Page:Popular Science Monthly Volume 25.djvu/831

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PROTECTION AGAINST LIGHTNING.

ference nothing whatever is said of the influence of length in reducing the efficacy of a conductor. This is the more strange, because, in speaking of the care required for the formation of joints in the "final decision of the conference on controverted points," the report categorically remarks that bad joints have the same effect as "lengthening a conductor," and a reference is incidentally made to one instance, in which a bad joint was found to have had the same effect on a discharge of electricity that the lengthening of a conductor to nineteen hundred miles would have had. This nevertheless was a point that was perfectly understood by the French investigators, and it is obviously one in which the London code is behind its predecessors. In the first French instructions, issued in 1823, there is a paragraph which says:

Among the conducting bodies there are none, however, which do not oppose some resistance to the passage of the electric force; this resistance to the passage, being repeated in every portion of the conductor, increases with its length, and may exceed that which would be offered by a worse but shorter conductor. Conductors of small diameter also conduct worse than those of larger diameter.

It follows, as a matter of absolute certainty from this increase of resistance with augmented length, that a conductor which was of ample dimensions for the protection of a building eighty feet high would not be of the same efficacy for a building four hundred feet high. It is for this reason that M. Melsens employed eight main conductors for the Hôtel de Ville at Brussels, and it is for this reason that eight half-inch copper ropes have been carried down from the lantern and cupola in St. Paul's. To use eight main conductors of a given size is obviously, in an electrical sense, the same thing as to use one conductor only of eight times the size.[1] The practice of the French engineers has hitherto been to double the sectional capacity of the rod for each additional eighty feet of the length that is to be protected by its instrumentality. This practice is a sound one, and certainly should be observed.

There is one other particular in reference to the conference report to which it seems desirable to draw attention on account of the erroneous doctrine to which it may possibly give a sanction. Among the appendices which have been added to the report there is a table, obviously prepared at the cost of some labor, which professes to give the sizes of lightning-conductors recommended by various authorities. In order to facilitate the comparison of the several sizes, all have been reduced to what has been termed the equivalent dimensions of copper. But the oversight has been made, in preparing this table, of treating all cases of galvanized iron as if the zinc in the combination had no other function than the protection of the iron from rust. In reality,

  1. The solid copper tape which is chiefly used by Mr. Anderson is, to meet the circumstances here alluded to, manufactured of four different sizes, the smallest being 58 inch wide and 112 inch thick, and the largest 112 inch wide and 18 inch thick.