These jars are also called Leyden's Death Jars because they killed not only their creator, but several other people as well. These jars are made by taking a jar made of some sort of non-conductive material and coating the outside and inside with tinfoil, taking care so that at no point the outside and inside tin foil meets. For best results the ends of the tin foil should be several inches away from each other. A non-conductive jar then had a metal rod with a brass top and a metal wire long enough to reach the bottom.
These jars work by attracting electrons using the brass top when put into a magnetic field. The magnetic field can be provided by a generator that puts electrons in to the atmosphere, or a natural field created during thunder storms. The electrons travel down the top through a dangeling wire that touches the tin foil covered bottom of the jar. Here the electrons are stored until they can be discharged. The tin foil on the outside is what helps to make it an actual capacitor as it keeps the electrons from simply leaving the object, it acts as a barrier so that only when the inside metal and outside metal meet in a circuit will the charge be dispelled.
Leyden's Jars can hold enough electricity to rival that of a small lightning bolt. Leyden's jars are completely harmless if treated correctly but can be harmful or even fatal when discharged after being filled. The best way to avoid being harmed by one of the death jars is to simply leave it without a top. This ensures that no electrons are submitted into the object. It is also perfectly fine to leave one of these jars with the top ajar so that the opening is not covered. This is only effective if the top of the jar is NOT grounded.
No matter how the jar is positioned, top on or off, grounded or not the jar will never ever cause harm if held the right way. What makes this jar dangerous is the discharge of electrons. The only way to discharge the jar is too complete the circuit allowing the electrons to flow through an intermediate object and become grounded.
If the jar is only held around the middle where the tin foil is then the holder is safe because the electrons inside can't get out.
If one touches the top of the jar, similarly the loop is not complete and the jar will do nothing.
Under No Circumstances, no matter how safe you think you are being, never touch the top of the jar and the middle when the jar is assembled. This releases the charge and can cause death.
A physics teacher at my school once showed his class a Leyden's jar after using a Van de Graaff generator all hour. The jar had been sitting with the top askew, but resting on a faucet that was grounded. Unbeknownst to him it had been collecting lots of electrons from the generator. He decided that while he had the jar in class he should explain its potential for harm. He placed the lid firmly on the jar, explained what it was and said, "Now never touch the jar like this....". A streak of electricity shot from the top of the jar and into him. Fortunately since the top had not been fully resting on the jar he was only unconscious for 10 minutes, but we all learned our lesson.
Please, for the love of all that is good and holy PLEASE DO NOT build your own Leyden jar using these instructions. It's all fun and games until there is a hot dry day and you are getting shocked by everything you touch and your Mother decides to clean your room and picks up the jar the wrong way! (OK so for most of us it'll probably be a roommate instead of a Mom, but you get the picture.)