c) Laws on electricity : Point Power

During the kite experiment, Benjamin Franklin put an iron stalk on the kite. As he wanted to prove the similarities between lightning and electricity, that was an exellent reasoning.

Indeed, a law regarding electricity was discovered after Franklin’s life-time. It tells us the smaller an object is, the stronger its electric field is near it. The law involved is Gauss’ law.

Where E is the value of the electric field in a point near the sphere,

1/(4πε) is a constant ( also called k),

Q the total electric charge of the sphere,

and r the radius from the source to the point.

E(S1)= k*Q*(1/R²)= (k*Q)/(2r)²= (k*Q)/r² * ¼

= ¼ E(S2)

E(S2)= k*Q*(1/r²)= (k*Q)/r²= (k*Q)/r²= 4 E(S1)

E is inversely proportional to r squared

That proves that with a radius twice smaller, S2 owns an electric field 4 times stronger near it.

If we apply that principle to an infinitely small object, its electric field will be infinitely strong near it. And if we identify a point as a small object, its electric field is strongly worth. It is an important factor that allows the ionization of the air particles wich determines the path of the lightning.

b) Benjamin Franklin’s thoughts and his letters

Benjamin Franklin wrote about his experiments dealing with the similarities between electricity and lightning in five formal letters from 1747 to 1752. He addressed his letter to a Fellow of the Royal Society of London Peter Collinson, who provided him with equipments he needed. The English man published them in a pamphlet entitled Experiments and Observations on Electricity made at Philadelphia in America, by Benjamin Franklin and communicated in several letters.

In his first letters, Benjamin Franklin described “the wonderful effect of pointed bodies, both in drawing off and throwing off the electrical fire”. He showed that sharp points work better that blunt bodies. He proved that it is more efficient to use metal points than dry wood. Besides, the pointed object would be grounded in order to obtain a maximum draw effect.

In the second letter, Benjamin Franklin explained the electrical mechanism of the leyden jar, the first electrical capacitor. He started to use the term “charge” and “discharge” when describing the Leyden jar in the third letter. And he described an electrical battery which he used later to simulate the effects of lightning in a variety of material.

Based on his previous experiments with the power of point, in his fourth letter, Benjamin Franklin speculated that when an electrified cloud passes over a region, it will draw electricity and discharge high hills and trees, towers, chimney etc.

In his last letter, he attempted to explicate the power of point. Sharp points discharge silently and produce large effects at greater distance contrary to blunt bodies. Then, he stated what he called “Law of Electricity”: the point will tend to “draw on and throw off the electrical fluid with more or less power and at a greater or smaller distances, and in larger or smaller quantities in the same time” as the angle of point is more or less acute.

From his earlier experiments, Benjamin Franklin knew that tall objects were preferred places for lightning to strike. Therefore, even if the point discharges did not neutralize the cloud, grounded conductor would provide a safe path for the lightning to go to ground.

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.