Popular Science Monthly/Volume 43/August 1893/Protection from Lightning
|PROTECTION FROM LIGHTNING.|
DURING the year 1801 two hundred and five lives were lost (that we know of) in the United States, east of the Rocky Mountains, directly through the action of lightning. How many were lost indirectly, and how many cases there were of shattered health and more or less permanent injury, we can only surmise. The financial loss due directly to lightning was certainly not below one and a half million dollars. To get at something like a commercial estimate of the damage done by lightning in the past few years, in this country, I have made use of the Chronicle Fire Tables for the six years 1885-1890, and find that some twenty-two hundred and twenty-three fires, or 1·3 per cent of the whole number, were caused by lightning, and the total loss was $3,386,826, or 1·25 per cent of the whole amount lost by fire. During 1892 we have a record of two hundred and ninety-two lives lost. The damage may be estimated at as high a figure as in 1891. These losses are the more appalling when we recall that the year is virtually less than six months. Over ninety-five per cent of the casualties due to lightning occur between the months of April and September. It is therefore pertinent at this time to discuss the question whether or not we are able to protect ourselves from lightning. Some five years ago the question would have been answered readily and with all sincerity, "Yes, a good electrical connection with the earth—a stout, continuous copper rod, for example—will suffice." To-day no such answer can pass unchallenged, for reasons which we shall see.
In 1888, after years of dispute, we had just settled down to the calm enjoyment of the belief that rods did protect. The Lightning-rod Conference had shown, in quite an exhaustive report, that Faraday's position (as opposed to the opinions of Harris) was correct, viz., that the problem was one of simple conductivity; that a solid rod was better than a tube or tape (which would give greater surface with less copper); that solid volume was everything, superficial area nothing; and that, provided the metallic passage afforded the Hash was continuous, any Hash might be successfully carried off and harmlessly conducted to the ground.
This conference, while not strictly an official body, was one that, from the character of its members, carried great weight. It was a joint committee of representative members of the Institute of British Architects, the Physical Society, the Society of Telegraph Engineers and Electricians, the Meteorological Society, and two co-opted members.
In 1888 came Dr. Oliver J. Lodge's remarkable course of lectures before the Society of Arts upon the oscillatory character of the lightning flash. . Then followed the famous number, height, or conductivity, bad points or bad earth connections; . . . and there was no authentic case on record where a properly constructed conductor failed to do its duty. . . . He personally had under his supervision at that present moment 500,000 lightning conductors, and, fixed throughout the offices (post-officeat the meeting of the British Association. As this debate was one in which quarter was neither asked nor given, and the question at issue was clearly understood by all to be whether a lightning conductor, when constructed in accordance with the directions of the conference, would absolutely protect, it may not be out of place to give here a synopsis of the arguments advanced. Mr. Preece, who opened the discussion, defined the functions of a lightning conductor as twofold. "It facilitates the discharge of the electricity to the earth, so as to carry it off harmlessly, and it tends to prevent disruptive discharges by silently neutralizing the conditions which determine such discharges in the neighborhood of the conductor. To effect the first object, a lightning conductor should offer a line of discharge more nearly perfect and more accessible than any other offered by the materials or contents of the edifice we wish to protect. To effect the second object, the conductor should be surmounted by a point or points; fine points and flames have the property of slowly and silently dissipating the electrical charges; they, in fact, act as safety valves. If all those conditions be fulfilled, if the points be high enough to be the most salient features of the building, no matter from what direction the storm cloud may come, be of ample dimensions and in thoroughly perfect electrical connection with the earth, the edifice, with all that it contains, will be safe, and the conductor might even be surrounded by gunpowder in the heaviest storm without risk of danger. All accidents may be said to be due to a neglect of these simple elementary principles. The most frequent sources of failure are conductors deficient either in
and telegraph), had apparatus, protected by about 30,000 or 40,000 lightning protectors."
