APPENDIX.RECENT CONTROVERSY ON LIGHTNING CONDUCTORS.
The lecture on lightning conductors contained in this volume fairly represents, I think, the theory hitherto received on the subject. It is, moreover, entirely in accord with the report of the Lightning Rod Conference, brought out in 1883, by a committee of most eminent men, representing several branches of science, who were specially chosen to consider this question some ten years ago.
Lectures of Professor Lodge.—But, in the month of March, 1888, two lectures were given before the Society of Arts, in London, by Professor Oliver Lodge, in which this theory was directly challenged, and attacked with cogent arguments, supported by striking and original experiments. These lectures gave rise to an animated controversy, which culminated in a formal discussion at the recent meeting of the British Association in Bath. The discussion was carried on with great spirit, and most of the leading representatives of physical and mechanical science took an active part in it. The greater portion of this volume was printed off before the meeting of the British Association took place. But the discussion on the theory of lightning conductors seemed to me so interesting and important that I thought it right, in the form of an Appendix, to give some account of the questions at issue, and of the opinions expressed upon them.
Professor Lodge maintains[41]that the received theory of lightning rods is open to two objections. First, it takes account only of the conducting power of the lightning rod, and takes no account of the phenomenon known as self-induction, or electrical inertia. Secondly, it assumes that the whole substance of a lightning rod acts as a conductor, in all cases of lightning discharge; whereas there is reason to believe that, in many cases, it is only a thin outer shell that really comes into action. I will deal with these two points separately.
The Effect of Self-Induction.—When an electric discharge begins to pass through a conductor, a momentary back electro-motive force is developed in the conductor, which obstructs its passage. This phenomenon is called by some self-induction, by others electrical inertia; but its existence is admitted by all. Now, when a flash of lightning, so to say, falls on a lightning rod, the back electro-motiveforce developed is very considerable; and it may offer so great an obstruction that the discharge will find an easier passage by some other route, such as the stone walls and woodwork, and furniture of the building.
According to this view, the obstruction which a flash of lightning encounters in a conductor consists partly of the resistance of the conductor, in the ordinary sense of the word resistance, and partly of the back electro-motive force due to self-induction. The sum of these two Professor Lodge calls theimpedanceof the lightning rod; and he considers that the impedance may be enormously great, even when the resistance, in the ordinary sense, is comparatively small.
In support of this view he has devised the following extremely ingenious and remarkable experiment. A large Leyden jar, L, was arranged in such a manner that, while it received a steady charge from an electrical machine, it discharged itself, at intervals, across the air space at A, between two brass balls. The discharge had then two alternative paths before it; one through a conducting wire, C, the other across a second air space, between two brass balls at B. During the experiment, the two balls at A were kept at a fixed distance of one inch apart; but the distance between the two balls at B was varied. The conductor, C, used in the first instance, was a stout copper wire, about forty feet long, and having a resistance of only one-fortieth of an ohm.
INDUCTION EFFECT OF LEYDEN JAR DISCHARGE.
INDUCTION EFFECT OF LEYDEN JAR DISCHARGE.M Electrical Machine.L Leyden Jar.A B Air Spaces between Brass Knobs.C Conducting Wire.
It was found that, so long as the distance between the B knobs was less than 1.43 inches, all the discharges passed across between theknobs, in the form of a spark. When the distance exceeded 1.43 inches, all the discharges passed through the conductor, C, and no spark appeared between the balls at B. And when the distance was exactly 1.43 inches, the discharge sometimes took place between the knobs, and sometimes followed the conductor, C. The interpretation given to these facts is that the obstruction offered by the conductor C was about equal to the resistance of 1.43 inches of air; and it is proposed to call this distance, under the conditions of the experiment, thecritical distance.
Coming now to the application of these results, Professor Lodge argues that the conductor C, in his experiment, represents a lightning rod of unimpeachable excellence; and yet, in certain cases, the discharge refuses to follow the conductor, and prefers to leap across a considerable space of air, notwithstanding the enormous resistance it there encounters. In like manner, he says, a flash of lightning may, in certain cases, leave a lightning rod fitted up in the most orthodox manner, and force its way to earth through resisting masses of mason work and such chance conductors as may come across its path.
