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.
Professor Melsens thinks it is worthy of note that, although copper is a better conductor of electricity than iron, it has less molecular strength. Where thin iron wire would simply be beaded--without losing its conductivity--by an exceptionally strong charge of electricity, copper wire of the same thickness would by a similar charge be dissipated to a black powder. Professor Melsens has verified this in some very interesting experiments. The large use of iron in his system of conductors on the Hôtel de Ville, Brussels, was rendered imperative by reason of the enormous cost of sufficient copper for such an extensive system. But Professor Melsen’s experiments, nevertheless, give some support to the selection of iron for large and complete works of this kind.
Sir William Snow Harris, in his arrangements for the protection of the Palace of Westminster from lightning, has endeavoured to perfect the general conductivity of the whole mass of the building, and so make it assume the same relation to the electric discharge as if it were a complete mass of metal. Westminster Palace differs in one important respect from the Brussels Hôtel de Ville--the general level of the roofs is covered with iron coated with zinc, and in many places directly connected with the earth by cast-iron water-pipes. The roofing thus constitutes, although imperfectly, and only to a limited extent, a protection of itself. Sir William Snow Harris had, therefore, chiefly to provide for those portions of the building which are above the general level of the roofs, and, by the use of ample conductors of copper, to make up for the comparatively low conductivity of the roofing and the iron pipes which connect it with the earth.