MIT Physics Demo
Leyden Jars, an early style of capacitor, can be connected to the two electrodes of the Wimshurst Machine. They will store charge as it is generated, giving a much stronger spark across the gap.
Next we have a three-piece dissectible Leyden Jar consisting of two metal cups separated by a glass cup. When charged with the Wimshurst machine, we see by touching it with the shorting rod that it holds a large amount of charge. However, when disassembled, the metal cups can be brought into contact with each other and no spark will be generated. When the jar is reassembled it can then be discharged. This demonstrates that, in this situation, the charge actually resides on the surface of the glass (a dielectric), not on the metal.
Source: Department of Physics
