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.

The system employed by Mr. R. S. Newall, F.R.S., for the construction and erection of lightning conductors is probably the most complete--and certainly the most representative--of the various methods in vogue in England. The special study Mr. Newall has made of the subject in all its bearings, both theoretical and practical, added to the fact of his possessing at his extensive cable works at Gateshead such exceptional facilities for the production of copper ropes and bands composed of the purest metal, render him one of the first authorities on all matters connected with the application of lightning conductors to buildings. In describing, therefore, the English method, reference will chiefly be made to this gentleman’s apparatus and inventions.

The function of a lightning conductor is twofold. In the first instance, it operates as a medium by which explosions of lightning, or, to speak more accurately, disruptive discharges of electricity, are led to the earth freely, and without the risk of their acting with mechanical force, as they invariably do when compelled to pass on their way to the earth through so-called non-conductors, that is to say, bodies possessed of low conductivity, such as the atmosphere, wood, stone, &c. In the second instance, the conductor acts as a means whereby the accumulation of electricity existing in the atmosphere is quietly drawn off and carried noiselessly into the earth, and dissipated in the subterraneous sheet of water beneath it. Now this accumulation of electricity, always greatly intensified during a thunderstorm, invariably seeks the easiest road to earth; this road is technically called ‘the line of least resistance.’ This line of least resistance is influenced by various circumstances; the resistance of any line may be lessened by the presence of streams of warm vapour or rarefied air such as would come from chimneys, from barns or stacks containing new hay; by a column of smoke, or by the presence of tall trees moist from rain. It is not always easy to find the reason why the lightning takes any particular path, but one thing is certain, that is, it acts under certain fixed principles, and does not take any particular route by chance, but always because it is the line of least resistance. What the lightning conductor really does is to prevent the possibility of an electric discharge within a certain district, for instance, in the interior of a house or other building.

From the above remarks, it will easily be seen that lightning conductors should be made of materials possessing the highest possible power of conductivity, and be large enough to carry off the heaviest electric discharge that is ever likely to fall upon them. The various metals being by far the best conductors of electricity, it follows that the lightning conductor must be constructed of metal of some kind. But even metals differ to a great extent in their conducting powers, as has been shown in a previous chapter. There are, however, only two metals which are practically available for use as lightning conductors, namely, iron and copper, and after repeated experiments Mr. E. S. Newall has arrived at the conclusion that a conductor made of copper of adequate size is the best--and, in the end, the cheapest--means of protecting buildings from the effects of lightning. The relative conductivity of iron and pure copper being as six to one, it follows that if a copper cable or bar of a given size be sufficient, an iron cable or bar ought to weigh six times as much per lineal foot in order to be equally safe. It may be added, that while copper is more expensive, weight for weight, than iron, it is not so liable to oxidise; nor, on account of its higher conducting power, is it so easily fused. The comparative smallness of its mass renders it far more manageable than iron, and does not interfere with the architectural features of the building on which it is used. On the contrary, it is readily adapted to curves and angles.

It may therefore be taken for granted that, almost without exception, pure copper is the best material that can be used in the construction of lightning conductors.

The size of the terminal rod or point used in Mr. Newall’s method varies in length and diameter according to the extent and height of the building to be protected. As a rule, they are from three to five feet in length, and from five-eighths to three-quarters of an inch in diameter; at the upper end they branch out as shown in fig. 19.

In conjunction with this terminal rod a short description of the ‘Auffangstange,’ or ‘reception rod’ of the Germans, may be given. This ‘reception rod’ (see fig. 21) is made of iron, and varies in length from ten to thirty feet. It consists of two parts, the higher part, which measures two-thirds of the whole length, is fastened by a flange to the lower part of the rod. In fixing this German ‘reception rod,’ its height and weight have to be taken into consideration. It is generally made fast by two strong staples, _b_ and _c_, as shown in fig. 20, which pass through the king post of the roof and are fastened behind by screw-nuts. The part marked _d_ rests in the lower ring _c_ so that it cannot sink, and the extreme end passes through this ring _c_ and is screwed tightly to the nut _e_; _f_ is a cap to prevent the rain getting into the roof.