This conclusion, he admits, is altogether at variance with the received views on the subject; but he contends that it is perfectly in accord with the scientific theory of an electrical discharge. The moment the discharge begins to pass in the conductor, it encounters the obstruction due to self-induction; and this obstruction is so great that the bad conductors offer, on the whole, an easier path to earth.
Variation of the Experiment.—When the experiment was varied by substituting a thin iron wire for the stout copper wire at first employed, a very curious result was obtained. The wire chosen was of the same length as the copper, but had a resistance about 1,300 times as great; its resistance being, in fact, 33.3 ohms. Nevertheless, in this experiment, when the B knobs were at a distance of 1.43 inches, no spark passed, which showed that the discharge always followed the line of the conductor, and therefore that the conductor offered less obstruction than 1.43 inches of air. The knobs were then brought gradually nearer and nearer; and it was not until the distance was considerably reduced that the sparks began to pass between them. When the distance was exactly 1.03 inches, the discharge sometimes passed between the knobs, and sometimes through the conductor; this was, therefore, thecritical distance, in the case of the iron wire. Thus it appeared that the obstruction offered to the discharge by the iron wire was much less than that offered by the copper, the one being equal to a resistance of only 1.03 inches of air, the other to a resistance of 1.43 inches.
It does not appear that Professor Lodge undertakes to offer any satisfactory explanation of this result. He has come to the conclusion, from his various experiments, that, in the case of a suddendischarge, difference of conducting power between fairly good conductors is a matter of practically no account; and that difference of sectional area is a matter of only trifling account. But he does not see why a thin iron wire should have asmallerimpedance than a much thicker wire of copper. He proposes to repeat the experiments so as to confirm or to modify the result, which for the present seems to him anomalous.[42]
The Outer Shell only of a Lightning Rod Acts as a Conductor.—As a consequence of self-induction or electrical inertia, Professor Lodge contends that a lightning discharge in a conductor consists of a series of oscillations. These oscillations follow one another with extraordinary rapidity—there may be a hundred thousand in a second, there may be a million. Now it has been shown that, when a current starts in a conductor, it does not start at once all through its section; it begins on the outside, and then gradually, but rapidly, penetrates to the interior. From this he infers that the extremely rapid oscillations of a lightning discharge have not time to penetrate to the interior of a conductor. The electricity keeps surging to and fro in the superficial layer or outer shell, while the interior substance of the rod remains inert and takes no part in the action. A conductor, therefore, will be most efficient for carrying off a flash of lightning if it present the greatest possible amount of surface; a thin, flat tape will be more efficient than a rod of the same mass; and a number of detached wires more efficient than a solid cylinder. As for existing lightning conductors, the greater part of their mass would, in many cases, have no efficacy whatever in carrying off a flash of lightning.
The Discussion.—The discussion at the meeting of the British Association was opened byMr.William H. Preece, F.R.S., Electrician to the Post Office, who claimed to have 500,000 lightning conductors under his control. He expressed his conviction that a lightning rod, properly erected and duly maintained, was a perfect protection against injury from lightning; and in support of this conviction he urged very strongly the report of the Lightning Rod Conference. This report represented the mature judgment of the most eminent scientific men, who had devoted years to the study of the question; and he wished particularly to bring before the meeting their clear and decisive assertion—an assertion he was there to defend—that “there is no authentic case on record where a properly constructed conductor failed to do its duty.”
The new views put forward by Professor Lodge were based, in great measure, on his theory that a lightning discharge consisted of a series of rapid oscillations. But this theory should be received with great caution. It seemed to be nothing more than a deduction fromcertain mathematical formulas, and was not supported by any solid basis of observation or experiment. Besides, there were many facts against it. They all knew that a flash of lightning magnetized steel bars, deranged the compasses of ships at sea, and transmitted signals on telegraph wires. But such effects could not be produced by a series of oscillations, which, being equal and opposite, would neutralize each other. It was alleged that these rapid oscillations occurred in the discharge of a Leyden jar. That might be true, and probably was true; but they were not dealing with Leyden jars, they were dealing with flashes of lightning. If there was any analogy between the discharge of a Leyden jar and a flash of lightning, it was to be found, not in the external discharge employed by Professor Lodge in his experiments, but in the bursting of the glass cylinder between the two coatings of the jar.
Lord Rayleigh thought the experiments of Professor Lodge were likely to have important practical applications to lightning conductors. But though these experiments were valuable as suggestions, they did not furnish a sufficient ground for adopting any new system of protection. It was only by experience with lightning conductors themselves that the question could be finally settled.
