  We measure the expansion of an ultracold plasma across the field lines of a
uniform magnetic field. We image the ion distribution by extracting the ions
with a high voltage pulse onto a position-sensitive detector. Early in the
lifetime of the plasma ($< 20$ $\mu$s), the size of the image is dominated by
the time-of-flight Coulomb explosion of the dense ion cloud. For later times,
we measure the 2-D Gaussian width of the ion image, obtaining the transverse
expansion velocity as a function of magnetic field (up to 70 G). We observe
that the expansion velocity scales as B$^{-1/2}$, explained by a nonlinear
ambipolar diffusion model with anisotropic diffusion in two different
directions.
