The coherence length &xgr; and penetration depth &lgr; set the characteristic length scales in superconductors, typically 100−5,000 A˚. A lattice of flux lines, each carrying a single quantum, can penetrate type II superconductors, i.e., those for which &kgr;≡&lgr;/&xgr;≳1/&sqrt;2. Inhomogeneities on the scale of the flux lattice spacing are required to pin the lattice to prevent dissipative flux motion. Recent work using voids as pinning centers has demonstrated this principle, but practical materials rely on cold‐work, inclusions of second phases, etc. to provide the inhomogeneity. For stability against thermal fluctuations, the superconductor should have the form of many filaments of diameter &angupr;10−100 &mgr;m imbedded in a highly conductive normal metal matrix. Such wire is made by drawing down billets of copper containing rods of the superconductor. An alternative approach is metallurgical one of Tsuei, which leads to thousands of superconducting filamentary segments in a copper matrix. The superconducting proximity effect causes the whole material to superconduct at low current densities. At high current densities, the range of the proximity effect is reduced so that the effective superconducting volume fraction falls below the percolation threshold, and a finite resistance arises from the copper matrix. But, because of the extremely elongated filaments, this resistance is orders of magnitude lower than that of the normal wire, and low enough to permit the possibility of technical applications.