In the past, many people believed that thunder
and lightning were punishments from God. Lightning
came from the sky, so they believed that it
came from God. It was powerful. It could cause
great damage. But no one knew what lightning
really was. That changed in 1752. Today’s
Spotlight is on the experiment that showed
the true nature of lightning.
Benjamin Franklin was a scientist and statesman.
He lived almost 300 years ago, in the United
States. Franklin is mainly known today as
a political person. His writings greatly influenced
the United States, and many other countries
too. But he is also known as a scientist and
inventor. His most important discoveries were
about electricity.
At that time, electricity was a new discovery.
Franklin did many experiments with electricity
in his home. He began to think that lightning
acted like electricity in many situations.
Franklin suspected that lightning was an electrical
current. But he wanted to test his theory.
He developed an experiment. The experiment
would discover if lightning would pass through
metal. He decided to use a metal key. But
first, he had to find a way of linking the
key to the lightning - high up in the sky.
Franklin’s experiment is quite famous. However,
Franklin did not record this experiment until
much later. So some experts believe that it
did not happen the way stories describe it.
Scientists debate the details of the experiment.
They say that it could not have happened exactly
as most people imagine. It is unclear how
Franklin actually did the experiment. But
Franklin’s results are clear. Later experiments
proved that he was correct. Most modern versions
agree that it happened something like this.
The story says that Franklin decided to use
a kite. We usually only see children playing
with kites. They like to make them using brightly
coloured paper, sticks, and very thin string.
They stretch the paper over the sticks and
attach the string to the sticks. And then
they hold onto the long string and run. The
wind carries the kite through the sky.
Franklin made his kite from light cloth. He
tied a metal key to the bottom. He expected
the lightning to strike the kite. He believed
that the lightning would flow down the kite
string to the key at the end.
The experiment took place in Philadelphia,
Pennsylvania. It was June, 1752. Franklin
and his son waited for a thunderstorm. When
the storm began, they carried the kite outside.
They flew it high. It entered a rain cloud.
Franklin put his hand on the key. And then,
bang! Electricity flowed through. The experiment
had worked! The lightning behaved like electricity.
It flowed through wet kite-string and the
metal key!
Some modern scientists say that the lightning
would have killed Franklin. But they do suggest
other possible ways that the kite experiment
could have happened. For example, they suggest
that lightning did not strike Franklin’s
kite. They say that the string got electricity
from the air in the storm.
No matter who is right about the details of
the experiment, Franklin’s theory still
caused many people to investigate lightning.
Now, scientists agree that lightning is electricity.
And scientists know much more about lightning.
They tell us that:
On average, there are 100 lightning strikes
every second around the world.
Lightning can send out a million volts of
electricity! The temperature in a lightning
bolt is about 30,000 degrees Celsius.
Lightning can strike in half a second. In
that time, the lightning heats the surrounding
air. It heats it to an extremely high temperature
- five times hotter than that on the sun’s
surface! The heat causes the air to expand.
This makes a sound. We hear this sound as
thunder. Although this happens at the same
time, we hear the thunder after the lightning.
The reason for this is simply because sound
travels more slowly than light.
Lightning disappears quickly in the air. But
it does sometimes leave something behind:
fulgurites. The word comes from the Latin
word for lightning, ‘fulgur’. The best-known
fulgurites are sand fulgurites. These are
found under the surface of sand.
Sand fulgurites form when lightning hits sand.
They are thin glass tubes under the sand.
The heat from the electrical current is very
intense. It melts the sand into glass. Fulgurites
usually take the shape of the roots of a tree.
The outside of the thin tubes is silica glass.
Scientists can easily make silica glass in
laboratories. But it is very rare to find
it in nature. Fulgurites are not usually longer
than a few centimetres. This is because movement
of the sand often breaks the fine tubes. They
are after all, glass.
Rock fulgurites form in a similar way. They
form when lightning strikes the surface of
rock. The lightning melts the rock’s surface.
It can also melt the inner part of the rock.
Rock fulgurites are even less common than
sand fulgurites.
The formation of fulgurites shows just how
powerful lightning is. Lightning can cause
great damage to buildings - especially tall
buildings. Benjamin Franklin invented something
simple to prevent this damage. His invention
is called a lightning rod. Lightning rods
are tall thin pieces of metal. A rod may have
a pointed end, or a ball at the end. People
place lightning rods at the top of a building.
The rod is the tallest thing on the building.
A wire goes from the rod to the ground. When
lightning strikes, the electricity goes through
the rod to the ground. The lightning does
not affect the building. Lightning rods became
very popular because of Franklin. They have
saved many buildings!
From electricity to a lightning rod that saves
buildings. Benjamin Franklin asked questions.
He changed ideas. And he invented something
that still helps people today! All discoveries
start in the same place. What questions do
you have? What can you create?