Sir William Thomson hoped for great fruit from the further investigation of self-induction in the case of sudden electrical discharges. He warmly encouraged Professor Lodge to continue his researches; but he expressed no decided opinion on the question at issue. Incidentally he observed that the best security for a gun-powder magazine was an iron house; no lightning conductor at all, but an iron roof, iron walls, and an iron floor. Wooden boards should, of course, be placed over the floor to prevent the danger of sparks from people walking on sheet-iron. This iron magazine might be placed on a dry granite rock, or on wet ground; it might even be placed on a foundation under water; it might be placed anywhere they pleased; no matter what the surroundings were, the interior would be safe. He thought that was an important practical conclusion which might safely be drawn from the consideration of these electrical oscillations and the experiments regarding them.
Professor Rowland, of the Johns Hopkins University, America, said that the question seemed to be whether the experiment of Professor Lodge actually represented the case of lightning. He was very much disposed to think it did not. In the experiment almost the whole circuit consisted of good conductors; whereas, in the case of lightning, the path of the discharge was, for the most part, through the air, and therefore it might be an entirely different phenomenon. The air being a very bad conductor, a flash of lightning might, perhaps, not consist of oscillations, but rather of a single swing. Moreover, it was not at all clear that the length of the spark, in the experiment, couldbe taken as a measure of the obstruction offered by the conductor. Professor George Forbes was greatly impressed with the beauty and significance of Professor Lodge’s experiments, but he did not think the result so clear that they should be warranted in abandoning the principles laid down by the Lightning Rod Conference.
M. de Fonvielle, of Paris, supported the views ofMr.Preece. He cited the example of Paris, where they had erected a sufficient number of lightning conductors, according to the received principles, and calamities from lightning were practically unknown. He suggested that the Eiffel Tower, which they were now building, and which would be raised to the height of a thousand feet, would furnish an unrivalled opportunity for experiments on lightning conductors.
Sir James Douglass, Chief Engineer to the Corporation of Trinity House, had a large experience with lighthouse towers. The lightning rods on these towers had been erected and maintained during the last fifty years entirely according to the advice of Faraday. They never had a serious accident; and such minor accidents as did occur from time to time were always traced to some defect in the conductor. They had now established a more rigid system of inspection, and he, for one, should feel perfectly safe in any tower where this system was carried out.
Mr.Symons, F.R.S., Secretary to the Meteorological Society, had taken part in a discussion on lightning conductors as long ago as 1859. It had been a hobby with him all his life to investigate the circumstances of every case he came across in which damage was done by lightning, and the general impression left by his investigations entirely coincided with the views just expressed by Sir James Douglass. He had been a member of the Lightning Rod Conference, and was the editor of their report; and he wished to enter his protest against the idea of rejecting all that had hitherto been done in connection with lightning conductors on the strength of mere laboratory experiments.
Professor Lodge, in reply, said he could perfectly understand the position of those who held that a lightning rod properly fitted up never failed to do its duty, because, whenever it failed, they said it was not properly fitted up. The great resource in such cases was to ascribe the failure to bad earth contact. He thought a good earth contact was a very good thing, but he could not understand why such extraordinary importance should be attached to it. A lightning rod had two ends—an earth end and a sky end—and he did not see why good contact was more necessary at one end than at the other. If a few sharp points sticking out from the conductor were sufficient for a good sky contact, why were they not sufficient also for a good earth contact?
Besides, though a bad earth contact might explain why a certain amount of disruption should take place at the earth where the badcontact existed, he did not see how it accounted for the flash shooting off sideways half-way down the conductor. Again, what does a bad earth contact mean? If an electrical engineer finds a resistance of a hundred ohms, he will rightly pronounce the earth contact to be very bad indeed. But why should the lightning flash leave a conductor with a resistance of a hundred ohms in order to follow a line of non-conductors where it encounters a resistance of many thousand ohms?
He accepted the statement ofMr.Preece that his whole theory depended on the existence of oscillations in the lightning discharge; but there was good reason to believe they existed, because they were proved to exist in the discharge of a Leyden jar.Mr.Preece objected that an oscillating discharge could not produce magnetic effects, as a flash of lightning was known to do. He confessed he was unable to explain how an oscillating discharge produced such effects;[43]but that it could produce them there was no doubt whatever, for the discharge of a Leyden jar produces magnetic effects, and we have ocular demonstration that the discharge of a Leyden jar is an oscillating discharge.
