  We study the persistent currents induced by the Aharonov-Bohm effect in a
closed ring which either embeds or is directly side coupled to a quantum dot at
Kondo resonance. We predict that in both cases, the persistent current is very
sensitive to the ratio between the length of the ring and the size of the Kondo
screening cloud which appears as a fundamental prediction of scaling theories
of the Kondo effect. Persistent current measurements provide therefore an
opportunity to detect this cloud which has so far never been observed
experimentally.
