That copper should not have been employed, long before brass and other metals, in serving mainly for lightning conductors, its pre-eminence for this purpose being undisputed, would seem a strange fact, were it not explicable on several grounds. The first was the cost of the metal, which, though varying in price, is seldom less than six or seven times that of iron. It was needless for the advocates of copper as conductors—and there were not a few from the time its high conductive power had been demonstrated—to say that if copper was six times as dear as iron, it was likewise six times better as a carrier of the electric force, and that consequently the price, in respect of applicability for lightning protection, was in reality the same. But the reply to this was that copper, being one of the most expensive metals, except the so-called ‘precious’ ones, was exposed to the temptation of theft, and ought therefore not to be employed, since it was possible that vagrants, or other people, might tear off at any time the, in more than one sense, valuable pieces of metal protecting buildings against destruction from lightning. The argument, perhaps, was not worth much, but a better one not mentioned was in the background. It was, till quite recent times, an achievement of the greatest difficulty to manufacture long rods or bands of sufficiently pure copper to serve as lightning conductors. Sir William Snow Harris attempted, as already related, to get over this impediment by taking short plates, and fastening them together, and over each other, by copper nails. But this process, besides being enormously expensive, was in many other respects unsatisfactory, notably in that it made a shifting of the plates possible, and by the destruction of a few of them ruined the whole system. The pith and substance of the whole was the technical difficulty of hammering or drawing pure copper out into great lengths. That it must be pure was essential, the fact being thoroughly established that the electric conductivity of copper, mixed with impurities such as arsenic, is often not two-thirds, and sometimes not as much as one-half, that of the pure metal. This was conclusively shown by Sir William Thomson in a series of researches, and likewise by that distinguished investigator in the conductivity of metals, Professor Matthiessen. The latter, while placing copper on the same rank with silver, and far above gold—100 to 78—furnished the following instructive list as to the relative value of different kinds of copper:—
| Pure copper | 100·00 |
| Best American copper | 92·57 |
| Australian copper | 88·86 |
| Russian copper | 59·34 |
| Spanish ‘Rio Tinto’ copper | 14·24 |
It will be seen that, according to the investigations of Professor Matthiessen, admitted on all hands to be correct, the copper lowest in the list, the ‘Rio Tinto,’ is barely equal to iron in electrical conductivity, and, not having the hardness of the latter metal, would be in every way inferior to it as a lightning protector. The employment of the purest copper therefore became an essential point in the manufacture of lightning conductors.
Fortunately, the difficulty was solved, at an earlier period than might have been expected, by the demand for submarine cables. These had to be made of wires of the highest possible electrical conductivity, and the matter being one of high financial and commercial importance, manufacturers soon began to use the utmost care in selecting ores containing the smallest amount of metallic impurities. We believe the lightning conductors now manufactured at the extensive works of Mr. R. S. Newall, F.R.S., established at Gateshead on Tyne about forty years ago, have generally a conductivity of 93 per cent. of pure copper. It was laid down by one of the most eminent scientific men of the day, not long ago, that the three principal qualities of a good lightning conductor ought to be a maximum of conductivity, of durability, and of flexibility that could be obtained, and there is nothing coming up to this standard so well as ropes of pure copper.
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.