THE BEST MATERIAL FOR CONDUCTORS.
‘The art of protection against lightning,’ says a recent German
writer, in a book on conductors, ‘is precisely the same now as it was
a hundred years ago: still, it has made immense progress since that
time.’ Though apparently involving a paradox, the words nevertheless
are literally true. The art, or rather science, of guarding objects
against the destructive effects of lightning is theoretically the same
as it was in the days of Benjamin Franklin; nevertheless, the practical
execution of the appliances necessary to attain this aim has undergone
extraordinary improvements since that time. This has been due simply to
the astounding progress of the metallurgical arts for the last forty or
fifty years. With the help of machinery on a colossal scale, such as
was never dreamt of before, our factories have come to produce metallic
masses of dimensions and shapes such as make all former achievements of
the kind appear utterly insignificant. We build huge iron ships, armed
with cannon of ponderous weight; we throw iron bridges across rivers
and arms of the sea; we lay metallic cables through the ocean and over
the earth, encircling the globe. All these wonderful achievements, in
which the development of engineering science went hand in hand with
that of tool-making and the ever-growing employment of the power of
steam, have gone to the constant improvement of lightning conductors.
They have benefited, indirectly, in the result of great inventions, and
of immense toil and labour, originally directed to other ends.
There is something half touching, half comical, in reading of the
troubles which Benjamin Franklin had to undergo before he was able to
set up his first lightning conductor. He could meet with no assistance
but that of the blacksmith of little Philadelphia; and the ability
of the latter in the art of forging iron rods more than a few feet
in length was of the most limited kind. The ingenuity of Franklin
overcame this difficulty by a variety of clever contrivances, such
as connecting a number of small rods by caps and joints, fitting
closely; but others were not so successful as he in the matter. Even
in Paris there were no artisans to be found, for many years after
lightning conductors were first recommended, able to make them, and
foreigners, chiefly English, had to be brought there for the purpose.
The difficulties arising from this backward state of the industrial
arts were greatly increased by the belief, prevalent for a long time,
that lightning conductors, to be efficient, ought to be of very great
height, their so-called ‘area of protection’ being in proportion to
their height. The supposition, originating in France, was carried to
extremes in that country, chiefly through the teachings of M. J. B.
Le Roy, a very able but eccentric man. Guided by vague analogies in
electrical phenomena, M. Le Roy, who enjoyed in his time--the latter
part of the eighteenth century--the reputation of being an authority on
the subject of lightning conductors, laid it down as an indisputable
fact that the ‘Franklin rods’ only protected buildings if rising high
above them. He recommended the length of the rods above the chimney, or
summit of any edifice, to be not less than fifteen feet, guaranteeing
that, if of this height, they would offer absolute protection against
lightning over an area of four times the same diameter--that is, sixty
feet. Modern experience has proved this to be an absurdity; still,
in the infancy of all knowledge about lightning conductors it was,
perhaps, not unnatural that even learned men should believe in such
fancies. Lightning was looked upon, not only in name but in reality,
as an electric ‘fluid’ and the conductor was supposed to draw this
‘fluid’ from the clouds. Therefore it was but cogent reasoning to raise
conductors as high above the roofs, and as near to the storm-clouds, as
could possibly be done. If possessed of modern means for manufacturing
pieces of metal of almost any length, M. Le Roy would not improbably
have recommended to elevate lightning conductors a couple of hundred
feet, instead of only fifteen, above the summit of buildings.
