THE EARTH CONNECTION.
To dwell too largely upon the importance of leading all lightning
conductors down into moist earth, or, as technically called, ‘good
earth,’ would be scarcely possible. It would perhaps not be too
strong an expression to say that the part of the conductor above
ground is a mere appendage to that under ground, the essential
function of the whole apparatus--that of dispersing the electric force
harmlessly--being accomplished by the subterranean portion. The clear
understanding of Benjamin Franklin perceived this at the outset;
but after him it seemed as if forgotten for a long time, and the
result showed itself in numerous disasters that occurred to buildings
protected with conductors, which brought the latter into disrepute with
many persons. While, no doubt, in many instances the cause of these
disasters was in the bad application of the conductors themselves,
their defective character, or their feebleness, still in the great
majority the underground connection may be taken to have been in
fault. It may be laid down as an absolute certainty that a really good
conductor--say, a copper rope from five-eighths to three-quarters of an
inch in thickness--cannot possibly fail to carry off the electric force
if the lower part reaches moist earth or water. Probably, in nine cases
out of ten, whenever a building provided with a conductor is struck by
lightning, it is for want of ‘good earth.’
Franklin’s own ideas were very clear on the subject. He laid them
down at various times, more particularly when residing in England,
during the years from 1764 to 1775, as colonial agent for Pennsylvania.
During the latter part of this period he took an active interest in the
proceedings of the Royal Society; and this learned body being requested
by the Government to give advice regarding the best protection against
lightning that could be provided for the great powder magazines at
Purfleet, he was nominated into a committee with three other members,
William Watson, H. Cavendish, and J. Robertson. The committee drew up
a report, dated August 21, 1772, signed by all the members, but known
to be written by Franklin alone. Dwelling strongly on the importance of
the underground connection, Franklin says in this report: ‘In common
cases it has been judged sufficient if the lower parts of the conductor
were sunk three or four feet into the ground, till it came to moist
earth; but this being a case of great consequence, we are of opinion
that greater precaution should be taken. Therefore we would advise that
at each end of each magazine a well should be dug, in or through the
chalk, so deep as to have in it at least four feet of standing water.
From the bottom of this water should rise a piece of leaden pipe to, or
near, the surface of the ground, where it should be joined to the end
of an upright bar.’ Franklin then goes on to recommend the usefulness
of having even more wells than the two, so as to avoid any possibility
of failure in protecting the powder magazines. ‘We also advise,’ he
says in his report, ‘in consideration of the great length of the
buildings, that two wells of the same depth with the others should be
dug within twelve feet of the doors of the two outside magazines--that
is to say, one of them on the north side of the north building, and the
other on the south side of the south building, from the bottom of which
wells similar conductors should be carried up.’ It is not on record
whether these recommendations were adopted by the Government, but it
seems likely that this was the case, as the fear of explosion of powder
magazines through a stroke of lightning was very great at the time. Not
long before, a magazine had been so destroyed at Brescia, in Italy,
with the appalling result of a considerable part of the city being laid
in ruins, burying many hundreds of persons. The destruction of the
Brescia powder magazine, like all similar events, had, it is scarcely
necessary to say, its due effect in spreading a desire for lightning
conductors, fear doing what was not effected by foresight.
Whether or not the English Government made the wells recommended by
Franklin for the Purfleet powder magazine, it is certain that the
sound advice given was not largely followed. On the contrary, there
grew a generally prevailing laxity in regard to the indispensableness
of a good underground connection, which led to numerous accidents.
They were seldom, however, ascribed to the right cause, others being
sought instead--such as particular forms of conductors and the
insufficient length of those phantoms called ‘reception-rods,’ which,
as many thought, could never be made high enough, in order to ‘draw
the electric fluid’ from the clouds. Height was sought where nothing
but depth was required, and the same unsightly rods, towering high
above buildings, would have very effectually carried off the electric
forces if brought from the top to the bottom of the conductor, being
taken out of the air and stuck into the earth. Still, there were
not wanting philosophical minds impressed with the truth that no
lightning conductor can discharge its functions unless rooted in
moisture, and who not only knew it, but did their best to spread this
knowledge in all directions. One of these philosophers, a singular
character in his way, was a German clergyman, the Rev. Dr. Hemmer,
who lived at Mannheim, on the Rhine, at the end of the last century.