Dr. Lodge said that "if his views were correct, very few buildings are effectively and thoroughly protected at the present time. . . . He had read carefully the Conference Report, and found a large number of entire failures; . . . one noteworthy one, a brass rod an inch thick on a steeple which was smashed to pieces and the spire destroyed. Again, the best protected building in the world, the Hôtel de Ville at Brussels, on which M. Melsens had spent so much time and trouble. It was elaborately protected; protected by innumerable conductors with admirable earths made in a variety of ways, bristling with points all over the top—everything carried out in the most approved style, regardless of expense. Yet in the month of June the building was struck and set on fire.... If a lightning conductor can prevent a flash from occurring by its repellent action, well and good; but he had
shown in his lectures that there are cases where a point has no protective action whatever, when a point could be struck by a thick and heavy flash. There were other cases where the point acts with a brush or fizz and neutralizes the electric charge without a flash. They could not always do it. And so the lightning rod has two functions; one is to be repellent if it can, and the other is to carry off a flash when it can not help receiving it. There was a certain amount of energy which they must dissipate somehow, and they could not expect to hocus pocus it out of existence by saying they could conduct it to the earth. The quicker they tried to conduct it down to the earth the more searching and ramifying disturbances they were likely to get. It might be better to let it trickle down slowly by using a moderately bad conductor than to rush it with extreme vehemence down a good conductor, just as it would be safer to let a heavy weight suspended in a dangerous position down slowly rather than let it drop as quickly as possible. . . . If a man holds a lightning conductor when a flash passes down it, he will most likely be killed. . . . It did not matter about the earth; . . . a spark was likely to occur. . . he had made experiments in the laboratory with a rod very thick and a yard long, in circuit with a Leyden-jar discharge. He took a platinum wire as fine as possible to make the contrast greater, and arranged it so as to make a kind of tapping circuit; if, then, the bottom end was arranged so as to be in contact with the rod and then let the top end be an eighth of an inch away, then they would have a splendid conductor, better than any lightning conductor ever was. They would have no trouble about earth. It seemed absurd for any portion of the discharge to leave this conductor to jump across and make for the little strip of wire. Nevertheless, a portion of it did, and from every spark that went to the conductor, a side branch went to that little wire. . . . What are the conditions of a flash? He assumed that a flash behaved like experiments in the laboratory, but it was a question whether a cloud discharge was of this kind, A cloud is not like a conductor; it consists of globules of water separated from one another by interspaces of air; it may be compared to a spangle jar: when a spangle jar discharges, you have no guarantee that the whole of it discharges—it discharges in a slowish manner. It might be that there was with a cloud first a bit of a discharge and then another bit, and so on, so that there might be a kind of dribbling of the charge out of it, and they might therefore fail to get these sudden and oscillatory rushes. . . But we must provide for the possibility of a sudden discharge."
Hon, Ralph Abercromby contributed to the discussion facts brought out by an examination of some ninety photographs of lightning flashes in different parts of the world. In one instance the whole air was filled with threads of lightning coming down like the roots of a tree from the sky. He thought it was very much a question where the area of protection would be when the whole air seemed to be pouring lightning down upon you. He would also like to find out whether buildings were struck during rain or when it was not raining. In connection with the fact that thunderstorms were confined to the lower ten thousand feet of the air, he mentioned the fact that in Norway there were two kinds of thunderstorms: one occurred in the summer, he believed, when the lightning clouds were high, and very little damage was done; on the contrary, in winter time the clouds were very low, and the churches were frequently struck.
Lord Rayleigh, Sir William Thomson, and Prof. Rowland discussed the questions whether or not the experiments actually represented the actual conditions. M. de Fonveille called attention to the most extraordinary lightning conductor in existence, the Eiffel Tower, and the fact that Paris was practically free from calamities produced by lightning, because a sufficient number of lightning rods had been erected according to the principles advocated by the many official boards, substantially the same as the conference. Prof. George Forbes thought a copper alternative path better than an iron one. Sir James Douglas, speaking from an experience of forty years with a large number of conductors, was of the opinion that the rods, when properly constructed, were entirely adequate. In the matter of copper versus iron he pointed out the practical consideration that iron corroded rapidly compared with copper. Mr. Walker further pointed out that this corrosion of iron was not a question of weather alone, but, as in the case of the top of the chimney of a factory, there would be some chemical action. Mr. Symons brought out with respect to iron conductors that galvanizing would not entirely overcome the difficulty.
