In 1782, nevertheless, at the reiterated request of Le Roi, a member of the Academie des Sciences, and friend and admirer of Franklin, the Louvre was endowed with the first lightning-conductor put up on a public building in France. Soon afterwards they became common.
In 1784 the Academie des Sciences drew up the first set of rules for the construction of lightning-conductors. It was revised and corrected in 1823, in accordance with the various improvements that had been introduced up till then, and it has been further added to in 1854, 1867, and 1903. These instructions point out that the most important metallic portions of the building should be placed in communication with the conductor, and this should sink into a well. Conductors that are not perfectly constructed are a source of danger, instead of being a protection, for the electric current is apt, instead of running down into the earth, to make for any kind of metallic substance, and cause great havoc.
The conductor ought really to communicate with a large body of water—a body of water of greater extent than the storm cloud from which the lightning comes. When the flow is insufficient, the water itself is apt to become electrically charged. It is dangerous to bury the conductor in merely damp soil; first, because one generally does not know whether there is enough of this soil; secondly, because one cannot be sure that the humidity will be sufficient at times of great drought —the very times when storms are most to be feared. Failing a river or great pond, the conductor should be put into wells issuing or having their source in inexhaustible supplies of water deep down in the soil.
In his table of statistics showing the number of cases in which lightning has struck either lightning-conductors, or buildings, or ships furnished with conductors, Quebelet gives a hundred and sixty-eight cases in which the conductor has been struck, and in only twenty-seven instances of these (one-sixth of the whole) have the conductors, from some grave flaw in their construction, failed to fulfil their office. These results are the best proof possible of the efficacy of conductors, and the best answer to those who decry them.
The area of protection covered by the conductor is not so great as is generally supposed. It is limited to a distance about three or four times the length of the conductor above the roof. Thus a conductor standing out five yards will protect an area stretching only about fifteen or twenty yards away. This depends also to some extent upon the nature of the place and the materials of which the house is constructed.
Buildings are often struck by lightning because the number of conductors has been insufficient for the extent of the edifice to be protected.
To remedy this defect, conductors are made with a number of separate stems—veritable wire traps in which to catch the lightning. This system, the invention of a Belgian physicist, M. Melsens, decreases considerably the risks of destruction, and is much more economical than the erection of a number of separate conductors.
A conductor of this kind has been installed on the Hotel de Ville at Brussels, which has been well protected from lightning ever since, whereas previously this building had been struck by lightning several times in spite of the single conductors with which it was supplied. The metallic trellis is in communication with the sewers.
The slaughter-houses of La Villette, the Hôtel Evigné, and other buildings in Paris, are provided with similar defences.
The Eiffel Tower boasts several such multiplex conductors. It has often been struck by lightning, but no one who has happened to be up it at the time has ever suffered any damage therefrom. The lightning strikes the conductor sometimes from out the actual cloud—curious photographs have been taken of this. The Eiffel Tower is in itself a gigantic lightning-conductor.