The Galaxy-corrected Faraday rotation measure of a distant quasar can be broken into three unrelated components in equation (1.3): (i) the RM which is intrinsic to the source (IRM), (ii) a component added by some intervening galaxy system (e.g. galaxy disk or halo, galaxy group, or cluster of galaxies), and (iii) hypothetically, that due to an all-pervading, cosmologically scaled magneto-ionic medium. Using measured or estimated values of ne, and the field reversal scale, an RM associated with any of these three components could give an estimate, or limit to the associated magnetic field (cf Kronberg and Perry 1982). The observed value of a rotation measure generated at redshift, z, will have its detectability reduced by (1 + z)-2. On the other hand due to cosmological expansion, ne ~ (1 + z)3, and |Bigm| ~ (1 + z)2, assuming flux conservation i.e. B in component (iii) is frozen in to the IGM plasma. This illustrates how ne B could, for large z, overwhelm the 1 + z)-2 watering down effect on the observed RMS and thereby produce a detectable RM signal, if the magneto-ionic strength follows the cosmological scale factor in some reasonable way. We briefly review the results of observational attempts to probe components (ii)-(iii) at significant cosmological lookback times.