Taking the deepest interest in Franklin’s great discovery, he made
many experiments with lightning conductors, which brought him to the
conviction that the electric force, in its chief tendency, seeks
the mass of water on the globe, and that where this is not on the
surface, it must be guided to it to become harmless. Consequently, he
recommended to sink the conductor invariably deep into the ground, so
as to reach water, and to subordinate everything else to this prime
necessity. To make the use of lightning conductors as general as
possible, Dr. Hemmer not only wrote a number of little books, which
he liberally distributed, but travelled about through many parts of
Germany, instigating the authorities to place conductors on all public
buildings, and the people to set them up over their own houses. Holding
that the earth connection was everything, he advocated simply to dig
a hole in the ground till water or very moist earth was reached, and
to stick a small iron bar, wrapped in lead to prevent rust, into it,
running up the roof. The bar any village blacksmith could forge, and
the hole any man or boy could dig, thus making the absolute cost of the
conductor under this arrangement very trifling. Dr. Hemmer was right,
no doubt, in his main argument, and most successful in spreading the
knowledge of lightning conductors, while he was able to boast that not
one of all the number he had set up had ever failed. However, he lived
in an age when as yet water and gas pipes were unknown, and iron, or
any other metal, scarcely entered into the construction of buildings.
Given a leaden roof and a network of metal tubes, and Dr. Hemmer’s
small iron rod could scarcely be expected to do its work of protection.
Together with Dr. Hemmer in Germany, Professor Landriani, of Milan,
drew attention to the paramount importance of a perfect earth
connection. He made it his special business to investigate cases in
which buildings with lightning conductors had been struck, and was able
to show in nearly every instance that it had been for want of ‘good
earth.’ A very striking case, which ought to have brought conviction
of the truth to all investigators of the subject, occurred in Genoa in
1779. The church of St. Mary in this city, standing in a very elevated
position, had been frequently struck by lightning, sometimes as often
as twice in one year, and it was noticed that the electric force always
followed precisely the same path, running along a certain portion
of masonry, partly secured by iron hoops, and finally demolishing a
wall at the bottom to get into the earth. At last, in November 1778,
a conductor, made of the most approved design, was placed over the
church, but, to the great surprise of the scientific men who had
superintended the work, the lightning fell once more upon the building
in the month of July of the following year, again following the old
path it had constantly taken before, and causing absolutely the same
damage as previously, even to the knocking out of certain portions of
the wall nearest the ground. Naturally, the event caused widespread
interest, leading to the closest examination of the church of St. Mary
by several experts, among them Professor Landriani. He had no great
trouble in discovering both the causes of the path of the lightning
having always been the same when falling upon the church, and of the
edifice having been struck again in the same manner when provided with
a lightning conductor. Being a somewhat peculiar structure, consisting
in part of hewn stones held together with iron cramps, there was a
large quantity of metal both in and outside; and it was found that
the path of the lightning had always been precisely in the direction
where the metal offered the greatest continuity, leaping over the short
intervals that existed by destroying the stone, and finally getting
into the ground to a place where there was always a collection of
water by knocking down a wall. If this accounted satisfactorily for
the former accidents, that which took place when a conductor had been
placed was not much more difficult of explanation. Professor Landriani
found that though the conductor itself was very good, it was useless
simply by having its roots in hard rock instead of moist ground. On
the one side of St. Mary’s Church there was a rill of water rippling
down from the hills, and forming a small pool near the church, while
on the other was the hard rock. It was into a crevice of the latter
that the conductor had been laid, thus leaving the electric force to
seek its old path into the water along the iron bars, which, although
disjointed, formed a far better road to earth than the planned road.
It was a convincing proof of the supreme necessity of a good earth
connection. Still, a long time yet was to elapse before conviction
became general.