We now come to Lodge's book upon Lightning Conductors and Lightning Guards, and shall get from it a more satisfactory understanding of his experiments and deductions. He believes that the current ideas on the character of the lightning discharge were not altogether correct, because the momentum of an electric current and the energy of an electrostatic charge had both been more or less overlooked. The application of the known fact of electrokinetic momentum revolutionized the treatment of certain phenomena. The old drain-pipe idea of conveying electricity gently from cloud to earth was thus proved fallacious, and the problem of protection became at the same time more complex and more interesting. His position, therefore, is not that lightning rods are useless, but that few or none of the present types are absolute and complete safeguards; and he believes it possible to so modify existing protective systems as to afford more certain protection. The problem is one, lie very clearly shows, far removed from the old idea of conduction.
To-day we know from the experiments of Hertz, Lodge, and others that when an electric current flows steadily in one direction in a conductor, its intensity is the same in all parts of the wire; but if it be of an oscillatory character—i. e., a current reversing rapidly in direction—the interior of the conductor may carry far less than the surface carries.
All of which goes to prove the correctness of Snow Harris's opinion (and he probably studied the effects of lightning more exhaustively than any one else) that surface was more of a consideration in the form of a protector than solid section. In this matter of the form of a conductor we follow Lodge, and prefer
the tape to the solid rod. Increase of surface diminishes impedance; and as impedance is probably at the bottom of side flashes and spittings, that conductor is to be "Think now," says Lodge, "of a cloud and of the earth under it as forming the two coats of a Leyden jar, in the dielectric of which houses and people exist; we now have to consider what determines a discharge, and what happens when a discharge occurs. The maximum tension which air can stand is one half gramme weight per square centimetre. At whatever point the electric tension rises to this value, smash goes the air. The breakage need not amount to a flash, it must give way along a great length to cause a Hash; if the break is only local, nothing more than a brush or fizz need be seen. But when a flash does occur it must be the weakest spot which gives way first—the place of maximum tension—and this is commonly on the smallest knob or surface which rears itself into the space between the dielectrics. If there be a number of small knobs or points, the glows and brushes become so numerous that the tension is greatly relieved and the whole of a moderate thunder-cloud might be discharged in this way without the least violence.... But sometimes a flash will descend so quickly or it will have such a tremendous store of energy to get rid of that no points are sufficiently rapid for the work, and crash it all comes at once. One specially noteworthy case is when one cloud sparks into another and thence to the ground; or in general whenever electric strain is thrown quite suddenly upon a layer of air."which offers less impedance, and hence tape appears to be preferable to rod. It is also more convenient, and it has also the advantage of being made either continuous or in very long lengths. The tape must be of dimensions sufficient to withstand melting or deflagration.
Thus, then, we begin to see that much will depend upon the character of the flash. There are many flashes, I believe, that the body could experience without very serious consequences; and there are many that will rive solid granite and shatter in splinters the heaviest masonry. The impulsive rush discharge shown on the preceding page was doubtless a flash of the latter character; and on the other hand, with a kite in air during thunderstorms with a wire connection to the ground I have experienced sharp shocks with lightning flashes, which were perhaps side branches or minor spitting-off discharges.
