  The transition to a type II proton superconductor which is believed to occur
in a cooling neutron star is accompanied by changes in the equation of
hydrostatic equilibrium and by the formation of proton vortices with quantized
magnetic flux. Analysis of the electron Boltzmann equation for this system and
of the proton supercurrent distribution formed at the transition leads to the
derivation of a simple expression for the transport velocity of magnetic flux
in the liquid interior of a neutron star. This shows that flux moves easily as
a consequence of the interaction between neutron and proton superfluid vortices
during intervals of spin-down or spin-up in binary systems. The differences
between the present analysis and those of previous workers are reviewed and an
error in the paper of Jones (1991) is corrected.
