Six epochs of linear polarization sensitive VLBI observations spanning more then 6 yr are used to study the evolution of the parsec-scale structure of the superluminal quasar 3C 345 at 5 GHz (λ = 6 cm). These images form the first few frames of a "magnetic movie" showing the structure and evolution of the magnetic field. Most of the polarized flux is associated with the moving components in the jet. New components emerge with a small degree of polarization that increases as they separate from the core. The most highly polarized part of the jet has fractional polarization m_max_ ~ 20%, so there is significant order in the magnetic field. The magnetic field orientation along the jet varies in a complicated fashion with both position and time. Near the core, where components have been observed to accelerate, the field orientation also changes rapidly. In the gently curving part of the jet the magnetic field is very nearly parallel to the local jet axis. At the outermost knot visible at this frequency and resolution, the polarization is confined to the leading edge of the component, and the magnetic field is offset from parallel to the jet axis by ~40^deg^. The difference between the polarization on milliarcsecond and arcsecond scales shows the "intermediate-scale" jet to be modestly polarized (4%) with its magnetic field lying roughly parallel to the jet direction. At all epochs a peak in the fractional polarization occurs between components, in the underlying jet. This is strong evidence that the knots are regions of compression in a jet containing both longitudinal and tangled components of magnetic field. Such a model is described in detail in a companion paper (Wardle et al. 1994).
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Brown et al. (1994) studied this question.