  A superconducting magnetometers based on the magnetic field dependence of the
Eck step voltage in long Josephson tunnel junctions (LJTJs) is demonstrated.
The field to be measured is applied perpendicular to a continuous
superconducting pickup loop. Wherever the loop has a narrow constriction, the
density of the flux-restoring circulating currents will become relatively high
and will locally create a magnetic field large enough to bring a biased LJTJ in
the flux-flow state, i.e., at a finite voltage proportional to the field
strength. This method allows the realization of a novel family of robust and
general-purpose superconducting devices which, despite their simplicity,
function as ultra-low-noise, wide-band and high-dynamics magnetometers. The
performances of low-T$_c$ sensor prototypes, among which a highly linear
voltage responsivity and a magnetic spectral density $S_B^{1/2}<
3\,fT/Hz^{1/2}$, promise to be competitive with those of the best
superconducting quantum interference devices.
