The Seebeck effect was discovered by
Thomas Johann Seebeck in 1821. He was a German physicist, and he found that when two different metals are connected at two junctions, and if there is a temperature difference between these junctions, an electric current flows through the circuit. This phenomenon is what we now call the Seebeck effect.
In simple terms, Seebeck showed that heat can generate electricity, which was a foundational discovery in understanding thermoelectric effects.