= Thermoelectric offset
A \b[thermoelectric offset] is an unwanted <electric potential difference> associated with <Seebeck effect> responses in a measurement circuit. Dissimilar conductors at different junction temperatures can generate a signal even when the intended electrical input is absent. Keithley's handbook discusses these offsets as a major limitation in low-voltage and low-resistance measurements.<Low Level Measurements Handbook, Seventh Edition by Keithley Instruments>[§3.2.2, thermoelectric EMFs; §3.3.2, offset compensation]{ref}
For a simple two-material circuit, a useful model is
$$
V_{\rm th}=\int_{T_1}^{T_2}[S_A(T)-S_B(T)]\,dT,
$$
where the <Seebeck coefficients> depend on conductor material and temperature. With a stipulated constant difference $S_A-S_B=20\,\mathrm{\mu V\,K^{-1}}$ and junction-temperature difference $0.5\,\mathrm K$, the offset is $10\,\mathrm{\mu V}$. These are teaching inputs, not coefficients assigned to a particular real connector pair.
That small offset can dominate an intended signal. For an original $1\,\mathrm{m\Omega}$ resistor tested at $10\,\mathrm{mA}$, the wanted drop is also $10\,\mathrm{\mu V}$. An additive offset of the same sign doubles the indication to $20\,\mathrm{\mu V}$ and gives an apparent $2\,\mathrm{m\Omega}$ if it is ignored. Reversing the current produces readings $20$ and $0\,\mathrm{\mu V}$ in this model; their half-difference recovers the wanted $10\,\mathrm{\mu V}$.
<Polarity reversal measurement> cancels the unchanged offset, not every temperature effect. Thermal changes during the cycle can leave a residual, while current-induced heating can alter the resistor itself. The handbook therefore connects reversal timing with thermal stability.<Low Level Measurements Handbook, Seventh Edition by Keithley Instruments>[§3.3.2, current-reversal method and thermal-time-constant condition]{ref}
An offset investigation records connector materials, junction temperatures, warm-up and handling, and compares readings before and after reversal. Reducing temperature gradients and avoiding unnecessary dissimilar-metal connections can reduce the generated signal. Keep residual correction uncertainty in the <measurement result>. A thermoelectric offset and random <thermal noise> have different roles in the measurement model; they are not interchangeable merely because both involve temperature.
Back to article page
