PROTECTING WOODEN ROOFS
If a metal roof is out of the question, the protection of the wooden roof must be provided for. Very little attention has been paid in this country to the proper erection and maintenance of lightning rods. It is not sufficient to put up a point in an out-of-the-way place, and with a careless ground connection, and then expect immunity from lightning. The lightning rod will protect a wooden-roofed building if it is properly installed; and in order that this simple but important piece of apparatus be thoroughly understood it will now be considered in detail.
In the first place, it should be noted that there are two forms of electric discharge or lightning which are provided for in equipping a building with lightning protection: the brush discharge and the disruptive discharge. The brush-form is so named because the fine streamers of sparks which are emitted have somewhat the appearance of a brush. This discharge is harmless, and one of the important functions of the bunch of points on the upper end of the lightning rod is to quietly take from the surrounding atmosphere the electricity there generated, and thus prevent its accumulation to a dangerous extent. Very high towers, such as steel windmills, high trees, and steeples do the community a good service in this respect. But sometimes the discharges cannot be dissipated through the brush form, but reach a high pressure, and exhibit themselves with great violence, producing the booming and crackling noise of thunder. This is the second form; and although the points may be useful in this case too, yet if they are too far apart the discharge may not seek them, but may take a shorter path through the moist hay from which the hot, damp air is rising to the roof and forming another lightning conductor. Protection from this can be partly provided by the use of several points, not over forty feet apart; but in cases in which lightning is very violent and frequent, the conductor should be run all around the edges of the roof, and in several places to the ground.
An experiment made by a noted electrician some years ago will illustrate this point: A frame was made of iron wire in the shape of a barn, the wire representing the edges of the walls and roof. The frame was connected to the ground, or “grounded,” as the electricians say, and then artificial lightning was allowed to play upon it from a distance of a foot or more above. This gave a model about in proportion to the real barn and actual lightning. All the discharge followed the wire frame, and did not ignite a dummy of gun-cotton which was placed inside. The instant that the metal barn frame was removed the dummy was struck and burned violently. One can draw his own conclusions from an experiment of this sort.
Fig. 134. Proper adjustment of lightning rods on a barn.
A barn properly fitted with lightning rods is shown in [Fig. 134]. The location of the points is such that there is not more than forty feet between two adjacent ones. The rod projects about six feet above the roof, and these projections are all connected by means of rod of the same form as the vertical conductors. Sharp turns are avoided in erecting the conductor, for an electric discharge would prefer to go straight through the air rather than turn a corner.
It will now be necessary to go into some practical details of the construction of lightning rods, and the suggestions that will be made have been included here because good points or rods may not always be readily obtainable. Their manufacture is easy and can be performed with the limited facilities of a small village. If the raw materials have to be bought at a distance, this can be easily done by correspondence.
Parts of the system: The equipment will consist of three parts—the conductor and its support, the points, and the ground connection.
