  We present five epochs of 15 GHz VLBA observations of 13 AGN. These
observations were specially calibrated to detect parsec scale circular
polarization and our calibration techniques are discussed and analyzed in
detail. We obtained reliable detections of parsec scale circular polarization
in the radio jets of 4 AGN: 3C84, PKS 0528+134, 3C273, and 3C279. For each of
these objects our detections are at the level of ~ 0.3-1% local fractional
circular polarization. Our observations are consistent across multiple epochs
(and different calibration techniques) in the sign and magnitude of the
circular polarization observed. 3C273 and 3C279 both undergo core outbursts
during our observations and changes in the circular polarization of both
sources are correlated with these outbursts. In general, we observe the
circular polarization to be nearly coincident with the strong VLBI cores of
these objects; however, in 3C84 the circular polarization is located a full
milli-arcsecond south of the source peak, and in the 1996.73 epoch of 3C273 the
circular polarization is predominately associated with the newly emerging jet
component. Circular polarization may be produced as an intrinsic component of
synchrotron radiation or by the Faraday conversion of linear to circular
polarization. Our single frequency observations do not easily distinguish
between these possible mechanisms, but independent of mechanism, the remarkable
consistency across epoch of the sign of the observed circular polarization
suggests the existence of a long term, stable, uni-directional magnetic field.
Single dish observations of 3C273 and 3C279 at 8 GHz by Hodge and Aller suggest
that this stability may persist for decades in our frame of observation.
