On basis of the Meissner effect, which assures the reversibility of the normal-superconducting state transition, thermodynamical relations describing the difference in superconducting and normal states were derived in the preceding chapter. In this chapter, a two-fluid theory is described where the free electrons in the superconducting state are assumed to be a mixture of superelectrons that flow freely without resistance and normal electrons that have Ohm resistance. The theory is phenomological in that the property of the superelectron is based on experiment and the theory does not attempt to describe the nature and origin of the superelectrons.
