Introduction

For a long time, humans have justified the lightning as god’s penalty. Then came the time they wanted to protect themselves from that natural phenomenon.

Benjamin Franklin was born in 1706 in Boston and died in 1790, in Philadelphia. He was a pioneer in the research about the lightning so there weren’t any precedent studies about the similarities between lightning and electricity. He had to lead a typically scientific step to advance the concern. As expected, his first question about the lightning was “What is lightning?” .He had in fact an intuition. Indeed He is the one who discovered the electric nature of the lightning. During the Age of Enlightenment, a century full of discoveries, Franklin’s lightning rod was one of the main inventions and never stopped evolving from 1752 to now.

What we are talking about is the initiative that took an inventor, at the risk of his life, to give out a shield to the humankind. We’ll study the lightning then, its similarities with electricity and Franklin’s legacy in the modern lightning rod.

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.

a) Components of a lightning protection system

Currently, the lightning conductor refers to all a protection system designed to protect people from the lightning, so the real lightning rod is only a small part of a complete lightning protection system. In fact, the rods may play the least important role in a system installation. A lightning protection system is composed of three main components:

Rods or air terminals: Rods can be found in different shapes, sizes and designs. Most are topped with a tall, pointed needle or a smooth polished sphere. The functionality of different types of lightning rods, and even the necessity of rods are subjects of many scientific debates.

Conductor cables: These heavy cables are made of copper or aluminum. They connect air terminals to the grounds cables and carry lightning current from the rods to the ground. Cables are run along the tops and around the limits of roofs, then down one or more corners of a building to the ground rod(s).

Ground rods or protection grounds: Long, heavy rods buried deep into the earth, around a protected structure. Main conductors are set at least 300 meters deep in the earth and are attached to metal grounding rods. Special requirements are sometimes necessary in sandy or rocky soil. The conductor cables are connected to these rods to complete a safe path for a lightning discharge around a structure. The energy is directed into the ground as current flows through the rods. Then the chance for injury or damage is eliminated.

The conductor cables and ground rods are the most important components of a lightning protection system, which  divert lightning current safely through the structure. A full protection setup composed of good cable coverage and good grounding, would still work sufficiently without the air terminals.

The French connection: Marly-la-Ville experiment

To improve Franklin’s thoughts about electricity, the physicists had to check thunderclouds are electrified. So three of them, Buffon, Dalibard and Delor carried out the experience and wait a thunderstorm. M. Dalibard chose for this purpose a large open area, situated at Marly-La-Ville, where he placed a pointed bar of iron, twenty-meter high. Silk ropes (g) and wine bottles (e) insulated a 13 meter iron rod (a) from ground, and covers (h) protected the ropes from rain. Then the 10th of May 1752, twenty minutes past two in the afternoon, a stormy cloud having passed over the place where the bar standing, people who were appointed to observe it, drew near and attracted from it sparks of fire, perceiving the same kind of reactions as in the common electrical experiments. The result of all the tests and observations were related in Dalibard’s mémoires and especially of the last test done at Marly-la-ville, is that the matter of thunder is incontestably the same as that of electricity. Franklin’s theory ceases to be a conjecture; it has become a reality. The sparks drawn at Marly-la-Ville proved, for the first time, that thunderclouds are electrified and that lightning is an electrical discharge.