The word "potato" commonly refers to the edible tuber of a potato plant. The tuber is a modified part of the plant's root structure used to store food for the plant in the form of starch. Potatoes are cultivated worldwide. Today, they are the world's fourth-largest food crop, behind rice, wheat and corn. The starch contained in the potato plant tuber is not only a food source. It can also be used to generate electricity.
Potatoes are stem tubers. These plants develop enlarged stems that thicken into the tuber for nutrient storage in the form of starch, although they retain all the parts of a normal stem. The center of the tuber has the same structures of a stem, including a cortex and pithy vascular zones.
Raw potatoes tend to be highly acidic. The average potato has a pH (a measure of acidity) of about 5. This is not highly acidic, but it is far from neutral. A potato's acidity is determined by a combination of the potato's strain, the pH of the soil in which it was grown, and the method of storage. The acid can be used in a chemical reaction to generate electricity, much in the same way a chemical battery functions.
Constructing a Potato Battery
Get a piece of copper, a galvanized nail, some wire and the small lightbulb you intend to light. This is most easily done with an LED, or light emitting diode, because they require little power and can be easily attached to the wires. Cut the wire into two pieces, and strip the ends so you have enough exposed wire to wrap around the copper, nail and contacts connected to the lightbulb. Cut the potato in half. Wrap one end of a wire to the nail, then wrap the other end to one of the blub's or LED's contacts. Do the same with the copper and the remaining wire. Stick the copper into one half of the potato and place the nail into the other.
Galvanized nails are covered in zinc, which reacts with the acid in the potato to release electrons. The active chemical in the potato's acid is positively charged hydrogen, and you can see hydrogen gas released from the potato near the nail as a byproduct. The freed electrons travel from the nail electrode through the wire, lighting the bulb or LED, then finally travel to the copper electrode. The reaction at the copper electrode liberates electrons from the copper, which produces the positively charged hydrogen atoms in the remaining half of the potato.
Any other highly acidic fruit or vegetable can be used in a similar fashion to generate small amounts of electricity. It is unlikely that any particular fruit or vegetable will be able to generate significant amounts of electricity. However, this same principle is largely used in synthetic batteries. A car battery, for instance, is a series of electrodes suspended in a highly acidic liquid.