It is much to be regretted that not only professors and amateurs studying the manifestations of the electric force, but even learned societies, such as the French ‘Académie des Sciences,’ should have spread so many imaginative theories about this ‘reception rod.’ At the bottom of all was the fancy, not often declared, but still visible in its expression, of the metallic conductor possessing some occult power of _attracting_ lightning. In France, as well as in Germany and Italy, there existed for a long time, and to some extent still exists, quite a mania for erecting huge rods, such as that shown in the engraving (see fig. 21), on the top of buildings, the general belief being that the more high-towering the greater would be the ‘area of protection.’ A little common sense, brought to the aid of fanciful imaginings, should have taught the supporters of this ‘area-of-protection’ theory that it was absolutely untenable. The electric force, seeking its nearest path to the earth, could not be expected to diverge from it through the action of a rod raised somewhat higher than the surrounding building; and the proper method clearly was to bring the metal everywhere as near to any possible emanation of the force, whether lateral or vertical, as could be done. Besides being really of no use, except in rare instances, such as the neighbourhood of high trees, these tall rods formerly employed, and still frequently seen on the roofs of buildings, had the detriment of being unsightly, while at times they were positively dangerous. Instances occurred in which a high wind threw them down from their elevated position into the road below, on the heads of passers-by. Thus two persons were killed in Paris in the summer of 1830 by the fall of a gigantic ‘tige’ from the steeple of the church of St. Gervais. Either at the same moment, or immediately before, a stroke of lightning fell upon the church in its lower part, away from the conductor, making a hole in one of the walls, and then escaping, without doing further damage, by some iron water-pipes running underground. The conductor in this case had been constructed on the model approved by the ‘Académie des Sciences,’ but the accident conclusively showed that there was no trust to be placed in any mere theoretical calculations as to the extent of the ‘area of protection.’

A noteworthy example of the fallacy of the ‘area-of-protection’ theory is to be found in the case of the explosion at the powder magazine at the Victoria Colliery, BurntclifFe, Yorkshire, which was struck by lightning and destroyed on August 6, 1878. The instance is also instructive as showing how important it is that copper conductors should possess the highest possible conductivity--i.e. be made of the best and purest copper.

The magazine was an oblong building of brick, nine feet long, five feet wide, and six feet high (internal dimensions), and it had a uniform thickness of three bricks. At one end was a heavy iron door, and at the other a lightning conductor, consisting of a copper-wire rope seven-sixteenths of an inch in diameter. The point of the terminal rod was about thirteen feet above the top of the building, and a similar length was carried into the ground and terminated in clayey soil. The conductor was fixed to a pole distant about two inches from the end of the building opposite to that in which the iron door was fixed. _It was not connected with the iron door in any way._ At the time of the explosion the magazine contained about 2,000 pounds of gunpowder.

Major Majendie, H.M.’s Chief Inspector of Explosives, in his official report ascribed the accident to the fact of the iron door being unconnected with the lightning conductor, and in doing this he was doubtless right, but only to a limited extent. The author of this work visited the colliery shortly after the explosion, and found that the conductor--the weight of which was about one pound per yard--had been fastened to the pole, which was about twenty-one feet high, by two glass insulators, and that the conductor was not connected with the building. On testing the copper rope which formed the conductor, its conductivity was found to be only 39·2 instead of 93 or 94 per cent. The conductor, therefore, was but little better than if it had been made of iron, and, even supposing it had been made of good copper, it was of too small a size. It should have been of double the weight, and _not_ insulated from the pole. In order to be thoroughly efficient it ought to have been brought down the pole, carried through under the roof, down the iron doorpost, and so into the ground.

