  We re-examine the nature of the turbulent magnetic diffusivity tensor of mean
field electrodynamics and show that an inconsistency arises if it is calculated
via consideration of time-independent magnetic fields. Specifically, the
predicted growth rate of the mean magnetic field is, in general, incorrect. We
describe how the traditional expansion procedure for the mean electromotive
force should be extended, and illustrate the consistency of this approach by
means of a perturbation analysis for a mean magnetic field varying on long
spatial scales. Finally we examine the magnitude of this new contribution to
the magnetic diffusion for a particular flow.
