This is a quick-and-dirty method for making a Leyden Jar (an early form of capacitor) from my old friend, the 35mm film cannister.
Step 1: Tools and materials.
35mm film cannister
Aluminium (kitchen) foil
Proper wire strippers.
Step 2: Make it.
Do the inside first - it's the most fiddly part, but once it's done you won't damage it while you do the outside.
Cut a rectangular piece of foil large enough to line the cannister. Add a few dabs of glue-stick, then slide it into place inside the cannister. Make sure it doesn't stick out of the top of the canister. Try and keep edges of foil smooth and straight, or they will "leak" charge.
Added 1st March 2007:
I've just made a load with a bunch of fumble-fingered kids, and I've worked out a method for putting the foil inside:
> Cut the foil to size, and glue the back with the stick.
> Roll it loosely around your thumb, glue side out.
> Put your foiled thumb into the pot, and gently roll the pot around your thumb several times. The foil will smooth against the inside of the pot and can be slid down into place.
Too much glue near the top of the pot is as bad as having the foil too close to the top - it turns out that it is a fairly good conductor until it is bone dry (which can be a long time inside a closed pot).
Step 3: Connect the inside.
Poke a small hole in the middle of the film cannister lid.
Cut a piece of wire about 7-10cm long. Strip the ends and thread the wire through the lid.
Turn the inside end into a loop, then use a piece of tape to stick it to the foil inside the cannister. Slide the lid along the wire and put it on.
If you are planning to add the discharge loop, carefully scrape away 1-2cm of the insulation from the wire that will be just outside the lid. Give it a twist to make the loop. When you put the lid in place, push the wire back into the lid until the discharge loop is just at the lid.
Step 4: Wrap the outside.
Now cover the outside of the cannister. Again, glue-stick is enough to hold the foil in place. Again, avoid rough edges, and make sure the foil doesn't get in the way of the lid.
Cut a second piece of wire about 4-5cm long and strip both ends.
Tape one end to the foil on the outside, so that the other end can be easily curved around to meet the other wire (or the discharge loop if you made one).
(Optional step: wrap the whole thing in sticky tape to prevent tears and scuffs.)
Step 5: Charge it.
To charge the Leyden Jar up, you need to connect it to a source of, well, charge.
If you don't have your own Van Der Graff generator or Wimshurst machine, try switching your CRT TV or monitor on and off whilst brushing the screen with the centre wire, or rub balloons on your jumper and stroke the bare wire along them them pick up charge.
When it's charged, hold the insulated parts of the wires and bend the ends together (or bend the outer wire towards the discharge loop): as they get close, you will (should) see, and possibly hear, a spark.
The one I made to illustrate this Instructable worked well when charged from a VdG on a humid evening, storing enough charge to get a spark about 3-4mm long.
No, I don't have a photo of it, you'll just have to take my word on it (like I'm really going play with high voltage and my digital camera at the same time...)
Step 6: Extend it.
To hold more charge, you need more surface area of foil. The only way to increase this is to use a larger container.
Try glass jars with plastic lids, or plastic food boxes (the ever-popular "Tupperware" is good). Maybe even try a plastic bucket!
The film-cannister-sized jar is a toy. It hardly carries any charge at all, and a shock would just give you a surprise. Larger jars carry larger charges, enough to cause injuries either indirectly (you twitch and smash your arm on the bench) or directly (why do you think they shout "Clear!" on medical dramas?).