Lightning is a dazzling bluish white light produced in the clouds. It is followed by a loud noise called thunder.
Till 1752, nobody really knew about the cause of lightning. Then, on 10 June, 1752, renowned physicist, Benjamin Franklin, by a brilliant experiment, proved that it is caused by the static electric charges in the clouds.
Benjamin Franklin’s Experiment
Benjamin Franklin made a kite of silk cloth. The central spar of the kite was made from iron wire about 30 cm long. The central spar was connected to a strong silk string. The other end of silk string was connected to an iron key. Next, he tied a thin metal wire from the key and inserted the wire into a Leyden jar, a jar that was used for storing an electrical charge in those days. He tied a silk ribbon to the key and held onto the kite via the silk ribbon.
Soon, the skies darkened with rain clouds and a thunderstorm approached. Franklin flew his kite from the safety of a shed, so that he would not get wet. The thunder storm caught his kite, and passed an electric charge onto his kite, down the wet silk string, to the key, and into the jar. Since Franklin had held onto the kite by a silk ribbon, which was dry, he remained unaffected by the negative charges that passed down the wet string of the kite, onto the key. However, when he brought his knuckles near the lower end of key, bright sparks were produced between the knuckles and key, and he received a jolt of an electric shock.
Why did this happen? The reason is that wet string became a conductor of electricity and hence allowed the electricity to flow from clouds to the key and then from the key to Benjamin’s knuckles. The negative charges in the key were so strongly attracted to the positive charges in his body that a spark jumped from the key to his hand.
Benjamin Franklin thus proved successfully that lightning consists of electrically charged particles.
Luckily, he was saved from death by the electric shock, because the dry silk ribbon lent him insulation. Several physicists after him, who tried the kite experiment, were electrocuted to death.
How do the clouds get electrically charged?
We know now that static electricity is produced by friction between two objects consisting of opposite charges. In the atmosphere, due to the heat of sun, the warm air and water vapours rise up. Similarly, the cold air above sinks down. When the air molecules rub against water molecules or the cold currents of air rub against the hot currents of air, then, owing to the friction, the electrons of air get transferred to water molecules or vice versa. All this depends upon atmospheric conditions.
When condensation takes place and clouds are formed, these clouds have a huge amount of static electric charges, which may be positive or negative.
How does lightning strike a building-
Imagine a negatively charged cloud passing over a high-rise building. This cloud induces positive charges on top of the building and negative charges at its base due to electric induction.
As the positive charges attract negative charges, the free electrons from the cloud start pushing their way through moist air. This forms a sort of conducting path. When these electrons strike the building, the charges from the cloud flow into the building. Thus, lightning strikes the building with a devastating effect, even setting it on fire.
Invention of the Lightning Rod-
Benjamin Franklin is credited with the invention of the lightning rod. Through his numerous experiments on electricity, Benjamin Franklin realized that this form of electricity could be passed through a conductor and into the ground. He then invented the lightning rod and conductor, which guides the lightning that strikes a building through an alternative path and safely into the earth. Much later, after he had invented his lightning rod, lightning struck his own house. However, his lightning rod saved his house from destruction.