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20
includes: (1) different electronic work function of two metals. (2) different electron concentration of two metals.
Suppose that metal A and metal B have work functions Va and Vb respectively. The electromotive force (emf) between A
and B is:
)
ln(
b
a
b
a
ab
N
N
q
kT
V
V
V
Where K=1.38
×
10
-23
J/K is Boltzmann's constant. T is the temperature in
°
Kelvin;q is the elementary charge of 1.60
×
10
-19
Coulomb. It's found that from the formula, the contact potential depends on the nature of the metal and the temperature of
the contact surface.
If two dissimilar metals contact, the electromotive force at the contact point will plus a slowly changing millivolt level.
This kind of noise is sensitive to temperature. This noise is typically at very low frequency because the temperature of the
detector and experiment generally changes slowly. This effect is large on the scale of many detector outputs and can be a
problem for low frequency measurements, especially in the mHz range.
Solutions to minimize thermocouple effects:
1) Hold the temperature of the detector or experiment constant.
2) Use a compensation junction.
1.11
Harmonic Detection
Harmonics represent waves at multiple frequencies. Periodic function can be described as the linear combination of one
constant and many sine or cosine functions with the base frequency and the multiple frequencies. The constant is the DC
component. The base frequency component is the base wave. The multiple frequencies are harmonics.
The SE1201 can only measure the base wave or one component of the harmonics at one time. The settings of harmonic
detection are at <Harmonic> in [REF/PHASE] menu.