Lightning bell
The Lightning bell is an electrostatic device invented circa 1742 by Andrew Gordon, Professor of Natural Philosophy at the University of Erfurt in Germany. The device converted electrical energy into mechanical energy in the form of a repeating mechanical motion.
Construction
It consisted of two metal bells, one electrically connected to the earth (grounded) and the other connected to a lightning rod. Hanging between the two bells was a metallic ball suspended by an insulating (dielectric) thread. The lightning rod allows an electric charge to build up on one bell, which then attracts the metallic ball. When the ball hits this charged bell it becomes charged to the same potential and is immediately repelled. Since the grounded bell is charged oppositely, this attracts the ball towards it. When the ball touches and rings the grounded bell, the charge is transferred and the process repeats.
Benjamin Franklin
In 1752 Benjamin Franklin used the set-up to warn him of approaching thunderstorms. Although invented by Gordon, it is most often referred to as Franklin’s (lightning) bells.