It was owing chiefly to the difficulty of forging long iron pieces,
and of welding them together in a satisfactory manner, that, for many
years after lightning conductors had been introduced into Europe,
there were constant attempts made to find substitutes for the rods
devised by Franklin. Chains were largely used towards the end of the
last and the beginning of the present century, both in France and
Germany, their employment having been suggested by the example of the
English navy, where they were introduced, as already mentioned, upon
the recommendation of Dr. Watson. The Continental mode of using iron
chains for the protection of buildings against lightning was to hang
them between the upper part of the conductor, surmounting the roof,
which continued to be a straight piece of metal or rod, and the lower
portion buried in the ground, sometimes, but not always, likewise a
chain, but thicker than the rest. The characteristic of this method,
and showing its long existence, is that it gave rise to a nomenclature
existing to this day in France and Germany, where in all books on
lightning conductors they are described as consisting of three distinct
parts. The French call the upper part of the rod, over the roof, ‘_la
tige_,’ the stem or stalk; and the Germans, ‘_die Auffangstange_’
literally the reception-rod. In both languages the middle part,
from the roof downwards to the earth’s surface, is described as the
conductor proper, ‘_le conducteur_’ and ‘_der Leiter_.’ Again, the
lowest underground part of the conductor is designated, by the French,
‘_la racine_,’ the root, and by the Germans as ‘_der Bodenleiter_,’
or the ground-conductor. It has often been said that, as language
springs from ideas, so it reacts upon them, and if the proposition
be true, as most will admit, the French and German designations of
the parts of lightning conductors--also to be found in Italian, and
adopted in a few of the older English treatises on the subject, mostly
translations--have a strongly misleading tendency. Nothing could be
further from the truth than the assertion that a conductor ought to
consist of three distinct parts. On the contrary, the more it is ‘one
and undivided,’ the better it will be as a lightning protector.
The use of iron chains as conductors gave rise to very many fatal
accidents, and for a time resulted in an outcry that the system
itself could not be depended upon, as it was known to be not always
efficacious. Lists were published of numerous instances in which
buildings with what were supposed to be the best conductors were struck
by lightning, from which it was argued that Franklin’s great discovery
of the electric force always seeking a metallic path to the earth
was a myth. It was not till some painstaking scientific men, deeply
interested in the subject, had set to work to discover the causes
of the failure, that the whole became plain enough. The chains, in
some of the instances in which they had proved inefficient lightning
conductors, were found to be corroded to such an extent as barely to
hang together. Of course this corrosion would impair the efficiency
of the conductor by reducing the quantity of metal; but the chief
objection to the use of chains lies in the fact long ago pointed out
by Mr. Newall, that even supposing a chain were formed of links of
half-inch copper rods, and were perfectly bright and clean, the area
of the conductor is reduced to a mere point where the links touch each
other, and the resistance becomes so great in such a small conductor
that instances have been recorded of the fusion of the links. In other
cases, as in that of H.M.S. ‘Ætna’ in 1830, the chain was boomed out,
and did not touch the water!
Simultaneously with the chains, there was trial made, in several
Continental states, and also in England, of several other metallic
conductors besides iron. Tin and lead had both their advocates, but
the latter more than the former, on account of its far lower price. As
regards tin, it had really no advantages whatever over iron, except
pliability and non-oxidation. Against this was to be set that it was
much more expensive than iron, with only about the same conducting
power, according to Becquerel, Ohm, and other investigators. Professor
Lenz, it is true, ranked tin very much higher, asserting, from
experiments of his own, that its power of conductivity was nearly twice
that of iron; and it was partly owing to his great influence that
the metal obtained a trial in several countries, more particularly
in Russia and in the United States of America. Still, the result was
not satisfactory on many accounts, and its price alone brought tin
to be soon abandoned as a conductor. Lead had a far longer trial.
Its cheapness recommended it strongly, and equally so its extreme
pliability. One of the greatest difficulties of the constructors of
‘Franklin rods,’ when first they came into demand, was to make the
iron pieces fit properly around sharp corners of buildings, either
by bending them in fire, or, as was more commonly done, soldering
them together, or employing screws and other joints. But it was early
discovered that these junctions, when occurring at acute angles, were
bad conductors, occasioning sometimes the electric force to leave its
traced course, and fly off in some other direction. It is probable
that, in several well-authenticated instances in which this really did
happen, the joints were eaten away by oxidation, as in the case of
the chains; still, the effect of such occurrences was all the same.
The joining of strips of lead together was a far easier task than
that of handling iron in the same way, particularly for inexperienced
workmen, and thus the employment of the metal continued for some
time. However, it had to be abandoned gradually, on account of its
manifest disadvantages. Its extreme softness, which made it liable to
be broken by any accident, was one of them, and, still more so, its
want of conducting power--only about one-half that of iron. Thus leaden
conductors slowly went out of use, except in the form in which they
still act often to great advantage, that of water-pipes.
Among all the experiments made for producing the most perfect lightning
conductors, the one which created the greatest attention, some fifty
years ago, both on the Continent and in England, was the employment
of ropes made of brass wire. They were first recommended about the
year 1815 by a professor at the University of Munich, J. C. von Yelin,
distinguished for his researches into the nature of thunderstorms.
