  Multi-frequency (4.6, 5, 5.5, 8, 8.8, 13, 15, 22 & 43 GHz) polarization
observations of 6 "blazars" were obtained on the American Very Long Baseline
Array (VLBA) over a 24-hr period on 2 July 2006. Observing at several
frequencies, separated by short and long intervals, enabled reliable
determination of the distribution of Faraday Rotation on a range of scales. In
all cases the magnitude of the RM increases in the higher frequency
observations, implying that the electron density and/or magnetic field strength
is increasing as we get closer to the central engine. After correcting for
Faraday rotation, the polarization orientation in the jet is either parallel or
perpendicular to the jet direction. A transverse Rotation Measure (RM) gradient
was detected in the jet of 0954+658, providing evidence for the presence of a
helical magnetic field surrounding the jet. For three of the sources (0954+658,
1418+546, 2200+420), the sign of the RM in the core region changes in different
frequency-intervals, indicating that the line-of-sight component of the
magnetic field is changing with distance from the base of the jet. We suggest
an explanation for this in terms of bends in a relativistic jet surrounded by a
helical magnetic field; where there is no clear evidence for pc-scale bends,
the same effect can be explained by an accelerating/decelerating jet.
