LIGHTNING CONDUCTORS.
Principles and Instructions relative to their application to Powder Magazines, by Sir W. Snow Harris, F.R.S. Extracted from Army List for July, 1859.
1.—Thunder and lightning result from the operation of a peculiar natural agency through an interval of the atmosphere contained between the surface of a certain area of clouds, and a corresponding area of the earth’s surface directly opposed to the clouds. It is always to be remembered that the earth’s surface and the clouds are the terminating planes of the action, and that buildings are only assailed by Lightning because they are points, as it were, in, or form part of, the earth’s surface, in which the whole action below finally vanishes. Hence buildings, under any circumstances, will be always open to strokes of Lightning, and no human power can prevent it, whether having Conductors or not, or whether having metals about them or not, as experience shows.
2.—Whenever the peculiar agency, (whatever it may be), active in this operation of nature, and characterized by the general term Electricity, or Electric Fluid, is confined to substances which are found to resist its progress, such, for example, as air, glass, resinous bodies, dry wood, stones, &c., then an explosive form of action is the result, attended by such an evolution of light and heat, and by such an enormous expansive force, that the most compact and massive bodies are rent in pieces, and inflammable matter ignited. Nothing appears to stand against it. Granite rocks are split open, oak and other trees, of enormous size, rent in shivers, and masonry of every kind frequently laid in ruins. The lower masts of ships of the line, 3 feet in diameter, and 110 feet long, bound with hoops of iron half an inch thick and 5 inches wide, the whole weighing about 18 tons, have been, in many instances, torn asunder, and the hoops of iron burst open and scattered on the decks. It is, in fact, this terrible expansive power which we have to dread in cases of buildings struck by Lightning, rather than the actual heat attendant on the discharge itself.
3.—When, however, the electrical agency is confined to bodies, such as the metals, which are found to oppose but small resistance to its progress, then this violent expansive or disruptive action is either greatly reduced, or avoided altogether. The explosive form of action we term Lightning, vanishes, and becomes, as it were, transformed into a sort of continuous current action, of a comparatively quiescent kind, which, if the metallic substance it traverses be of certain known dimensions, will not be productive of any damage to the metal. If, however, it be of small capacity, as in the case of a small wire, it may become heated and fused. In this case, the electrical agency, as before, is so resisted in its course as to admit of its taking on a greater or less degree of explosive and heating effect, as in the former case. It is to be here observed, that all kinds of matter oppose some resistance to the progress of what is termed the Electrical Discharge, but the resistance through capacious metallic bodies is comparatively so small, as to admit of being neglected under ordinary circumstances; hence it is that such bodies have been termed Conductors of Electricity, whilst bodies such as air, glass, &c., which are found to oppose very considerable resistance to electrical action, are placed at the opposite extremity of the scale, and termed Non-conductors or Insulators.
The resistance of a metallic copper wire to an ordinary electrical discharge from a battery, was found so small, that the shock traversed the wire at the rate of 576,000 miles in a second. The resistance however, through a metallic line of Conduction, small as it be, increases with the length, and diminishes with the area of the section of the Conductor, or as the quantity of metal increases.
4.—It follows from these established facts, that if a building were metallic in all its parts, an iron magazine for example, then no damage could possibly arise to it from any stroke of Lightning which has come within the experience of mankind; e.g., a man in armour is safe from damage by Lightning; in fact, from the instant the electrical discharge in breaking with disruptive and explosive violence through the resisting air, seizes upon the mass in any point of it, from that instant the explosive action vanishes, and the forces in operation are neutralized upon the terminating planes of action, viz., the surface of the earth, and opposed clouds.
5.—All this plainly teaches us, that in order to guard a building effectually against damage by Lightning, we must endeavour to bring the general structure as nearly as may be, into that passive or non-resisting state it would assume, supposing the whole were a mass of metal.
6.—To this end, one or more conducting channels of copper depending upon the magnitude and extent of the building should be systematically applied to the walls; these conducting channels should consist either of double copper plates united in series one over the other, as in the method of fixing such Conductors to the masts of Her Majesty’s Ships, the plates being not less than 31⁄2 inches wide, and of 1⁄16th and 1⁄8th of an inch in thickness, or the Conductors may with advantage be constructed of stout copper pipe not less than 3⁄16ths of an inch thick, and 11⁄2 to 2 inches in diameter: in either case the Conductors should be securely fixed to the walls of the building, either by braces, or copper nails, or clamps; they should terminate in solid metal rods above, projecting freely into the air, at a moderate and convenient height above the point to which they are fixed, and below they should terminate in one or two branches leading outward about a foot under the surface of the earth; if possible, they should be connected with a spring of water or other moist ground.
It would be proper in certain dry situations, to lead out in several directions under the ground, old iron or other metallic chains, so as to expose a large extent of metallic contact in the surface of the earth.
7.—All the metals in the roof and other parts of the building of whatever kind, should so far as possible have metallic communication with these Alarm Conductors, and in case of any prominent elevated chimney, it would be desirable to lead a pointed conducting tube along it to the metals of the roof; all of which satisfies the conditions above specified.
