Use of the cgs system was entirely acceptable and magnetism and superconductivity did not coexist.
In the recent past, the differentiation of B and H was a matter left to those who worried about the basic physics of magnetism. Definitions of the various critical fields are given both in the discussion of the theory and of the extrapolation techniques. The terminology used here for the general references to the field of superconductivity is that presented in the several review documents on terminology prepared by our group and in the new ASTM standard for superconductor terminology. Thus, although our concentration is on the conductors that are commercially available, these new materials with great potential are also considered. However, that situation could easily change. Similarly, the more exotic materials, such as the Chevrel-phase conductors, are not of immediate practical importance. There are, of course, exceptions such as pinning studies.
However, for practical purposes, effects that take place below a critical current value of about 10 4 A/cm 2 are seldom of interest, which removes us from the more esoteric regions of the J-H and T-H curves. Thus, we have included most of the effects that have been observed to date during attempts to determine the critical fields. Yet, our survey of the literature indicates that frequently these effects do appear in rather mundane measurements and can alter the results significantly. This criterion might seem to allow us to ignore many of the more difficult aspects of the problem, such as the effect of surface superconductivity. 1 We consider only superconductive materials that are either commercial items now or those that have the potential of being so in the foreseeable future. Our goal is to evaluate whether there is a need for a measurement standard for critical field similar to one developed recently for critical current. Several ( H c, H c1, H c2) have universal application and acceptance, while many others are defined for subtle and specific purposes. The literature of the field of superconductivity is replete with numerous critical magnetic fields.
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