Franklin’s Bells: Converting Electrical Energy Into Continuous Mechanical Motion

Abstract

In a letter to his friend, Benjamin Franklin writes about an experiment he conducted. He writes, ”In September 1752, I erected an iron rod to draw the lightning down into my house, in order to make some experiments on it, with two bells to give notice when the rod should be electrified.” He adapted a setup made in 1742 by German professor, Andrew Gordon. Franklin’s Bells was an early demonstration of converting static electrical energy into continuous mechanical motion. Franklin’s setup consists of two metal vessels with a metal clapper hanging on a non-conductive line between the two. One vessel is placed on a non-conductive surface and the other is connected to a static electricity source. How Franklin’s Bells works is that when a positive charge is brought to one of the vessels, it attracts the negatively charged clapper. The clapper hits the vessel and then absorbs that charge and gets repelled. The now positively charged clapper gets attracted to the negatively charged vessel and the process repeats. It repeats until the charge has evened out again. The original Franklin’s Bells used an electrical rod for the electrical current and the setup rang during thunderstorms. Occasionally it would ring without thunder, hinting at electrical charges in the air. Franklin’s Bells is also easily replicated using two aluminum cans, a soda tab, some string, a straw, a plastic lid, and a static electricity source. Ground one aluminum can by placing it on top of a plastic lid. Place another can on the table, not on the plastic, about $1\frac{1}{2}$ inches away from the grounded can. Tie a soda tab to one end of the string, and secure the other end of the string around the middle of a straw. Balance the straw between the two cans so the tab hangs freely between the cans. To get the tab moving, bring a static electricity source near one can. Rub a pvc pipe with wool to collect static electricity and hold the pipe close to a can without touching it. Alternatively, take a balloon and rub it on your hair and then bring the balloon close to a can without touching it. The goal of this experiment was to help students understand how static electricity affects mechanical motion. There are not any modern examples for this experiment yet it still provides a good model for education. It is easy to learn about the history of how people came to learn about the effects with the model. Teachers can use this demonstration in their classroom to show how electrical energy can be converted into a continuous mechanical energy. This experiment is fun, low cost, highly effective, and can easily be replicated by students. I have been researching and recreating this experiment for three months. The opportunity was provided by a program run by Professor Littman at Princeton University called Community Project Studios (CPS). The letter mentioned in paragraph one is Letter XII, written by Benjamin Franklin to Peter Collinson in September, 1753.