  The current-voltage curves and magnetic dependence of the critical current of
asymmetric superconducting loops are measured. It was found that sign and value
of the asymmetry of the current-voltage curves changes with value of magnetic
field, periodically for single loop and system of identical loops. The obtained
results allow to explain the quantum oscillation of the dc voltage, observed
below superconducting transition in the previous works, as rectification of ac
current or noise.
