How can a potato run a light bulb?

I think this question was answered best by Lee Ellen Benjamin, M.A., Technical Theatre, San Fancisco State University in an article where he said:

“Yes, a potato(e) can be used to power a lamp. Usually a penny & a galvanized nail are used for this ‘battery’. Copper & zinc are REQUIRED for this process, but not a penny & a nail per se. A potato works well, but a tomato, lemon or other citrus fruit can be substituted. The zinc and the copper are the anode and cathode terminals of your potato battery. Using ordinary hook-up electrical wire, you can use the potato to create a voltaic cell, which will power a VERY small bulb. A light emitting diode (LED) will work fine. A side note here about voltage & current. This process will produce less than 1.5 volts DC (AA/AAA battery). However, producing 1.5 volts does not necessarily produce enough current to make the lamp actually power up to full use. Voltage is only the POTENTIAL to do work. (See Ohm’s law: V = I x R) This kind of battery generally produces only a few milliamps. Even multiple potatoes may not generate enough amperage. Most assuredly, it will NOT power a household light, but a small flashlight lamp will GLOW.

Cut the potato in half. Wrap the end of a piece of wire around a galvanized nail and wrap the end of a second piece of wire around a penny. Stick the copper side into one piece of potato and the nail into the other. The zinc and copper electrodes should not touch each other. If a wire is connected between the Zinc nail and the copper penny, electrons will flow. However, direct contact of the two electrodes will only produce heat.

Electric current is the movement of electrons from one atom to another in a conductor. Inserting the two common metal electrodes into the potato causes a chemical reaction to occur resulting in current. The potato does not participate directly in the reaction. It is there rather as an electrolyte to facilitate the transport of the zinc and copper ions in the solution, while keeping the copper and zinc electrodes apart. The potato contains phosphoric acid (H3PO4), which facilitates the electro-chemical reaction of zinc with copper.

Zinc is an active metal, which reacts readily with acid to liberate electrons. The acid’s active ingredient is positively charged hydrogen, so a transfer of electrons takes place between the zinc and the acid. The zinc (Zn0) is oxidized (Zn++ ) and the acid (H+) is reduced to hydrogen gas (H2), which you can see bubbling out around the electrodes. The reaction at the penny electrode depletes the electrons from the copper and attaches them to the hydrogen ions in the phosphoric acid.

Oxidation: Zn –> Zn++ + 2e- (Zinc looses 2 electrons)

Reduction: 2H+ + 2e- –> H2 (Hydrogen ions gain electrons)

Net Reaction: Zn + 2H+ –> Zn++ + H2 (Hydrogen gas and ‘power’) “