= Galvanometer current sensitivity
{title2=$S_I=d\theta/dI=NAB/k$}
The <galvanometer current sensitivity> is the change in settled deflection <angle> per change in <electric current>. In the linear model its <derivative> is constant, $S_I=NAB/k$. The illustrative design has $S_I=3000\,\mathrm{rad/A}$, or $0.1719^\circ$ per $\mathrm{\mu A}$. A chosen full-scale <angle> $\theta_{\max}=0.180\,\mathrm{rad}$ corresponds to $|I|=60.0\,\mathrm{\mu A}$; a straight-line extrapolation beyond that range does not establish real instrument behavior.
\Image[/galvanometer-calibrated-deflection-current.png]
{title=Illustrative moving-coil calibration, current reversal and finite zero resolution}
{id=galvanometer-calibrated-deflection-current}
{description=Calculated deflection for the original design, with selected calibration points and a close view of zero. The shaded band is a stipulated $\pm0.10^\circ$ readout tolerance, corresponding to about $\pm0.582\,\mathrm{\mu A}$; it is not measured noise. Values and points are illustrative model predictions, not observations from a real instrument.}
{source=galvanometer-calibrated-deflection-current.py}
Finite sensitivity and finite readable <angle> set different limits. Under the stipulated $0.10^\circ$ tolerance, a reading indistinguishable from zero means $|I|\lesssim0.582\,\mathrm{\mu A}$ only if the zero is stable and the model applies. More turns can increase <galvanometer current sensitivity> while also increasing winding <electrical resistance>; the response to an applied <electric potential difference> depends on both. <Friction>, zero drift, departures from the radial <magnetic field>, and spring nonlinearity can spoil the calibration. <Damping> sets how long one must wait for a settled reading. A rapidly reversing <electric current> can produce little mean pointer deflection even when its instantaneous magnitude is large.
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