A possible energy source for active galactic nuclei (AGN) jets is the extraction of the rotational energy stored in a Kerr black hole by a large-scale magnetohydrodynamic (MHD) wind. Current models of black hole magnetospheres assume that pair plasma is photoproduced on field lines which thread the event horizon. In analogy with the Goldreich-Julian unipolar inductor model of pulsars, strongly magnetized, relativistic MHD winds, which flow into the event horizon and out to asymptotic infinity, transport a net energy and angular momentum flux from the black hole. For pulsars, unipolar induction of the central star drives a global current system which generates and sustains the outgoing wind. Current black hole models assume, either explicitly or implicitly, that the event horizon also acts as a unipolar inductor. However, since the ingoing wind must flow faster than any MHD wave signal, the event horizon is fundamentally out of causal contact with both the plasma source region and the outgoing wind; thus, the black hole cannot be a unipolar inductor. Furthermore, the ingoing wind does not, by itself, drive a global unipolar current system. The electrodynamics of the ingoing wind near the event horizon are exactly analogous to the properties of the pulsar wind at asymptotic infinity. Hence, both the ingoing and outgoing winds are best viewed as propagating toward asymptotic infinities and transporting the conserved mass, angular momentum, and energy fluxes established by the source region. Current black hole wind theories determine the fourth conserved quantity - the rotation rate of the magnetic field - by equating the Poynting fluxes at the horizon and infinity. However, since these two regions cannot communicate, those solutions are acausal and are therefore not unique. The field line roation rate or total voltage drop across the system must also be determined by the dissipative process which injects plasma onto the magnetospheric field lines.
