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_Effects of Lightning_.
The analogy between the electric spark, and more especially of the
explosive discharge of the Leyden jar, with atmospheric lightning
and thunder, is too obvious to have escaped notice, even in the
early periods of electrical research. It had been observed by Dr.
Wall and by Gray, and still more pointedly remarked by the Abbé
Nollet. Dr. Franklin was so impressed with the many points of
resemblance between lightning and electricity, that he was convinced
of their identity, and determined to ascertain by direct experiment
the truth of his bold conjecture. A spire which was erecting at
Philadelphia he conceived might assist him in this inquiry; but,
while waiting for its completion, the sight of a boy's kite, which
had been raised for amusement, immediately suggested to him a more
ready method of attaining his object. Having constructed a kite by
stretching a large silk handkerchief over two sticks in the form of
a cross, on the first appearance of an approaching storm, in June
1752, he went out into a field, accompanied by his son, to whom
alone he had imparted his design. Having raised his kite, and
attached a key to the lower end of the hempen string, he insulated
it by fastening it to a post, by means of silk, and waited with
intense anxiety for the result. A considerable time elapsed without
the apparatus giving any sign of electricity, even although a dense
cloud, apparently charged with lightning, had passed over the spot
on which they stood. Franklin was just beginning to despair of
success, when his attention was caught by the bristling up of some
loose fibres on the hempen cord; he immediately presented his
knuckle to the key, and received an electric spark. Overcome with
the emotion {42} inspired by this decisive evidence of the great
discovery he had achieved, he heaved a deep sigh, and conscious of
an immortal name, felt that he could have been content if that
moment had been his last. The rain now fell in torrents, and wetting
the string, rendered it conducting in its whole length; so that
electric sparks were now collected from it in great abundance.
It should be noticed, however, that about a month before Franklin
had made these successful trials, some philosophers, in particular
Dalibard and De Lors, had obtained similar results in France, by
following the plan recommended by Franklin. But the glory of the
discovery is universally given to Franklin, as it was from his
suggestions that the methods of attaining it were originally
derived.
This important discovery was prosecuted with great ardour by
philosophers in every part of Europe. The first experimenters
incurred considerable risk in their attempts to draw down
electricity from the clouds, as was soon proved by the fatal
catastrophe, which, on the 6th of August, 1753, befel Professor
Richman, of Petersburg. He had constructed an apparatus for
observations on atmospherical electricity, and was attending a
meeting of the Academy of Sciences, when the sound of distant
thunder caught his ear. He immediately hastened home, taking with
him his engraver, Sokolow, in order that he might delineate the
appearances that should present themselves. While intent upon
examining the electrometer, a large globe of fire flashed from the
conducting rod, which was insulated, to the head of Richman, and
passing through his body, instantly deprived him of life. A red spot
was found on his forehead, where the electricity had entered, his
shoe was burst open, and part of his clothes singed. His companion
was struck down, and remained senseless for some time; the door-case
of the room was split, and the door itself torn off its hinges.
The protection of buildings from the effects of lightning, is the
most important practical application of the theory of electricity.
Conductors for this should be formed of metallic rods, pointed at
the upper extremity, and placed so as to project a few feet above
the highest part of the building they are intended to secure; they
should be continued without interruption till they descend into the
ground, below the foundation of the house. Copper is preferable to
iron as the material for their construction, being less liable to
destruction by rust, or by fusion, and possessing also a greater
conducting power. The size of the rods should be from half an inch
to an inch in diameter, and the point should be gilt, or made of
platina, that it may be more effectually preserved from corrosion.
An important condition in the protecting conductor is, that no
interruption should exist in its continuity from top to bottom; and
advantage will result from connecting together by strips of metal
all the leaden water pipes, or other considerable masses of metal in
or about the building, so as to form one continuous system of
conductors, for carrying the electricity by different channels to
the ground. The lower end of the conductors should be carried down
into the earth till it reaches either water, or at least a moist
stratum.--_Library of Useful Knowledge._
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The Sketch-Book.