National High Magnetic Field Laboratory
Magnet Academy: Leyden Jar
These devices, though quite humble, represented a tremendous breakthrough in the history of electricity; they were the first capacitors, and as such were able to store electric charge. (Java tutorial)
National High Magnetic Field Laboratory
Magnet Academy: Leyden Jars 1745
Because they could store significant amounts of charge, Leyden jars allowed scientists to experiment with electricity in a way never before possible.