Progress in superconductivity

Superconductors are in a quantum state, extending over macroscopic distances. Consequently, they trap magnetic flux in multiples of the flux quantum. This point of view will be taken for a discussion of some recent developments related mainly to induction phenomena in superconductors. 1) In superconductors of the second kind magnetic flux penetrates at external magnetic field values within a certain interval (mixed state). This state violates the traditional characteristics of superconductivity. Not only does it fail to show the perfect Meissner diamagnetism, but it is also, in principle, a resistive state. The latter feature is correlated with the instability of the magnetic structure in the mixed state when a current is injected, the induced flux flow phenomenon being responsible for a variety of effects recently studied, including the Hall effect and a mode of magnetic coupling which makes a dc transformer feasible. 2) It is well known by now that superconductors of the second kind may obtain excellent current-carrying capacities if the mixed state magnetic pattern is stabilized, or pinned. This process is still not fully understood. The recent manufacture of some interesting and practicable wires and cables and the problems of coil construction and their stabilization will be briefly surveyed. 3) By proper manipulation of magnetic flux in superconducting systems, electromagnetic induction of heavy direct currents may be achieved. Fully superconducting dc generators or dynamos have recently been developed, the qualities of which will be discussed. The results of some of the author´s recent investigations will be given, including those on multikiloampere dynamos for energizing supermagnets.