  Measurement of the current-voltage characteristics of point contacts makes it
possible to study the properties of individual crystallites in a
superconducting ceramic. The critical current density in the superconducting
regions of the ceramic $\rm La_{1.8}Sr_{0.2}CuO_4$, with a size of the order of
several tens of angstroms, is found to attain values of $10^8\ A/cm^2$, which
are of the same order of magnitude as the pair-breaking current density, as
evaluated from the formulas of the standard theory of superconductivity.
