  We quantitatively study the stationary, axisymmetric, force-free
magnetospheres of spinning (Kerr) black holes (BHs) and the conditions needed
for relativistic jets to be powered by the Blandford-Znajek mechanism. These
jets could be from active galactic nuclei, blazars, quasars, micro-quasars,
radio active galaxies, and other systems that host Kerr BHs. The structure of
the magnetosphere determines how the BH energy is extracted, e.g., via
Blandford-Znajek mechanism, which converts the BH rotational energy into
Poynting flux. The key assumption is the force-free condition, which requires
the presence of plasma with the density being above the Goldreich-Julian
density. Unlike neutron stars, which in principle can supply electrons from the
surface, BH cannot supply plasma at all. The plasma must be generated \em{in
situ} via an electron-positron cascade, presumably in the gap region. Here we
study varying conditions that provide a sufficient amount of plasma for the
Blandford-Znajek mechanism to work effectively.
