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curves for large Q, which must persist to some extent when noise is included, is the occurrence of a sharp rise in voltage ("instability") at a value of X considerably smaller than xmity.It should be noted that the parameter Q is independent of the area of the junction, depending only on the thickness of the junction, the temperature, and the materials involved. The parameter y, by contrast, is proportional to the area of the junction.We are grateful to Professor M. J. Stephen for posing this problem to us, and for discussions, and to Mr. F. Kardon for the numerical computations. One of the authors (V.A.) would like to thank the Bell Telephone Laboratories for hospitality during the month of January 1969 when this work was begun.

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Direct Observation of Single Magnetic Flux-Quanta in Superconducting Tubes. III By use of electron interferometry the trapped magnetic flux was investigated in superconducting microcylinders (inner diameter < 1 gm). Even in thin walled cylinders (wall thickness < penetration depth) at temperatures near the superconducting transition the flux was found to be quantized in integer multiples hc/2e.In the appendix an interpretation of the magnetic Aharonov-Bohm effect as a local interaction between the excluded field and the electromagnetic field of the electron is discussed.