An Account of the Kite Experiment
From Carl Van Doren's "Benjamin Franklin," ©1938 by Carl Van Doren
Before that he had
thought of another way of proving his theory, and with the help of his
electrical kite had drawn lightning from a cloud. The episode of the kite, so
firm and fixed in legend, turns out to be dim and mystifying in fact. Franklin
himself never wrote the story of the most dramatic of his experiments. All that
is known about what he did on that famous day, of no known date, comes from
Joseph Priestley's account, published fifteen years afterwards but read in
manuscript by Franklin, who must have given Priestley the precise, familiar details.
"As every
circumstance relating to so capital a discovery (the greatest, perhaps, since
the time of Sir Isaac Newton) cannot but give pleasure to all my readers, I
shall endeavour to gratify them with the communication of a few particulars
which I have from the best authority.
"The Doctor, having
published his method of verifying his hypothesis concerning the sameness of
electricity with the matter of lightning, was waiting for the erection of a
spire [on Christ Church] in Philadelphia to carry his views into execution; not
imagining that a pointed rod of a moderate height could answer the purpose;
when it occurred to him that by means of a common kite he could have better
access to the regions of thunder than by any spire whatever. Preparing,
therefore, a large silk
handkerchief and two
cross-sticks of a proper length on which to extend it, he took the opportunity
of the first approaching thunderstorm to take a walk in the fields, in which
there was a shed convenient for his purpose. But, dreading the ridicule which
too commonly attends unsuccessful attempts in science, he communicated his
intended experiment to nobody but his son" — then twenty-one, not a child
as in the traditional illustrations of the scene — "who assisted him in
raising the kite.
"The kite being raised, a
considerable time elapsed before there was any appearance of its being
electrified. One very promising cloud had passed over it without any effect;
when, at length, just as he was beginning to despair of his contrivance, he
observed some loose threads of the hempen string to stand erect, and to avoid
one another, just as if they had been suspended on a common conductor. Struck
with this promising appearance, he immediately presented his knuckle to the
key, and (let the reader judge of the exquisite pleasure he must have felt at
that moment) the discovery was complete. He perceived a very evident electric
spark. Others succeeded, even before the string was wet, so as to put the
matter past all dispute, and when the rain had wet the string he collected
electric fire very copiously. This happened in June 1752, a month after the electricians in France had
verified the same theory, but before he heard of anything they had done."
How Franklin
Made His Kite
Written by Benjamin Franklin to Peter Collinson, October 19, 1752
Make a small cross of
two light
strips of cedar, 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 wet and wind 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 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 of the twine, will stand out every way, and be attracted by an
approaching finger. And when the rain has wetted 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. At this key the phial may be
charged: and from electric fire thus obtained, spirits may be kindled, and all
the other electric experiments be 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.
For
Whom the Bells Toll
In the early 1750s, Franklin erected a
lightning rod on top of his house for the purposes of experimentation,
protection and, perhaps, to get electricity for experimentation without having
to go through the laborious process of creating it himself via a primitive
battery.
"fixed to the top
of my chimney and extending abut nine feet above it. From the foot of this rod
a wire (the thickness of a goose-quill) came through a covered glass tube in
the roof and down through the well of the staircase; the lower end connected
with the iron spear of a pump. On the staircase opposite to my chamber door the
wire was divided; the ends separated about six inches, a little bell on each
end; and between the bells a little brass ball, suspended by a silk thread, to
play between and strike the bells when clouds passed with electricity in them.
After having frequently
drawn sparks and charged bottles from the bell of the upper wire, I was one
night awakened by aloud cracks on the staircase. Starting up and opening the
door, I perceived that the brass ball, instead of vibrating as usual between
the bells was repelled and kept at a distance from both; while the fire passed,
sometimes in very large, quick cracks from bell to bell, and sometimes in a
continued, dense, white stream, seemingly as large as my finger, whereby the
whole staircase was enlightened with sunshine, so that one might see to pick up
a pin."
Legend has it that
Franklin's wife, Deborah, was so flustered by the ringing bells and flashing lights,
that she wrote to her husband who was off in London, asking him how to
disconnect the experiment.
Assault on Batteries
Before Franklin started his scientific
experimentation, it was thought that electricity consisted of two opposing
forces. Franklin
showed that electricity consisted of a "common element" which he
named "electric fire." Further, electricity was "fluid"
like a liquid. It passed from one body to another — however it was never
destroyed. In a letter to Peter Collinson, Franklin wrote that the:
"fire only
circulates. Hence have arisen some new items among us. We say B (and other
Bodies alike circumstanced) are electrolyzed positively;
A negatively; Or rather B
is electrolyzed plus and A minus ... These
terms we may use till
philosophers give us better."
- battery
- charge
- condenser
- conductor
- plus
- minus
- positively
- negatively
- armature
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