  Differential rotation occurs in conducting flows in accretion disks and
planetary cores. In such systems, the magnetorotational instability can arise
from coupling Lorentz and centrifugal forces to cause large radial angular
momentum fluxes. We present the first experimental observation of the
magnetorotational instability. Our system consists of liquid sodium between
differentially rotating spheres, with an imposed coaxial magnetic field. We
characterize the observed patterns, dynamics and torque increases, and
establish that this instability can occur from a hydrodynamic turbulent
background.
