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How Fruit and Vegetable Batteries Work
The vegetable itself does not have significant electricity in it. Rather, two metals with different electron charges -- typically copper and zinc -- are inserted into each end of the vegetable. The electrons of the two metals push each other through the mass of the vegetable and this movement is electrical current. This same principle is how commercial batteries work, although they use a mass of acid that serves as a more efficient conductor than vegetable innards.
Vegetable Density
To maximize electrical flow, the matter inside the vegetable should be dense (packed together) and fairly uniform. This creates fewer barriers for the electrons to overcome in their travel from one metal wire to the other. Vegetables with gaps, such as lettuce, have lots of air pockets that cannot conduct electricity and thus make poor batteries.
Vegetable Chemical Makeup
The ingredients of the vegetable also play a role. Water and acid make for the best electrical conductors. We're all aware of water's ability to conduct electricity -- that's why you don't use a hair dryer in the bathtub. Acid also introduces more charged ions into the material, which have the effect of "pushing" the metal electrons along in their journey, thus speeding up the current. Vegetables high in potassium make for optimal conductors.
Advantages of Potatoes
Potatoes are considered the best conductors because they are uniformly solid, about 80 percent water and rich in potassium. Other solid vegetables, such as carrots and broccoli stems, can conduct electricity but are not rich enough in potassium to move current very well. (Some acidic fruits, such as lemons and limes, make good batteries for this reason.) Potatoes also represent a good balance of size and density: they are just big and non-dense enough to slow the electrons down without impeding their travel, thus making the battery last longer.