According to the French theory, that the ‘area of protection’ afforded by a lightning conductor is the space contained within the circular area of a radius double the height of the conductor, the magazine was thoroughly secured, for the conductor was twenty-one feet high, and the building only nine feet long, five feet broad, and six feet high. This case, however, with many others, entirely controverts this theory, and shows very forcibly the fallacy of an argument that at one time was accepted almost as an axiom.

One other case of more recent date may be instanced. At Cromer, in Norfolk, the church--a fine perpendicular building of flint and freestone, having a tower 159 feet high--was damaged by lightning in August 1879. During a thunderstorm the lightning struck one of the pinnacles with considerable force, although on another pinnacle, only twenty-seven feet six inches distant, a good copper conductor, having a diameter of five-eighths of an inch, was fixed. On testing the conductor by means of a galvanometer, it and the earth connection were found to be in thoroughly good order. After what has been said, comment on this last example is needless.

The general disposition and adjustment of a lightning conductor demands the greatest care and consideration. No hard and fast rules can be laid down, for each individual case must be studied and elaborated by itself, especially in the instance of large structures, where much depends upon style, outline, and other details. The main point is that _every_ part of the building shall be placed beyond the possibility of being damaged by a disruptive discharge of electricity.

It has been stated previously that the lightning invariably follows the line of least resistance, and that this line may be influenced by the presence of streams of warm vapour, columns of smoke, &c., which, escaping into the air, furnish a ready path for the electric discharge. Consequently it sometimes happens that a building or barn may be struck although it be provided with a lightning conductor. In order to explain this it must be borne in mind that the line of least resistance is not always the shortest line mathematically. The accompanying illustration (fig. 22) is an example to the point. It represents a barn furnished with a lightning conductor and filled with new-made hay, which is a better conductor of electricity than the material of which the barn is constructed. This hay is giving off the stream of warm vapour which is pouring out of the opening at the end, and forms an invisible band of conducting matter between the thunder-cloud and the barn, as marked out in the engraving by the dotted lines, the direction of the wind being shown by the arrow and the trees. Under these circumstances the discharge of lightning would naturally follow the path between _c_ and _d_ in preference to the shorter route between _a_ and _b_, because the former is the line of least resistance between the cloud and the earth. Thus the barn--although furnished with a conductor in good condition--would most likely be set on fire, or otherwise damaged. The same deflection of the lightning-stroke might be caused by a column of smoke, or by the fact of one portion of the building being moistened by the rain and the other kept dry; an occurrence that might easily happen when a strong wind is blowing during a storm.

In order to ensure complete protection, the conductor on the barn should have been carried along the ridge and down the edges of the roof at each gable. By this means the stroke of lightning would have been intercepted.

The engraving on the next page shows a design for the protection of a large detached mansion by means of a multiplication of short points or terminal rods fixed on all the prominent features of the building. The conductor is carried along the ridges in every direction, and down the edges of the roof at each gable. Generally it is sufficient to have two descending conductors, but occasionally the conformation of the building or the nature of the ground renders necessary the use of even more. It is imperative, for obvious reasons, that the descending portion of the lightning conductor shall be carried from the roof to the ground by the shortest possible route, and placed in perfect electrical contact with the earth in the manner to be indicated in a succeeding chapter.

The projecting points of the conductor are drawn in fig. 23 larger than they need be, in order to show them more clearly, distinguishing them from the rest of the building. The same has been done with the copper rod, running from the roof to the ground and thence into the earth. In reality a conductor may be made perfectly safe, and yet all but invisible to the naked eye. For private houses and buildings, a rope made of copper ought to be at least five-eighths of an inch in diameter, for a copper rod of half an inch in diameter has never been known to be fused. For chimneys of manufactories, where gases are liable to corrode the rope, it had better be a little thicker. Such copper ropes as those manufactured by Messrs. R. S. Newall and Co., five-eighths of an inch in diameter, weighing two-thirds of a pound per foot, and having a conductivity of 93 per cent., have never been known to fail in protecting even the largest buildings. It is supposed by some writers that the value of the conductor is in proportion to the amount of surface of metal exposed. This, however, is a mistake, for the conductivity depends on the weight per foot of metal used, the purity in both being equal. Wire-rope is used simply because it is so pliable that it is easily handled, and can be made of any length required without joints.

