Instrument Encyclopedia | The Four Fundamental Laws of Thermocouples

The fundamental laws of thermocouples include the Homogeneous Conductor Law, the Intermediate Conductor Law, the Intermediate Temperature Law, and the Reference Electrode Law.

I. Homogeneous Conductor Law
A closed circuit formed by welding both ends of the same homogeneous material (conductor or semiconductor) will generate zero net electromotive force (EMF) regardless of the conductor’s cross‑section or temperature distribution. This is because:

· No contact potential (also known as contact EMF or Peltier EMF) is produced. Contact potential arises from electrochemical effects at the interface between two dissimilar conductors; identical homogeneous materials do not generate such a potential.
· The thermoelectric potentials (also known as Thomson potentials) cancel each other out. The Thomson effect refers to the phenomenon where a temperature difference along a single conductor creates a potential difference between its ends.

Thus, a thermocouple must consist of two different homogeneous conductors or semiconductors. If the thermoelectrode materials are inhomogeneous, an additional thermal EMF will be generated due to the temperature gradient.

✅ Application: Based on this law, it is possible to verify whether two thermoelectrode materials have the same composition (known as the identical‑polarity verification method) and to inspect the uniformity of thermoelectrode materials.

II. Intermediate Conductor Law
When an intermediate conductor (a third conductor) is introduced into a thermocouple circuit, as long as both ends of the intermediate conductor are at the same temperature, its presence will not affect the total EMF of the thermocouple circuit.

According to the Intermediate Conductor Law, in practical temperature measurement with thermocouples, it is common to weld the hot junction while leaving the cold junction open. The cold junction is then connected via wires to a display instrument to form a temperature measurement system.

For example, when copper wires are used to connect the cold junction of a thermocouple to an instrument for reading millivolt values, the contact potential generated at the connection points between the wires and the thermocouple will not introduce additional error into the measurement. Similarly, thermocouple ends can be directly immersed into molten metal or welded onto a metal surface for temperature measurement without being welded together.

III. Intermediate Temperature Law
The thermoelectric potential between the two junctions of a thermocouple circuit (at temperatures T₁ and T₃) equals the algebraic sum of the thermoelectric potential of the thermocouple at temperatures T₁ and T₂ and that at temperatures T₂ and T₃. Here, T₂ is called the intermediate temperature.

EAB（T1，T2）+ EAB（T2，T3）=EAB（T1，T3）

✅ Application:

1. Based on the Intermediate Temperature Law, thermocouple reference tables (also called “thermocouple calibration tables” or “thermoelectric EMF tables”) can be compiled.
2. In practical applications, the cold‑junction temperature of a thermocouple is often not 0 °C but either higher or lower. The Intermediate Temperature Law enables the calculation of the thermoelectric EMF when the cold‑junction temperature differs from 0 °C, thereby facilitating temperature compensation.

✅ Definition: If the thermoelectromotive forces (EMF) produced by two conductors separately paired with a third conductor are known, then the thermoelectromotive force generated by a thermocouple made from these two conductors can also be determined.

If conductors A and B are separately paired with a standard electrode C to form thermocouples, and their respective EMF values are known, then the thermoelectromotive force of the thermocouple composed of conductors A and B is given by:

EAB（T，T0） =EAC（T，T0）- EBC（T，T0）

✅ Application: The Standard Electrode Law is an extremely practical rule. There are many types of pure metals, and even more varieties of alloys. Consequently, determining the thermoelectromotive forces (EMF) for all possible thermocouple combinations among these metals would require an enormous amount of work.

Due to platinum’s stable physical and chemical properties, high melting point, and ease of purification, high‑purity platinum wire is commonly chosen as the standard electrode. Once the EMF values of thermocouples formed between various metals and pure platinum are measured, the EMF values for thermocouples made from any combination of those metals can be derived.