Lightning Conductors: Their History, Nature, and Mode of Application

by F.C.S. Richard Anderson

Fig. 15.

In larger buildings what is termed a ‘ridge-circuit’ is often used. It consists of an unbroken metallic connection running along the ridges of the building to be protected, and connected with the conductors and terminal rods, and consequently with the subterraneous sheet of water which forms the common reservoir. It is made of lengths of square iron bars or rods having a thickness of about three quarters of an inch square, and fastened together by overlaying the ends, bolting them together with two bolts, and covering them well with solder in the manner shown in [fig. 13]. New branches are formed by T-shaped connections, the cross-piece of the T overlaying the original ridge-circuit, and the stem making the first length of the new branch. In some cases the ridge-circuit rests directly on the ridge of the roof; but in order to avoid injury during the repairs to the roof or in other ways, the plan adopted in good work is to raise it some distance above the ridge on supports at suitable distances, and thus prevent the possibility of damaging the joints and solderings.

The form and arrangement of these supports depend on the nature of the roof. Sometimes forked uprights are used—these allow for the expansion and contraction due to changes of temperature; in other cases simple cast-iron bearings, weighing from ten to twelve pounds each, are laid upon the ridge, their upper surfaces being grooved to receive the bars of the ridge-circuit.

All masses of metal used in the construction of the building are metallically connected with the paratonnerre. As a rule, this is done by pieces of iron about half an inch square, which are strongly soldered to the metal surfaces, and then connected with some part of the conductor or ridge-circuit.

Although in France, as elsewhere, all experts are agreed as to the prime importance of the disposition and arrangement of the racine or earth-end of the paratonnerre, a difference of opinion prevails as to the best means of insuring a good earth-contact, and many methods have been tried, all of them similar in principle, but differing somewhat in application. It is proposed to give here a brief outline of the best contrivances employed for this purpose.

One main object, in arranging the earth terminal of a lightning conductor, is to avoid the gradual destruction of the racine by the action of alternate dryness and moisture which, unless the iron is protected in some way, corrodes, and eventually eats it entirely through. There are several ways of remedying this evil. In France it is common to find used for this purpose a vertical spout of tarred, boucherised, or creosoted wood, rising a few inches above the soil. Some authorities recommend the simple plan of covering this part of the conductor with a strong coating of tar, others covering it with a wrapper of sheet lead, and this last method is probably the best. With regard to the extreme end of the conductor, the system approved of by the Paris Academy of Sciences is generally used in good work. This system is the use of a trough filled with broken charcoal, through which the conductor runs; charcoal preventing the too rapid oxidation of the iron. For charcoal, coke may be substituted. The trough (see [fig. 16]) is made either of wood, gutter tiles, or ordinary bricks without mortar, so as to allow the moisture of the soil to permeate through. It is preferable, even at the expense of lengthening the conductor, to carry it through the lowest and dampest plots of ground around the building.

Fig. 16.

To obtain a perfect contact between the end of the conductor and the earth, or common reservoir, the French use several methods. One of the earliest ones was the multiplication of the iron bars attached to the end of the conductor, and inserting them for some distance into well-water. Theoretically this arrangement is good, but it has been found that the decay of these terminals by the action of rust was so rapid that, unless they were carefully watched and periodically repaired, they soon became insufficient, if not useless. In addition to this, it is the opinion of many French savants that a mere water contact is not enough, a soil that is always moist being in their judgment far preferable. The simplest plan adopted for attaining this end is that of inserting into the moist ground to a certain depth, regulated by the nature of the soil, one or several metallic branching stems, which are connected with the conductor. By another arrangement, invented by M. Callard, the conductor is terminated by a kind of galvanised iron grapnel, placed in a wicker basket filled with pieces of coke. Where the soil is dry, and moist ground cannot easily be got at, the harrow or grating shown in [fig. 17] is often used. It is placed between two layers of horn embers, or charcoal, and sunk as deeply as it conveniently can be, the end of the conductor being carefully connected with it by soldering or by a quantity of melted zinc.

In towns, the water-pipes and gas-mains, possessing as they do, large metallic surfaces, are generally utilised for making the ‘earth-contact.’