In fig. 24 is given an illustration of a small detached house, in which the arrangement of the lightning conductor is indicated by the dark lines. The method followed is exactly the same in principle as that employed for the mansion just described. A terminal rod is placed upon each chimney. These terminal rods are connected with each other by a copper-rope conductor which is carried along the ridges and gables of the roof, thus constituting a similar arrangement to the French ‘ridge-circuit’ (_circuit des faîtes_), with the additional advantage of being far lighter and more sightly. The copper conductor descends to the earth down the angle formed by the projecting entrance to the house. By this means every corner of the building is protected; an important matter in all detached buildings, and especially when they happen to stand among trees. The preference of the electric force for trees as its path to the earth in the absence of metal or other bodies of higher conductivity than trees, has probably no other ground than their being full of moisture; still this is a disputed question.

Fig. 25 exhibits a slightly different method of arranging the lightning conductor. In this case the ridges of the roof are surmounted by ornamental iron-work, instead of the usual terra-cotta, or earthenware, tiles. This iron-work is utilised and carefully connected with the conductor. The chimneys, in place of being fitted with terminal rods, are provided with cast-iron caps--as shown in the engraving--to which the conductor is attached. The conductor, after descending to the ridge, is led along it and down the edges of every gable, and is finally carried down to the ground and connected with the earth in the usual manner. It is of course absolutely necessary that all masses of metal, such as gutters, waterspouts, rain-pipes, &c., should be brought into connection with each other and with the conductor, in order that the house may constitute one electrically homogeneous body.

It was for a long time held that the protection of churches against lightning offered special difficulties. This arose mainly from the constant reports of churches being struck, often when they were believed to be protected, whereas the accidents arose from the conductor not being properly fitted. It is even now too often forgotten that all so-called ‘conductors’ of the electric force are only so in relation to ‘non-conductors,’ and that, strictly speaking, all things on earth are to some extent conductors and to some extent non-conductors. This being kept clearly in view, there is no more difficulty in protecting the largest cathedral against lightning in the most efficient manner than in similarly guarding the smallest cottage.

A case in point occurred in May 1879. The steeple of the church at Laughton-en-le-Morthen was struck by lightning and damaged, the lightning conductor being thrown down and broken into two pieces. A correspondence on the subject ensued in the _Times_, and Mr. R. S. Newall had the remains of the conductor examined, with the following result:

‘The spire is 175 feet in height, and it had attached to it a thin tube, made of corrugated copper, about seven-eighths of an inch in external diameter and five-eighths internal. The copper is about one-thirty-second of an inch in thickness, and it weighs about one and a quarter pound per yard. It is made in short lengths, joined together by screws and coupling pieces, but there is no metallic contact whatever between the pieces, which are much corroded.

‘The conductor appeared to be fastened to the vane. It was not in contact with the building, which it ought to have been, but it was kept at a distance of about two-and-a-half inches from it by twenty-one insulators. The earth contact was obtained by bending the tube and burying it in the ground at a depth of from six inches to eighteen inches, the soil being dry loose rubbish; the length of the earth end was only three feet, with two short pieces of about a foot in length each tied to the tube by thin wires, thus forming altogether a most inefficient conductor. It was placed in a corner formed by a double stone buttress, which came between the conductor and a lead-covered roof attached to the spire, the distance between the conductor and the lead roof being about six feet six inches.

‘The lightning appears to have come down the conductor a certain distance, and, finding the road to earth bad, it passed through the buttress, dislodging about two cart-loads of stone, and then came down the cast-iron down pipes leading from the lead-covered roof and so to earth.’

