An NMR spectrometer includes a superconducting, solenoid coil formed of superconducting wire having a rectangular cross-section. The wire is formed of a non superconducting metal surrounding superconducting material having a small enough cross sectional area that a magnetic field resulting from current flowing in the superconductor material has a relatively constant position relative to the cross section of the wire. In one embodiment, the wire has a monofilament configuration including a core of the superconducting material surrounded by a sheath of the metal. In a second embodiment, the wire includes a copper block of the metal through which a multiplicity of superconductive strands extend.