  We report on the design and performance of a highly sensitive combined
$^3$He/Cs magnetometer for the absolute measurement of magnetic fields. The
magnetometer relies on the magnetometric detection of the free spin precession
of nuclear spin polarized $^3$He gas by optically pumped cesium magnetometers.
We plan to deploy this type of combined magnetometer in an experiment searching
for a permanent electric dipole moment of ultracold neutrons at the Paul
Scherrer Institute (Switzerland). A prototype magnetometer was built at the
University of Fribourg (Switzerland) and tested at Physikalisch-Technische
Bundesanstalt (Berlin, Germany). We demonstrate that the combined magnetometer
allows Cram\'er-Rao- limited field determinations with recording times in the
range of $\sim 500\mathrm{s}$, measurements above $500\mathrm{s}$ being limited
by the stability of the applied magnetic field. % With a $100\mathrm{s}$
recording time we were able to perform an absolute measurement of a magnetic
field of $\approx1\mathrm{\mu T}$ with a standard uncertainty of $\Delta
B\sim60\mathrm{fT}$, corresponding to $\Delta B/B<$6$\times$10$^{-8}$.
