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We present the first 86 GHz VLBI observations of the radio galaxy 3C~120,together with contemporaneous 43 and 22 GHz polarimetric VLBA observations. Thevery high angular resolution obtained at 86 GHz provides an upper limit to thesize of the core of 54 $\mu$as (0.025 $h^{-1}$pc). This represents a directdetermination of the base of the jet which is independent of variabilityarguments (which depend on uncertain estimates of the Doppler factor), andplaces it below approximately one light-month. Comparison with previous VLBAobservations after a one-year interval shows pronounced changes in thestructure and polarization of the jet. Most of the components are found tofollow a curved path while undergoing a steepening of their spectra accompaniedby a decrease in total and polarized emission. However, at least one componentis observed to follow a quasi-ballistic motion, accompanied by a flattening ofits spectrum, as well as an increase in total and polarized flux. This may beexplained by its interaction with the external medium, resulting in a shockthat enhances the emission and aligns the magnetic field perpendicular to thecomponent motion, thereby producing an increase of the degree of polarizationfrom undetected values to as high as 15%. A second strong component, with thehighest degree of polarization (23%), is found to have experienced adisplacement from the ridge line of the structural position angle of the jet asit moved downstream. We have found a mean swing to the south of the positionangle of the innermost components of $\sim 6^{\circ}$ between late 1996 and1997, which may be responsible for the jet curvature observed at parsec andkiloparsec scales.Comment: 12 pages, 1 figure. Accepted for publication in ApJ Letter

86, 43, and 22 GH[CLC]z[/CLC] VLBI Observations of 3C 120