Mr. Newall, in writing to the _Times_, goes on to say:—

‘Now if the conductor had been made of copper-wire rope, weighing about two pounds per yard, and fixed in contact with the spire, without insulators and with a proper earth contact, no damage whatever would have been sustained by the building; and if the conductor had been tested periodically by an expert he would have shown whether the conductor was good or useless. This examination ought to be insisted on, as the earth connection is often wilfully destroyed; but I have never in all my experience known a building which had a conductor properly fixed to suffer damage from lightning.’

What is really required is to make a lightning conductor of sufficient calibre to carry down the electric discharge, however great it may be, from the summit of the building into the earth, and that the earth contact should be above suspicion and thoroughly good in all seasons.

Fig. 26 shows a plain and simple design for protecting an ordinary church. The conductor in the case of churches and all other high or extensive buildings ought invariably to be made of copper rope, other metals of less conductivity, such as iron, being inadmissible, since their employment would necessitate the use of ponderous masses of metal, which would be not only unsightly, but extremely heavy, and difficult to manipulate successfully. In the accompanying engraving (fig. 27) lent by the Society for Promoting Christian Knowledge, is shown a somewhat more complex structure and the method of arranging the conductors thereon. In this case there is a conductor attached to each spire, leading to and connected with the metal-work of the roof and gutters. On the gable _c_, and the transept gables _d e_, there are fixed three conductors which unite in the centre of the roof, from which they are carried down to the gutters. The same arrangement is followed for the smaller gables _f g h_. The water-pipes and gutters being connected with the conductors, these latter are carried down the side to the earth. It need scarcely be explained how important it is that all metal ornaments on the ridges of churches, as well as other buildings, should either be connected with the general conductors or, in the case of extensive buildings, with a conductor that is carried straight to the earth, as shown in fig. 28. In the case of the finials so often found on Gothic structures, it is necessary to splice the conductor round the bottom of the finial, as shown in fig. 29. If, instead of placing terra-cotta tiles along the ridges, a cresting of fancy iron-work is fixed there, the expense of running a conductor along the ridges will be saved.

The various methods of fixing weathercocks on to the terminal rod are fully explained in another chapter. Fig. 30 shows the best arrangement for connecting the conductor to the terminal rod on a church spire. The copper rope which forms the conductor is spliced round the terminal rod at the bottom of the finial, and as an additional security round the base of the vane rod, which in this instance also serves as the terminal rod of the lightning conductor.

There has been much controversy as to whether it is better to carry the conductor from the roof to the ground inside a building than outside the walls. As a matter of fact, it is a question of very small importance which way the conductor is carried, so long as it arrives at the ground by the shortest possible route. Benjamin Franklin, to judge from many expressions in his works, seems to have been decidedly in favour of the inside plan, which was adopted almost universally in France and on the Continent in general on the first introduction of lightning conductors. But the method was soon abandoned, owing partly to a witty saying of Voltaire, constantly quoted to this day. Speaking of the death of the unfortunate Professor Richman, of St. Petersburg, killed while experimenting with electric discharges from the clouds, Voltaire remarked, ‘There are some great lords whom one should only approach with extreme precaution: lightning is such a one.’ A mere jocular exclamation, it would have had no great force except in France, where a _bon mot_ may cause the fall of a king and the dethronement of a dynasty. In regard to Voltaire’s pleasantry about not approaching too close to lightning, it really had in great part the effect of preventing conductors to be laid inside the houses. Even such calm philosophers and men of science as Professor Arago quote Voltaire with approval. ‘I feel inclined,’ he remarks in his ‘Meteorological Essays,’ ‘to admit that the illustrious author (Voltaire) may be right, when I remember a case that occurred in the United States.’ The case relied upon, a very curious one, was as follows, in Arago’s own words.

