Observations with the Very Long Baseline Array (VLBA) have revealed extreme parsec scale rest frame rotation measures of several thousand rad m -2 within the flat spectrum cores of quasars. A rotation measure of 1000 rad m-2 will turn the polarization angle at 5 GHz by 200°. Therefore knowledge of the parsec scale rotation measure is required to properly interpret polarization vector orientations in AGN at frequencies less than 15 GHz. Additionally, the rotation measure serves as a probe of the physical conditions in the gas which acts as the Faraday screen. Rotation measure maps on parsec scales yield information on the magnetic field topology and strength within a few hundred parsecs of the supermassive black hole responsible for the observed AGN activity. Unification schemes make predictions of the rotation measure properties of different AGN classes, and these may be tested observationally. A survey of the rotation measure properties of 40 quasars, radio galaxies and BL Lacertae objects has been performed with the VLBA. Quasars have rest frame rotation measures in their optically thick cores on average at least four times those found for cores of BL Lac objects. Beyond a projected distance of approximately 10 parsecs the rotation measures of the optically thin jets of BL Lacs and quasars are indistinguishable with rotation measures of a few hundred rad m-2 or less. Radio galaxies generally have depolarized cores, and only one out of four radio galaxies in the sample has a detectable core rotation measure. The jet of the radio galaxy M87 has a rotation measure of nearly 10,000 rad m-2, comparable to those found in quasar cores. The Faraday screen is most likely located in close proximity to the relativistic jet, based on the observed rotation measure properties. If correct, path lengths through the Faraday rotating medium are of order 1--10 parsecs. Line of sight magnetic fields of less than 50 muG can account for the observed RM distributions.
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