  We consider the Blandford-Znajek (BZ) mechanism for extracting black hole
spin energy to drive astrophysical jets. Analyses of the BZ mechanism generally
take no account of any electric charge on the black hole. But, as noted by Wald
and others, if the medium surrounding the black hole is an ionized plasma with
mobile charges, then a spinning hole quickly acquires an electric charge. The
effect of this charge is to nullify the electric field structures which drive
the BZ mechanism. Since jets are now observed in a wide variety of classes of
accreting objects, most of which do not contain a central black hole, it seems
likely that the jet driving mechanism in all astrophysical objects uses energy
directly from the accretion disc, rather than black hole spin.
