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Table of Contents

Points.

Material for Conductor.

Size of Rod.

Shape of Rod.

(Rods, Tubes, Tape, Rope, Plait.)

Joints.

Protection of Rod.

Attachment to Building.

Earth Plates.

Space Protected.

Height of Upper Terminal.

Testing Conductors.

Internal Masses of Metal.

External Masses of Metal.

POINTS.—Starting with the extreme top, we have first to deal with the question of points. The utility of points was hotly contested rather more than a century since, and an abstract of the discussion will be found in Appendix F, page (79), and difference of opinion still exists as to their precise functions and value. The decision as to the best form of points is complicated by two opposing requirements (1), the sharper the point the more rapid the silent discharge of electricity, and, therefore, the more effective the conductor; but (2) the sharper the point the more easily is it destroyed by oxidation, or fused, should a heavy disruptive discharge fall upon it.

Attempts have been made by the use of gold, silver, and platinum, to obtain a sharp point which should not only be durable, but, owing to its high melting point, resist fusion by a disruptive discharge. But such metals are very expensive, and the statements in Appendix F, pages (67, 69, 73, 103, 123, 128, and 139) prove that even platinum points are often damaged. Copper points whose sectional area is less than ·05 of a square inch are very liable to be melted. Lightning has even fused a copper rod ·10 sq. in. in sectional area, i.e., 0·35 in. in diameter, and there are many rods still standing of which the extremity has been melted into a button or knob.

For these reasons it seems best to separate the double functions of the point, prolonging the upper terminal to the very summit, and merely bevelling it off, so that, if a disruptive discharge does take place, the full conducting power of the rod may be ready to receive it, and, therefore, that there may be no risk of melted particles of metal setting fire to the building, as has occurred. [Appendix F, p. (93).]

At the same time, having regard to the importance of silent discharge from sharp points, we suggest that at one foot below the extreme top of the upper terminal there be firmly attached, by screws and solder, a copper ring, bearing three or four copper needles, each 6 inches long and tapering from ¼ inch diameter to as fine a point as can be made; and with the object of rendering the sharpness as permanent as possible, we advise that they be platinized, gilded, or nickel plated.

Vanes, finials, and ornamental ironwork so frequently form the upper portion of edifices, that it is essential to consider their relation to the conductor. They should always be in perfect metallic connection with the conductor. The possibility of such metal work inducing the charge to desert the conductor for some other path is sometimes suggested, but it could not happen unless the conductor were out of order, e.g., of inadequate conducting power, or had an imperfect earth-contact.

With respect to factory chimneys, a different practice prevails in England from that which is nearly universal on the Continent. In this country one straight rod is usually carried up on one side of the chimney to a height above the top about equal to the diameter of the chimney. On the Continent two arches of iron are put crosswise over the aperture of the chimney, and a vertical rod is carried up from the intersection. In both systems the upper terminal suffers from the corroding effect of the fumes from the chimney. Dr. Mann thought, Appendix F, p. (132), that considering the ready path for lightning afforded by the heated smoke discharged from chimneys, a coronal conductor should be placed upon them, as well as a multiple point. Messrs. Gray say, p. (9): “For high chimney shafts we fit a copper band round the top, and four points thereon connected to main down rod.” The Edinburgh Gas Works chimney, 341 feet high and 14 feet across at the top, was fitted with a conductor under the advice of Faraday, Appendix F, p. (89). It had an iron plate on the top; Faraday directed that the rod should be connected with this plate, and the upper terminal should rise vertically 6 feet above it.

We are of opinion that a coronal or copper band, with stout copper points, each about 1 ft. long, at intervals of 2 or 3 ft. throughout the circumference, will make the most durable and generally useful protector for a factory chimney, but these points should be gilded or otherwise protected against corrosion.

MATERIAL FOR CONDUCTOR.—Iron and copper are practically the only two metals which need consideration; brass, which has sometimes been used is so perishable that its employment is a self-evident error. We will assume the conductivity of equal lengths and weights of iron to be, in the case of steady currents of electricity,⅙th that of copper, and the cost of iron to be ⅑th that of copper, this would make the cost of copper for equal conducting power ⁹⁄₆ths, or 50 per cent. dearer than iron. But there are other matters to be considered: (1) the great weight and bulk of iron rods; (2) their deterioration by rust; (3) the serious obstruction offered by a rusty joint; (4) the suddenness of lightning discharge which modifies the conductivity; and lastly, that iron is so much more rigid than copper that (except in the form of iron wire rope, of which we shall speak hereafter) it can rarely be used in greater lengths than 20 feet, and thus numerous joints become necessary, whereas every needless joint should be avoided.

