Benjamin Franklin was born in Boston in January, 1706, and died in April, 1790. 20,000 people attended his funeral in Philadelphia, so he was clearly a special individual. During his 84 years he was a printer, a statesman, an inventor, the Postmaster General, and an experimenter in static electricity.
For us in SC526, Ben Franklin is relevant for a number of reasons, including:
- Franklin's first year of school was at the age of eight when he attended the South Grammar School in Boston, which is now the Boston Latin School.
- He came up with many of the terms we still use today in talking about electricity, including charge, plus, minus, positive, negative, battery, conductor, and electric shock. He also came up with the idea that the charged particles are not used up, but instead flow from one place to another much like a fluid. This fluid model is one we will come back to when we talk about circuits.
- In June of 1752 he carried out his famous kite experiment, although most likely he did not do the experiment in a lightning storm, which would have been extremely dangerous. He did build a simple kite, however, with a metal rod mounted on it. The kite was tied to a string, and the string had a key tied to the other end. Franklin actually held a piece of silk ribbon that was also tied to the bottom end of the string, to make sure the charge (or the "electric fire", as he called it) he drained out of the cloud would not go directly to him, and he realized the importance of keeping the silk ribbon dry so it would be a good insulator. Franklin proved without a doubt that the charge he drained from the cloud was going to the key by drawing sparks from the key to his knuckle, and he also used the charge to do some electric experiments. By showing that the experiments worked the same way when he did them with the charge from the cloud as when he did them with rubbed glass rods, Franklin proved that the two phenomena were the same.
- Franklin also invented the lightning rod, which is a simple concept but a very important safety device. A typical lightning rod is a vertical pointed metal rod that is mounted on the roof of a house, and then electrically connected to the Earth (this is called a ground connection). Without a lightning rod a house is in danger of being struck by lightning, whereas with a lightning rod the electric charge in the cloud is safely drained away into the ground.
The lightning rod is an important safety device unless it is used as Franklin used it, in fact! Instead of being connected to ground his lightning rod was connected to a bell, with another bell connected to ground a short distance away. When charge built up on the first bell a brass ball between the bells would go back and forth between the bells, discharging the system and causing the bells to ring. Occasionally, however, there would be large sparks passing directly between the two bells.
Brief descriptions of the web links
Bob Morse, a high-school teacher who had a Wright Fellowship at Tufts earlier this year, has put together a terrific site about Benjamin Franklin, including Franklin's own writings about electricity, and a large collection of experiments using everyday items like aluminum foil, film canisters, and styrofoam cups. These are experiments, and building projects, you could consider doing with your own students.
"The Electric Ben Franklin" is a general site that discusses all sorts of different aspects of Franklin's life, and even has Franklin's autobiography on-line.
In the PBS site look for the "How Shocking" part, which has three interactive components that you could show to your students to learn about static electricity, Franklin's kite experiment, and lightning rods. There are also some other good resources, including a teachers guide, on the PBS site, about Franklin.