LAWS OF THERMOELECTRONICITY
1. Laws of Homogeneous Metals
A thermoelectric current cannot be sustained in a circuit of a single homogeneous material, however, varying in x-section, by the application of heat alone. A consequence of this law is that two different materials are required for any thermocouple circuit.
2. Law of Intermediate Metals
This law states that the insertion of a new metal into a circuit of a thermocouple does not alter the existing emf as long as its extremities are at the same temperature.
If metal C is inserted into a thermocouple formed by joining A and B, then according to this law
If metal C is inserted into a thermocouple formed by joining metals A and B, then according to this law
If the cold junction is at , and the hot junction is at any temperature say , then equation of thermo-emf can be written as
where and are characteristic constants of combination of metals A and B under consideration and are known as thermoelectric coefficients and E is emf in microvolts.
However, if cold junction instead of being at C, then the above equation is written as
As discussed above (Art. 1.2) the emf is maximum at neutral temperature and emf will be maximum when is zero.
Differentiation Eq. T we get
The temperature T, at which will be neutral temperature.
The temperature of inversion is the temperature at which the emf is zero and starts reversing. Hence substituting E=0 in eq. we get