As regards galvanizing, we think it scarcely judicious to trust entirely to it for protection against oxidation, for many instances of imperfect galvanizing have come to our knowledge.

On the other hand copper becomes brittle, not only when exposed to the air, but also by the passage through it of powerful charges of atmospheric electricity. Franklin used iron, and it is employed in America and on the Continent much more generally than copper, and it is less tempting to the thief.

Nevertheless, as the cost of erection bears a considerable ratio to the cost of the rod itself, and as iron possesses the disadvantages above stated, we think that in all ordinary cases a copper rod will in the end prove the cheapest, as it will certainly be the most durable.

SIZE OF ROD.—This is perhaps the most difficult subject which has to be determined. We greatly regret the shortness of Table I. in Appendix K; but we think that it must be assumed from it that lightning has fused a copper rod ·10 in. (⅒th) in area, i.e., weighing 6 ounces to the foot. We have also the Caterham case, Appendix I, p. (214), where a copper tube weighing 5¾ ounces per foot was heated to redness.

The saving of cost which might be effected by using, for very low buildings, rather slighter rods than for ordinary edifices is not worth considering. In a 30 feet rod it could hardly amount to 10s. We therefore recommend as the minimum to be used:—

Material.	Pattern.	Diameter.	Sectional Area of Metal.	Weight per foot.
		in.	sq. in.
Copper	Rope	½	·10	6 oz.
Copper	Round Rod	⅜	·11	7 oz.
Copper	Tape	¾ × ⅛	·09	6 oz.
Iron	Round Rod	9/10	·64	35 oz.

SHAPE OF ROD.—This depends upon a subject which until lately was warmly discussed, viz., upon the relative importance of the sectional area, and of the superficial area of a conductor; a matter which has been the subject of active discussion among electrical authorities. Faraday and Sir W. Snow Harris, for example, held diametrically opposite views respecting it. [Appendix F, p. (89), and I, p. (195).]

There is abundant and conclusive evidence that in the case of steady electric currents, conductivity depends upon sectional area alone, and not at all upon extent of surface, and experiments by Mr. Preece and Dr. Warren De la Rue tend to show that, in the case of sudden discharges from condensers, to which lightning discharges are probably analogous, the influence of form is not considerable. On the other hand, there is equally conclusive evidence that the facility with which currents of short duration pass through conductors is affected by the form and arrangement, as well as by the sectional area of the conductors. Upon the whole we agree with the opinion quoted below, from a writer recognized in the United States as a high authority on lightning conductors, who, after describing and engraving more than fifty patterns of rods, says^[1]:—

1. Spang, “A Practical Treatise on Lightning Protection,” p. 121.

“The alleged improvements in the said conductors are, in nearly all cases, worthless, or of a trifling and unimportant character. The fact is, the said conductors are quite inferior, and contain no essential improvement upon the ordinary round iron rod used during the days of Franklin.”

In Europe the only forms at all generally employed are:—

Rods (round or square); Tubes; Tape; Ropes (wire, or wire with hemp centres); Plait.

Rods (round or square).—The advantages and disadvantages of rods are easily stated. The advantages are their durability and their rigidity, the latter being of importance for long upper terminals. The disadvantages are the necessity for numerous joints, and the difficulty of avoiding serious disfigurement to the building to which they are attached.

Tubes have much the same merits and demerits, with the additional objection that they are necessarily of larger diameter than solid rods, and therefore more conspicuous. They have also an additional disadvantage in that they are generally joined together by screw collars. The cutting of the thread in the tube seriously diminishes the sectional area, and the joint so made is electrically defective. If tubes are used, the joints should be made as directed in the code of rules under the head of joints.

