Operation
3-19
In the unlikely event that the two junction temperatures
are identical, the thermal EMFs will exactly cancel
because the generated potentials oppose one another.
More often, the two junction temperatures will differ, and
considerable thermal EMFs will be generated.
A typical test setup will probably have several copper-to-
copper junctions. As pointed out earlier, each junction can
have a thermoelectric coefficient as high as 0.2µV/°C.
Since the two materials will frequently have a several
degree temperature differential, it is easy to see how ther-
mal potentials of several microvolts can be generated even
if reasonable precautions are taken.
Figure 3-7
Thermal EMF generation
Nanovolt
Preamplifier
T
2
HI
LO
T
1
A
B
A
E
AB
= Q
AB
( T
1
– T
2
)
The thermal EMF developed by dissimilar metals A
and B in a series circuit is:
Temperature of the A to B
junction in
°
C
Temperature of the B to A
junction in
°
C
Thermoelectric voltage
coefficient of material A with
respect to B,
µ
V/
°
C
E
AB
Thermoelectric generation
Figure 3-7 shows a representation of how thermal EMFs
are generated. The test leads are made of the A material,
while the source under test is the B material. The temper-
atures between the junctions are T
1
and T
2
. To determine
the thermal EMF generated, the following relationship
may be used:
Where: E
AB
= Generated thermal EMF
Q
AB
= Thermoelectric coefficient of material A
with respect to material B (µV/°C)
T
1
= Temperature of B to A junction (°C)
T
2
= Temperature of A to B junction (°C)
E
AB
Q
AB
T
1
T
2
)
–
(
=