, buried in ground which is permanently damp, form usually the best arrangement. A. well makes a good earth; a carefully constructed water tank (of stone or cement) is not an earth at all.
The third condition, so far as experience can guide us, seems to be effectually realized by making the conductor throughout of iron rod of an inch in diameter, or of copper rod not less in diameter than 4ths of an inch. Such rods of equal length have nearly the same conducting power, and therefore would have equal amounts of heat developed in them by a given discharge. But if such a discharge took place, the copper would be heated much more than the iron, in consequence of its smaller mass per foot (the specific heat being approximately the same in the two materials). Hence iron is, in this respect, preferable to copper, if the conducting powers of the rods are equal. Another advantage possessed by the iron rod is that it is much less likely to be wilfully damaged or stolen. Against this may be set the objections that it is easily injured by rust, and is not nearly so flexible as the equivalent copper rod. Conductors are now usually made of wire-rope, so that the question of flexibility is no longer of serious importance ; but when iron is used it should always be protected by zinc, i.e., be what is absurdly called "galvanized." Many fantastic forms of lightning rods were devised in consequence of the old erroneous notion that their efficiency depended on their surface and not on their cross section. In reality all conductors of equal length, and of the same material, are equally efficient if their cross sections be equal.' Thus, instead of stating the diameter of a rod, we may speak of its weight per foot, and say that a copper conductor should weigh at least about half a pound, and an iron one at least two pounds and a half per foot, - provided the materials be of good conducting quality.
The points need not be very sharp, but they ought to be protected by a coating of platinum or other nou.oxidizable metal. And they should be in a group of two or three at the end of each branch of the rod, lest one of them should be fused and impaired in efficiency by an accumulation of electricity so rapid as to make the silent continuous discharge impossible. Joints should be avoided as far as possible ; where they are unavoidable they should be made, not by screws or brazing, but by means of a large mass of solder completely enveloping the ends to be connected.
Another point to be carefully attended to is that all large metallic bodies, such as lead or zinc roofing, metal tanks, &c., should be in good conducting connexion with the rod, so as to prevent discharges of electricity inside the ship or building. In many buildings we see the lightning rods attached by means of glass or porcelain insulators, such as are employed for telegraph wires. This is a perfectly needless, expensive, and possibly dangerous practice.
The literature of this subject is very extensive, as may be seen from Ronalds's Catalogue of Works on Electricity. The reader may also consult Anderson on Lightning Conductors (1880), and the Report of the Lightning-Rod Conference (1882). In the latter work will be found abstracts of many valuable papers, especially the reports on lightuing-rods made to the French Academy by some of its most distinguished members, including Coulomb, Laplace, Poisson, Gay Lussac, Fresnel, Pouillet, Cagniard de la Tour, Regnault, Sec. There will also be found hints about some of the most ludicrous devices employed by men ignorant of the laws of electrical phenomena. One of the most singular of these was the so-called "Repeller." A lightning-rod, in all respects sufficient, was wont to be capped by a piece of glass like a thick soda-water bottle, inverted upon its point. The effect of this could only have been to prevent the possibility of the silent discharge, to produce which is the proper function of the rod, and to make probable a lightning flash, just as if the rod had been. terminated by a ball instead of a point. One of these dangerous monuments of ignorance was removed from a British lighthouse within the last ten years. In an Irish lighthouse, which was recently examined after suffering serious damage, it was found that the lower end of the lightning-rod was jumped into the solid rock - a truly original form of " earth "!
In 1876 Clerk Maxwell suggested to the British Association the idea (based on Faraday's experiments) of protecting a building from the effects of lightning by surrounding it with a sort of cage of rods or stout wire. Here an "earth" would not be absolutely required. The present writer had some mouths previously suggested the same idea in a Report to the Board of Northern Lights. It is possible, though not certain, that this form of defence might be useful against globe-lightning, which undoubtedly occurs, and against which ordinary lightning-rods would probably be of little use.
These brief remarks contain all that is yet known to be necessary to the complete solution of an important practical problem about which many treatises have been written. (P. G. T.)
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