Tape is a form of rod which is of comparatively recent introduction, and possesses many advantages. Foremost among these is the length which can be supplied in a single piece. Where, as at the junction with an upper terminal, a joint is needed, it is easily made by clamping or rivetting the two surfaces together and then imbedding the whole in a mass of solder. No kind of coupling known to us is, in our opinion, equal to this very simple one. Owing to the flexibility of the tape it can be made to follow closely the outlines of a building, or may be countersunk in it, and painted over, but, as stated further on, abrupt bends should be avoided, and the precautions and instruction set forth on page 18 should be followed. The objections to tape, Appendix A, pages (5) and (16) will be found to be objections, not to tape per se, but to bad practice on the part of some persons who have fitted it up and availed themselves unduly of its flexibility.

Ropes.—For many years past rope constructed of twisted strands of copper or of iron wires has been largely employed for lightning rods. There is on record a very remarkable case of the complete destruction of a brass wire rope, an event which, if it had been repeated, might justly have been regarded as a serious objection to the use of ropes. This case is fully reported in Appendix F, pages (62–63); and from it some French electricians have concluded that lightning may single out some wires from a rope and travel along them in preference to the rest, even when the whole of them are hardly sufficient to give it a free passage. Whatever may have been the explanation, this accident seems to be unique, and even if we accept the explanation given, the only extra precaution which it calls for, is the soldering of each extremity of the several wires forming the rod, and at every joint, into a single mass.

We agree with M. Borrel in thinking that serious evil arises from using wire of too small diameter, which involves an additional number of interstices for the lodgement of dirt, smoke, and water, and at the same time renders the wires too thin effectually to resist oxidation. We have had before us rope ⅜ in. in diameter, composed of 49 strands of a copper wire about No. 19 B.W.G., say 0·04 in. in diameter. On the contrary, one firm speaks of employing No. 10 B.W.G., i.e. 0·14 in. diameter, and in special cases Nos. 8 and even 7, which would be about 0·17 in. and 0·19 in. diameter respectively: these would not be open to the objection we have raised.

The objection to thin wires is necessarily greater with iron ropes, even if galvanized, than with copper, for irrespective of the doubt as to the perfect galvanizing of every part, there is the greater brittleness, and consequent risk of damage from defective continuity.

Ropes with Hemp Centres.—One English firm sent us a specimen of 6–strand copper rope with a hemp core, and we understand that the same pattern is occasionally used both in iron and copper in France. We do not know the precise object aimed at—probably flexibility—but considering the perishableness of such a core, its variation in length with the hygrometric state of the air, and its invariability when the copper is varying with temperature, we cannot regard it as a wise construction.

Plait.—This form of rod was probably designed in the belief that the essential element in a lightning rod was plenty of surface. It is made in two sizes, with copper wire, about No. 16 B.W.G., plaited into a sort of ribbon. It invites oxidation as much as is possible, and is in our opinion neither durable nor trustworthy. The original form of this rod was ridiculously bad; for it consisted of 13 copper wires and 1 zinc one. Every time that it became wet, feeble electric action was set up, and the zinc wire was gradually destroyed, without the slightest benefit to anybody.

JOINTS.—The most fruitful sources of danger in rods are bad joints, not necessarily those that are mechanically bad, but those that are so electrically. A joint is said to be electrically bad when it offers resistance to the passage of electricity through it. There should be no resistance whatever. A careful inspection by Capt. Bucknill, R.E. (Appendix M, p. 243), has proved that bad joints in lightning rods are very abundant, though they appear perfectly sound; and everyone who has measured the electrical condition of conductors confirms this fact. Bad joints have the same effect as lengthening the conductor; and, in one case, one bad joint was found to have the same effect on a discharge of electricity as a conductor 1,900 miles long. It is evident that such rods may be worse than useless, for other parts of the building may offer easier paths for the discharge to the earth. If the joint be imperfect, and the rod convey a charge to earth, heat will be generated at the joint, the rod may be fused, and the discharge be diverted to the building.

Screwed, scarfed, and rivetted joints, however well they may be made mechanically, are certain to rust and corrode in time, owing to the expansions and contractions due to changes of temperature admitting moisture, and thus causing corrosion and resistance. No joint can possibly be electrically perfect that is not metallically continuous, and careful soldering, in addition to screwing, scarfing, or rivetting, is the only certain mode of securing this. Soldering is a method that has borne the test of experience, and its success as a means of securing perfect joints leaves no excuse for its omission. The fewer joints the better, but where there are joints they can only be made electrically secure by careful soldering.