Probably, the matter was more studied by Italian scientific men than
any others, the study of electricity having always been a favourite
pursuit in that country; yet there, too, the matter was not understood
till quite recently. This is proved by a letter of the celebrated
astronomer and meteorologist, Father Secchi, addressed to the French
scientific journal ‘Les Mondes,’ in October 1872, in which he tells
the story of an accident that befel a building protected by lightning
conductors set up under his own direction, the earth connection being
made after rules laid down by Professor Matteucci, considered the
leading authority on the subject. The letter of Father Secchi, though
of some length, is given here entirely, both on account of the great
fame of the writer, but recently deceased, and because it throws a
flood of light on some of the most important points connected with the
art of designing and applying lightning conductors.
‘Eight years ago,’ says Father Secchi, writing, as just mentioned, in
1872, ‘some lightning conductors had been erected under my direction
on the cathedral and on the Bishop’s palace of Alatri, situated at
the summit of the Acropolis of that town, which, by its elevated and
solitary position, was exposed to frequent ravages from storms. It was
not long ago that a flash of lightning demolished a great part of the
belfry, and damaged the organ of the church. In the erection of this
lightning conductor there arose a great difficulty proceeding from the
nature of the soil, which at the depth of some centimetres turns out to
be entirely of solid calcareous rock.
‘In order to remedy this defect, that part of the conductor which
enters the ground has been made very long, more than 4 metres [13
feet], and has been provided with a great many couples of points, 5
centimetres [2 inches] broad, 5 millimetres [⅛ inch] thick, indentated
on the edges, with the addition of a thick copper wire twisted among
the same points, to help to multiply the points of contact between the
rod and the carbon. The foot of the lightning conductor is entirely
of copper. The rod is also of copper up to a metre [3¼ feet] above
the ground; and there is joined to it the iron conductor, in the
ordinary receptacle made in the heart of the wall, to preserve it from
disturbances of the inferior parts. The ditch into which the foot of
the lightning conductor was sunk is 5 metres [·16 feet] long, and
half-a-metre [1⅝ feet] wide, and it was dug into the ground as far
as to touch the roots of some neighbouring trees, from which point
upwards a layer of cinders was placed, covering the greater part of the
ditch. Thus the surface of contact between the metal and the carbon,
and of the latter with the soil, was such that one would have supposed
it to be more than sufficient, while the presence of trees, although
they were not very large, made it highly probable that the ground did
always contain sufficient moisture. Moreover, as the edifice had two
culminating points--namely, the belfry and the raised back portion of
the choir--two rods were placed on them, each having an independent
connection with the earth, so that, in the case of a discharge on one
of the points, the electric force might find two ways in its course
towards the earth.
‘These arrangements produced, on the whole, a good result, since,
although the edifice was struck at least four times after conductors
had been placed on it, it suffered no damage of any kind. Nevertheless
a very curious accident, highly interesting as a scientific study,
happened on October 2. Early on the morning of this day several flashes
of lightning fell down from the clouds during a terrific storm,
which lasted over two hours. The belfry was struck at first by weak
discharges twice; but the third flash was so appalling in its strength
as to terrify the whole town below. The injuries it caused were not
great, still they seemed to me to be extremely noteworthy. But before
I describe them I must give some necessary details as to place and
position of the lightning conductor.
‘It so happened that four years after the erection of the conductor
a line of pipes was laid down to carry water to the towns of Alatri
and Ferentino, passing at a short distance from the belfry of the
cathedral. The lightning conductor was not placed in communication with
the pipes, because it seemed established, from previous experiments
and observations, that it was needless to do so, the ground containing
apparently sufficient moisture, the head of the waterworks being close,
and there existing also a running fountain. I was not asked at the time
whether it was necessary to establish this communication, but, had the
question been put to me, I should probably have answered it in the
negative, considering, from what I then knew, the work as superfluous.
That I was in error then as to the necessities of a perfect underground
connection is shown by what happened during the great storm in the
early morning of October 2. The heavy flash of lightning before
referred to did not go its appointed path underground, but passed off
into the waterworks, with the following results:—
‘1. It made in the earth a perfectly rectilinear excavation, which,
from the lower part of the conductor, went to the tube of the
waterworks running to Ferentino, and in traversing the wall destroyed
the angle of that structure. The earth of the ditch thus dug was
disposed regularly to right and left with great symmetry. The length
of the ditch was about 10 metres, the depth about 70 centimetres [28
inches].