8.—Remark 1.—It is now proved beyond all questions, that the electrical discharge never leaves perfect conducting lines of small resistance, in order to pass out upon bad conducting circuits, in which the resistance is very great, that is an established law of nature; hence a stroke of Lightning upon such conducting lines will be confined to the Conductors as constituting a line of discharge of less resistance than any other line of discharge through the building, which can be assigned. The apprehension of “Lateral Discharge” therefore, from the Conductor, is quite absurd; and is not countenanced by any fact whatever; if any doubt could possibly exist, it would be now most completely set at rest by the experience of the permanent Conductors, applied to the masts of Her Majesty’s ships. In very many instances furious discharges of Lightning have fallen on the masts with a crash as if the ship’s broadside had been fired, and the solid point aloft has been found melted; in all these cases electrical discharge robbed by the Conductor of its explosive violence, has traversed the line of action to the sea, through the ship, and through the copper bolts, driven through the ship’s solid timbers, without the least damage to the surrounding masses, whether metallic, as in the case of the massive iron hoops on the lower masts, or not. Persons have either been close by or actually leaning against the Conductors at the time, without experiencing any ill consequence.
9.—Remark 2.—It has also been incontestably shown, that metallic bodies have not any specific attractive force or affinity for the matter of Lightning; metals are as little attractive of lightning as wood or stone. All matter is equally indifferent to Electricity so far as regards a specific attraction, hence the idea that metals attract or invite Lightning is a popular but very unlearned error contradicted by the most satisfactory evidence, and the whole course of experience; in short, we find that Lightning falls indiscriminately upon trees, rocks, and buildings, whether the buildings have metals about them or not.
10.—Remark 3.—A building that is hence clear, may be struck and damaged by Lightning without having a particle of metal in its construction; if there be metals in it, however, and they happen to be in such situations as will enable them to facilitate the progress of the electrical discharge, so far as they go, then the discharge will fall on them in preference to other bodies offering more resistance, but not otherwise; if metallic substances be not present, or if present, they happen to occupy places in which they cannot be of any use in helping on the discharge in the course it wants to go, then the electricity seizes upon other bodies, which lie in that course, or which can help it, however small their power of doing so, and in this attempt such bodies are commonly, but not always, shattered in pieces. The great law of the discharge is,—progress between the terminating planes of action, viz:—the clouds and earth, and in such line or lines as upon the whole, offer the least mechanical impediment or resistance to this operation, just as water falling over the side of a hill in a rain storm, picks out or selects as it were by the force of gravity, all the little furrows or channels which lie convenient to its course, and avoids those which do not. If in the case of Lightning you provide through the instrumentality of efficient Conductors, a free and uninterrupted course for the electrical discharge, then it will follow that course without damage to the general structure; if you do not, then this irresistible agency will find a course for itself through the edifice in some line or lines of least resistance to it, and will shake all imperfect conducting matter in pieces in doing so; moreover it is to be specially remarked in this case, that the damage ensues, not where the metals are, but where they cease to be continued, the more metal in a building therefore the better, more especially when connected by an uninterrupted circuit with any medium of communication with the earth.
Such is, in fact, the great condition to be satisfied in the application of Lightning Conductors, which is virtually nothing more than the perfecting a line or lines of small resistance in given directions, less than the resistance in any other lines in the building, which can be assigned in any other direction, and in which by a law of nature the electrical agency will move in preference to any others.
11.—It follows from the foregoing principles, that a magazine constructed entirely of iron or other metal, would be infinitely more safe in Lightning storms than if built with masonry in the usual way; metallic roofs for magazines, with capacious metallic Conductors to the earth, would be unobjectionable, and a source of security.
Metallic gutters and ridges having continuous metallic connection with the earth are also unobjectionable.
A good method of Conductors for magazines built of masonry, would be such as already described, regard being had to the position of the building, its extent, and most prominent points, also to the nature, state, and condition of the soil, whether it be moist or dry, alluvial calcareous, or of hard rock; we must also consider the extent, disposition, and peculiar position of the metallic bodies entering into the general structure of the building, whether the roof be flat, pointed, or angular in various parts.
The pointed projecting extremities of the two Conductors, one or more as the case may be, will be commonly sufficient; but, in buildings having tall chimneys or other elevated prominent points, at a distance from the Main Conductor, it will be requisite to guard such chimneys or other parts, by a pointed rod, led along them to the metals of the roof, or directly connected with the Main Conductors, by metallic connections.
12.—Pointed terminations of the Conductors in the air, are so far important that they tend to break the force of a discharge of Lightning when it falls on them. In fact, before the great shock actually takes place, under the form of a dense explosion, a very large amount of the discharge, which otherwise would be concentrated, runs off, as it were, through the pointed Conductor; but they have no other influence.
With respect to these pointed terminations, no great care need be taken about them, except that they should consist of solid copper rod, of about three-quarters of an inch in diameter, and about a foot in length, and be united by brazing to the conducting tube, elevated at such convenient height above the walls of the building as the case may suggest.
As a support to the Conductor, when raised above the wall, we may employ a small staff or spar of wood fixed to the masonry.
13.—Copper linings to the doors and window shutters of magazines are not objectionable, if requisite, as a precaution against fire; but they are useless as a means of keeping out Lightning; on the other hand, it is not easy to conceive a case in which the explosion of the gunpowder is to be apprehended from the action of Lightning on the doors or windows. Supposing, however, such metallic linings desirable as a precaution against common cases of fire, then the masses of metal should, according to the principles already laid down, have metallic communication with the general system of conduction in the building and the Main Conductor.