  We study rectification of a current of particles moving in a spatially
periodic potential under the influence of time-periodic forces with zero mean
value. If certain time-space symmetries are broken a non-zero directed current
of particles is possible. We investigate this phenomenon in the framework of
the kinetic Boltzmann equation. We find that the attractor of the Boltzmann
equation completely reflects the symmetries of the original one-particle
equation of motion. Especially, we analyse the limits of weak and strong
relaxation. The dc current increases by several orders of magnitude with
decreasing dissipation.
