Leyden jar

The Leyden jar was the original capacitor, developed in the 18th century and used to conduct many early experiments in electricity.

The "dissectible Leyden jar" myth

A popular demonstration of the Leyden jar involves taking one apart after it has been charged and purportedly showing that the energy is stored on the dielectric, and not the plates:

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A Leyden jar is constructed out of a plastic cup nested between two snugly-fitting metal cups. When the jar is charged with a high voltage and carefully dismantled, it is discovered that all the parts may be freely handled without discharging the jar. If the pieces are reassembled, a large spark may still be obtained.

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This demonstration shows that the charge has been transferred to the surface of the dielectric, and is not on the metal conductors. When the jar is taken apart, simply touching the cup does not give you enough surface area to discharge it. The conductors provide this surface area.

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This is not typical of capacitors, however, and doesn't happen at lower voltages. In a typical capacitor, the charge is on the surface of the conductors. It results from the high voltages present when the conductors are separated from the dielectric "spraying" charge onto the dielectric. If the experiment were performed in an insulating fluid (like oil) instead of air, the effect would no longer be present.

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