Fig. 135. Supporting a rod.
The conductor, or so-called “rod,” first demands attention. All metals conduct electricity to some extent, but certain ones are very much better than others. For example, lead, platinum, brass, and iron are poor conductors, which is equivalent to saying that they heat up readily on the passage of an electric current. On the other hand, silver, copper, and aluminum are good conductors. In making a lightning rod, the best all-round conductor should be used, when cost and conductivity are the basis for the selection. As an example, take the metals iron, copper, and aluminum for comparison. Iron is cheapest in price per pound, but its electrical conductivity is small, while copper, though more expensive, has so much more conductivity that to get rid of a certain charge of electricity requires much less of it. So with aluminum, which has slightly less conductivity and which costs more than copper, but which is so light that a rod having the same conducting ability when made of this metal actually costs less than one made of copper, and the price of aluminum is constantly lessening, while that of copper cannot fall much on account of the limited supply. To compare actual figures, call the conductivity of copper 100, then that of steel or iron will be about 18, and that of aluminum about 60. As to relative weights, copper weighs about 550 pounds per cubic foot, iron or steel 480, and aluminum 160. As the prices of these materials are constantly varying, it would be impossible to say at this time what the relative costs would be at any other time; but it can be said that on the score of cost there is little choice among them. For a number of reasons aside from cost, copper is at present the best material, and these reasons are: That it is smaller than the others for a given conducting ability, and thus is more sightly; that it is easier to support on account of this small size, and that it can be readily soldered to the ground plate, which will be considered later.
In addition to the material of the lightning rod, its form is a matter of considerable importance. The cable forms have been used extensively and successfully, but the ribbon or flat form is better on account of the smaller cost, and because there is a greater area exposed for the dissipation of the heat generated by the lightning in passing from the points to the ground. A rectangular section of three-quarters by one-eighth of an inch is recommended.
In supporting the conductor from the wall or roof, it should be separated or “insulated” from these surfaces. There is a slight chance that the lightning might leave the conductor if the building were wet. A more important reason for the use of the insulator is that the heat which is generated on the surface of the rod when a heavy discharge occurs will not be able, if supported away from the wall, to heat up any inflammable material near it. [Fig. 135] shows a method of support in which one of the standard insulators used in running electric light and other wires is employed. These insulators, which are made of porcelain and iron, can be screwed into the wood or into a plug driven into the joints between the stones very readily. The insulator shown is manufactured by the General Electric Company, of Schenectady, New York, and similar ones are made by other manufacturers of electrical materials.
Fig. 136. Efficient points for a lightning rod.
In order to attract the discharge, the rods must project some distance above the roof, about 6 feet being the proper height. This projection must be supported, and there are two ways to do this. The first is to screw or nail a piece of timber to the side of the building, projecting about 5 feet above the roof. Two insulators on this will provide the necessary support for the rod. As this might be considered unsightly in some places, a neater but more expensive method is to use a piece of ³⁄₄-inch copper, brass or iron rod for the upper 9 or 10 feet of the rod. This can be easily joined and soldered to the copper ribbon and is strong enough to support itself in any wind. A brace from the vertical to the horizontal rod will provide additional support if desired, and will give a more substantial appearance. At the point at which the horizontal rod passes through a timber support, in case such plan is used, a hole 1¹⁄₂ inches should be bored in the timber to avoid any risk of its being burned. In joining the horizontal to the vertical rod, the former should be bent up at right angles for an inch, and the surfaces should then be well cleaned and soldered.
The points for attracting the discharge should be made very carefully, and with a view to accommodating the brush discharge particularly. As a rule, the more points in the bunch at the head of the rod the better will the brush discharge be attracted; and for the same reason these points should be sharp and bright. These facts have been determined by experiment, from which it has been learned that the discharge is quieter and at a lower pressure from sharp, bright terminals than from others. Aluminum wire fulfils the requirements for the points better than any other metal of reasonable cost. Unfortunately this metal is difficult to solder, but if the directions here given are carefully followed there will be no difficulty in producing a good bunch.
The sketch ([Fig. 136]) shows the general construction. In the end of a block of copper of the dimensions shown, drill a hole ⁵⁄₈ of an inch in diameter and 1 inch deep. Cut off a number of pieces of aluminum wire, of about ¹⁄₁₆ of an inch in diameter, about 4 inches long. This wire can be obtained from the Pittsburg Reduction Company, of Pittsburg, Pennsylvania. These wires must then be filed to sharp points on one end, the opposite ends being roughened with coarse sandpaper. Push as many of the wires into the hole in the block as it will hold and bend the points back so as to form a brush. Now heat some solder in a ladle and pour in around the lower ends of the aluminum wires, having first taken the precaution to heat the copper block so that the solder will flow well. The conductor rod is then soldered into a slot filed in the lower end of the block, and the bunch of points is complete.
The ground connection is the most important part of the whole equipment. With poor ground connections, the rods become a menace to a building rather than a protection. Examples could be cited where buildings were actually struck and destroyed, even though “apparently properly rodded.” In one case the wire entered but two inches into dry soil, while in another the lower end was buried in concrete. It is absolutely essential that the lower end of the rod be connected with moist earth in some way, as this is the only method which will insure safety. If there are water pipes in the building, they should be attached to the rod in the basement in addition to the main ground connection.