‘Lightning,’ Professor Arago tells his story, ‘having struck a rather thick rod erected on a Mr. Raven’s house, in Carolina, United States, afterwards ran along a wire carried down the outside of the house to connect the rod on the roof with an iron bar stuck in the ground. The lightning in its descent melted all the part of the wire extending from the roof to the ground-storey, without injuring in the least the wall down which the wire was carried. But at a point intermediate between the ceiling and the floor of the lower storey things were changed: from thence to the ground the wire was not melted, and at the spot where the fusion ceased the lightning altered its course altogether, and, striking off at right angles, made a rather large hole in the wall and entered the kitchen. The cause of this singular divergence was readily perceived, when it was remarked that the hole in the wall was precisely on a level with the upper part of the barrel of a gun which had been left standing on the floor leaning against the wall. The gun barrel was uninjured, but the trigger was broken, and a little further on some damage was done in the fire-place.’ Commenting upon this case, Professor Arago goes on: ‘Here the lightning went off horizontally through the wall, in order to strike a fowling-piece standing upright in the kitchen. How much injury might not have resulted from this lateral movement, if the lightning had not had to traverse a thick wall?’ Consequently, he argues, Voltaire is right in his jocular-oracular declaration about the perils of indoor lightning conductors, in their being ‘great lords’ dangerous to approach.

It is really difficult to understand how a man like Professor Arago could be misled into such false reasoning as this about an accident which, in itself, was of the simplest, and of the very easiest explanation. That the stroke of lightning falling upon Mr. Raven’s house, in Carolina, should have melted the wire of the conductor points to one cause, and to one only, namely, that there was no proper earth connection. Had it existed, the wire, although thin, could not possibly have been ‘melted all the part extending from the roof to the ground-storey,’ nor could the electric force have left its appointed path to seek a better one through a wall, and, still more astounding, ‘striking off at right angles.’ It is abundantly clear that such cases, and others to the same effect, brought against the fixing of lightning-conductors inside the walls of buildings, prove absolutely nothing. What is beyond controversy is, that a good conductor, in proper condition, is absolutely harmless to surrounding objects, including human beings. A man, even with a ‘fowling-piece’ in his hands, might lean full length against half-an-inch copper rod carrying off a heavy stroke of lightning into ‘good earth’ without so much as becoming aware of the passing of the electric discharge. If certainly a ‘grand seigneur,’ as Voltaire remarks, the electric force has this in common with some of the greatest of men, of not wasting its time, but following a clear aim.

A very common, and, it may be added, a very mischievous opinion is prevalent, that lightning conductors should be carefully insulated from the buildings to which they are attached, and consequently many conductors are made to pass through insulators of glass and other materials of low conductivity. This practice of separating the building from the lightning conductor is not only utterly useless but positively dangerous. It is not unusually thought that by insulating the conductor the electric discharge will be prevented from entering the building. Such an idea is _ipso facto_ absurd, for it is preposterous to suppose that a flash of lightning which can travel through thousands of feet of air--itself a very bad conductor of electricity--and then shatter to pieces the most compact bodies, would be stopped in its course by means of a few inches of glass, or a few feet of air. It may therefore be confidently asserted that no insulator can possibly be made that would be capable of preventing the electric discharge leaving the lightning conductor provided it could find an easier path leading to the earth. Mr. Phin, in his work on ‘Lightning-Rods’ says very pertinently:--But not only are insulators worthless--they are positively dangerous if the principle upon which they are adopted is fully carried out, which, however, is but rarely done. A little consideration will show this. Thus, if a house be furnished with a carefully-insulated lightning-rod, and should also have any large surface of metal, such as a tin roof, an extensive system of gutters, or such like, connected with it, it is easy to see that the house must resemble a large Leyden jar, of which the tin roof, or other mass of metal, constitutes one coating, and the lightning-rod and the earth constitute the other, while the insulators and the dry material of the house represent the glass of the jar. If both the outside and the inside of this jar (the tin roof and the earth) had been connected together, it would have been impossible to have brought one coating into a condition opposite to that of the other. But the rod being carefully insulated from the roof, it is obvious that the inductive action of the cloud will bring the roof and the earth into opposite conditions; and if a man were to form the path of least resistance between them, the discharge would take place through his body, and he would probably be destroyed. It is obvious, then, in the first place, that lightning-rods should be connected with all large masses of metal which may exist in or upon the house, such as metallic roofs, tin or iron gutters, or pipes, iron railings, &c. In the second place, the rod should be attached to the house in the neatest and least obtrusive manner possible.’

