  We consider force-free plasma waves launched by the motion of conducting
material through a magnetic field. We develop a spacetime-covariant formalism
for perturbations of a uniform magnetic field and show how the transverse
motion of a conducting fluid acts as a source. We show that fast-mode waves are
sourced by the compressibility of the fluid, with incompressible fluids
launching a pure-Alfv\'en outflow. Remarkably, this outflow can be written down
in closed form for an arbitrary time-dependent, nonaxisymmetric incompressible
flow. The instantaneous flow velocity is imprinted on the magnetic field and
transmitted away at the speed of light, carrying detailed information about the
conducting source at the time of emission. These results can be applied to
transients in pulsar outflows and to jets from neutron stars orbiting in the
magnetosphere of another compact object. We discuss jets from moving conductors
in some detail.