‘2. The lightning struck the water-pipe of Ferentino, broke it
completely, throwing the pieces to a distance of about 80 centimetres
[32 inches]. The lead which soldered the joint of the broken tube with
the tube beyond was found melted. In consequence of this rupture the
water ceased flowing to Ferentino, and poured into the waterworks.
‘3. Another part of the discharge spread itself by the pipe which goes
to Alatri, and traversing the reservoir threw to a great distance
some wooden plugs which stopped up the discharging tubes, the plugs
being forcibly hammered in. It arrived at the town in a tank, where it
damaged and twisted in a strange manner a leaden slab which was in the
tank, made some other little injuries, and finally left the trace of
its passage at the spouts of the public fountain.
‘4. The point of the lightning conductor was examined, and it was found
very blunt; it was found impossible to unscrew it, and it could not be
removed without breaking the screw. It was found broken to a length of
more than 3 centimetres [1¼ inch], and the section of fusion was nearly
flat, as though it had been cut. The gold of the gilding had nearly all
disappeared. In the church, and in the edifice which is attached to
it, no injury was detected. These facts appear to me important both as
regards practice and theory: in respect to theory, because they give
an idea of the quantity and of the immense force of the discharge.
The melting of the point down to a section 1 centimetre [½ inch] in
diameter proves that it would have been melted down much further if it
had been slighter. It is not prudent, then, to use very slender points;
it is best that they should thicken quickly.
‘The excavation of the ditch at the foot of the lightning conductor
could not be the direct effect of electricity, but would be the result
of the sudden evaporation of the moisture of the ground, generating
steam, and forming, as it were, a mine.
‘The breaking of the tube is most singular. It seems to me that it
can with difficulty be attributed to the mechanical shock of the
electricity itself. As the lead which united the broken tube to the
one beyond was found melted, it is evident that, in spite of the water
which flowed in this tube, it was raised to an enormous temperature in
the place where it was struck, and probably it was the instantaneous
evaporation of the water inside which caused the breaking of the tube.
‘But the most singular fact, in a certain respect, is what was observed
in the tube which descends to Alatri--that is to say, the alteration
in form of the leaden slab. The little interruption which necessarily
exists in this tank between the conducting-pipe and the metallic
receptacle evidently gave occasion for a discharge by a flash, and, in
consequence, for an explosion of steam. But we see at the same time
by that that the distance traversed in the tube from the building to
the slab, a distance of more than 200 metres [650 feet], in which the
pipe is buried underground, did not suffice for the charge to lose
itself in the ground, although during the passage it had to cross the
reservoir, and might there have distributed itself. Our surprise is
still greater when we reflect that it was only part of the discharge,
since the greater portion had to flow by the water-pipe of Ferentino,
which was the first struck in a direct manner, and that these pipes
are joined together with lead. The quantity of electricity must have
been enormous, in order to be able to have so much force and to run
another 300 metres [975 feet] to reach the public fountain, and leave
its traces there. A circumstance which deserves attention is, that this
storm took place after a long and constant drought; and consequently
the earth was less moist, and could offer little facility for
dispersion.
‘These cases are not so rare among us as one might suppose. Not very
long ago, at Lavinia, a flash of lightning destroyed a great part of
the belfry, passed to the bell, broke and melted it in its passage in
such a manner that the metal had run away like wax. I do not believe
this breakage of the bell to have been a mechanical effect of the
lightning in a rigorous sense, for the bell could have been broken by
the instantaneous expansion produced by the heat at the point of the
passage, an expansion which had had no time to disperse, as a glass
vase breaks when touched with a red-hot iron.
