Franklin's Kite
Probably the most famous experiment to do with lightning is that of Benjamin Franklin and his famous kite.
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!
