Electromagnetic extraction of energy from Kerr black holes: revisited
  Blandford \& Znajek (1977) suggested that "physical observers traveling
around the hole with the `frame-dragging-angular-velocity' $\omega$ will see
the electric field reverse direction on the surface $\omega=\Omega_{\rm F}$.
Inside \emph{this surface} they see a Poynting flux of energy going toward the
hole". Based on thermodynamics, frame-dragging, and unipolar-induction, that
when $0<\Omega_{\rm F}<\Omega_{\rm H}$, i.e., when the hole is losing energy
electromagnetically, \emph{this surface} (S$_{\rm N}$) always exists, where
$\Omega_{\rm F}$ and $\Omega_{\rm H}$ are the field-lines' and the hole's
angular velocities. The `inner domain' inside S$_{\rm N}$ counter-rotates
against the `outer domain,' and the Poynting flux ${\bf S}_{\rm EM}$ reverses
direction from outward to inward on S$_{\rm N}$. This means that the force-free
and freezing-in conditions `break down' on \emph{this surface}, severing
current- and stream-lines, when the magnetic field-lines thread S$_{\rm N}$.
\emph{This surface} is, therefore, the `magneto-centrifugal divider' of the
force-free magnetosphere into the two domains by $\Omega_{\rm F}$<=>$0$, where
$\Omega_{{\rm F}_{\omega}}=\Omega_{\rm F}-\omega$. We also argue that a pair of
surface batteries with each EMF oppositely-directed are existent at both sides
of S$_{\rm N}$. The voltage drop between the two EMFs proportional to
$\Omega_{\rm H}$ will construct a steady particle source on S$_{\rm N}$,
widening S$_{\rm N}$ to a `zero-angular-momentum' gap circulating the hole with
$\omega_{\rm N}=\Omega_{\rm F}$.