It is indeed desirable for various reasons that the copper rope or band forming the lightning conductor should be affixed to the building in the neatest and least obtrusive manner possible. The conductor may be fastened by means of ordinary metal staples made of stout copper wire. A better method however is indicated in figs. 31 and 32, one showing the rope conductor formed of forty-nine wires, usually employed by Messrs. R. S. Newall and Co. for the protection of ordinary houses and buildings, and the other the copper band used by them for the same purpose. This fastening is simply a strap of copper bent to the required shape and pierced with two holes, by means of which it is fixed to any building by copper nails or screws. This method possesses several advantages; it is very sightly and neat, it can be easily applied without injury to any building, and as it allows the conductor a certain freedom of movement, it readily permits the contraction and expansion caused by the variations of temperature. The band conductor shown here is one inch wide by one-eighth of an inch thick, and weighs ·44 pound per foot. The rope conductor, although it appears less, has more metal in it; it measures five-eighths of an inch in diameter, and weighs ·67 pound. Fig. 33 shows a different mode of attaching the lightning conductor. It is generally used for the heavier ropes.

Fig. 34 exhibits an apparatus called a ‘tightening screw.’ It is used for making the conductor taut when it gets loose from any cause. The diagram explains itself, so there is no necessity for describing it.

The tall chimney shafts of factories and similar buildings, from which smoke or rarefied air escapes, are peculiarly liable to be struck by lightning. This is principally due to the current of smoke or warmed air forming, with the soot in the chimney, a medium conductor leading to the iron-work of the furnace or stove beneath, but ending there--a result that must be carefully avoided; for although a conductor that leads past any object is a protection (provided always that it has a good earth connection), a conductor that leads to an object, and ends in that object, is a distinct danger. It is therefore necessary to offer to the electric discharge a better conductor, able to intercept it and convey it safely to earth on the outside of the shaft.

The mode by which this is generally accomplished in England is by fixing a copper terminal rod (four or five feet long), on to the side of the top of the chimney shaft. This method is open to one serious objection: if the wind should happen to blow the stream of smoke or heated vapour in a direction opposite to the terminal rod, the electric discharge might go down the chimney shaft and effect considerable damage. By far the best plan is that shown in fig. 35. It consists simply of an iron or copper cap, to the centre of which is attached the terminal rod. This latter, however, is by no means essential, and may be said to be merely placed on the top for ornament. A structure of such small circumference really wants no terminal rod, the most important thing being to provide a copper rope or band conductor of sufficient size to carry any electric discharge in safety to the ground. It will conduce greatly to the strength and stability of such a conductor if it be built up together with the chimney shaft, and fastened into the brickwork by clamps on the plan shown in fig. 36. A conductor of this kind should be made of copper rope or band of much greater calibre and weight than that used for ordinary buildings. That made of seven solid wires twisted together (see fig. 37) being the best.

A theory propounded some years ago by the late Prof. Clerk Maxwell, F.R.S., one of the most eminent physicists in Europe, deserves some notice here, perhaps more from its ingenuity than its practical accuracy. On investigation, it proves to be a revival of an old presumption that it is possible to protect a powder magazine or other building from the effects of lightning by having its roof, walls, and ground floor surrounded with a covering of sheet metal, or a network of lightning conductors, and disconnecting the said covering or network from the earth, or even insulating it by means of a layer of asphalt or some similar substance. Prof. Clerk Maxwell argues that the presence of a lightning conductor induces a larger number of electric discharges in its immediate neighbourhood than would occur provided no conductor was present, although at the same time these discharges are rendered less intense and smaller by reason of the existence of the conductor. Therefore, it is possible that fewer discharges take place in the area just outside the radius of the conductor. Reasoning from this, Prof. Clerk Maxwell considers that an ordinary lightning conductor tends rather to mitigate the accumulation of electricity in the clouds than to protect the building on which it is placed.

