  We present a detailed examination of the relationship between the magnetic
field structures and the variations in Faraday Rotation across PKS1246-410, a
radio source in the Centaurus cluster of galaxies, using data from Taylor,
Fabian and Allen. We find a significant relationship between the intrinsic
position angle of the polarization and the local amount of Faraday Rotation.
The most plausible explanation is that most or all of the rotation is local to
the source. We suggest that the rotations local to cluster radio galaxies may
result from either thermal material mixed with the radio plasma, or from thin
skins of warm, ionized gas in pressure balance with the observed galaxy or hot
cluster atmospheres. We find that the contribution of any unrelated cluster
Rotation Measure variations on scales of 2 - 10 arcsec are less than 25
rad/m^2; the standard, although model dependent, derivation of cluster fields
would then lead to an upper limit of approximately 0.4 microGauss on these
scales. Inspection of the distributions of Rotation Measure, polarisation angle
and total intensity in 3C75, 3C465 and Cygnus A also shows source-related
Faraday effects in some locations. Many effects can mask the signatures of
locally-dominated RMs, so the detection of even isolated correlations can be
important, although difficult to quantify statistically. In order to use radio
sources such as shown here to derive {\it cluster-wide} magnetic fields, as is
commonly done, one must first remove the local contributions; this is not
possible at present.
