  We present a system developed to premagnetize liquid samples in an
ultra-low-field Nuclear Magnetic Resonance (NMR) experiment. Liquid samples of
a few milliliter are exposed to a magnetic field of about 70~mT, which is
abruptely switched off, so to leave a transverse microtesla field where nuclei
start precessing. An accurate characterization of the transients and
intermediate field level enables a reliable operation of the detection system,
which is based on an optical magnetometer.
