It is pointed out that if a superconducting ring containing a weak link is placed in a cavity, which is in resonant with the current flowing in the ring, i.e., when the frequency of the Josephson current or one of its n-th harmonies is equal to the cavity frequency, and if the cavity has sufficiently high Q-value, then the feedback of the stationary electric-magnetic wave will act on the ring itself and produce a small sinusoidal oscillation superposing on the primary sawtooth wave oscillating current. The period of the small oscillation will be φ0/n and its amplitude that of the sawtooth wave multiplied by 1/nπ.
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