A simple copper oxide thermoelectric generator can generate hundreds of millivolts. Sixteen of them in series can generate enough voltage to light an LED.
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What is happening? I have spent my entire life reading whatever technical books and articles I could find and can recall seeing only one book that says tells how to make something like this and none that explain how this device works. It would be safe to say however, that an explanation could be found in the institutionalized literature. Not being presently affiliated with any institutions, I do not have ready access to most of their literature which makes the cost of reading it prohibitive. So, I will offer my own take on what is happening.
A thermocouple is generally thought of as being a junction between two dissimilar metals but it might be better to say that it is a junction between two dissimilar conductors. Touching the two oxidized wires together forms a junction of copper oxide to copper oxide. This is not where the action is. The copper oxide on both wires should be thought of as one solid conductor between the two copper wires - a very short one at that. This can now be seen as the classic two thermocouples back to back circuit. We have a copper - copper oxide junction on the hot wire and an opposing copper oxide - copper junction on the cold wire. With this in mind it is now easy to view this device as being a normal thermocouple circuit.
It is easy to wonder how this device could work at all because of the copper oxide, that is between the two wires, being almost an insulator. Copper oxide however, also acts like a thermistor with a very high negative temperature coefficient. Even the "cold" wire still gets hot enough that the resistance of the copper oxide drops relatively to a very low value - enabling current to flow.
I must give credit to an older book "Simple Scientific Experiments" by Aurel de Ratti as being the one and only book I have ever seen that makes the reader aware of this copper oxide thermoelectric generator. This book also contains other fascinating stuff. It is reprinted and sold by Lindsay Publications Inc.