  We consider the evolution of electromagnetic fields coupled to conduction
currents during the reheating era after inflation, and prior to the
establishing of the proton-electron plasma. We assume that the currents may be
described by second order causal hydrodynamics. The resulting theory is not
conformally invariant. The expansion of the Universe produces temperature
gradients which couple to the current and generally oppose Ohmic dissipation.
Although the effect is not strong, it suggests that the unfolding of
hydrodynamic instabilities in these models may follow a different pattern than
in first order theories, and even than in second order theories on non
expanding backgrounds.
