Potato batteries use the acids in the potato to start a reaction with two electrodes made of different metals that cause electrons to flow from one to the other through the potato, producing power. The potato acts as a salt bridge, connecting the anode, or negative electrode, to the cathode, or positive electrode. The potato is not a source of electricity by itself.
Potato batteries require two different metal electrodes with different electrical qualities to work. The most common materials are zinc and copper. The acids in the potato react with the metals, creating an electron imbalance at each electrode. Because it keeps the ions formed by the reaction separate, but conducts electricity via its water and electrolytes, the reaction forces the electrons in the copper electrode to move. This movement of electrons is an electric current and is sufficient to power small devices.
Some scientists have actually researched potatoes as a practical form of power. While many different plants can serve as batteries, potatoes are especially durable due to their high starch content. They do not rot easily or attract pests to the same degree as fruits and other alternatives. Cooking potatoes reduces their electrical resistance and actually makes them much more powerful as batteries.