  We present a mechanism for thermalizing a moving particle by microscopic
deterministic scattering. As an example, we consider the periodic Lorentz gas.
We modify the collision rules by including energy transfer between particle and
scatterer such that the scatterer mimics a thermal reservoir with arbitrarily
many degrees of freedom. The complete system is deterministic, time-reversible,
and provides a microcanonical density in equilibrium. In the limit of the disk
representing infinitely many degrees of freedom and by applying an electric
field the system goes into a nonequilibrium steady state.
