straddlethemoon:
wow thank you very much Aplonis for your clear answer!
I have another question, obviously stupid but since I can't understand it I need an explanation
how come the base metal to which the two thermocouple wires are soldered doesn't set them to the same potential?
Of course they are at the same potential, at the point they are joined!
so their voltage output would be zero all the time?
The voltage is actually due to the thermal gradient along the conductors. Different metals develop different voltages. See Thermoelectric effect - Wikipedia