The risk, then, will vary with the discharge, and this is influenced somewhat by locality, and therefore the methods of protection to be employed in the Mississippi Valley would be somewhat different in character from the methods appropriate for, say, the New England coast; the frequency of thunderstorms in the one place compared with the other being about four to one. The character of the storm is also somewhat different. Then again the liability for places comparatively near is not the same. "If I urge on Glasgow manufacturers," said Sir William Thomson, "to put up lightning conductors, they say that it is cheaper to insure." These manufacturers answered the man of science more wisely perhaps than they themselves knew. Thanks to the investigations of the Prussian Bureau of Statistics, we know now that in the main in thickly settled communities the risk is small. We can state with some certainty that there is but little need for the erection of expensive or elaborate lightning rods upon buildings standing among others in city blocks. We do not say that such buildings are never struck. As we have seen above, under some conditions, "exposure" seems to have little to do with the determination of the path of discharge. The case is somewhat analogous to that of the trees and rocks upon a mountain side. However much they may determine the course of small streams and "trickles" down the mountain side, they are powerless to influence the course of an avalanche or land-slide. Sometimes, therefore, such buildings are struck and severely injured (and as might be expected, with seemingly good protectors, do not entirely escape); but these cases are rare, and it may, we think, be safely set down that rods upon city houses are not (as hitherto insisted upon by some) necessary. With country houses the conditions are different.
Our next question is. In flashes of ordinary intensity how much confidence may be placed in the protection afforded by a good conductor, rod or tape? Few questions have been so thoroughly discussed from a practical standpoint, and the verdict may be given in Sir William Thomson's words: "There is a very comfortable degree of security, if not of absolute safety, given to us by lightning conductors made according to orthodox rules."
If the reader is contemplating the erection of a lightning protector, these points may be of service to him:
1. Get a good iron or copper conductor of rod or tape form, preferably the latter. If copper, have it weigh about six ounces to the foot; if iron, about two pounds to the foot.
2. The nature of the locality will determine to a great degree the need of a rod. In some localities rods are imperative; in others, needless.
3. The very best ground you can get is, after all, for some flashes, none too good; therefore do not imagine that you can overdo it in making a good ground. For most flashes ordinary "grounds" suffice; but the small resistance of even one ohm may be dangerous with an intense flash.
4. "If the conductor at any part of its course goes near water or gas mains it is best to connect it to them. Wherever one metal ramification approaches another it is best to connect them metallically. The neighborhood of small-bore fusible pipes and indoor gas pipes in general should be avoided."—Lodge.
5. The top of the rod should be plated, or in some way protected from corrosion and rust.
6. Independent grounds are preferable to water and gas mains.
7. Clusters of points or groups of two or three along the ridge rod are recommended.
8. Chain or link conductors are of little use.
9. Area of protection.' Very little faith is to be placed in the so-called area of protection.
10. Indifference of lightning to the path of least resistance. Nearly every one who has written in late years has taken it for granted that lightning always follows the path of least resistance. This is not true. "It is simply hopeless to pretend to be able to make the lightning conductor so much the easier path, that all others are out of the question," says Lodge. This, however, requires modification. For the path will depend largely upon the character of the flash; and without doubt, for almost all flashes, a good lightning rod well earthed is the most appropriate path to earth.
11. Any part of a building, under certain conditions, may be struck, whether there is a protector on it or not. There are cases on record where edifices seemingly amply protected have been struck below the rods (not cases of defective connection), and it is now beginning to dawn upon us that (paradox of paradoxes) a building may be seriously damaged by lightning without having been struck at all. The Hôtel de Ville, at Brussels, perhaps the best protected building in the world against lightning, was damaged by fire, caused by a small induced spark near escaping gas. During the thunderstorm some one flash started up "surgings" in a piece of metal not connected in any way with the protective train of metal. "The building probably did not receive," says Dr. Lodge, "even a side flash, yet the induced surgings set up in it were so violent as to ignite some gas and cause a small fire." In other words, we had the condition of an "oscillator" in the cloud-flash-earth and a "resonator" that responded with its little spark. In some experiments which we made last summer, simultaneously with some flashes, we got little sparks from a wire in the air.
12. So many people suffer so keenly from a kind of nervous alarm during thunderstorms that it is a great pleasure to be able to point out that the danger is vastly overestimated. "Heaven has more thunders to alarm than thunderbolts to punish" was the old irreverent way of putting it. One who lives to see lightning need not worry about the results.
13. The notion that lightning never strikes twice in the same place is erroneous. We have numerous cases disproving this.
14. If you are near a person who has been struck by lightning, go to work at once to try and restore consciousness. Try to stimulate the respiration and circulation, and do not cease in the effort to restore animation for at least one hour.