CHAPTER X.

HÔTEL DE VILLE, BRUSSELS, AND WESTMINSTER PALACE.

The systems of lightning-conductors used for the protection of the Hôtel de Ville and Westminster Palace seem worthy of separate description, as showing the methods employed by Professor Melsens and the late Sir William Snow Harris, both eminent authorities in their respective countries. The two buildings are so entirely distinct in their character, that it will be seen at once that very different methods had to be employed in rendering them safe from the effects of thunderstorms.

The Hôtel de Ville, Brussels, one of the finest Gothic structures in the Netherlands, is fitted with an elaborate system of lightning-conductors, erected under the superintendence of Professor Melsens, a distinguished electrician and scientist. He has for many years advocated the method of employing a great number of small lightning-rods, in preference to one rod of large size, for the protection of buildings from the effects of lightning; the main characteristic of his system being that of covering the building with a network of metal furnished with very many points, combined with numerous and ample earth-contacts. This idea has been thoroughly worked out at the Hôtel de Ville, Brussels; and probably no other building is so completely guarded from the dangers of thunderstorms. The principal feature of the Hôtel is a large central building, with a pinnacled turret, from which rises a lofty spire, nearly three hundred feet high, and adorned with four galleries, each with corner pinnacles. Upon the top of this spire is a gilded colossal figure, seventeen feet high, of St. Michael, holding a naked sword and standing upon a dragon. This acts as a vane, and the point of the sword forms the highest terminal conductor of the system. The main block of the Hôtel is ornamented with six turrets, from each of which springs a small spire. In the rear is a courtyard formed by buildings annexed to the front main block, and composing the remaining three sides of this inner quadrangle.

The figure of St. Michael, all the parts of which are rivetted and soldered together, rests on a pivot of iron, three and a half inches in diameter, which is deeply embedded in the stone-work of the spire. The weight of the vane produces a metallic connection with the pivot, and the top of the platform in which the pivot is fixed is covered with sheet-copper. Around this and in connection with the pivot are fixed eight perpendicular galvanised iron conductors, two-fifths of an inch in diameter, and provided with five points each. A flash of lightning striking the statue would thus reach the pivot and then be divided between the eight conductors. Just below the platform are placed, at an angle of 45 degrees, eight large points six and a half feet long. These are fastened to an iron band which encircles the spire, and are connected with the eight conductors by means of a mass of zinc. Thus the pivot of the statue, and consequently the statue itself, the eight conductors, the eight large points, and the forty small points on the conductors, constitute a protection which dominates the edifice, and represents a circular space of about five and a half yards in diameter; that is, between the extremities of the large points which project from under the platform. In this manner a flash of lightning is instantly distributed and conveyed by the conductors to the ground. It may be mentioned here that a thin copper wire, insulated by three coatings, is fixed on the north side of the iron band in which the large points are fastened; the other end of this wire is left free, and can be utilised as a conductor for a rheometer or any other electric machine which it might be thought proper to use permanently for the registration of lightning striking the conductors.

The eight conductors have each an unbroken continuity of about 310 feet; and they collectively show a continuous section of nearly one inch--almost half as much again as the limit of safety given in the ‘Instruction’ of the Paris Academy. Although, in Professor Melsen’s opinion, rods of somewhat less diameter would have been amply sufficient for security, he chose the largest size which could be easily bent to the varying contours of the building, and also as allowing for the expansion and contraction caused by changes of temperature. If conductors of only one quarter of an inch diameter had been used they would, it is true, have shown a total section just above the limit of the ‘Instruction;’ but, since Coulomb has demonstrated that tensional electricity is more particularly carried on the surface of bodies, M. Melsens thinks it is necessary to consider the action that this surface might exercise in the easy transmission of electricity. Some old German writers on this subject went so far as to assert that the conductivity was proportioned to this surface. They therefore recommended flat bands or hollow tubes in place of rods. Although exact figures cannot be given as to the effect due to the area of the surface, M. Melsens considers that it is unquestionable that the relation of the section to the surface has a marked and definite, although at present unknown, result. In the case of the Hôtel de Ville, Brussels, he thinks the eight conductors possess a signal advantage over one conductor, even though it had a larger section--say one inch. Experience will doubtless teach how to determine more precisely the extent of this surface-action.

