Monday, July 6, 2009

The Nature of Electricity

Electricity is one of our greatest energy resources and one of the few natural energy sources. Franklin’s electricity experiments were the first scientific ventures into the nature and use of electricity and uncovered its true nature. They set the stage for much of the scientific and engineering development in the nineteenth century and for the explosion of electrical development—batteries, motors, generators, lights, etc.

How Was It Discovered?


All that was known about electricity in the mid-eighteenth century was that there were two kinds of it: playful static and deadly lightning. Benjamin Franklin was the first scientist to begin serious electrical experiments (in 1746). He was also the first to suspect that static and lightning were two forms of the same thing.

Franklin had been experimenting with Leyden jars—large glass jars, partially filled with water and wrapped with tin foil both inside and out. A rod extended through an insulating cork out the top of the jar to a metal knob. Once a Leyden jar was charged with a hand crank, any one who grabbed the knob got a resounding shock.

Franklin found ways to more than double the amount of electricity his Leyden jars carried, and he invented a way to connect them in series so that they could, collectively, carry an almost deadly punch.

During a 1752 demonstration for friends, Franklin accidentally touched a Leyden jar’s knob. With a sharp crack, a sizzling blue arc leapt from jar to Franklin’s hand. He shot back half a dozen feet and crashed to the floor. Franklin realized that that jolt looked exactly like a mini lightning bolt.

He decided to prove that static and lightning were the same by designing a Leyden jar–like electric circuit to let electricity flow from clouds just as it did into a jar.

Franklin’s “circuit” was made of a thin metal wire fixed to a kite (to gather electricity from the clouds) and tied to a twine kite string. Electricity would flow down the twine to a large iron key tied to the bottom. Franklin tied the other end of the key to a nonconducting silk ribbon that he would hold. Thus, electricity would be trapped in the key, just as it was in a Leyden jar.

When an after noon storm brewed up dark and threatening a few weeks later, Franklin rushed to launch his kite. The wind howled and the clouds boiled. A cold rain pounded down about Franklin’s up turned collar. The kite twisted and tore at the air like a ram paging bull.

Then it happened. No, a lightning bolt did not strike the kite, as has of ten been reported. And a good thing, too. A French scientist was killed a few months later by a lightning strike when he tried to repeat Franklin’s experiment. No, what hap pened that stormy after noon was that the twine began to glow a faint blue. The twine’s fibers lifted and bristled straight out. Franklin could almost see electricity trickling down the twine as if electricity were liquid.

Franklin reached out a cautious hand closer and closer to the key. And pop! A spark leapt to his knuckle and shocked him—just like a Leyden jar.

Lightning and static were all the same, fluid electricity!

The practical out come of this experiment was Franklin’s invention of the lightning rod, credited with saving thousands of houses and lives over the next 100 years. More important, Franklin’s work inspired experiments by Volta, Faraday, Oersted, and others in early part of the nineteenth century that further unraveled electricity’s nature.

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