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

by F.C.S. Richard Anderson

Fig. 17.

Sometimes, instead of iron bars, galvanised iron cables of about an inch in diameter are used for the conductors of paratonnerre, and occasionally red copper cables of half-an-inch only in diameter, but the use of these latter is uncommon.

Fig. 18.

[Fig. 18] exhibits a modification of the point of the terminal rod which is advocated by M. R. F. Michel. The arrangement is based on the principle that, on the approach of a tempest cloud, the more points there are, the greater will be the neutralising effect. M. Michel considers that when a terminal rod has only one point, it acts only in one direction; but if there is a large number of points branching in all directions, the preventive action is materially increased; he therefore proposes the use of this contrivance, which is carried out by having the ordinary conical trunk copper point on the top of the terminal rod melted down, and moulded so that it presents in its middle a circular swelling. Into this swelling arrows are fixed, inclined at each side of the horizontal plane to an angle of 45 degrees, as shown in the engraving. These arrows radiating in all directions are supposed to ‘hasten the neutralisation of the electrified cloud; and in the event of a discharge, the discharge, by dividing amongst them, will prevent their fusion.’

Before quitting the French system, mention should be made of a novel form of lightning conductor devised by M. Jarriant. This gentleman proceeds on the hypothesis that the most essential requisites of a lightning conductor are:—a terminal rod metallically homogeneous, which should rise to a good height; that it be sufficiently light to avoid damage to the roof, and yet be strong enough to resist the violence of the wind. To attain these requirements, M. Jarriant secures his conductor with three or four stays, which are firmly fixed to the roof and converge to the top of the terminal rod, to which is fastened the ordinary copper point, recommended in the ‘Instruction’ of the Academy of Sciences. Iron supports are placed at different heights in order to ensure the perfect solidity of the system. Galvanised iron is employed, and all the various stays and supports are metallically connected with each other. The angles of the irons are all acute, and placed so as to offer the least resistance to the wind. The advantages claimed for this method are that the upper part of the conductor bristles with spikes and aigrettes, which he considers a great advantage in regard to the preventive effect produced by the conductor; it allows of the conductor being raised much higher above the building; it presents a large surface to the electrified cloud; the joints are so arranged that they cannot be dislocated by the expansion and contraction caused by variations of temperature; and, lastly, it is affirmed that these conductors cost thirty per cent. less than those erected under the ordinary system.

America stands pre-eminent above other countries in the numerous and extraordinary schemes that have there been promulgated in regard to the protection of buildings from the effects of lightning, and probably no other nation has been so systematically victimised and swindled in this matter. The tramping ‘lightning-rod men’ of the United States are notorious for extortion and ignorance: they use all kinds of fantastic and peculiar shaped terminal rods and conductors, the main object being to make as great a show with as little metal as possible. Their work is almost entirely confined to the upper portion of the conductor, to the neglect of the most important part—the earth terminal. In consequence, the majority of the lightning conductors in America are untrustworthy; very often they are practically insulated from the ‘common reservoir’ or subterraneous water, and are therefore more often a source of danger than a protection. Unhappily, these peripatetic mechanics are by no means extinct, although increased knowledge is gradually driving them from the field.

In America, a strong point is made of utilising, as far as possible, all the existing natural conductors that are to be found upon a building, such as gutters, rain-pipes, and other metal surfaces. During a tempest, the opposite electricities of the earth and the air often select, by their inductive influence, a rain-pipe, gutter, metal roof, &c., for the passage of an electric discharge between them, and unless these metallic surfaces are connected with the earth, they are apt to be dangerous. But if they are properly connected together, and provided with a good earth-contact, they materially assist to diminish the intensity of a discharge.

In the case of a building with a roof of slate, wood, or other material of low conductivity, a conductor made of either bar iron or stranded cable is placed along the ridge and gable ends, and carefully connected with the gutters and rain-pipes; where the rain-pipes are less than three inches in diameter, the bar or cable conductor is often extended from the roof down the side of the building, and connected with the earth terminal. When this is done, the bar or cable conductor is placed between the rain-pipe and the wall of the building, or at any rate close to the rain-pipe, and connected with it by solder or bolts.