Wednesday, June 10, 2015
Ben Franklin was standing out in a thunderstorm. He was flying a kite and the string holding it was wet (allowing it to conduct electricity) with a metal key at the bottom of the string to accumulate electrical change. There was a wire (or more wet string?) connecting the key to a Leyden Jar (an early form of capacitor which can store electrical charge). While some people say the experiment never really happened (or, they say, Franklin would have been electrocuted), the kite was almost certainly NOT struck by lightning or Mr. Franklin would have died. As it was, other people who tried the experiment did die. So, don't try this yourself. Lightning and static electricity is notoriously unpredictable.
But lightning didn't have to strike the kite to prove Franklin's conjecture. His idea (built up from the ideas of others) was that lightning is a form of electricity and it is. That electric charge builds up in the atmosphere as clouds move through the atmosphere "rubbing" against the atmosphere kind of like your feet rub on a carpet and build up a static charge. So, just the fact that the clouds were moving fast and building up charge in the atmosphere (in preparation for a lightning discharge), Franklin's Leyden Jar did accumulate a charge from what was in the atnosphere, transferring to the kite, running down the wet string, into the key and on into the jar and he was able to prove his point. He went on to do a lot more experiments with electricity even setting the precedent that electrical current flows from positive to negative which is still used in most electrical engineering texts. In reality, the flow of electrons (the current) is from negative to positive but that's OK. We know what's really going on. And the preponderance of texts and writing with current flowing from positive to negative is too great to change now. Just keep it in the back of your mind when you're tracing electrons.
The picture I showed above is from the Bureau of Engraving and Printing and can be found at this link. That's Franklin's son William helping him. It reminds me of the time my own son helped me put up some fence in our backyard and it began to rain. He stayed out with me to help because we were rushing to get it done for a deadline. He got wet and muddy and didn't complain. He stayed to the end and seemed very happy when we finished.