[en] We have investigated experimentally the penetration of low magnetic fields into Bi-based ceramics. The behavior of the harmonics of the induction was investigated in ac and dc fields in a wide range of temperatures and field amplitudes. From the experimental results obtained in the asymptotic case of a weak field the values of a number of quantities characterizing the low-field electrodynamics of HTSC and their temperature dependences as well as an explicit expression for the critical current density j_c(H, T)=j₀(T)H₀(T)/[vertical strokeHvertical stroke+H₀(T)] were derived. (orig.)