  We present sub-milliarcsecond resolution total intensity and linear
polarization VLBI images of 3C273, using concurrent 43 and 86 GHz data taken
with the Very Long Baseline Array in May 2002. The structure seen in the
innermost jet suggest that we have fortuitously caught the jet in the act of
changing direction. The polarization images confirm that the core is
unpolarized (fractional polarization m < 1 %) at 86 GHz, but also show well
ordered magnetic fields (m ~ 15 %) in the inner jet, at a projected distance of
2.3 pc from the core. In this strongly polarized region, the rotation measure
changes across the jet by 4.2 x 10^{4} rad m^{-2} over an angular width of
about 0.3 milliarcseconds. If the lack of polarization in the core is also
attributed to a Faraday screen, then a rotation measure dispersion > 5.2 x
10^{4} rad m^{-2} must be present in or in front of that region. These are
among the highest rotation measures reported so far in the nucleus of any
active galaxy or quasar, and must occur outside (but probably close to) the
radio emitting region. The transverse rotation measure gradient is in the same
sense as that observed by Asada et al and by Zavala and Taylor at greater core
distances. The magnitude of the transverse gradient decreases rapidly with
distance down the jet, and appears to be variable.
