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, could be 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 of Mr. 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 bad contact 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 of Mr. 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, as Mr. 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.