Ben:
After the laughter, realities. the voltage generated is around 0.3 to 0.4 volts at few microamps, you will need several in series to read greater than 2 volts but not enough energy for LED lighting.
make several batteries from one potato if you cut the potato in pieces to isolate the "batteries" common electrolyte -- you can make batteries using orange, apples, lemons, any acidic fruit.
As well using disimilar metals
Regards
Nando