As to the assurances we had received from electrical engineers that a properly fitted lightning conductor never fails, he should like to ask them how the Hotel de Ville, in Brussels, had been set on fire by lightning on the 1st of last June. The system of lightning conductors on this building had been erected in accordance with the received theory, and had been held up by writers on the subject as the most perfect in Europe. Unless some explanation were forthcoming to account for its failure, we could no longer regard lightning conductors as a perfect security against danger.
The President of Section A, Professor Fitzgerald, in bringing the discussion to a close, observed that one result of this meeting would be to give a new interest to the phenomena of static electricity and its practical applications. He was inclined himself to think that the experiments of Professor Lodge were not quite analogous to the case of a flash of lightning. In comparing the discharge of a Leyden jar with a flash of lightning they should look for the analogy, not so much in the external discharge through a series of conductors, but rather, asMr.Preece had observed, in the bursting of the glass between the two coatings of the jar. As regarded the oscillations in a Leyden jar discharge, he did not think such oscillations were at all necessary to account for the phenomena observed in the experiments. Many of the results which Professor Lodge seemed to think would require some millions of oscillations per second would be produced by a single discharge lasting for a millionth of a second. Improvements, perhaps, were possible in our present system of lightning conductors,but practical experience had shown, however we might reason on the matter, that, on the whole, lightning conductors had been a great protection to mankind from the dangers of lightning.
Summary.—I will now try to sum up the results of this interesting discussion, and state briefly the conclusions which, as it seems to me, may be deduced from it. First, I would remind my readers that a lightning rod has two functions to fulfill. Its first function is to promote a gradual, but rapid, discharge of electricity according as it is developed, and thus to prevent such an accumulation as would lead to a flash of lightning. Its second function is to convey the flash of lightning, when it does come, harmless to the earth. Now, the new views advanced by Professor Lodge in no way impugn the efficiency of lightning rods as regards their first function; and it is evident that the greater the number of lightning rods distributed over a given area, the more perfectly will this function be fulfilled. This is a point of great practical importance which seemed to me, in some degree, lost sight of during the progress of the discussion.
Secondly, it was practically admitted by the highest authorities that the experiments and reasoning of Professor Lodge afford good grounds for reconsidering the received theory of lightning conductors as regards their second function—that of carrying the lightning flash harmless to the earth. But there was undoubtedly a general feeling that it would be rash to set aside, all at once, the received theory on the strength of laboratory experiments made under conditions widely different from those which actually exist in a lightning discharge. Experiments are wanted on a larger scale; and, if possible, experiments with lightning rods themselves.
Thirdly, the testimony of electrical engineers who have had large experience with lightning conductors seems almost unanimous that a lightning conductor erected and maintained in accordance with the conditions prescribed by the Lightning Rod Conference gives perfect protection. It was certainly unfortunate that the Hotel de Ville, in Brussels, which was reputed the best protected building in Europe, should have been damaged by lightning just two months before the discussion took place; but no certain conclusion can be drawn from this catastrophe until we know exactly the conditions under which it occurred.
So the matter stands, awaiting further investigation.
FOOTNOTES:[41]See his Lectures, published in theElectrician, June 22, June 29, July 6, and July 13, 1888.[42]See paper read at the meeting of the British Association, in Bath, 1888, published in theElectrician, page 607. September 14.[43]See a very ingenious hypothesis, to account for this phenomenon, suggested by Professor Ewing in theElectrician,p.712. October 5, 1888.
[41]See his Lectures, published in theElectrician, June 22, June 29, July 6, and July 13, 1888.
[41]See his Lectures, published in theElectrician, June 22, June 29, July 6, and July 13, 1888.
[42]See paper read at the meeting of the British Association, in Bath, 1888, published in theElectrician, page 607. September 14.
[42]See paper read at the meeting of the British Association, in Bath, 1888, published in theElectrician, page 607. September 14.
[43]See a very ingenious hypothesis, to account for this phenomenon, suggested by Professor Ewing in theElectrician,p.712. October 5, 1888.
[43]See a very ingenious hypothesis, to account for this phenomenon, suggested by Professor Ewing in theElectrician,p.712. October 5, 1888.