Lightning Conductors: Their History, Nature, and Mode of Application

by F.C.S. Richard Anderson

Together with Dr. Hemmer in Germany, Professor Landriani, of Milan, drew attention to the paramount importance of a perfect earth connection. He made it his special business to investigate cases in which buildings with lightning conductors had been struck, and was able to show in nearly every instance that it had been for want of ‘good earth.’ A very striking case, which ought to have brought conviction of the truth to all investigators of the subject, occurred in Genoa in 1779. The church of St. Mary in this city, standing in a very elevated position, had been frequently struck by lightning, sometimes as often as twice in one year, and it was noticed that the electric force always followed precisely the same path, running along a certain portion of masonry, partly secured by iron hoops, and finally demolishing a wall at the bottom to get into the earth. At last, in November 1778, a conductor, made of the most approved design, was placed over the church, but, to the great surprise of the scientific men who had superintended the work, the lightning fell once more upon the building in the month of July of the following year, again following the old path it had constantly taken before, and causing absolutely the same damage as previously, even to the knocking out of certain portions of the wall nearest the ground. Naturally, the event caused widespread interest, leading to the closest examination of the church of St. Mary by several experts, among them Professor Landriani. He had no great trouble in discovering both the causes of the path of the lightning having always been the same when falling upon the church, and of the edifice having been struck again in the same manner when provided with a lightning conductor. Being a somewhat peculiar structure, consisting in part of hewn stones held together with iron cramps, there was a large quantity of metal both in and outside; and it was found that the path of the lightning had always been precisely in the direction where the metal offered the greatest continuity, leaping over the short intervals that existed by destroying the stone, and finally getting into the ground to a place where there was always a collection of water by knocking down a wall. If this accounted satisfactorily for the former accidents, that which took place when a conductor had been placed was not much more difficult of explanation. Professor Landriani found that though the conductor itself was very good, it was useless simply by having its roots in hard rock instead of moist ground. On the one side of St. Mary’s Church there was a rill of water rippling down from the hills, and forming a small pool near the church, while on the other was the hard rock. It was into a crevice of the latter that the conductor had been laid, thus leaving the electric force to seek its old path into the water along the iron bars, which, although disjointed, formed a far better road to earth than the planned road. It was a convincing proof of the supreme necessity of a good earth connection. Still, a long time yet was to elapse before conviction became general.

Probably, the matter was more studied by Italian scientific men than any others, the study of electricity having always been a favourite pursuit in that country; yet there, too, the matter was not understood till quite recently. This is proved by a letter of the celebrated astronomer and meteorologist, Father Secchi, addressed to the French scientific journal ‘Les Mondes,’ in October 1872, in which he tells the story of an accident that befel a building protected by lightning conductors set up under his own direction, the earth connection being made after rules laid down by Professor Matteucci, considered the leading authority on the subject. The letter of Father Secchi, though of some length, is given here entirely, both on account of the great fame of the writer, but recently deceased, and because it throws a flood of light on some of the most important points connected with the art of designing and applying lightning conductors.

‘Eight years ago,’ says Father Secchi, writing, as just mentioned, in 1872, ‘some lightning conductors had been erected under my direction on the cathedral and on the Bishop’s palace of Alatri, situated at the summit of the Acropolis of that town, which, by its elevated and solitary position, was exposed to frequent ravages from storms. It was not long ago that a flash of lightning demolished a great part of the belfry, and damaged the organ of the church. In the erection of this lightning conductor there arose a great difficulty proceeding from the nature of the soil, which at the depth of some centimetres turns out to be entirely of solid calcareous rock.

