There is strong observational evidence that AGN, Galactic X-ray transients
and (probably) γ-ray bursts are associated with black holes, and that
these sources are able to form collimated, ultrarelativistic outflows. There is
much interest in trying to understand how these prime movers are able to
release energy from accreting gas and their own spin energy. Electromagnetic
field plays a large role in many of the mechanisms under active consideration.
In this article, several of the many possible ``metabolic pathways'' through
which mass, angular momentum and energy can flow around and away from black
hole magnetospheres are discussed. Particular importance is attached to the
interactions between the inflowing disk, the outflowing wind, the black hole
and the jet. Some important unresolved questions are identified and it is
argued that large scale numerical computation will almost certainly be
necessary to address them.Comment: 21 pages Progress of Theoretical Physics Supplement in pres