PROTECTION OF ROD.—The lower part of copper rods is sometimes stolen for the sake of the metal. This can be guarded against by putting it inside a length of iron gas-barrel, extending from some distance below ground to 10 ft. above it.

PAINTING.—Iron conductors, even if they are galvanized, should be painted throughout, except at the points, which should be gilded or nickel-plated.

In France and Belgium painting is resorted to to a considerable extent, and the practice was recommended by the late Professor Joseph Henry, and followed very largely in America. [Appendix F, pages (99) and (113).]

ATTACHMENT TO BUILDINGS.—The evidence against the use of glass or other material in order to insulate the conductor, is overwhelming, and insulation may be regarded as unnecessary and mischievous. The essentials are (1) that the rod be attached to the building by fastenings of the same metal as itself, (2) that the fastenings be of adequate strength, (3) that they be of such form as not to compress or distort the rod, (4) that they allow play for its expansion and contraction, (5) that they hold it firmly enough to prevent all the weight falling on any one bearing.

Where practicable it is well to take the rod down that face of the house which is most exposed to rain.

EARTH PLATES.—This portion of the lightning conductor is of the utmost importance, but has hitherto been the most neglected. The majority of cases in which lightning has caused injury very near to or upon conductors are traceable to those conductors having imperfect earth terminals. We know of many cases in which the earth terminals have been miserably imperfect, or entirely neglected, when the above-ground portion has been perfectly satisfactory. In fact, though it may be admitted that the case found by Dr. Mann,^[2] of the lightning rod of a church tower, the lower end of which was thrust into an empty glass bottle, is an exceptionally bad one; yet there are sadly too many, of which the Middlesboro’ case, Appendix I, page (217) is a perfectly fair type.

2. Quarterly Journ. Met. Soc., Vol. II., p. 420.

A convenient earth connection is often afforded in towns by the iron mains for gas and water—arguments both for and against the utilisation of both water and gas mains will be found in the Appendix—we, therefore, need only state our opinion in favour of connection with both. But no connection should ever be made with soft metal pipes, because of the risk of their fusion; and the conductor should be kept as far as possible from internal gas pipes on account of the risk of lighting the gas at an imperfect joint.

As a general rule we advise the soldering of a plate of metal, copper to copper, iron to iron, to the lower end of the conductor. The earth plate should always be of the same metal as the rod, otherwise destructive galvanic action sets in. This plate, which may be flat or cylindrical, must not have less surface than 18 square feet, i.e., 9 square feet on each face; there is no advantage in notching or pointing it. A hole must be dug, or well sunk, to receive this plate, and the hole must be so deep that the earth surrounding the plate shall never be dry. Any available drain or other water should be allowed to soak into the earth, over the site of the plate. After the hole has been dug, and the plate lowered into position, it should be filled with cinders, or coke. In extremely dry rocky localities, it is sometimes impossible to fulfil these conditions: then the best thing to do is to bury three or four hundredweight of iron at the foot of the conductor, still using the earth plate and the coke, and taking especial care that the rain-water and sink pipes discharge over it.

All drains, water-courses, in fact, everything which will assist in distributing the charge over a large extent of moist earth should be utilized by leading branches from the earth plate to them, or a long length of the rod may be laid in a drain if it be one which will be constantly wet.

SPACE PROTECTED.—The question as to the extent of the space which will probably be protected by a lightning rod is one which is of very great practical importance, because it governs the number and height of the upper terminals which are required for the protection of any given building. The index to the Appendix shows that “Protection, Area of,” is discussed upon twenty-nine pages in different parts of the Appendix. It has been laid down that the space protected was a cone, having the point for its apex, and a base whose radius was equal to twice the height of the point, while the latest French official instructions, Appendix F, p. (67), state that a point will “effectively protect a cone having the point for its apex, and a base whose radius is 1·75 of its height.” The English War Department instructions considerably reduce this space by asserting, Appendix F, p. (71), that “no precise limit can be fixed to the protecting power of conductors. In England the base of the protected cone is usually assumed to have a radius equal to the height from the ground; but though this may be sufficiently correct for practical purposes, it cannot always be relied upon.”^[3]

3. On page (96) two instances are recorded in which, if the evidence can be trusted, the stroke fell within a radius equal to the height, but it is only right to say that the facts are not very clearly recorded.

