Europe,
hitherto somewhat sceptical of
this time convinced of the identity of lightning and electricity.
It was
now
a rod, one hundred feet high, became electrified during a storm
did not necessarily prove that the storm-clouds were electrified.
A rod of that length was not really projected into the cloud, for
even a very low thunder-cloud was more than a 16316l111q hundred feet above
the ground. Irrefutable proof could only be had, as he saw it, by
"extracting" the lightning with something actually sent up into
the
storm-cloud; and to accomplish this
kite, with which he finally demonstrated to his own and the
world's satisfaction that his theory was correct.
Taking his kite out into an open common on the approach of a
thunder-storm, he flew it well up into the threatening clouds,
and then, touching, the suspended key with his knuckle, received
the electric spark; and a little later he charged a Leyden jar
from the electricity drawn from the clouds with his kite.
In a brief but direct letter, he sent an account of his kite and
his experiment to England:
"Make a small cross of two light strips of cedar," he wrote, "the
arms so long as to reach to the four corners of a large, thin,
silk handkerchief when extended; tie the corners of the
handkerchief to the extremities of the cross so you have the body
of a kite; which being properly accommodated with a tail, loop,
and string, will rise in the air like those made of paper; but
this being of silk is fitter to bear the wind and wet of a
thunder-gust without tearing. To the top of the upright stick of
the cross is to be fixed a very sharp-pointed wire, rising a foot
or more above the wood. To the end of the twine, next the hand,
is to be tied a silk ribbon; where the silk and twine join a key
may be fastened. This kite is to be raised when a thunder-gust
appears to be coming on, and the person who holds the string must
stand within a door or window or under some cover, so that the
silk ribbon may not be wet; and care must be taken that the twine
does not touch the frame of the door or window. As soon as any of
the thunder-clouds come over the kite, the pointed wire will draw
the electric fire from them, and the kite, with all the twine,
will be electrified and the loose filaments will stand out
everywhere and be attracted by the approaching finger, and when
the rain has wet the kite and twine so that it can conduct the
electric fire freely, you will find it stream out plentifully
from the key on the approach of your knuckle, and with this key
the phial may be charged; and from electric fire thus obtained
spirits may be kindled and all other electric experiments
performed which are usually done by the help of a rubbed glass
globe or tube, and thereby the sameness of the electric matter
with that of lightning completely demonstrated."[5]
In experimenting with lightning and Franklin's pointed rods in
Europe, several scientists received severe shocks, in one case
with a fatal result. Professor Richman, of St. Petersburg, while
experimenting during a thunder-storm, with an iron rod which he
had erected on his house, received a shock that killed him
instantly.
About 1733, as we have seen, Dufay had demonstrated that there
were two apparently different kinds of electricity; one called
VITREOUS because produced by rubbing glass, and the other
RESINOUS because produced by rubbed resinous bodies. Dufay
supposed that these two apparently different electricities could
only be produced by their respective substances; but twenty years
later, John Canton (1715-1772), an Englishman, demonstrated that
under certain conditions both might be produced by rubbing the
same substance. Canton's experiment, made upon a glass tube with
a roughened surface, proved that if the surface of the tube were
rubbed with oiled silk, vitreous or positive electricity was
produced, but if rubbed with flannel, resinous electricity was
produced. He discovered still further that both kinds could be
excited on the same tube simultaneously with a single rubber. To
demonstrate this he used a tube, one-half of which had a
roughened the other a glazed surface. With a single stroke of the
rubber he was able to excite both kinds of electricity on this
tube. He found also that certain substances, such as glass and
amber, were electrified positively when taken out of mercury, and
this led to his important discovery that an amalgam of mercury
and tin, when used on the surface of the rubber, was very
effective in exciting glass.
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