A Leyden jar (or capacitor) is electrostatically charged with a Wimshurst generator and then disassembled. In its disassembled state, the parts can be handled freely and touched together with no observable result. However, when the Leyden jar is reassembled and electrical contact is made between the inner and outer conductors a dramatic electrical discharge results.This is used to demonstrate that the charge resides on the surface of the dielectric.

To demonstrate the presence of charge, simply use the Wimshurst generator (rotate crank in direction on the label and put the leads close; sparks will soon appear as charge is accumulated in the generator's Leyden jars). Now connect one lead to the outer copper container of the capacitor through the foil underneath, insert the glass and then insert the inner cylinder. Connect the other lead to the hook of the inner container and charge with the generator. After charging, you can disconnect the leads (you may get a small shock). Then take the inner cylinder and glass out off the copper container and short the inner cylinder to the copper container. Nothing will happen. When you reassemble the jar and use the semicircle electrode to short the inner cylinder to the outer container with the glass in between, you will hear and see a spark.

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PIRA DCS 5C20.30