LIGHTNING CONDUCTORS
Until comparatively recent times, as we have seen, all that was known
about thunderstorms was that they occurred pretty well all over the
world, and generally in either spring or summer.
While efforts were being made on our old continent to establish by
long and ingenious dissertations the exact degrees of relationship
between lightning and the sparks given out by machines, in America
practical experiments were being set about towards solving the
problems of electricity.
Franklin it was who hit upon the idea of extracting electricity from
the clouds for the purpose of investigation.
This man of immortal genius, who by his achievements in science, his
noble character, and his devotion to his country, has won the
admiration and gratitude of posterity, was of humble origin.
The son of a soap manufacturer in a small way of business, Benjamin
Franklin was born at Boston in 1706. His parents had intended him to
go in for science. He was successively an apprentice to a candle
manufacturer, a journeyman printer, the head of a big printing firm in
Philadelphia, deputy to Congress, an ambassador, and finally President
of the Assembly of the States of Pennsylvania. His political record
was a great one. No one ever rendered greater services to his country
than the diplomatist who signed the peace of 1783, and insured the
independence of the United States.
It was towards the age of forty that Franklin began his study of
electricity. Here is his own account of the memorable experiments to
which he owed the greater part of his immense fame:--
"In 1746 I met at Boston a certain Dr. Spence, who came from Scotland.
He performed some electrical experiments before me. They were not very
perfect, as he was not a man of great ability; but as the subject was
new to me they surprised me and interested me in an equal degree.
Shortly after my return to Philadelphia, our librarian received as a
gift from Pierre Collinson, a member of the Royal Society of London, a
tube of glass, together with certain written instructions as to the
way in which it should be used for experiments. I seized eagerly on
the chance of reproducing what I had seen done at Boston, and with
practice I acquired a great facility in performing the experiments
indicated to us from England and in devising other ones. I say 'with
practice,' because many people came to my house to witness these
marvels."
After making several discoveries in regard to electricity, Franklin
took it into his head to extract the fluid direct from the clouds. He
had established the fact that a stem of pointed metal, placed at a
great height--on the summit of a building, for instance--served as an
attraction to lightning and guided it into the way prepared for it. He
had been looking eagerly to the erection of a clock-tower which was
being built at this time at Philadelphia; but, tired of waiting and
anxious to carry out experiments which should solve all doubts, he had
recourse to a more expeditious instrument, and one, as events proved,
not less efficacious, for getting into touch with the region of
thunder--a kite such as children play with.
He prepared two sticks in the form of a cross, with a silk
handkerchief stretched upon them, and with a string attached of
suitable length, and set forth on his mission the first time there was
a storm. He was accompanied only by his son. Fearing the ridicule that
is showered upon failure, he did not take any one else into his
confidence. The kite was set flying. A cloud which looked promising
passed without result. Others followed, and the excitement with which
they were awaited can be imagined.
At first there was no spark and no sign of electricity. Presently some
filaments of the string began to move, as though they had been pushed
out, and a slight rustling could be heard. Franklin now touched the
end of the string with his finger, and instantly a spark was given
out, followed quickly by others. Thus for the first time the genius of
man may be said to have come to grips with lightning, and begun to
learn the secret of its existence.
This experiment took place in June, 1752, and made an immense
sensation throughout the world, and was repeated in other countries,
always with the same success.
A French magistrate, named de Romas, making use of Franklin's idea as
soon as it was known in France, took it into his head to use a kite
with raised cross-bars, and in June, 1753, before the full results of
Franklin's experiments were made public, secured still more remarkable
signs of electricity, having inserted a thread of metal throughout the
whole length of the string, which was 260 metres. Later, in 1757, de
Romas repeating his experiments during a storm, secured sparks of a
surprising size. "Imagine before you," he said, "lances of fire nine
or ten feet in length and an inch thick, and making as much noise as
pistol shots. In less than an hour I had certainly thirty lances of
this length, without reckoning a thousand shorter ones of seven feet
and under." Numbers of people, ladies among them, were present at
these experiments. They were not without danger, as may be imagined;
de Romas was once knocked over by an unusually heavy discharge, but
without being seriously hurt.
Franklin was the first to turn his experiments to practical account,
attaching lightning-conductors to public and private buildings for
their protection, and achieving marvellous results; the lightning
being caught by the metallic stem and following it obediently into the
ground.
