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The observed quantization of magnetic flux enclosed in a superconducting annulus is discussed. It is shown that the size of the quantum unit (hc/2e) implies a pairing effect between the electrons. The suggestion that flux quantization is an intrinsic property of electromagnetic fields is analyzed. The effect of flux quantization on the observed fact, that the magnetic field in a superconductor = 0, is discussed. (T.F.H.)
Lars Onsager (Sat,) studied this question.