What is a thermocouple? How do they work?

A thermocouple is a device for measuring temperature. It comprises two dissimilar metallic wires joined together to form a junction.
A front and back diagram to show what a thermocouple is and explain how it measures temperatures using two metallic wires.
A Process Parameters type K thermocouple made from Nickel-Chromium or Nickel-Alumel with green casing.
A Process Parameters type J thermocouple made from Iron / Copper-Nickel (aka Constantin) with black casing.

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