‘Let these facts come about how they may, they enable us to see that it
is necessary to devote great attention in the erection of lightning
conductors, that we must allow them a large surface for discharge, _and
that there can never be too much of it_. The surface of the foot of
our lightning conductor was certainly superior to what has been judged
sufficient by Matteucci for the discharges of telegraphic conductors,
and yet it has not sufficed. Further, it is a confirmation of the
necessity of making the neighbouring metallic masses communicate, and
especially with water and gas-pipes.’
From out of the almost endless number of cases in which lightning
conductors failed for want of a good earth connection, another one or
two may be given, illustrated as having happened quite recently in
England, and as such showing, in a very striking manner, in what a
neglected state the knowledge of the subject still is at this moment.
A thunderstorm passed over the town of Clevedon, Somersetshire, in the
afternoon of March 15, 1876, and a flash of lightning fell upon the
steeple of Christchurch, provided, as was generally thought, with a
most efficient conductor of recent construction, made of good copper
rope. What happened is graphically and minutely told in a letter
addressed to the ‘Journal of the Society of Telegraph Engineers,’ by
Mr. Eustace Buttor, of Lewesfell, near Clevedon. ‘There was but a
single flash,’ Mr. Buttor relates, ‘which appeared to many observers
to travel horizontally through the air. However, the lightning passed
down the lightning conductor of Christchurch. The flag-staff, about
100 feet high, and the four pinnacles, about 90 feet high, have each a
conductor, the flag-staff having the usual conical point, the pinnacles
having the copper rope attached to their vanes. The five copper ropes
unite inside the tower in the neighbourhood of the clock. Lower down
the conductor passes through a slanting hole to the outside, and for
the lowest 12 feet is encased in a pipe. On reaching the ground it
passes into a dry freestone channel for about a dozen feet, and then
dips down into the drain which carries rain-water from the roof. As no
rain preceded or accompanied the flash, it may be presumed that _the
drain was dry_.
‘The protector is copper throughout, and, with the exception of the
termination, seems to have been carefully and efficiently placed.
The diameter I estimate to be half-inch, or it may be a trifle more.
Just at the point where it leaves the pipe and enters the ground, the
electric charge left it, dashed through three feet or more of solid
wall supporting the tower, in order to reach the gas-meter inside, then
it passed safely along the gas-pipe. The cavity made was considerable,
but very irregular. I was unable to ascertain when the workmen were
engaged in repairs, and therefore cannot give their estimate of the
weight of stone displaced, but it must have been many hundredweights,
though only a few pounds were actually thrown out on to the path, or
inside into the vault. A large quantity of stone was pulverised, and
the whole gave one the idea of the explosion of a charge of gunpowder
under great compression. In a house about 100 yards from the church,
the inmates felt the shock intensely, but did not know that the house
had been touched. Some hours after, however, on going to turn on the
gas, a hissing noise was heard, and a hole was found in the composition
gas-pipe, about five-eighth inch diameter, just where the pipe passed
within an inch of a water-pipe. The lightning must have come along
the main from the church gas-pipe to this house, and then passed to
the water-pipe as the readiest way to moist earth. The whole soil in
the neighbourhood is mountain limestone, very dry. There is not the
slightest evidence of displaced plaster, or any other sign of the
passage of an electrical discharge through the house.’ There need be
little comment on the facts stated in this letter, notable though they
are. It is the old delusion that a lightning conductor need be brought
down underground only, and that then all is right. In this case, those
who protected Christchurch, Clevedon, thought it quite sufficient to
bring the conductor down into a drain-pipe carrying rain-water from the
roof, without reflecting for a moment that an earthenware drain-pipe
would insulate the conductor from ‘earth.’ A similar instance came
under the writer’s notice about a year ago. One of the pinnacles
of Cromer Church, in Norfolk, was struck by lightning, although
fitted with a conductor on one of the pinnacles. On examination it
was discovered that the earth terminal had been inserted into an
earthenware drain.
