  The generation of harmonics of the voltage response is considered when an AC
current is applied through a superconducting film above T_c. It is shown that
almost at all temperatures the mechanism of the temperature oscillations
created by the AC current and the temperature dependence of the resistance
dominates over the isothermal nonlinear electric conductivity. Only in a narrow
critical region close to T_c the latter is essential for the generation of the
harmonics. A detailed investigation of harmonics generation provides an
accurate method for measuring the thermal boundary conductance between the film
and the insulating substrate. The critical behaviour of the third harmonic will
give a new method for the determination of the lifetime of metastable Cooper
pairs above T_c. The comparison of the calculated fifth harmonics of the
voltage with the experiment is proposed as an important test for the
applicability of the employed theoretical models.
