  The c-axis penetration depth $\lambda_c$ in
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (BSCCO) single crystals as a function of
temperature has been determined using three high-frequency techniques, namely:
(i) measurements of the ac-susceptibility at a frequency of 100 kHz for
different sample alignments with respect to the ac magnetic field; (ii)
measurements of the surface impedance in both superconducting and normal states
of BSCCO crystals at 9.4 GHz; (iii) measurements of the surface barrier field
$H_J(T)\propto 1/\lambda_c(T)$ at which Josephson vortices penetrate into the
sample. Careful analysis of these measurements, including both numerical
solution of the electrodynamic problem of the magnetic field distribution in an
anisotropic plate at an arbitrary temperature and influence of defects in the
sample, has allowed us to estimate $\lambda_c(0)\approx 50 \mu$m in BSCCO
crystals overdoped with oxygen ($T_c\approx 84$ K) and $\lambda_c(0)\approx 150
\mu$m at the optimal doping level ($T_c\approx 90$ K). The results obtained by
different techniques are in reasonable agreement.
