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A null detector indicates whether a residual signal has been brought sufficiently close to zero in a null measurement method. A galvanometer between the middle junctions of a Wheatstone bridge is a concrete example: equal junction potentials give zero detector current.^[ref] Its useful property is the ability to distinguish small residuals and their sign near balance.

For an original electronic model, let the detector output bewhere $ϵ$ is the residual electric potential difference, $G$ is gain, $z$ an output zero offset and $n$ an output noise term. With known zero offset and negligible noise, an output threshold $|d|\le 1\mathrm{mV}$ at $G=1000$ corresponds to $|ϵ|\le 1\mu \mathrm{V}$. It is a resolution criterion for this model, not a total measurement uncertainty.

A false zero exposes another limit. If $z=2\mathrm{mV}$, a displayed $d=0$ can occur at $ϵ=-2\mu \mathrm{V}$ for the same gain. Increasing gain improves the output response to a given residual, but measurement calibration must still address offset and noise. Polarity reversal measurement can help separate signal from an unchanged internal offset: ifthen $ϵ=(d_{+}-d_{-})/(2G)$ and $z=(d_{+}+d_{-})/2$. The separation assumes reversal changes the signal sign while leaving the offset unchanged.

Detector loading also matters away from null. For a residual source with series electrical resistance $R_s$ connected to detector input resistance $R_d$, the detector receives $ϵR_d/(R_s+R_d)$. The ideal zero stays at $ϵ=0$, but a low input resistance reduces the response to a small departure from it. Verify the response with known small positive and negative signals, allow settling, and investigate persistent offsets. A quiet zero may otherwise reflect a broken connection or insufficient sensitivity rather than actual balance. Check the working range when approaching null because detector saturation invalidates the assumed linear response.

A 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.^[ref]

For a simple two-material circuit, a useful model iswhere the Seebeck coefficients depend on conductor material and temperature. With a stipulated constant difference $S_A-S_B=20\mu \mathrm{V}{\mathrm{K}}^{-1}$ and junction-temperature difference $0.5\mathrm{K}$, the offset is $10\mu \mathrm{V}$. 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 $10\mu \mathrm{V}$. An additive offset of the same sign doubles the indication to $20\mu \mathrm{V}$ and gives an apparent $2\mathrm{m\Omega }$ if it is ignored. Reversing the current produces readings $20$ and $0\mu \mathrm{V}$ in this model; their half-difference recovers the wanted $10\mu \mathrm{V}$.

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.^[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.