1. THERMOCOUPLES THEORY
7
Many investigations of thermo electric circuits have been made
and have resulted in the establishment of several basic pre-
cepts. These precepts, while stated in many different ways, can
be reduced to three fundamental laws:
• Law of Homogeneous Circuits
• Law of Intermediate Metals
• Law of Successive or Intermediate Temperatures
Law of Homogeneous Circuits
This law states that ‘an electric current cannot be sustained
in a circuit of a single homogeneous metal, however varying in
section, by the application of heat alone’. If a junction of two
dissimilar metals is maintained at T1, while the other is at T2,
the developed thermal EMF is independent and unaffected by
any temperature distribution along the wires T3 and T4 (fg. 4).
It can be shown that the magnitude of the ‘Peltier Effect’ is given by:
- the product of the absolute temperature (°K) of the junction;
- and the rate of change of the thermal EMF of the junction at
that temperature (fg. 3).
If a complete analysis is done, you will fnd that the ‘Peltier
Effect’ produces no measurable change in temperature of the
junction if the only current through it is due to the thermal EMF.
THERMO ELECTRIC LAWS
MAGNITUDE OF PELTIER EFFECT
Fig. 3
Junction
Temp. in
˚K
Rate of emf
Change at
Junction
Temp.
x
=
Magnitude
of
Peltier Effect
Fig. 4 Law of Homogeneous Circuits
B (–)
T 1 < T 2
E Unaffected by T 3 and T 4
E = EMF
A (+)
T 1
T 3
T 4
T 2