  We explore the possibility of obtaining unidirectional current in a symmetric
(periodic) potential system without the application of any obvious (apparent)
externally applied bias. There are many physical models proposed to accomplish
this nonequilibrium effect. In the present work we consider inhomogeneous
systems so that the friction coefficient and/or temperature could vary in
space. We find out a model with minimal conditions that the inhomogeneous
system assisted by fluctuating forces must satisfy, in order to obtain
unidirectional current. In the process we discuss about thermal and frictional
ratchets that are of current interest. We argue that different models of
frictional ratchets work under the same basic principle of alteration of
relative stability of otherwise locally stable states in the presence of
temperature inhomogeneity. We also discuss in detail the nature of currents in
rocked frictional ratchets. In particular we analyse a novel phenomenon of
multiple current reversals and the efficiency of the energy transduction in
these systems.
