  When magnetic field lines thread a rotating black hole's horizon and connect
with remote astrophysical loads, the rotational energy of the black hole can be
extracted through the Blandford-Znajek mechanism. Due to the rotation of the
black hole, the magnetic field lines are twisted and toroidal components are
generated. So poloidal electric currents are induced and the black hole's
rotational energy is transported to the astrophysical loads through Poynting
flux. The Blandford-Znajek mechanism has been considered to be a possible
process for powering extragalactic jets.
  In this paper we show that due to the screw instability of magnetic field,
the toroidal components of the magnetic field, and thus the poloidal currents,
cannot exceed the limits given by the Kruskal-Shafranov criterion. This
significantly lowers the power of the Blandford-Znajek mechanism when the loads
are far from the black hole. So the Blandford-Znajek mechanism can only work
efficiently within the neighborhood of the black hole. The implications of the
results for the scenario of extragalactic jets powered by the Blandford-Znajek
mechanism are discussed.
