The Nature of Jets: Evidence from Circular Polarization Observations
  We review recent observations of circularly polarized radiation from AGN made with the VLBA and with the ATCA. We also discuss briefly the detections of the Galactic sources Sag A* and SS433. The origin of the circular polarization is still an open question in most cases, and we discuss four possible mechanisms. Detectable circular polarization is a common property of quasars, but not of radio galaxies, and is always associated with the compact core. There is growing evidence that the sign of the circular polarization stays the same over at least two or three decades in time, suggesting it is a fundamental property of the jet. 
John F. C. Wardle and Daniel C. Homan