The eight conductors descend the length of the octagon of the spire until they reach the first gallery; going round this they pass over the balustrade, and then converge towards each other; are carried over a prominence in the roof; and as they pass along gather up other conductors of similar size from the ridges and parapets of the buildings which form the quadrangle. Projecting vertically from these horizontal lengths of the conductors are a large number of points and aigrettes. The summits of the lower tower are also furnished with a great many points. These eight main conductors are then taken down the wall of the building into the courtyard, and at about three feet from the ground are carried into a box constructed of galvanised iron, and in it are connected into one solid mass by zinc, which has been poured molten into the box. Almost throughout their length the conductors are left loose, so as to remove all complication arising from dilatation; the play of this dilatation being rendered easy on account of the small section of the conductors, which bend readily.

In accounts of lightning striking buildings which have been provided with lightning-conductors, it is almost invariably found that these conductors are incomplete, and have generally been fixed by persons ignorant of the scientific questions involved. When the facts in such cases are carefully examined it is found, as a rule, that the defect is in the connection with the water underground, or in the bad conductivity of the earth in which the conductors terminate. In establishing a perfect communication with the earth, M. Melsens considers it is necessary, not only to place the conductors in contact with water, but also to see that the contact extends over a large surface. The Paris Academy ‘Instruction’ recommends this precaution, but in a very vague and too succinct a manner. To the above rule may be added another condition, namely, that the earth-connection should be large in proportion as the site of the building is redundant in metal products in direct or indirect contact with the ground, the subsoil, or the damp earth of the foundations, and sometimes even with water itself. With regard to the metal contained in the materials of buildings, it is not sufficient to establish a connection at one point only, as is generally supposed. On the contrary, it is important that all the metal-work should be connected with the conductor at least at two points, in order to realise closed metallic circuits, and thus offer an entry and exit, or a free metallic course, for the current of electricity. The foregoing statements have been placed here chiefly because the principles they convey have been so rigidly, and at the same time successfully, carried out by Professor Melsens at the Hôtel de Ville, Brussels.

To return to the eight conductors and the earth-connections provided for them. It has been shown that these conductors, after descending the wall of the building, reach a point about three feet from the ground, where they are embedded in a rectangular box of galvanised iron, which is eight inches long, three inches broad, and three and a half inches high. In the bottom of the box are three holes, through which pass three series of eight conductors, each series being of the same diameter as those which descend from above; the conductivity being thus increased threefold. All of these are formed into one mass by the zinc, which has been poured into the box in a molten state, so that they constitute with the eight rods from above, one integral conducting system. In the illustration which is here given the box is represented by B, and the eight main conductors coming down from the building by C C. The three series of rods numbered 1, 2, 3 show the triplicated conductors issuing from the box. The first series is placed in communication with the water by means of an iron pipe, which carries it underground to a well. Here the rods are inserted in a large tube six and a half feet long and nearly two feet in diameter (see engraving). This tube is let down almost four feet below the level of the earth, and sustained by two chains hung on two iron holdfasts fixed in the side. The conductors C C are fastened to this tube in the following manner:--A small length of straight iron cylinder is placed outside the flange of the tube; and the ends of the conductors being arranged between the cylinder and the flange, the space _a a_ is filled with molten zinc; thus rendering the substance of the iron tube and that of the conductors metallically continuous. The well into which the tube is sunk furnishes perpetually a contact of eleven square yards between the water and the iron of the tube. Into the space _a a_ is also introduced a large number of small galvanised iron wires to act as auxiliary conductors; these are terminated by being brought to a point and soldered to the mass of zinc. In order to prevent as far as possible the formation of rust, a large quantity of lime is thrown into the well, in order to make the water alkaline. The second series of conductors, painted with coal-tar, is placed in a covered metal gutter and carried some distance to a gas-main in a spot where the earth is moist. The conductors are fixed by means of a large copper plate, which is soldered to the gas-pipe or main. On the copper plate are fastened sixteen large-headed brass screws, to which the conductors are secured. This arrangement is enclosed in brickwork, the wires being painted with coal-tar; and a quantity of boiling tar is poured on the copper plate, over which is laid a cloth, thus preserving the whole from oxidation. The third series of conductors is carried in a gutter, similar to that which contains the second series, to a water-pipe in the Place de l’Hôtel de Ville, and the wires are fixed to it in the same way.

