Fig. 23.
The projecting points of the conductor are drawn in [fig. 23] larger than they need be, in order to show them more clearly, distinguishing them from the rest of the building. The same has been done with the copper rod, running from the roof to the ground and thence into the earth. In reality a conductor may be made perfectly safe, and yet all but invisible to the naked eye. For private houses and buildings, a rope made of copper ought to be at least five-eighths of an inch in diameter, for a copper rod of half an inch in diameter has never been known to be fused. For chimneys of manufactories, where gases are liable to corrode the rope, it had better be a little thicker. Such copper ropes as those manufactured by Messrs. R. S. Newall and Co., five-eighths of an inch in diameter, weighing two-thirds of a pound per foot, and having a conductivity of 93 per cent., have never been known to fail in protecting even the largest buildings. It is supposed by some writers that the value of the conductor is in proportion to the amount of surface of metal exposed. This, however, is a mistake, for the conductivity depends on the weight per foot of metal used, the purity in both being equal. Wire-rope is used simply because it is so pliable that it is easily handled, and can be made of any length required without joints.
Fig. 24.
In [fig. 24] is given an illustration of a small detached house, in which the arrangement of the lightning conductor is indicated by the dark lines. The method followed is exactly the same in principle as that employed for the mansion just described. A terminal rod is placed upon each chimney. These terminal rods are connected with each other by a copper-rope conductor which is carried along the ridges and gables of the roof, thus constituting a similar arrangement to the French ‘ridge-circuit’ (circuit des faîtes), with the additional advantage of being far lighter and more sightly. The copper conductor descends to the earth down the angle formed by the projecting entrance to the house. By this means every corner of the building is protected; an important matter in all detached buildings, and especially when they happen to stand among trees. The preference of the electric force for trees as its path to the earth in the absence of metal or other bodies of higher conductivity than trees, has probably no other ground than their being full of moisture; still this is a disputed question.
[Fig. 25] exhibits a slightly different method of arranging the lightning conductor. In this case the ridges of the roof are surmounted by ornamental iron-work, instead of the usual terra-cotta, or earthenware, tiles. This iron-work is utilised and carefully connected with the conductor. The chimneys, in place of being fitted with terminal rods, are provided with cast-iron caps—as shown in the engraving—to which the conductor is attached. The conductor, after descending to the ridge, is led along it and down the edges of every gable, and is finally carried down to the ground and connected with the earth in the usual manner. It is of course absolutely necessary that all masses of metal, such as gutters, waterspouts, rain-pipes, &c., should be brought into connection with each other and with the conductor, in order that the house may constitute one electrically homogeneous body.
It was for a long time held that the protection of churches against lightning offered special difficulties. This arose mainly from the constant reports of churches being struck, often when they were believed to be protected, whereas the accidents arose from the conductor not being properly fitted. It is even now too often forgotten that all so-called ‘conductors’ of the electric force are only so in relation to ‘non-conductors,’ and that, strictly speaking, all things on earth are to some extent conductors and to some extent non-conductors. This being kept clearly in view, there is no more difficulty in protecting the largest cathedral against lightning in the most efficient manner than in similarly guarding the smallest cottage.
Fig. 25.