‘In order to remedy this defect, that part of the conductor which enters the ground has been made very long, more than 4 metres [13 feet], and has been provided with a great many couples of points, 5 centimetres [2 inches] broad, 5 millimetres [⅛ inch] thick, indentated on the edges, with the addition of a thick copper wire twisted among the same points, to help to multiply the points of contact between the rod and the carbon. The foot of the lightning conductor is entirely of copper. The rod is also of copper up to a metre [3¼ feet] above the ground; and there is joined to it the iron conductor, in the ordinary receptacle made in the heart of the wall, to preserve it from disturbances of the inferior parts. The ditch into which the foot of the lightning conductor was sunk is 5 metres [·16 feet] long, and half-a-metre [1⅝ feet] wide, and it was dug into the ground as far as to touch the roots of some neighbouring trees, from which point upwards a layer of cinders was placed, covering the greater part of the ditch. Thus the surface of contact between the metal and the carbon, and of the latter with the soil, was such that one would have supposed it to be more than sufficient, while the presence of trees, although they were not very large, made it highly probable that the ground did always contain sufficient moisture. Moreover, as the edifice had two culminating points—namely, the belfry and the raised back portion of the choir—two rods were placed on them, each having an independent connection with the earth, so that, in the case of a discharge on one of the points, the electric force might find two ways in its course towards the earth.

‘These arrangements produced, on the whole, a good result, since, although the edifice was struck at least four times after conductors had been placed on it, it suffered no damage of any kind. Nevertheless a very curious accident, highly interesting as a scientific study, happened on October 2. Early on the morning of this day several flashes of lightning fell down from the clouds during a terrific storm, which lasted over two hours. The belfry was struck at first by weak discharges twice; but the third flash was so appalling in its strength as to terrify the whole town below. The injuries it caused were not great, still they seemed to me to be extremely noteworthy. But before I describe them I must give some necessary details as to place and position of the lightning conductor.

‘It so happened that four years after the erection of the conductor a line of pipes was laid down to carry water to the towns of Alatri and Ferentino, passing at a short distance from the belfry of the cathedral. The lightning conductor was not placed in communication with the pipes, because it seemed established, from previous experiments and observations, that it was needless to do so, the ground containing apparently sufficient moisture, the head of the waterworks being close, and there existing also a running fountain. I was not asked at the time whether it was necessary to establish this communication, but, had the question been put to me, I should probably have answered it in the negative, considering, from what I then knew, the work as superfluous. That I was in error then as to the necessities of a perfect underground connection is shown by what happened during the great storm in the early morning of October 2. The heavy flash of lightning before referred to did not go its appointed path underground, but passed off into the waterworks, with the following results:—

‘1. It made in the earth a perfectly rectilinear excavation, which, from the lower part of the conductor, went to the tube of the waterworks running to Ferentino, and in traversing the wall destroyed the angle of that structure. The earth of the ditch thus dug was disposed regularly to right and left with great symmetry. The length of the ditch was about 10 metres, the depth about 70 centimetres [28 inches].

‘2. The lightning struck the water-pipe of Ferentino, broke it completely, throwing the pieces to a distance of about 80 centimetres [32 inches]. The lead which soldered the joint of the broken tube with the tube beyond was found melted. In consequence of this rupture the water ceased flowing to Ferentino, and poured into the waterworks.

‘3. Another part of the discharge spread itself by the pipe which goes to Alatri, and traversing the reservoir threw to a great distance some wooden plugs which stopped up the discharging tubes, the plugs being forcibly hammered in. It arrived at the town in a tank, where it damaged and twisted in a strange manner a leaden slab which was in the tank, made some other little injuries, and finally left the trace of its passage at the spouts of the public fountain.

‘4. The point of the lightning conductor was examined, and it was found very blunt; it was found impossible to unscrew it, and it could not be removed without breaking the screw. It was found broken to a length of more than 3 centimetres [1¼ inch], and the section of fusion was nearly flat, as though it had been cut. The gold of the gilding had nearly all disappeared. In the church, and in the edifice which is attached to it, no injury was detected. These facts appear to me important both as regards practice and theory: in respect to theory, because they give an idea of the quantity and of the immense force of the discharge. The melting of the point down to a section 1 centimetre [½ inch] in diameter proves that it would have been melted down much further if it had been slighter. It is not prudent, then, to use very slender points; it is best that they should thicken quickly.