In my own
experiments, I have concocted the perfect way to build a Leyden jar on a small scale so that its
capacitance is limited to a charge that is not large enough to kill, although it will make the average arm
twitch rather strongly.
I took one plastic spice shaker about four inches tall and coated the outside in tin foil. I then filled the inside with water. To charge up the capacitor, I use my Wimshurst machine, placing one lead in the water and turning the other so it is nowhere near the jar. I used to have a small metal rod in the plastic lid of the jar which functioned as the bridge to the water for easier charging. This rod, however, acted as a lightning rod and leaked away much of the charge when left standing and ungrounded.
Once the water is charged, the capacitor can be set on any ungrounded surface such as a wooden desk for substantially longer than the design of Leyden jar with the rod and maintains its charge. Mine still delivered an arm-twitching jolt after sitting overnight.
The interesting part about the water leyden jar is that the water inside undergoes electrolysis and slowly is converted to hydrochloric acid thanks to the chlorine in the tap water.
NOTE: Not recommended for people with chronic heart trouble or pacemakers.