  We re-examine the characteristic polarization angle sweep of rotation-powered
pulsars and calculate the expected deviations from this sweep caused by
aberrational effects and by polar-cap current flow. We find that in addition to
the previously known phase shift of the entire sweep by $\Delta \Phi = -4
r/R_L$, aberration shifts the polarization angle itself by $\Delta \Psi =
-(10/3) (r/R_L) \cos \alpha$. Similarly, current flow above the polar cap
shifts the polarization sweep by $\Delta \Psi = (10/3) (r/R_L) (J/J_{GJ}) \cos
\alpha$, potentially providing a method of directly measuring the magnitude of
the current. The competition between these two effects produces a potentially
observable signature in the polarization angle sweep. Although these effects
may appear similar to orthogonal mode shifts, they are an independent
phenomenon with distinct observational characteristics.
