| MadSci Network: Chemistry |
It seems to me that what you want is to stick a piece of zinc into one side of the potato, a piece of copper into the other side, and connect the two metals to a light bulb. I can't vouch for that being the correct answer, but I can point you to some useful resources:
CHEMED-L has a threaded archive. This is a discussion list for teachers of chemistry at all levels, elementary through graduate school.
There is also a scientific mail-order company which advertises regularly in Science News, but I am sorry to say I can't dig up a reference to them. They advertise just such a "potato-powered" setup, in this case to run an alarm clock.
Try the links in the MadSci Library for more information on Chemistry.