  We examine the time-dependent non-equilibrium current in a mesoscopic metal
ring threaded by a static magnetic flux phi that is generated by a
time-dependent electric field oscillating with frequency omega. We show that in
quadratic order in the field there are three fundamentally different
contributions to the current. (a) A time-independent contribution which can be
obtained from a thermodynamic derivative. (b) A term increasing linearly in
time that can be understood in terms of Fermi's golden rule. The derivation of
this term requires a careful treatment of the infinitesimal imaginary parts
that are added to the real frequency omega when the electric field is
adiabatically switched on. (c) Finally, there is also a time-dependent current
oscillating with frequency 2 omega. We suggest an experiment to test our
results.
