a)The kite experiment

During summer 1752, in Philadelphia, Benjamin Franklin carry out the kite experiment to prove the similarities between electricity and lightning.

Benjamin Franklin supposed that he could withdraw electricity from a charged body with another point sharped one. So if the clouds are electrified, we could safely discharge them and protect people against lightning disasters.

We are going to explain his arguments with other terms.

(a) There is a separation of electrical charges when the metallic point is isolated.

(b) When the rod is linked to the ground, a steady current is flowing through the rod.

The picture (a) represents a metal rod which is negatively charged at the bottom and isolated under a stormy cloud. The electric field, vertically directed to the bottom, induces a separation of the charges in the rod. The negative charges in the air neutralize some positive charges at the top of the rod. Therefore, it gets negative charges in excess. When a conductor body linked to the ground, as a person, touches the isolated rod, it produces a spark. Whereas in picture (b), when a rod is linked to the ground, the negative charges flow from the rod to the ground. One can see a glimmer at the top of the stick.

Franklin decided to make a kite with two crossed-sticks and a silk handkerchief. He put an iron point on the vertical silk. At the end of the rope, he fixed a key.

During a thunderstorm, he went in the meadow with his son. He sheltered from the rain in a hangar and let the kite. Then, Benjamin Franklin noticed that some wisp of the rode moved away from each other and stiffened. He touched the key and a spark appeared. The phenomenon repeated. He proved then that the clouds are charged and that lightning is an electrical disaster.