  We study the dependence of the transport properties of square Josephson
Junctions arrays with the direction of the applied dc current, both
experimentally and numerically. We present computational simulations of
current-voltage curves at finite temperatures for a single vortex in the array
($Ha^2/\Phi_0=f=1/L^2$), and experimental measurements in $100\times1000$
arrays under a low magnetic field corresponding to $f\approx0.02$. We find that
the transverse voltage vanishes only in the directions of maximum symmetry of
the square lattice: the [10] and [01] direction (parallel bias) and the [11]
direction (diagonal bias). For orientations different than the symmetry
directions, we find a finite transverse voltage which depends strongly on the
angle $\phi$ of the current. We find that vortex motion is pinned in the [10]
direction ($\phi=0$), meaning that the voltage response is insensitive to small
changes in the orientation of the current near $\phi=0$. We call this
phenomenon orientational pinning. This leads to a finite transverse critical
current for a bias at $\phi=0$ and to a transverse voltage for a bias at
$\phi\not=0$. On the other hand, for diagonal bias in the [11] direction the
behavior is highly unstable against small variations of $\phi$, leading to a
rapid change from zero transverse voltage to a large transverse voltage within
a few degrees. This last behavior is in good agreement with our measurements in
arrays with a quasi-diagonal current drive.
