(republished on 08/02/06)
It is quite hard to get much power from these. Do you have access to a volt/ohm/amp meter? They're cheap and easily available. You can use them to find out how much voltage and current you get from one battery. You can also check what the bulb needs, and then figure out how many batteries you need in series to get enough voltage times how many in parallel to get the current.
Are you sure the nails are zinc-coated? If they don't have the right coating, you may not be getting much of any power from the battery.
With your meter you can test the voltage. You can also test how much current flows when the bulb is connected, and how much the voltage then drops.
What sort of bulb are you using? If you'd like to see some light without using too big a set of potato batteries, you should get a single LED bulb, which works on low current and voltage. It will only work with one direction of current, but that's not much problem, since there are only two directions to test.
(published on 10/26/13)