It is not very easy to give exact prescriptions as to the best manner
in which the underground connection should be effected. The means
vary entirely with the circumstances, and the matter should in all
cases be intrusted to an expert. Simple as is the whole theory of
lightning protection, consisting in nothing else but laying a good
metallic path from the top of a building down into moist earth, as an
unfailing path for the electric force, the practical execution of it
is not the less often very complicated. It is especially so as regards
the most important of points, that of the underground connection. Of
course, wherever there is running water at hand, a river, or even a
tiny stream that never dries, the matter is easy enough, but as in the
great majority of buildings to be protected such water does not exist,
the solution of the question becomes more difficult, and frequently
one of the greatest perplexity. It tends even to be more and more so
in consequence of the progress of sanitary arrangements under which
towns and villages are ‘drained’ until the soil has been made as dry
as a rock. Immense as the benefit is to public health, it is, like all
benefits, attended by certain drawbacks. One of these certainly is a
greater danger from lightning. It is often proposed by builders to
use the drain-pipes themselves in making ‘good earth’ for lightning
conductors, but the fallacy of this recommendation need scarcely be
exposed, seeing that these conduits are generally made of earthenware,
as happened when Christchurch, Clevedon, was struck by lightning.
While broad rules cannot be laid down, still it may be affirmed that a
good earth connection, sufficient to carry off the heaven’s electric
discharges, may always be obtained by either of two means. The first,
and in all cases most preferable, is to lay the conductor deep enough
into the ground to reach permanent moisture. When this exists in a
considerable mass, the single conducting rope, touching it, will be
quite sufficient; but when the quantity is deficient, or doubtful, it
will certainly be advisable to spread out the rope, so as to run in
various directions, similar to the root of a tree, likewise in search
of moisture. There are various modes of accomplishing this, shown in
figs. 46 and 47.
[Illustration: Fig. 46.]
[Illustration: Fig. 47.]
A variety of methods have been proposed for the dispersion of the
electric force underground where the soil contains little or no
moisture, except at great depths, to be reached only by a vast amount
of labour and expenditure. In France, the system most generally adopted
in these cases is to place at the bottom of the underground connection
an apparatus, made either of iron or copper, shaped somewhat in the
form of a harrow, and to embed it thickly in charcoal. Fig. 48 will
illustrate this system of earth connection.
[Illustration: Fig. 48.]
The apparatus is as simple as it may be useful, and the more so, of
course, the thicker the mass of charcoal in which it is embedded. But
it may be doubted whether it is sufficient to make ‘good earth’ under
all circumstances. Perhaps it will do so in ninety-nine cases and fail
in the hundredth. The amount of electric force discharged in ordinary
thunderstorms does not seem to vary much, and, according to all
observations, such an artificial connection as this of the charcoal bed
is sufficient to disperse it safely beneath the surface. But now and
then there come storms of extraordinary violence, or, in other words,
extraordinary accumulations of atmosphere electricity, which demand
precautions such as are not fulfilled by the subterranean harrow,
however thickly embedded in charcoal, or, as oftener done, in gas coke
or cinders. It is certain that there have been cases in which buildings
with otherwise excellent conductors, but provided with such an
artificial earth connection, have been damaged by lightning. However,
it may be stated, as the net result of all observations and known facts
upon the subject, that small private houses can be well protected by
this means against lightning, but that the system cannot be recommended
as absolutely safe for large edifices and public buildings.
To protect any structure of great extent, it is absolutely necessary to
bring the conductor, or conductors, deep enough into the earth to reach
water. It is all the more indispensable with modern buildings, as they
contain large masses of metal, not only in gas and water-pipes, but
often in staircases and iron columns, towards which the electric force
has the strongest tendency to direct itself unless drawn to the earth
by an immediate and unfailing connection with the great sheet of water
below its surface. It is considered by German electricians that there
is no necessity, if a large edifice has a number of conductors, to let
each have a separate earth connection; it is quite sufficient to bring
them all into one, provided only that this is absolutely perfect at all
seasons and under all circumstances. Fig. 49 will show how this can be
done.
[Illustration: Fig. 49.]
It will be seen that for the protection of this edifice there are six
conductors, with four elevated points marked A, B, C and _c_. Two of
these points, A and C, expand from the roof to the ground into double
conductors, so as to protect the sides of the building against possible
lateral discharges of lightning, and all the six conductors meet a
little below the surface in the earth connection prepared for them. To
form this one connection, either by digging or boring, may sometimes
be costly, but whether the expenses be more or less, the protection
against lightning thus effected will be so absolute as to be invaluable.
