The French method of carrying the conductor to the bottom of a neighbouring well is frequently adopted where it is practicable, and the water of the well is not required for drinking or cooking purposes.
A few words may be added here on the method of protecting the large mineral oil tanks which are to be found in the United States. Many of these oil tanks are of very large capacity, some of them containing a million gallons of oil. They are generally constructed of thick iron plates rivetted together. The roofs are usually made of wood coated with tar, but in some cases iron is adopted. As a rule, several of the tanks are grouped together and connected with each other—and in some instances with distilleries—by means of subterraneous iron pipes.
One method of protecting these tanks is to erect around them, at a distance of some ten feet, wooden supports, on which are placed upright metallic conductors which overlook the tank, and are connected with each other near their tops by stout iron wires, thus forming a network of conductor which is supposed to intercept any discharge of electricity from a tempest-cloud, and prevent it from reaching the oil tank. This method, however, has failed in several notorious instances, and is not countenanced by the best authorities.
A better and less complex arrangement is now usually adopted by the best firms. The chief object of this arrangement is to prevent the temperature of the oil tank, and of the atmosphere above and around it, being raised by means of an electric discharge. This is accomplished by using large conductors, which are carried some distance above the oil tank. These conductors, of which there should be at least four, are formed of flat iron bars about one and a half inches wide and half an inch thick; they are securely fastened to the sides of the tank at equal distances from each other, and metallically connected with it. About thirty feet above the roof of the tank they meet, and are carefully and substantially joined together, and supported, if necessary, by a wooden post extending from the centre of the roof of the oil tank.
The earth terminals, of which there must be one to every two conductors, consist of perforated iron pipes, as before described, three inches in diameter and fifteen feet long. They are sunk into thoroughly moist earth, and metallically connected with the lower part of the tank. These perforated pipes are so arranged that they catch the rain water from the roof of the tank; by this means the surrounding earth is kept moist. It may be mentioned that by utilising the tank as a portion of the system of conductors, the electric discharge is distributed and much weakened.
CHAPTER XIII.
LIGHTNING PROTECTION IN ENGLAND.
In its essence there cannot be anything more elementary than the theory of protection against lightning. It is simply to lay a metallic line from the top of a building, or other object to be protected, into moist ground, so as to make a path for the electric force, along which, not finding impediments, it will travel freely, without causing the least damage. But, like many other simple theories, their practical execution is not without perplexities. The first of these, in regard to conductors, arises from the existence of more or less considerable quantities of metals, to be found in almost every building which requires protection against lightning. As the use of metals, especially iron, in the construction of dwellings, both exterior and interior, is rapidly extending, this becomes a very important consideration in planning the design of lightning conductors. Of equal moment is a second point—that of the existence of water or great moisture under the buildings, or part of them. This must decide invariably the direction of the conductor towards the earth, and its depth underground. There are many minor matters to be taken into account, but these two may be laid down as the chief questions to be kept in view in settling the best mode of application of any conductors under given circumstances. It happens often enough that a proper solution as to what is best is not a little difficult. Still, it can always be arrived at by careful study, which must, however, be aided by experience.
Keeping always in view the fact that there is nothing whatever that may be called ‘erratic’ in the manifestations of the electric force, but that it acts under a ruling principle as strict as that governing the law of gravity, the first point in designing the protection of any building will be to clearly ascertain what path the lightning will take on its course from the clouds to the earth. It is absolutely certain that the electric force will make its way through materials, termed good conductors, which allow it free passage, and avoid those of the opposite class, or bad conductors, the character of every substance on earth being well known as regards these qualifications, although it would not be easy to draw sharp lines of demarcation, all conductivity being relative and not absolute. Looked at in this way, the fundamental one in the application of lightning conductors, the simplest object for protection will be a pyramid of stone, such as the Egyptian obelisk, popularly called ‘Cleopatra’s Needle,’ erected on the Thames Embankment in the summer of 1878. Stone being a bad conducting material, all that is necessary to protect it against lightning, provided there is no metal whatever near it, is to run a thin strip or rope of copper from the summit to the base, and down into moist earth. Although fragile, the strip of copper, if uninterrupted and rooted in moisture, will in this case form an absolute protection. The question assumes another aspect if, instead of a stone pyramid, a tall factory chimney, not very dissimilar in outward form, is given as an object for protection. Here there enters another element. A tall pile of bricks is as bad a conductor of electricity as a solid mass of stone, but the mass of bricks constituting a factory chimney is hollow, and the cavity being filled with smoke and mineral fumes, which are more or less good conductors of the electric force, the artificial path laid for the free passage of lightning has to surpass in acceptability the natural one. In other words, the copper rod laid alongside the factory chimney, to secure it against damage from lightning, must be considerably thicker than the one which will protect the simple stone pyramid. It is this principle which has to be followed all through in the application of conductors. They must form, in one word, the best path which can possibly be made for the electric force.