|I found this on the web. Hope it helps
What you need is a spud. Idaho potatos work best. (Well, I don't know that for a fact. That was just a plug on my part for Idaho. You can substitute any other potato -- or any other vegetable for that matter. The potato is the traditional, hands-down-favorite vegetable battery electrolyte). You also
need two lengths of copper and zinc metal. These you cannot substitute. It has to be copper and it has to be zinc. It is the chemical reaction between these metals that powers the battery.
Finger-length is long enough for these metals. Stick the copper in on one side of the potato, and the zinc on the other side. The zinc and copper fingers should not touch each other in the potato. The zinc and the copper are the anode and cathode terminals of your potato battery. Using ordinary hook-up electrical wire, you can use the potato battery to power a small bulb (and I do mean small! This battery will only put out about half a volt!)
The power of the battery comes from the electro-chemical reaction of zinc with copper. The potato does not participate directly in the reaction. It is there rather as an electrolyte to facilitate the transport of the zinc and copper ions in the solution while keeping the copper and zinc fingers apart. If the zinc and copper are in contact with each other, their reaction will still occur, but only heat will be generated. By keeping them apart, we force the electron transfer that occurs in the reaction to take place over the wires of our circuit.
(republished on 08/02/06)
(published on 03/03/07)