According to this rule, the church of Ste. Croix (see Appendix F, p. (141)), would require four upper terminals, one on steeple, one on chancel, and one in the middle of each half of the transept.

From theoretical considerations stated by Mr. Preece, Appendix F, p. (137), he arrives at the conclusion that “A lightning rod protects a conic space whose height is the length of the rod, whose base is a circle having its radius equal to the height of the rod, and whose side is the quadrant of a circle, whose radius is equal to the height of the rod.”

At present we have not sufficient data to enable us theoretically to calculate the space protected by a lightning rod, and therefore we are compelled to draw up our rules upon the question entirely from experience, and here we find, that with the doubtful exceptions already mentioned, there is no recorded instance of a building being struck by lightning within a conical space, the radius of whose base was equal to its height, and we think that the adoption of this rule may reasonably be expected to yield that security in the future, which as far as we know, it has done in the past.

HEIGHT OF UPPER TERMINAL.—This matter is one which may be left entirely to the option of individual architects and engineers, subject, of course, to the opinions expressed under the heading “Space Protected.” In France extremely long tiges, or upper terminals, generally 33 feet long, are used; but it is obvious that they are necessarily very strong and heavy, and both by their weight and by the great leverage which they exert when there is any wind, they must produce serious vibrations in the roof. In England hitherto the opposite error is almost universal, and we seldom see a conductor carried high enough to protect all the building to which it is attached. The question of appearance comes in here, but concerning it we need only remark that while in England care seems generally taken to conceal the conductors, in France they are, to a certain extent, made features of the edifice. With a proper exercise of taste, the terminals of the lightning conductors can be made to assist the ornamentation of the building, as has been done in many cases.

TESTING CONDUCTORS.—Periodical examination and careful testing of the lightning conductor are requisite to maintain the system in efficient order. Points will corrode from oxidation and fusion; joints will get loose and bad through the action of weather and workmen; connections will decay both above and below ground; imperfections will develope themselves; alterations will be made by landlords and tenants; and, in spite of every precaution during erection, the conductor will thus lose its efficiency if it be not maintained in thorough order. For this purpose inspection should be both visual and electrical. In order to facilitate the electrical examination of the conductor, some firms have erected a double rod, connected with one upper terminal, one on each side of a chimney or shaft; this is a very efficient arrangement, for it provides a means for testing from the ground. It has also been proposed to carry an insulated wire alongside or even within the rod, connected to the terminal at the top, and to the testing apparatus at the bottom.

A testing apparatus has been devised by Mr. Anderson (Lightning Conductors, p. 60). M. Borrell, Appendix K, p. (226), Captain Bucknill, R.E., Appendix M, p. (244), and Mr. Vyle, Appendix M, p. (244), have also introduced apparatus for the purpose. The system in use in Paris, Appendix K, p. (225), and M, p. (245), is perhaps the simplest and cheapest, and is effective as regards testing the efficiency of the conductor, but not that of the earth connection.

The efficiency both of the conductor and of its earth terminal should be annually tested. As this testing involves some skill and familiarity with electrical apparatus it would be advantageous if some competent person were officially appointed, either by the government or by some recognised authority, to perform this duty.

INTERNAL MASSES OF METAL.—All large and long masses of metal, such as beams, girders, pipes, hot water systems, and large ventilators fixed in the interior of buildings, should be electrically connected with the earth, or with the conductor; but the soft metal gas pipes should never be used as conductors. The inlet and outlet pipes of large meters should always be, independently of the meter, electrically connected with each other, for two remarkable cases of the explosion of a meter have occurred through the presence of a joint in the pipe electrically bad owing to the use of India-rubber packing. Appendix M, p. (239).

EXTERNAL MASSES OF METAL.—Large constructive and decorative ironwork, such as guttering, flashings, railings, finials, vanes, &c., and all masses of metals used in building, should be connected to each other, and to the earth direct, or to the conductor. In fact, the gutters and water pipes are already frequently utilized as a partially protective system. The ventilators of soil pipes may also be employed in this way, and even made sightly by the addition of an ornamental finial fitted with points, but care must be taken that the joints are metallic and not made with red lead or putty; and it must not be forgotten that the conductivity of lead is very small, so that undue reliance must not be placed upon pipes made of that metal.