  This paper describes a novel method for calibrating dc-precise magnetometers
in the low field range (100 uT), which gives acceptable results even in
laboratory conditions with significant magnetic interference. By introducing a
closely mounted reference magnetometer and a specific calibration procedure, it
is possible to compensate for the external magnetic field disturbances caused,
e.g., by the local transportation operated with dc power supplies. The field
compensation occurs only shortly after the calibrating coils are energized. In
this case, the leakage of the coil's magnetic flux to the reference sensor due
to the cancellation of the time-varying compensating field was negligible. When
using 60 cm coils and reference sensor in 2.5 m distance, we were able to
calculate magnetometer gains with a standard deviation of 91 ppm. We show that
an overall uncertainty of 0.1% can be achieved.
