Coherent population trapping resonances in cesium vapor can be used to
determine DC flux densities in the range from 1 mu T to 1 mT with typ
ically 3.10(-5) relative uncertainty. For fields modulated at a few kH
z, we find sensitivities of below 10 pT within 0.5 s integration time.
From the signal-to-noise ratio the sensitivity can be extrapolated to
500 fT/root Hz.. A quantitative understanding of the lineshape allows
to detect DC fields of several nT even when the Zeeman components of
the resonance are not resolved.