In a similar manner as the large edifice, with its many gables, a
church may be fitted with lightning conductors. Fig. 50 scarcely needs
much explanation.
[Illustration: Fig. 50.]
There is one thing, however, regarding churches, that must be well
borne in mind in establishing their protection against lightning.
Besides containing great masses of metal, in bells, organs, and other
contents, they are frequently placed in high situations, exposed to
the most violent discharges of the electric force. It often happens
also that they stand on rocky ground, with the subterranean waters
far below the surface. To ensure absolute protection under these
circumstances, it is indispensable to connect the conductors with
water, wherever it is to be found, by a solid channel, into which
the copper rods may run, if possibly some distance below the surface
of the earth. The form such a channel may take is indicated on the
engraving. It will be seen that the protection against lightning
indicated here is not only for the church, but the adjoining parsonage,
the conductors spreading over both, with points on the most prominent
and exposed places. It would be possible to carry out this principle
in ensuring the protection of a whole block of private buildings.
German electricians think that one channel or well, sufficiently broad,
leading from the surface of the earth to layers always moist, or to
perennial springs, would suffice to carry the electric force discharged
upon a hundred conductors, and all the easier as it would be impossible
that many would be struck at one and the same time by lightning.
Perhaps some such arrangements will be made in the future, when both
houses and towns are built upon a more systematic plan than is followed
at the present time.
If, as a rule, one channel of underground connection is amply
sufficient for the protection of even the largest buildings, there
may be cases in which it is indispensable to spread the conductors
into several directions. It may be laid down, broadly, that when
there is water to be reached, the one channel is sufficient, but that
when this is not possible, or expedient, more lines of underground
connection must be formed. Fig. 51 may serve to illustrate a case of
the latter kind. It shows a powder-magazine, partly above and partly
underground, standing on dry soil, with trees in the neighbourhood,
likely to add to the danger of atmospheric discharges of electricity,
and with no stream, or permanent moisture, into which to guide them.
Nothing remains, under these circumstances, to ensure safety, but to
multiply the lines of underground connection to the utmost extent. To
add to the facility of the dispersion of the electric force, the main
channels may be filled with charcoal, broken coke, or cinders, and if
large quantities of these substances can be placed in one or two pits,
it is possible to make thus an artificial connection as nearly as can
be responding to ‘good earth.’ Still, it must never be forgotten that,
absolutely, ‘good earth’ in reference to lightning conductors means
moisture, or water.
If permanent moisture cannot be obtained and iron water-mains are
within reach, it is desirable to connect the ground terminal with them
by means of good solder, as from the large mass of metal they generally
form very good ‘earths.’
[Illustration: Fig. 51.]
In giving directions, or rather suggestions, about the design and
application of conductors, and, what is most important in regard to
them, their connection with the subterranean mass of waters, the
idea that persons may construct their own conductors is left aside
altogether as absurd. It is a good old proverb which says that a man
who is his own lawyer is certain to lose his cause; another has it
that a man who is his own doctor is sure to succumb to his illness.
With regard to the setting-up of lightning conductors, it is precisely
the same. Simple enough as is the theory of ‘drawing lightning’ from
the clouds, the practical execution of it is, as mentioned more than
once, not a little complicated. The formation of the underground
connection, in particular, is a matter requiring very great experience,
and very frequently one of the utmost difficulty. Vast sums of money
are often thrown away needlessly in making a connection which in the
end proves useless, while, on the other hand, a trifling addition to
the expenditure in setting-up a conductor would procure its efficiency,
not attained simply from want of ‘good earth.’ A recent writer on
lightning conductors whimsically, yet with much truth, expresses it by
remarking that ‘people spend money upon gilded points on the top of
the house, while they ought rather to sink it in water at the bottom.’
Undoubtedly, the efficiency of conductors lies, even more than at the
top, on ‘the bottom.’ The earth connection may be called ‘the alpha and
omega’ of lightning protection.