= Null detector
A \b[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.<Wheatstone bridge by PHYWE (P2410200)>[p4, Theory (1/3)–(2/3), Figure2 and balance discussion]{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 be
$$
d=G\epsilon+z+n,
$$
where $\epsilon$ 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 $|d|\leq1\,\mathrm{mV}$ at $G=1000$ corresponds to $|\epsilon|\leq1\,\mathrm{\mu V}$. 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 $\epsilon=-2\,\mathrm{\mu V}$ 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: if
$$
d_+=G\epsilon+z,\qquad d_-=-G\epsilon+z,
$$
then $\epsilon=(d_+-d_-)/(2G)$ 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 $\epsilon R_d/(R_s+R_d)$. The ideal zero stays at $\epsilon=0$, 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.
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