CHAPTER XII.
LIGHTNING PROTECTION IN FRANCE AND AMERICA.
In this chapter it is proposed to give a brief résumé of the different systems of constructing, erecting, and repairing lightning conductors in France and America. The laws of electricity being the same all the world over, the methods employed in these countries are necessarily similar in their essential principles; nevertheless they vary somewhat in detail, both from each other and from the work of the best firms in England.
Until a very few years ago the lightning conductors throughout France, although many in number, were in a very neglected state. Badly constructed in many cases, their original faults had grown worse from want of attention. The connection of the terminal rod with the conductor was generally made by means of a strap or iron collar, which, after a short time, rusted to such an extent that the continuity was practically reduced to nothing, and the conductor, so far from being a protection to the building, was a positive danger to it.
Latterly, however, a reaction has taken place, and a more careful method of connecting the various joints of the conductor has been contrived, or rather, revived, and a better system of periodical inspection and testing is carried out.
Fig. 10. Fig. 11.
Under the French system, what is called in England a lightning conductor, and to which the French give the name Paratonnerre, is nominally divided into three parts: the terminal rod, the conductor, and the racine, or root, i.e. the earth connection. With regard to the terminal rod, the ‘area of protection’ theory is, in France at any rate, still believed in by a great many people. In that country, as a rule, it is made of wrought iron in a single length, and polygonal or slightly conical; its height depends upon the size and area of the building it protects, the general presumption being that, under ordinary circumstances, a terminal rod will protect effectually a cone of revolution, of which the apex is the point of the rod, and the radius of the base a distance equal to the height of the said rod above the ridge, multiplied by 1·75. Thus a rod rising eight yards above the ridge of a building would effectually protect a cone-shaped space, the base of which, at the level of the ridge, has a radius of 8 × 1·75 = 14 yards. In actual practice somewhat wider limits are allowed. The height of the terminal rod having been determined according to the circumstances under which it is erected, it is then galvanised with zinc in order to prevent oxidation, and the connection between the terminal rod and the conductor is formed by means of the following arrangement. A little above the base of the terminal rod, say about eight inches from the roof of the building, a flange A (see [fig. 10]) is welded with a hole pierced through it. Through this hole the conductor, previously filed down to the proper dimensions, must be tightly passed. After scraping the iron around the hole, a washer of lead is placed at P and P´ (see [fig. 11]), and the button B, by means of a strong layer of solder, thoroughly binds everything together. In this way an excellent joint is obtained; the contact surface is considerable, and, if the work is carefully done, the joint is completely preserved from rust.
Fig. 12.
The point of the terminal rod, although sometimes made of platinum, generally consists of either pure red copper, or, what is considered still better, an alloy of 835 parts silver and 165 parts copper. It is fastened to the terminal rod in the manner shown in [fig. 12].