Franklin bells are an early demonstration of electric charge designed to work with a Leyden jar. They were invented by Benjamin Franklin in the 18th century during his experimentation with electricity. Franklin bells are only a qualitative indicator of electric charge and were used for simple demonstrations rather than research.
The bells consist of a metal stand with a crossbar, from which hang three bells. The outer two bells hang from conductive metal chains, while the central bell hangs from a nonconductive thread. In the spaces between these bells hang two metal clappers, small pendulums, which hang from nonconductive threads. A short metal chain hangs from the central bell.
The central bell's chain is put in contact with the inner surface of a Leyden jar, while the outside surface of the jar is put in contact with the metal stand. The central bell takes its charge from the inner surface of the jar, while the outer surface charges the two bells on the conductive chains; this causes the bells to have a potential difference equal to that between the inner and outer surfaces of the jar. The hanging metal clappers will be attracted to one bell, will touch it, pick up its charge, and be repelled; they will then swing across to the other bell, and do the same there. Each time the clappers touch a bell, charge is transferred between the inner and outer surfaces of the Leyden jar. When the jar is completely discharged, the bells will stop ringing.