  We consider the power of a relativistic jet accelerated by the magnetic field
of an accretion disc. It is found that the power extracted from the disc is
mainly determined by the field strength and configuration of the field far from
the disc. Comparing with the power extracted from a rotating black hole, we
find that the jet power extracted from a disc can dominate over that from the
rotating black hole. But in some cases, the jet power extracted from a rapidly
rotating hole can be more important than that from the disc even if the
poloidal field threading the hole is not significantly larger than that
threading the inner edge of the disc. The results imply that the radio-loudness
of quasars may be governed by its accretion rate which might be regulated by
the central black hole mass. It is proposed that the different disc field
generation mechanisms might be tested against observations of radio-loud
quasars if their black hole masses are available.