He says: ‘What we really wish to prevent is the possibility of an electric discharge taking place within a certain region--say, in the inside of a gunpowder manufactory. If this is clearly laid down as our object, the method of securing it is equally clear.

‘An electric discharge cannot occur between two bodies unless the difference of their potentials (i.e. their electrical conditions) is sufficiently great, compared with the distance between them. If, therefore, we can keep the potentials of all bodies within a certain region equal, or nearly equal, no discharge will take place between them. We may secure this by connecting all these bodies by means of good conductors, such as copper wire ropes, but it is not necessary to do so, for it may be shown by experiment that if every part of the surface surrounding a certain region is at the same potential, every point within that region must be at the same potential, provided no charged body is placed within the region.

‘It would therefore be sufficient to surround our powder-mill with a conducting material, to sheath its roof, walls, and ground-floor with thick sheet-copper, and then no electrical effect could occur within it on account of any thunderstorm outside. There would be no need of any earth connection. We might even place a layer of asphalt between the copper floor and the ground, so as to insulate the building. If the mill were then struck with lightning, it would remain charged for some time, and a person standing on the ground outside and touching the wall might receive a shock, but no electrical effect would be perceived inside, even on the most delicate electrometer. The potential of everything inside with respect to the earth would be suddenly raised or lowered as the case might be; but electric potential is not a physical condition, but only a mathematical conception, so that no physical effect would be perceived.

‘It is therefore not necessary to connect large masses of metal, such as engines, tanks, &c., to the walls, if they are entirely within the building. If, however, any conductor, such as a telegraph-wire, or a metallic supply-pipe for water or gas, comes into the building from without, the potential of this conductor may be different from that of the building, unless it is connected with the conducting shell of the building. Hence the water or gas supply-pipes, if any enter the building, must be connected to the system of lightning conductors; and since to connect a telegraph-wire with the conductor would render the telegraph useless, no telegraph from without should be allowed to enter a powder-mill, though there may be electric bells and other telegraphic apparatus within the building. I have supposed the powder-mill to be entirely sheathed in thick sheet copper. This, however, is by no means necessary in order to prevent any sensible electrical effect taking place within it, supposing it struck by lightning. It is quite sufficient to enclose the building with a network of a good conducting substance. For instance, if a copper wire, say No. 4, B. W. G. (0·238 inch diameter) were carried round the foundation of the house, up each of the corners and gables, and along the ridges, this would probably be a sufficient protection for an ordinary building against any thunderstorm in this climate. The copper wire may be built into the wall to prevent theft, but should be connected to any outside metal, such as lead or zinc on the roof, and to metal rain-water pipes. In the case of a powder-mill, it might be advisable to make the network closer by carrying one or two additional wires over the roof and down the walls to the wire of the foundation. If there are water or gas-pipes which enter the building from without, these must be connected with the system of conducting wires; but if there are no such metallic connections with distant points, it is not necessary to take any pains to facilitate the escape of the electricity into the earth; still less is it advisable to erect a tall conductor with a sharp point in order to relieve the thunder-clouds of their charge.

‘It is hardly necessary to add, that it is not advisable, during a thunderstorm, to stand on the roof of a house so protected, or to stand on the ground outside, and lean against the wall.’

Prof. Clerk Maxwell, in a letter to Mr. Charles Tomlinson, F.R.S., the author of ‘The Thunderstorm,’ says: ‘My plan is to convert a building into a closed conducting vessel by a sufficient number of wires enclosing it. For an ordinary house, a skeleton of its edge is quite enough. A _a_ may be a zinc ridge, B _b_ and C _c_ water-gutters of zinc or iron; but the pieces A B D, A C E, _a b d_, _a c e_, and the circuit D E _e d_ should be of stout copper wire or rope, built into the wall as a security against theft, but connected to every other piece of metal on the outer surface of the house, and to every gas or water-pipe which enters the house from without, but _not_ to any masses of metal wholly within the whole, unless this is desirable for other purposes.’