So, if you have four potatoes and a bunch of pennies and household screws/nails, you will be well on your way to spending an hour trying to use some potatoes to light one extremely tiny LED. WHAT FRUITFUL USE OF YOUR PRECIOUS TIME! Fortunately, if you do not wish to replicate this experiment, you can take it on my good authority (and based on the following documentation) that it works. Do note that the smaller the LED, the likelier you will be to be able to use all your combined potato power to light it. Also, I actually only had FOUR potatoes so it was a good thing that four potatoes was exactly what it takes to light this one LED. (3 baby potatoes seem to be insufficient...)
4 baby potatoes
4 shiny 1p or 2p coins
4 galvanised screws/nails
1 very tiny led
Some random bits of wire or a crocodile clip
A rolled towel to hold the potatoes in place
Poke the nail/screw on one end of each potato, and jam a penny halfway into the other end of each potato. Gingerly arrange the four baby potatoes in a sort of daisychain in which the penny of the previous touches the galvanised nail/screw of the next potato. Make a wire connection between a very very low power LED - and connect the longer leg to the first penny in the potato chain and the short leg to the last galvanised nails/screw.
The electricity is generated because there are two electrodes - the copper coin and the galvanized (zinc coated) screw - and there is an electrolyte (the potato) between them. [FYI: The british 2p and 1p are technically made of bronze, which is 97% copper, so that is why it still works in this experiment]
From Wikipedia: "The energy for the battery comes from the chemical change in the zinc (or other metal) when it dissolves into the acid. The energy does not come from the lemon or potato. The zinc is oxidized inside the lemon, exchanging some of its electrons with the acid in order to reach a lower energy state, and the energy released provides the power. In current practice, zinc is produced by electrowinning of zinc sulfate or pyrometallurgic reduction of zinc with carbon, which requires an energy input. The energy produced in the lemon battery comes from reversing this reaction, recovering some of the energy input during the zinc production."
Apparently, all this electrochemical reaction can also lead to galvanic corrosion. The same principle for building a potato battery can apply to other foods such as lasagna. Apparently, if you cook a lasagna in a steel pan (cathode) and wrap it in aluminum foil (anode), the tasty and salty tomato-acid-laden lasagna will act as the electrolyte. The few points at which the aluminum foil touches the lasagna will cause a concentrated reaction at the point and eventually result in galvanic corrosion at that spot alone, where the aluminum foil will eventually appear to have melted onto the lasagna.
Example of Lasagna Cell (Image Source: Flickr - Tom Arthur)
Finally, I've realised that this also explains why I have never enjoyed chewing aluminium foil or putting it into my mouth whilst I'm eating, as I have permanent retainers at the back of a number of my teeth. The steel wire ends can be said to be slightly exposed since it has been many years since I last saw an orthodontist and I have also rigorously chewed on it over time. The tasty salty tinfoil wrapped food reacts in my mouth with the salty and acidic food and saliva being the electrolyte, the tinfoil being the anode, and the surgical steel retainers (or any of my other potentially metal fillings) at the back of my teeth being the cathode, culminating in a strange taste/unpleasant sensation of a small potential difference in one's mouth.