Through his influence most of the public edifices of Bavaria, more
particularly the churches, were provided with conductors of brass
ropes; and within a few years their employment became so popular,
owing to the ease with which they could be attached to all buildings,
that even the Roman Catholic clergy changed their attitude, and,
from being opposed to ‘heretical rods,’ advocated their extension in
every direction. But it was not long before the trust in brass ropes
as protectors against lightning was rudely shaken. Several instances
occurred in which buildings so protected were struck and damaged by
lightning, and at last there came a case which attracted the widest
attention, leading, on account of its supposed importance, to the
institution of a Royal Commission to report thereon. The little town
of Rosstall, in Franconia, Bavaria, had a church the steeple of which
was 156 feet high; and, standing on the brow of a hill, it overlooked
the country far and wide, visible for many miles. Necessarily much
exposed to the influence of lightning clouds, it had been provided
with one of the best brass-wire conductors, designed by Professor von
Yelin himself, and made of unusual thickness, being over an inch in
diameter. Nevertheless, on the evening of April 30, 1822, while a dark
storm-cloud, of extraordinary thickness, was passing over Rosstall, a
heavy flash of lightning was seen to fall vertically upon the church
steeple, followed by a terrible peal of thunder, which seemed to shake
the earth. When people looked up they beheld the church clock thrown
from its place, and part of a lower wall of the edifice thrown to the
ground. It was clear that the electric discharge from the atmosphere
had been one of unusual energy, but equally clear that the trusted
conductor had not done its work.
It was partly through scientific controversies about the relative
conducting value of metals, and partly through the action then
taken by several German Governments of providing all buildings with
lightning conductors, that the Rosstall case excited an extraordinary
interest at the time. The Royal Commission appointed by the King
of Bavaria, presided over by an eminent savant, Professor Kastner,
went to Rosstall to inspect the effects of the lightning discharge,
and Professor von Yelin did the same, as an independent, though not
disinterested witness. Their reports as to actual facts were the same.
The lightning, after striking the steeple of the church, had melted
the top of the ‘_Auffangstange_,’ or highest part of the conductor,
and further down had passed along the brass rope till coming to the
clock, only a few inches distance from it. Here the electric force had
evidently divided itself into several streams--the one exerting its
disastrous effects upon the clock and brickwork, and several metallic
objects underneath, and the other passing down the rope conductor,
but not without bending it, and, in one or two places, tearing it to
pieces. Such were the facts, visible to all eyes. But the conclusion
drawn therefrom differed widely. The members of the Royal Commission
made it public that the reason of the Rosstall lightning conductor not
having been efficient had simply arisen from its nature. Brass-wire
ropes, they declared, though perhaps useful against small discharges
of electricity, formed no reliable safeguards against powerful ones;
and they therefore strongly advised a return to the old-fashioned
iron rods. The conclusion was vehemently disputed by Professor von
Yelin. He admitted that it might be better, to provide for the proper
discharge of extraordinary masses of the electric force, to make his
brass ropes, when applied to high churches and other large edifices,
even thicker than they had been at Rosstall; but at the same time he
utterly denied that, even in this case, they had been the origin of the
disaster. He showed that the real cause of it was that the conductor
had not been laid deep enough into the ground, so as to touch moist
earth. The church stood upon sandy soil, on an eminence, and to touch
‘good earth’ the brass rope ought to have been sunk down to a depth of
at least fifty feet, whereas it did not reach one-third of that depth.
The professor was undoubtedly right, but his antagonists nevertheless
prevailed. A public prejudice, which no argument could overcome, set in
against brass-wire conductors, and they were pulled down from nearly
all buildings on which they had been laid, to be replaced by iron rods.
Some time had to elapse before real justice was done to metallic ropes
as lightning conductors.
