Critical state of Josephson medium

A model of the critical state of a Josephson medium is developed on the basis of the Sonin theory of averaged Josephson medium. The model is used to explain the experimental data on the differential magnetic susceptibility $\chi_d(H)$ and magnetoresistance $R(H)$ of polycrystalline YBa$_2$Cu$_3$O$_{7-x}$ samples in fields $H$<100 Oe.