Seebeck effect in thermocouples — what does the generated emf depend on? In practical thermocouple measurements, the electromotive force (emf) is determined by which temperature relationship between the two junctions?

Introduction / Context: Thermocouples operate on the Seebeck effect: when two dissimilar metals are joined to form two junctions held at different temperatures, a voltage is generated. Understanding exactly what this emf depends on is essential for correct cold-junction compensation and accurate temperature measurement in industry and laboratories.

Given Data / Assumptions:

• One junction is commonly called the “measurement” or hot junction.
• The other is the “reference” or cold junction (often at a known or compensated temperature).
• Lead materials and calibration tables are standard; parasitic thermal gradients are neglected.

Concept / Approach: The thermocouple emf is a function of the temperatures of both junctions, often expressed as E = f(T_hot) − f(T_cold). Hence, it strictly depends on their temperature difference referenced to a standard polynomial relation, not the absolute value of either junction alone. This is why cold-junction compensation (measuring or regulating T_cold) is mandatory for accuracy.

Step-by-Step Solution:

Verification / Alternative check: Cold-junction compensation modules add the measured ambient/reference temperature to the indicated emf so the instrument can report the actual hot junction temperature, reinforcing the dependence on ΔT.

Why Other Options Are Wrong:

Common Pitfalls: Forgetting cold-junction compensation or assuming that holding the cold junction at 0°C is the only valid method; modern instruments measure and compensate T_cold electronically.

Final Answer: difference between the hot and cold junction temperatures