It may be added that the whole of the conductors above-ground--with the exception of the points--are painted with oil.

Although it is correct that the coke generally placed around the earth-connection of conductors aids by its good conductivity to bring them in contact with a large surface of earth, Professor Melsens has preferred to employ tar, which, it is true, is insulating, but helps materially to preserve the conductors. It is estimated that the entire contact between the earth and the underground surface of iron is about 300,000 square yards.

Professor Melsens thinks it is worthy of note that, although copper is a better conductor of electricity than iron, it has less molecular strength. Where thin iron wire would simply be beaded--without losing its conductivity--by an exceptionally strong charge of electricity, copper wire of the same thickness would by a similar charge be dissipated to a black powder. Professor Melsens has verified this in some very interesting experiments. The large use of iron in his system of conductors on the Hôtel de Ville, Brussels, was rendered imperative by reason of the enormous cost of sufficient copper for such an extensive system. But Professor Melsen’s experiments, nevertheless, give some support to the selection of iron for large and complete works of this kind.

Sir William Snow Harris, in his arrangements for the protection of the Palace of Westminster from lightning, has endeavoured to perfect the general conductivity of the whole mass of the building, and so make it assume the same relation to the electric discharge as if it were a complete mass of metal. Westminster Palace differs in one important respect from the Brussels Hôtel de Ville--the general level of the roofs is covered with iron coated with zinc, and in many places directly connected with the earth by cast-iron water-pipes. The roofing thus constitutes, although imperfectly, and only to a limited extent, a protection of itself. Sir William Snow Harris had, therefore, chiefly to provide for those portions of the building which are above the general level of the roofs, and, by the use of ample conductors of copper, to make up for the comparatively low conductivity of the roofing and the iron pipes which connect it with the earth.

From the terminal which forms the highest point of the large central tower is brought a copper tube of two inches diameter and one-eighth of an inch in thickness, the joints of which are secured by solid screw-plugs and coupling-pieces. This tube is carried down in the south-west angle of the tower and fastened to the masonry by metallic staples. At the junction of the tower with the roofs the tubing is, or at any rate was, thoroughly connected with the metal of the roof, and then continued to the earth in as straight a course as practicable, and there terminates in two projecting branches made of solid copper rod. By carrying this copper tubing direct to the earth, instead of terminating it in the metal-work of the roof, the electrical discharge obtains a conducting medium of the same power throughout, in place of having to leave a high power for one of lower conductivity.

The Victoria and Clock Towers, which are each 300 feet high, are both fitted with a copper band, five inches wide and a quarter of an inch thick. These run down the walls, and are connected with the metal of the roof, and also with the metallic rail of the staircase within each tower. The ornamental turrets and pinnacles of St. Stephen’s Porch are protected by small bands of sheet-copper, two inches wide and one-eighth of an inch thick; these are also placed in connection with the metal of the roof.

The north and south towers of the central block, and the north and south wing towers of the front facing the Thames, have attached to them bands of sheet-copper running from their respective vanes to the roofing below. The bands are connected with the metal of the roof, and are then carried down independently to the earth, in a similar manner to that adopted on the large central tower.

The only other prominent portion of the edifice is the ventilating shaft of the House of Commons, where, during the sitting of Parliament, a coke-fire is generally burning, and from which, therefore, a stream of warm and rarefied air is constantly being emitted. The conductivity of an ascending column of warm vapour is known to be great, and accidents from this cause are of frequent occurrence, although very often they are not ascribed to their true source. To obviate this danger, the ventilating shaft is provided with a copper tube conductor, fixed on its eastern side, and connected with the metal of the roof.

This short description of the measures adopted by Sir William Snow Harris for the protection of Westminster Palace contains all the salient points of the system which at that time, some twenty years ago, was doubtless the best that could be devised. But, although nearly 4,000_l._ was spent upon this work, from that time to this, as far as can be ascertained, these lightning-conductors have never been tested! It is therefore very possible, and indeed probable, that on the occurrence of any very heavy thunderstorm they would be found wanting, and considerable damage would ensue, the extent of which no one can estimate.