  We propose an inductive method to measure critical current density $J_c$ in
bulk superconductors. In this method, an ac magnetic field is generated by a
drive current $I_0$ flowing in a small coil mounted just above the flat surface
of superconductors, and the third-harmonic voltage $V_3$ induced in the coil is
detected. We present theoretical calculation based on the critical state model
for the ac response of bulk superconductors, and we show that the
third-harmonic voltage detected in the inductive measurements is expressed as
$V_3= G_3\omega I_0^2/J_c$, where $\omega/2\pi$ is the frequency of the drive
current, and $G_3$ is a factor determined by the configuration of the coil. We
measured the $I_0$-$V_3$ curves of a melt-textured $\rm YBa_2Cu_3O_{7-\delta}$
bulk sample, and evaluated the $J_c$ by using the theoretical results.
