  Accretion discs are composed of ionized gas in motion around a central
object. Sometimes, the disc is the source of powerful bipolar jets along its
rotation axis. Theoretical models invoke the existence of a bipolar magnetic
field crossing the disc and require two conditions to produce powerful jets:
field lines need to be bent enough at the disc surface and the magnetic field
needs to be close to equipartition. The work of Petrucci et al (2008) on the
variability of X-ray binaries supposes that transitions between pure accretion
phases and accretion-ejection phases are due to some variations of the disc
magnetization. This rises the problem of the magnetic field dragging in
accretion discs. We revisit the method developed by Lubow et al (1994) by
including momentum and mass conservation equations in a time-dependent 1D MHD
code.
