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A null measurement method adjusts a reference or a system setting until a difference signal is brought to zero. A null detector tests the residual mismatch; the final measured quantity value follows from the reference and the balance condition. The VIM lists null measurement among measurement methods.^[ref]

A Wheatstone bridge provides a concrete electrical apparatus. The PHYWE manual connects a source across two resistor dividers and a current detector between their middle junctions; balance occurs when those junction potentials agree and detector current vanishes.^[ref] A sliding-wire implementation adjusts the divider ratio continuously.

For an original notation, let the left branch contain upper resistance $R_a$ and lower $R_b$, while the right contains upper unknown $R_x$ and lower reference $R_s$. With source electric potential difference $V_s$ and negligible loading at balance, the middle-junction potentials above the lower supply terminal areSetting $V_L-V_R=0$ yieldsFor $R_a=2000\Omega$, $R_b=1000\Omega$ and $R_s=500\Omega$, the balance identifies $R_x=1000\Omega$ independently of the nonzero source magnitude in this ideal model.

Zero on a display has finite meaning. With $V_s=3\mathrm{V}$, the derivative of residual $V_L-V_R$ with respect to $R_x$ at this balance is $1/1500\mathrm{V}{\mathrm{\Omega }}^{-1}$. A residual magnitude below $10\mu \mathrm{V}$ therefore corresponds locally to about $0.015\Omega$ of unresolved mismatch. A detector zero offset can also shift the apparent balance. The null measurement method reduces reliance on calibrated detector gain at exact balance, but reference uncertainties, lead effects, detector sensitivity and instrumental drift remain relevant to the complete measurement result.