= Dielectric-rise voltmeter
{title2=$\Delta h=\epsilon_0(\epsilon_r-1)V^2/[\rho g(b^2-a^2)\ln(b/a)]$}
At fixed applied <voltage>, the mechanical potential of a <capacitor> and its source includes $-CV^2/2$. Increasing the liquid overlap of a <partially filled coaxial capacitor> therefore pulls the liquid upward. Balancing $V^2(dC/dh)/2$ against the excess <hydrostatic pressure> times the annular area gives the title formula for a wide external reservoir. Its differential sensitivity is $2\Delta h/V$; it vanishes at zero bias and measures the magnitude, rather than the sign, of <voltage>.
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