= Four-terminal sensing
{title2=$R=[V(+I)-V(-I)]/(2I)$}
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= Four-terminal resistance measurement
{synonym}
<Four-terminal sensing> uses one pair of contacts to carry <electric current> and a separate pair to sense <electric potential difference>. The sensing instrument draws negligible <electric current>, so the voltage drops along its own leads are negligible, and the current-contact drops outside the sensing interval are excluded. A measurement using just two shared leads includes their <electrical resistance> and contact drops, which can overwhelm the small sample signal in a <resistance-versus-temperature experiment for superconductivity>. Four contacts do not automatically remove offsets from the <thermoelectric effect>. https://www.tek.com/en/documents/technical-article/low-voltage-measurement-techniques[Tektronix, Low-Voltage Measurement Techniques, four-lead arrangement and thermoelectric offsets]{ref}
For a stable offset $V_0$, let $I>0$ denote the magnitude of the reversed <electric current>. The measured <electric potential differences> are $V(+I)=IR+V_0$ and $V(-I)=-IR+V_0$, so subtraction gives the displayed <electrical resistance>. For example, with stipulated $I=1\,\mathrm{mA}$, $R=0.2\,\mathrm{m\Omega}$ and $V_0=3\,\mathrm{\mu V}$, the two readings are $3.2$ and $2.8\,\mathrm{\mu V}$; their difference correctly recovers $R$. If the offset changes between readings, its change leaves a residual <measurement error>; the sample and measuring circuit must remain sufficiently stable during reversal.
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