  We have applied the mutual inductive method to study higher-order harmonics
of complex ac susceptibility $\chi_n=\chi'_n-i\chi"_n$ for
$$YB_2C_3O_{7-\delta}$ thin films as function of the temperature and the
applied field. The experimental results were compared with analytical and
numerical results obtained from the Ishida-Mazaki model and the solution of the
integral equation for the current density, respectively. Both models allow us
to reproduced the main experimental features, however, as $n$ increases the
numerical model shows notable discrepancies. This failure can be attributed to
the current-voltage characteristics. Also this investigation yields the
activation energy $U_c$ and the critical current density $J_c$ for two samples
both at T=0.