From this time, lightning-conductors came into almost universal use,
and their value was not long in being generally recognized. Curiously
enough, France, which had been ahead of all other countries in the
study of electricity, was not one of the earliest to go in for
lightning-conductors. There were, indeed, signs of strong hostility to
their introduction. It was held even that they went against the
designs of Providence. In 1766, the Abbé Poncelet, in his work
entitled "La Nature dans la formation du tonnerre et la reproduction
des êtres vivants," in which he sets out to demonstrate that the force
which produces lightning is the same as that which causes the earth to
fructify, makes a strong protest against the construction of
lightning-conductors.
In 1782, nevertheless, at the reiterated request of Le Roi, a member
of the Academie des Sciences, and friend and admirer of Franklin, the
Louvre was endowed with the first lightning-conductor put up on a
public building in France. Soon afterwards they became common.
In 1784 the Academie des Sciences drew up the first set of rules for
the construction of lightning-conductors. It was revised and corrected
in 1823, in accordance with the various improvements that had been
introduced up till then, and it has been further added to in 1854,
1867, and 1903. These instructions point out that the most important
metallic portions of the building should be placed in communication
with the conductor, and this should sink into a well. Conductors that
are not perfectly constructed are a source of danger, instead of being
a protection, for the electric current is apt, instead of running down
into the earth, to make for any kind of metallic substance, and cause
great havoc.
The conductor ought really to communicate with a large body of
water--a body of water of greater extent than the storm cloud from
which the lightning comes. When the flow is insufficient, the water
itself is apt to become electrically charged. It is dangerous to bury
the conductor in merely damp soil; first, because one generally does
not know whether there is enough of this soil; secondly, because one
cannot be sure that the humidity will be sufficient at times of great
drought--the very times when storms are most to be feared. Failing a
river or great pond, the conductor should be put into wells issuing or
having their source in inexhaustible supplies of water deep down in
the soil.
In his table of statistics showing the number of cases in which
lightning has struck either lightning-conductors, or buildings, or
ships furnished with conductors, Quebelet gives a hundred and
sixty-eight cases in which the conductor has been struck, and in only
twenty-seven instances of these (one-sixth of the whole) have the
conductors, from some grave flaw in their construction, failed to
fulfil their office. These results are the best proof possible of the
efficacy of conductors, and the best answer to those who decry them.
The area of protection covered by the conductor is not so great as is
generally supposed. It is limited to a distance about three or four
times the length of the conductor above the roof. Thus a conductor
standing out five yards will protect an area stretching only about
fifteen or twenty yards away. This depends also to some extent upon
the nature of the place and the materials of which the house is
constructed.
Buildings are often struck by lightning because the number of
conductors has been insufficient for the extent of the edifice to be
protected.
To remedy this defect, conductors are made with a number of separate
stems--veritable wire traps in which to catch the lightning. This
system, the invention of a Belgian physicist, M. Melsens, decreases
considerably the risks of destruction, and is much more economical
than the erection of a number of separate conductors.
A conductor of this kind has been installed on the Hotel de Ville at
Brussels, which has been well protected from lightning ever since,
whereas previously this building had been struck by lightning several
times in spite of the single conductors with which it was supplied.
The metallic trellis is in communication with the sewers.
The slaughter-houses of La Villette, the Hôtel Evigné, and other
buildings in Paris, are provided with similar defences.
The Eiffel Tower boasts several such multiplex conductors. It has
often been struck by lightning, but no one who has happened to be up
it at the time has ever suffered any damage therefrom. The lightning
strikes the conductor sometimes from out the actual cloud--curious
photographs have been taken of this. The Eiffel Tower is in itself a
gigantic lightning-conductor.
Portable conductors have been invented from time to time--silk
umbrellas without iron ribs, and clothes of indiarubber and
such-like; but they have all been childish things.
* * * * *
Without allowing one's self to get lightning, so to speak, on the
brain, it is well to take certain precautions during a storm.
The first and principal one is not to get under a tree.
The second is to give a wide berth to telegraph posts, so as to avoid
contact with the sparks that may issue from them.
Movements of the air having the effect of preparing an excellent route
for the fluid, it is well not to run in a storm. It is well also not
to ring a bell.
It is well, also, to avoid being in the neighbourhood of animals, in
view of their attraction for lightning.
In houses, doors and windows should be closed in order to avoid
draughts. It is well to keep away from the chimney, too, as well as
from metallic objects.
But lightning always has its caprices. It is this that makes its study
so interesting.