With our present knowledge of electrical phenomena, and the practical
art of making conductors, it may safely be affirmed that the Munich
professor was right in recommending ropes, though not in approving
of brass as the best metal. In its very nature, brass, a compound,
can never be thoroughly reliable, because its conducting power varies
according to its composition. The facility with which it allows the
electric force to pass through it depends, in fact, entirely on
the amount of copper which brass contains, and is greater or less
accordingly, since the other metal entering into its composition,
zinc, has less than one-third the same conductivity. Now brass is, for
various purposes, made sometimes of 70 parts of copper and 30 parts of
zinc, and again, only equal amounts of both metals, setting calculation
as to its conducting power entirely at nought. But besides this, brass
has the great fault of being excessively liable to destruction by
atmospheric influences, and it was found, among others, in Germany,
that while brass ropes were used as lightning conductors, they were
frequently destroyed, in a comparatively short space of time, by the
action of smoke alone. It is true, the Continental mode, existing
both in France and Germany, of spanning conductors over the tops of
chimneys--illustrated in the engraving here as a warning ‘how _not_ to
do it’--had much to answer for this atmospheric deterioration, since
even tougher metals than brass could not be expected to stand the
constant action of smoke, often containing sulphurous fumes. But even
without such an evidently absurd arrangement as that of running any
conductors, whether in the form of ropes or cords, across the orifices
of chimneys, brass could never have answered all the requirements of
a lightning conductor. It was with justice that brass-wire ropes were
nearly altogether discarded some thirty or forty years ago, after
having had a short-lived reputation.
[Illustration]
That copper should not have been employed, long before brass and other
metals, in serving mainly for lightning conductors, its pre-eminence
for this purpose being undisputed, would seem a strange fact, were
it not explicable on several grounds. The first was the cost of
the metal, which, though varying in price, is seldom less than six
or seven times that of iron. It was needless for the advocates of
copper as conductors--and there were not a few from the time its
high conductive power had been demonstrated--to say that if copper
was six times as dear as iron, it was likewise six times better as a
carrier of the electric force, and that consequently the price, in
respect of applicability for lightning protection, was in reality the
same. But the reply to this was that copper, being one of the most
expensive metals, except the so-called ‘precious’ ones, was exposed
to the temptation of theft, and ought therefore not to be employed,
since it was possible that vagrants, or other people, might tear off
at any time the, in more than one sense, valuable pieces of metal
protecting buildings against destruction from lightning. The argument,
perhaps, was not worth much, but a better one not mentioned was in the
background. It was, till quite recent times, an achievement of the
greatest difficulty to manufacture long rods or bands of sufficiently
pure copper to serve as lightning conductors. Sir William Snow Harris
attempted, as already related, to get over this impediment by taking
short plates, and fastening them together, and over each other, by
copper nails. But this process, besides being enormously expensive,
was in many other respects unsatisfactory, notably in that it made a
shifting of the plates possible, and by the destruction of a few of
them ruined the whole system. The pith and substance of the whole was
the technical difficulty of hammering or drawing pure copper out into
great lengths. That it must be pure was essential, the fact being
thoroughly established that the electric conductivity of copper,
mixed with impurities such as arsenic, is often not two-thirds, and
sometimes not as much as one-half, that of the pure metal. This was
conclusively shown by Sir William Thomson in a series of researches,
and likewise by that distinguished investigator in the conductivity of
metals, Professor Matthiessen. The latter, while placing copper on the
same rank with silver, and far above gold--100 to 78--furnished the
following instructive list as to the relative value of different kinds
of copper:—
Pure copper 100·00
Best American copper 92·57
Australian copper 88·86
Russian copper 59·34
Spanish ‘Rio Tinto’ copper 14·24
It will be seen that, according to the investigations of Professor
Matthiessen, admitted on all hands to be correct, the copper lowest
in the list, the ‘Rio Tinto,’ is barely equal to iron in electrical
conductivity, and, not having the hardness of the latter metal, would
be in every way inferior to it as a lightning protector. The employment
of the purest copper therefore became an essential point in the
manufacture of lightning conductors.
Fortunately, the difficulty was solved, at an earlier period than might
have been expected, by the demand for submarine cables. These had to
be made of wires of the highest possible electrical conductivity, and
the matter being one of high financial and commercial importance,
manufacturers soon began to use the utmost care in selecting ores
containing the smallest amount of metallic impurities. We believe
the lightning conductors now manufactured at the extensive works of
Mr. R. S. Newall, F.R.S., established at Gateshead on Tyne about
forty years ago, have generally a conductivity of 93 per cent. of
pure copper. It was laid down by one of the most eminent scientific
men of the day, not long ago, that the three principal qualities of
a good lightning conductor ought to be a maximum of conductivity, of
durability, and of flexibility that could be obtained, and there is
nothing coming up to this standard so well as ropes of pure copper.