We introduce the first simple mechanical system that shows fully realistic
transport behavior while still being exactly solvable at the level of equil
ibrium statistical mechanics. The system is a Lorentz gas with fixed freely
rotating circular scatterers which scatter point particles via perfectly r
ough collisions. Upon imposing either a temperature gradient and/or a chemi
cal potential gradient, a stationary state is attained for which local ther
mal equilibrium holds. Transport in this system is normal in the sense that
the transport coefficients which characterize the flow of heal and matter
are finite in the thermodynamic limit. Moreover, the two hows are nontrivia
lly coupled, satisfying Onsager's reciprocity relations.