What component is formed by two dissimilar metal wires welded at one end?

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This item tests understanding of a thermocouple—a sensing element created by pairing two dissimilar metals welded together at a junction. When those metals meet at the hot junction and the other ends lead to a measurement instrument, a temperature difference between the hot junction and the reference ends generates a small voltage due to the Seebeck effect. That voltage is what the thermometer uses to infer temperature, so the described setup—two different metals welded at one end—precisely defines a thermocouple junction.

The other options don’t describe this metal-junction sensor: a bead on a single wire points to a different type of sensor (not a two-metal thermocouple), a semiconductor junction refers to PN or similar junctions in diodes or transistors, and a glass bulb with mercury is a traditional liquid-in-glass thermometer.