Lightning Rod Conference. VariousЧитать онлайн книгу.
electrically for their resistance.
But it has been pointed out by the late Mr. Brough (Phil. Mag., May, 1879), that by regarding (1) the influence of the rise of temperature, (2) the difference between the specific heats, and (3) the relative dimensions, iron conductors can be made much smaller than was formerly supposed: and, that as iron is so much cheaper, iron rods can be made equally efficient for a much less sum than copper. Moreover, the use of iron enables the architect to use one kind of metal throughout his structure, and thus avoid anywhere the contact of dissimilar metals, which always results in decay.
On the third point, the writer is clearly of opinion that a galvanized iron rope is amply sufficient for country residences and buildings free from chemical actions. In such places, and in towns, copper should be used. A rope, whether of iron or copper, is easily handled, it can be made of any size, it can be led in any direction without bends or angles, it is neat and easily jointed, diverted, or lengthened.
The writer refrains from expressing any opinion on its dimensions here, for this is a point that will require most careful examination by the Conference.
3. Is there any definite proportion between the length and sectional area of the conductor?
The majority of the manufacturers increase the size of the conductors for high buildings—one making the limit 120 feet, another 150 feet, while a third “varies the sectional area with the length.” One firm does not consider any difference necessary, while another takes it that the sectional area should be the same irrespective of length, for “lightning does not vary in intensity while passing through a conductor of greater or less length.”
Now, the laws of electricity clearly show that to maintain equal efficiency we must vary the sectional area as we increase the length of the conductor; but it is a question for the Conference to decide whether we should not recommend a rope of uniform dimensions that would be equally applicable for high and low buildings. Within ordinary limits the necessity for increased thickness for increased height is scarcely evident, but the remedy of an increased sectional area, with the number of separate points erected, is very clear. Indeed, each point should be the terminal of a conductor, whose sectional area should be uniform to the earth. For if it be not so, and each conductor be fully charged with electricity, then when the sectional area diminishes there will be congestion, resulting in heat and discharge to the building. Hence the thickness of the main conductor must increase with the number of separate points erected.
4. Joints, how made.
Some are rivetted, others are screwed, others are coupled by right and left-handed screws. Tubes are socketed into each other. In one case “the end of the conductor is knotted and drawn through a cup-shaped ring of metal.”
There can be no doubt that joints are the greatest source of danger in lightning conductors. If a joint be imperfect, and the conductor be conveying a charge to earth, heat will be generated there, the conductor may be fused and rendered useless, and the discharge will be diverted to the building. Or the joint may be so bad—that is, its resistance may be so great—that it renders the conductor practically useless, for other parts of the building will offer easier paths to the earth. Though the use of solder is pretty general, it is not universal. Indeed, one manufacturer objects to it because “it must interfere with surface conduction!” It certainly should be imperatively used. No joint can possibly be perfect that is not metallically continuous. Careful soldering is the only certain mode of securing this, and that this is practicable is evident from the millions of perfect joints in telegraph wires. To scrape the ends of wire bright, and cover the whole with thin sheet lead, as is done by one firm, is simply to court danger. The absence of joints in wire rope is one great element in its favour.
5. Attachment to building, how made.
Some attach the conductor to the building by copper straps and nails; some use holdfasts, either of copper wire or gun metal; others use staples; one uses metallic ties. Several pass the conductor through insulators of glass, porcelain, or earthenware. But the majority discard insulators as useless.
In the opinion of the writer they are quite right, for it is difficult to understand what useful function the insulator performs. One fact that occurred in 1837 is given as a reason for their use, but the fact militates against the efficiency of the conductor rather than the absence of insulators. If the conductor were perfect there could have been no concussion at the point of attachment. If it were imperfect there may have been, for the discharge would seek other paths to earth. Some manufacturers use holdfasts of a different metal to that of the conductor. This is wrong, for where different metals are used galvanic action sets in, tending to decay and rupture. The attachments for this reason should always be of the same metal as the conductor.
6. Ground connection, how formed, and of what extent.
The necessity for reaching moist ground is generally recognised, but various curious ways for making earth connection are suggested. One firm considers that a band cut into strips 18 inches long would suffice, while another says that not less than 30 feet, in two or three branches, with fork at the end of each band, should be used. One firm is very brief: “Ground end is coiled loosely in damp earth or a well.” The use of coke, powdered charcoal, or carbonaceous materials, is insisted upon by others.
It is questionable whether the difficulty of fitting a good connection with the earth is fully realized. None but telegraphists know the great difficulty there is in doing this. The first object to be secured is a good damp soil, and the next as large a conducting surface as possible. Metal pumps, iron, gas, or water pipes, wells in which plates of metal 2 or 3 feet square are placed, or similar plates may be buried in perpetually damp ground, or in holes well filled with powdered coke. Moisture in some form is essential, and without it a lightning protector is of small service.
7. Extent of area supposed to be protected.
The majority of the firms consider that the area protected has a radius equal to the height of the conductor; but one firm considers that this should be multiplied by five or six times; while another asserts, “that no appreciable extent is protected by a single rod conductor;” and another, that “many instances may be related of buildings being struck much within the radius of well protected churches or chimneys.”
We have no experience at present to enable us to form a definite opinion on this point. The Committee of the French Academy, gave the radius as equal to twice the height of the conductor from the ground, but buildings have undoubtedly been injured within this limit. The writer does not think that a greater radius than the height should be taken: but thinks that this is one of the most important questions that the Conference could determine. Calculation might, to a certain extent, settle the point: but it is more a case for experience.
8. If there is more than one terminal, is the size of the conductor increased?
This question has been partially considered. (See No. 3.)
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