Differential resistance and critical-current distribution in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ ceramics

V-I curves of several Y-Ba-Cu-O samples have been measured in the temperature range 77–91 K in weak magnetic fields (H<150 Oe). For the well prepared low-resistivity samples ${\mathit{R}}_{\mathit{f}}$==dV/dI reaches a value of about 0.1${\mathit{R}}_{\mathit{n}}$ (${\mathit{R}}_{\mathit{n}}$ is the normal resistance just above transition) in the linear V-I regime. This is consistent with a picture of decoupled superconducting grains and does not yield any direct information about the flux-flow effects. Because of self-field effects, critical-current density (${\mathit{J}}_{\mathit{c}}$) obtained from transport (and magnetization) measurements does not provide clear insight on the nature of intergrain coupling. Information on the distribution of critical currents, obtained from ${\mathit{d}}^2$V/${\mathit{dI}}^2$, supports the tunneling and/or proximity-effect coupling between the grains and seems to rule out large enhancements of ${\mathit{J}}_{\mathit{c}}$ in sintered samples with nonoriented grains.