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.

Thermocouple Explained Working Principles - RealPars

A Comprehensive Guide to Thermocouples: Types, Uses & Installation

Thermocouple - Construction, Working Principle & Its Applications

Thermocouple- Definition, Working Principle, Types, and Applications

The ABCs of Thermocouples

How Thermocouples Works and Basic Working Principle

How Does a Thermocouple Work in a Boiler?

Using the correct thermocouple

What is a thermocouple? How do they work?

How Does a Thermocouple Work?