  Many radio sources like quasars, blazars, radio galaxies, and micro-quasars
exhibit circular polarisation (CP) with surprising temporal persistent
handedness. As a possible explanation we propose that the CP is due to Faraday
conversion (FC) of linear polarisation (LP) synchrotron light which propagates
along a line-of-sight (LOS) through twisted magnetic fields. The rotational
nature of accretion flows onto black holes naturally generates the required
magnetic twist in the emission region, independent of whether it is a jet or an
ADAF. The expected twist in both types of flows is of the order of what is
required for optimal CP generation. This scenario requires that Faraday
rotation (FR) is insignificant in the emission region. Although this is an
assumption, it relaxes constraints on the plasma parameters, that were given in
scenarios which rely on FR, since there the strength of FR can not be too far
from the optimum. The proposed mechanism works in electron-positron (e+-) as
well as electron-proton (e/p) plasma. In the latter case, the emission region
should consist of individual flux tubes with independent polarities in order to
suppress too strong FR. The predominant CP is expected to mostly counter-rotate
(rotation is measured here in sky-projection) with respect to the central
engine in all cases (jet or ADAF, e+- or e/p plasma). If the proposed mechanism
is indeed operating, it will allow to measure the sense of rotation of quasar
engines. The engine of SgrA* is then expected to rotate clockwise and therefore
counter-Galactic, as do the young hot stars in its vicinity, which are thought
to feed SgrA* by their winds. Similarly, we expect the microquasars SS 443 and
GRS 1915+105 to rotate clockwise. Generally, sources with Stokes-V<0 (V>0) are
expected to rotate clockwise (counter-clockwise) in this scenario.
