  The obtained periodic magnetic-field dependencies of the critical current,
measured in opposite directions on asymmetric superconducting aluminum rings,
allow to explain observed earlier quantum oscillations of a dc voltage as a
result of alternating current rectification. It is found, that the high
efficiency of the rectification of both individual rings and ring systems is
connected to a hysteresis of the current-voltage characteristics. The asymmetry
of the current-voltage characteristics providing the rectification effect is
due to the relative shifts of the magnetic dependencies of the critical current
measured in opposite directions. This shift means that position of the critical
current minimums does not correspond to n + 0.5 magnetic flux quantum which is
in the direct contradiction with measurement results of the Little-Parks
resistance oscillations. Despite of this contradiction the amplitude of the
critical current anisotropy oscillations and its variations with temperature
correspond to expected amplitude of the persistent current oscillations and to
its variations with temperature.
