Superconducting Critical Currents in Wire Samples and Some Experimental Coils
For a given hard superconductor, the transition boundary between the superconducting and the normal state is known to be a function of many variables. Three primary variables are the temperature, the magnetic field, and the current density. Two additional variables are the orientation of the current relative to the magnetic field and the history of the sample since it became superconducting. In short-wire samples which have been cooled in zero magnetic field and are held in a fixed orientation with respect to the field, a fairly well-defined transition boundary can be determined which depends only upon the temperature, the magnetic field strength, and the amount of current passed through the wire. When the wire from which the sample was taken is wound in a coil, the conditions are apparently the same as for the short-sample measurement. The coil and sample are both cooled in liquid helium in zero magnetic field, and the field generated by the coil is in the same transverse orientation to the current. One would expect to find that transition from the superconducting state occurs at the same value of current passed through the wire. In fact, the transition occurs at some much lower value of current. Evidently, the expected conditions are not realized in the local wire environment in the coil. Perhaps the temperature of the wire has somehow been raised. Perhaps the particular history of the wire during the rise of current and field has resulted in eddy currents that raise the current density to the critical value for low values of input current. Or there may be undiscovered variables. This paper can only present experimental results to more completely describe the problem.
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