Abstract

Following Kamerlingh Onnes’ discovery of superconductivity, in 1933 Walther Meissner and his collaborator Robert Ochsenfeld at the Physikalisch Technische Reichsanstalt in Berlin discovered the most fundamental property of a superconductor, namely its ability to expel magnetic flux from its interior [1]. The perfect diamagnetism associated with this Meissner–Ochsenfeld effect results from electric shielding currents flowing without resistance near the surface of the superconductor. In Figure A2.4.1, we show schematically how the superposition of the applied magnetic field and the field generated by the shielding current result in zero magnetic flux density B inside the superconductor. We define B(r) in terms of the local field H(r) produced by the superposition of the external field H _a, produced by external currents (e.g., the field by a long solenoid), and the field generated by currents flowing within the superconductor. From Ampere's theorem, we can see that for B(r) = µ₀ H(r) = 0, the total current I per unit length flowing around the outer surface of a superconducting cylinder in a parallel field is I = –H. Since the supercurrent-flow without resistance is a necessary consequence of the existence of the Meissner–Ochsenfeld effect, whereas the inverse conclusion does not hold, the Meissner–Ochsenfeld effect is clearly more fundamental than just the disappearance of the electric resistance (although only the latter phenomenon is suggested by the name ‘superconductivity’).