What Franklin was investigating was whether or not lightning was an electric phenomenon. This seems fairly obvious to most of us today, but we must remember that in Franklin's day the largest sparks they could make were under an inch long! Since lightning is several miles long it is not so obvious that they can be the same.
The
question often arises whether or not Franklin actually did this
experiment, and the answer is we do not know for sure. One thing,
however, is certain: if he did do an experiment like this, he did not
do it the way it is often shown. That is, he didn't tie a key to the
kite string, fly it in a thunderstorm, and wait for it to be struck
by lightning! Such an experiment would be very dramatic--and quite
fatal.
There are safe ways to do similar things, however, and Franklin, in
his various writtings, shows that he was quite aware of both the
dangers and the alternatives.
Franklin
realized that if lightning was electricity, then it must be an awful
lot of the stuff, and that it must take a long time to amass in the
storm. Therefore, he suggested, fly the kite early in the storm
before the lightning comes near you.
He had several variations on how to show electricity was
present--you could draw sparks from a key tied to the string, or you
could attach the string to a Leyden Jar, which is a device for
collecting electricity (a capacitor). If the jar was empty before
flying the kite and full afterwards then that is good evidence that
thunderclouds contain electricity.
In our
demonstration the kite is suspended from a plastic rod (since there
is no wind). The string goes down into a jar (not a Leyden Jar) where
it is attached to a key. Below the key is a gap of 1-2 inches before
a grounded metal ball.
Notice in the picture on the left (which was taken with the
machine running, but not strongly enough to make sparks) that the
kite's tail is being pulled towards the dome of the machine. This is
a big example of static cling--the same thing that sticks your
clothes together in a clothes drier. Notice also that there is a
spark jumping between the key and the ball inside the jar.
There is
one other effect that we demonstrate using our kite, but it is too
faint to photograph easily. It is called Saint Elmo's Fire, or, to
scientists, corona. It shows up as a faint purple glow around the
edges of the kite, and would have been seen in the rigging and masts
by sailors on the old wooden sailing ships.
To the sailors, who named it after their patron saint, it was a sign of protection, but you can see in this picture how little it protects the kite! In fact it is a sign of great danger. So how did the sailors get it wrong?
The reason is probably that all sailors in thunderstorms saw
coronas, but only those whose ships were not struck and destroyed
made it back to tell anyone about it!
A
video clip of the kite being struck (417k, 5
seconds). It works best if you play it looped.