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

by F.C.S. Richard Anderson

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.

Fig. 3.

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.

Fig. 4.

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.