  Following the early paper of Goldreich & Julian (1969), polar-cap models have
usually assumed that the closed sector of a pulsar magnetosphere corotates with
the neutron star. Recent work by Melrose & Yuen has been a reminder that in an
oblique rotator, the induction field arising from the time-varying magnetic
flux density cannot be completely screened. The principal consequence is that
the plasma does not corotate with the star. Here it is shown that the physics
of the polar cap is not changed at the altitudes of the radio emission source.
But the presence of a plasma drift velocity in the corotating frame of
reference does provide a mechanism whereby the net charge of the star can be
maintained within a stable band of values. It also shows directly how electron
injection and acceleration occur in the outer gap of the magnetosphere. It is
consistent with radio-loud pulsars in the Fermi LAT catalogue of gamma-emitters
all having positive polar-cap charge density.
