  The standard approach to non-equilibrium thermodynamics describes transport
in terms of generalised forces and coupled currents, a typical example being
the Fourier law that relates temperature gradient to the heat flux. Here we
demonstrate the presence of persistent energy currents in a lattice of
classical nonlinear oscillators with uniform temperature and chemical
potential. In strong analogy with the well known Josephson effect, the currents
are generated only by the phase differences between the oscillators. The phases
play the role of additional thermodynamical forces, that drive the system out
of equilibrium. Our results apply to a large class of oscillators and indicate
novel ways to practically control the propagation of coupled currents and
rectification effects in many different devices. They also suggest a simple,
macroscopic setup for studying a phenomenon which hitherto have only been
observed in microscopic, quantum-mechanical systems.
