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The general flux line distribution in a type II superconductor in the mixed state is curved in three dimensions. In this paper flux line motion is treated using the methods of irreversible thermodynamics; a general condition for the flux line distribution to be stationary is obtained. The effects of pinning centres are included and forces acting in the mixed state are analysed. The macroscopic critical state equation in three dimensions is derived and its solution in some special cases discussed.
Evetts et al. (1968) studied this question.