  In the context of an Abelian gauge symmetry, spontaneously broken at a
first-order transition, we discuss the evolution of the phase difference
between the Higgs fields in colliding bubbles. We show that the effect of
dissipation, represented by a finite plasma conductivity, is to cause the
phases to equlibrate on a time-scale, determined by the conductivity, which can
be much smaller than the bubble radii at the time of collision. Currents
induced during the phase equilibration generate a magnetic flux, which is
determined by the initial phase difference. In a three-bubble collision, the
fluxes produced by each pair of bubbles combine, and a vortex can be formed. We
find that, under most conditions, the probability of trapping magnetic flux to
form a vortex is correctly given by the ``geodesic rule''.
