I was discussing about the theory that claims that "every emitter also behaves like a receptor": Are emitters always receptors? I was brilliantly told that this theory would be false for fluorescent lights and also for resistors, because of entropy messing with time-reversed operations (condensed version!).
However, as I'm curious by nature, I made this little experiment: heating a 330 Ohm resistor with a flame and measure its voltage with a cheap multimeter. What a surprise to discover that some current flowed from this hot resistor!
At 20°C, I measured 0.0mV, after ±3 seconds of heating it was 0.6 mV (current around 1μA).
Have I just discovered some kind of "reversed" first Joule's law? :)
Or did I made some logical, methodological or experimental mistakes?
Answer
Nice experiment!
But think: why would a current flow in a particular direction and not the other? Is the system somehow asymmetric? As the other answers have suggested, this is probably a thermo-electric current due to the Seebeck effect (a complication I didn't want to get into for your other question).
But now that we're there, here's another thing to try: can you change which direction the current flows by how you are heating the resistor? Does the magnitude or direction of current change when you heat one wire of the resistor versus the other? The Seebeck effect comes into play at electrical interfaces, where there's asymmetry, so the direction of the interface would determine the direction of the current!
The asymmetry issue is another way how you can see that you can't reverse the standard Joule heating: You will get the same heating for a current running either direction through the resistor. But if you cool the resistor to try to undo it, which way would the resulting current flow?
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