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.
[Illustration: Fig. 19.]
[Illustration: Fig. 20.]
[Illustration: Fig. 21.]
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.
[Illustration: Fig. 22.]
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.
[Illustration: Fig. 23.]
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.
[Illustration: Fig. 24.]
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.
[Illustration: Fig. 25.]
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.
[Illustration: Fig. 26.]
[Illustration: Fig. 27.]
[Illustration: Fig. 28.]
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.
[Illustration: Fig. 29.]
[Illustration: Fig. 30.]
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.
[Illustration: Fig. 31.]
[Illustration: Fig. 32.]
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.’
[Illustration: Fig. 33.]
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.
[Illustration: Fig. 34.]
[Illustration: Fig. 35.]
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.
[Illustration: Fig. 36.]
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.
[Illustration: Fig. 37.]
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.
[Illustration: Fig. 38. ARRANGEMENT OF PROFESSOR CLERK MAXWELL’S
LIGHTNING CONDUCTORS.]
‘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.’
[Illustration: Fig. 39.]