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Cool a thick superconducting ring through its transition in a controlled external flux, remove or change the applied field while keeping the ring superconducting, and measure the persistent trapped flux using a calibrated pickup coil and sensitive magnetometer. Repeating while varying the cooling flux selects successive winding sectors. The measured flux is close to steps separated by ${\Phi }_0$ when the kinetic-inductance correction is negligible. A SQUID-based flux readout can resolve that spacing; scanning local magnetic imaging offers another way to observe the trapped flux without identifying the ring current with an ordinary dissipative transport current.

The experiment tests the phase-winding/fluxoid relation in a. Self-inductance, London penetration depth and the size of the ring determine how nearly the measured geometric flux follows integer steps. A periodic response alone should not be used to infer an absolute trapped flux without calibration and the relevant inductive corrections.

TODO — source figure gap: the surviving ordinary-ring and two-junction-ring outlines and labels are readable, but page 9's figure form calls 52 absent raster objects, /R514 through /R565. The original archived parent has identical damage. An intact figure is needed to recover any missing image-specific details; those raster components have not been inferred or recreated. The observation method above answers the physical request without requiring the unavailable artwork; the TODO concerns verification of the input illustration rather than a missing part of the solution. The original circuit schematic used in c is an independent diagram of the stated model, not a claimed reconstruction of the unavailable artwork.

The AC Josephson effect is $V=({\Phi }_0/2\pi )\dot{\delta }$. Since the imposed flux is held fixed when defining a small-signal inductance, differentiate c to obtainOn the stable small-current branch with $\cos \varphi >0$, $\delta \simeq 0$ and the result reduces to the printed expression,The stable small-signal inductance of a symmetric DC SQUID requires $|I|\ll 2I_C|\cos \varphi |$, stronger than $I\ll I_C$ near a half-integer flux quantum. If $\cos \varphi <0$, the stable zero-current phase is $\delta \simeq \pi$ and $\cos \delta \simeq -1$. Its inductance is positive:For example, $\Phi ={\Phi }_0$ makes the printed signed formula negative when expanded about $\delta =0$, but that phase is an energy maximum. Expanding about the stable $\delta =\pi$ corrects the sign. At ideal half-integer flux the critical current vanishes and the linear inductance formally diverges; a finite transport current cannot satisfy the small-current approximation there. Geometric inductance and other nonidealities then have to be retained instead of extending the simple formula through its pole.