Encyclopaedia Britannica > Volume 8, DIA-England

ELECTRICITY.
] ory. plus and minus electricity which was afterwards made by
Franklin, was distinctly announced by Sir W. Watson.
He lays it down as a law, that in electrical operations
there is an afflux of electric fluid to the globe and the con¬
ductor, and also an efflux of the same matter from them.
In the case of two insulated persons, the one in contact
with the rubber and the other with the conductor, he
observed that either of them would communicate a much
stronger spark to the other than to any bystander. The elec¬
tricity of the one, he says, became more rare than it is natu¬
rally, and that of the other more dense, so that the density
of the electricity in the two insulated persons differed
more than that between either of them and a bystander.
Our limits will not permit us to give a detailed account
of the various ingenious experiments which were about
this time made by Le Monnier, Nollet, Winckler, Ellicott,
Jallabert, Boze, Menon, Smeaton, and Miles. In 1746
Le Monnier confirmed the result previously obtained by
Mr Gray, that electricity is communicated to homogene¬
ous bodies in proportion to their surfaces only. M. Boze
discovered that capillary tubes which discharged water by
drops afforded a continued stream when electrified. The
Abbe Nollet ascertained that electricity increases the
natural evaporation of fluids, and that the evaporation is
hastened by placing them in non-electric vessels. M.
Jallabert confirmed the result previously obtained by
Watson, that electricity passes through the substance of
a conducting wire, and not along its surface. Smeaton
found that the red-hot part of an iron bar could be as
strongly electrified as the cold parts on each side of it.
Dr Miles kindled common lamp spirits by a stick of black
sealing-wax excited by dry flannel. Mr Ellicott conceiv¬
ed that the particles of the electric fluid repel each other,
while they attract those of all other bodies. Mr Mowbray
discovered that the vegetation of two myrtles was hasten¬
ed by electrifying them ; a result which Nollet confirmed
in the case of vegetating seeds. The Abbe Menon found
that cats, pigeons, sparrows, and chaffinches, lost weight
by being electrified for five or six hours, and that the same
result was true of the human body; and hence it was con¬
cluded that electricity augments the insensible perspira¬
tion of animals.
1 o. Passing over the scientific fables of John Pivati of Ve-
j nice, we arrive at that auspicious period when Dr Frank-
l' lin raised electricity to the dignity of a science, and con-
1.1, ' nected it with that tremendous agency which had so of¬
ten terrified the moral and convulsed the physical world.
The thunderbolt had frequently descended from heaven
upon its victims; but mortal genius had now learned to
bring it down in chains, to disarm its fury, and to convert
it into an useful and even a friendly element.
One of the first labours of the American philosopher
was to present, in a more distinct form, the theory of plus
and minus electricity, which Sir W. Watson had been
the first to suggest. He showed that electricity is not
created by friction, but merely collected from its state of
diffusion through other matter by which it is attracted.
He asserted that the glass globe, when rubbed, attracted
the electrical fire, and took it from the rubber, the same
globe being disposed, when the friction ceases, to give out
its electricity to any body which has less. In the case of
the charged Leyden jar, the inner coating of tinfoil had
received more than its ordinary quantity of electricity,
and was therefore electrified positively or plus, while the
outer coating of tinfoil having had its ordinary quantity of
electricity diminished, was electrified negatively or minus.
Hence the cause of the shock and spark when the jar is
discharged, or wdien the superabundant plus electricity of
the inside is transferred by a conducting body to the de¬
fective or minus electricity of the outside. This theory
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569
of the Leyden phial Franklin established in the clearest History,
manner, by showing that the outside and the inside coat-
ing possessed opposite electricities, and that, in charging
it, exactly as much electricity is added on one side as is
subtracted from the other. The copious discharge of
electricity by points was observed by Franklin in his ear¬
liest experiments, and also the power of points to conduct
it copiously from an electrified body. Hence he was fur¬
nished with a simple method of collecting electricity
from other bodies; and he was thus enabled to perform
those remarkable experiments which we shall now pro¬
ceed to explain.
Hawksbee, Wall, and Nollet had successively suggested
the similarity between lightning and the electric spark,
and between the artificial snap and the natural thunder.
Previous to the year 1750 Dr Franklin drew up a state¬
ment, in which he showed that all the general phenomena
and effects which were produced by electricity had their
counterpart in lightning. Like the electric spark, light¬
ning moves in a crooked and irregular direction. Light¬
ning strikes the highest and most pointed bodies, and
electricity does the same. They both inflame combus¬
tibles, fuse metals, destroy animal life, produce blindness
in animals, render common sewing needles magnetic, and
reverse the polarity of needles that have been magnetis¬
ed. Notwithstanding these points of resemblance, direct
experiment was still necessary to establish his views. He
waited anxiously for the erection of a spire at Philadel¬
phia, by means of which he might bring down the elec¬
tricity of a thunder-storm; but his patience being ex¬
hausted, he conceived the idea of sending up a kite among
the clouds themselves. With this view he made a small
cross of two light strips of cedar, the arms being sufficient¬
ly long to reach to the four corners of a large thin silk
handkerchief when extended. The corners of the hand¬
kerchief w'ere tied to the extremities of the cross, and
when the body of the kite was thus formed, a tail, loop,
and string were added to it. The body was made of silk
to enable it to bear the violence and wet of a thunder¬
storm. A very sharp pointed wire was fixed at the top
of the upright stick of the cross, so as to rise a foot or
more above the wood. A silk ribband was tied to the
end of the twine next the hand, and a key suspended at
the junction of the twine and silk. In company with his
son, Franklin raised the kite like a common one, in the
first thunder-storm, which happened in the month of June
1752. To keep the silk ribband dry, he stood within a
door, taking care that the twine did not touch the frame
of the door ; and when the thunder-clouds came over the
kite he watched the state of the string. A cloud passed
without any electrical indications, and he began to despair
of success. He saw, however, the loose filaments of the
twine standing out every way, and he found them to be at¬
tracted by the approach of his finger. The suspended key
gave a spark on the application of his knuckle, and when
the string had become wet with the rain, the electricity
became abundant; a Leyden jar was charged at the key,
and by the electric fire thus obtained spirits were inflam¬
ed, and all the other electrical experiments performed
which had been formerly made by excited electrics. In
subsequent trials with another apparatus, he found that
the clouds were sometimes positively and sometimes ne¬
gatively electrified, and thus demonstrated the perfect
identity of lightning and electricity.
Having thus succeeded in drawing the electric fire from
the clouds, Franklin immediately conceived the idea of
protecting buildings from lightning, by erecting on their
highest parts pointed iron wires or conductors communi¬
cating with the ground. The electricity of a hovering
or a passing cloud would thus be carried off’ slowly and