What governs the bulk velocity of the jet components in active galactic
  nuclei?
  We use a sample of radio-loud active galactic nuclei (AGNs) with measured
black hole masses to explore the jet formation mechanisms in these sources.
Based on the K\"{o}nigl's inhomogeneous jet model, the jet parameters, such as
the bulk motion Lorentz factor, magnetic field strength, and electron density
in the jet, can be estimated with the very long-baseline interferometry and
X-ray data. We find a significant correlation between black hole mass and the
bulk Lorentz factor of the jet components for this sample, while no significant
correlation is present between the bulk Lorentz factor and the Eddington ratio.
The massive black holes will be spun up through accretion, as the black holes
acquire mass and angular momentum simultaneously through accretion. Recent
investigation indeed suggested that most supermassive black holes in elliptical
galaxies have on average higher spins than the black holes in spiral galaxies,
where random, small accretion episodes (e.g., tidally disrupted stars,
accretion of molecular clouds) might have played a more important role. If this
is true, the correlation between black hole mass and the bulk Lorentz factor of
the jet components found in this work implies that the motion velocity of the
jet components is probably governed by the black hole spin. No correlation is
found between the magnetic field strength at $10R_{\rm S}$ ($R_{\rm S}=2GM/c^2$
is the Schwarzschild radius) in the jets and the bulk Lorentz factor of the jet
components for this sample. This is consistent with the black hole spin
scenario, i.e., the faster moving jets are magnetically accelerated by the
magnetic fields threading the horizon of more rapidly rotating black holes. The
results imply that the Blandford-Znajek (BZ) mechanism may dominate over the
Blandford-Payne (BP) mechanism for the jet acceleration at least in these
radio-loud AGNs.
