The ESR spectra of oxygen-deficient and doped rutile have been investigated at liquid helium temperatures. Niobium and tantalum are shown to give rise to the donors Nb4+ and Ta4+ rather than Ti3+. The spectrum of reduced rutile depends on the method of reduction. Reasons for this are discussed. Under certain circumstances, involving hydrogen reduction, a particularly simple spectrum is observed. This degenerates and is replaced by a single line as the temperature is raised. A similar effect is obtained by increasing the concentration of centers. Possible assignments are discussed. Vacuum reduction results in distinctly different spectra which persist to higher temperatures. Resistivity measurements indicate a higher activation energy, by a factor of three, for vacuum-reduced samples.

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The EPR spectra of nonstoichiometric rutile and rutile doped with tantalum, niobium, and cerium have been examined at helium temperatures. Ta4+ and Nb4+ have short spin-lattice relaxation times and appear to be suitable for cross doping in maser applications.

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T h e properties of practical high-field' superconductors are outlined and related to the theory of type I1 superconductors. Their unexpected and disappointing behaviour when first wound into coils for the production of high fields is described. This problem is shown to arise from the nature of the current distribution in such materials and the consequent dissipation under conditions of changing current. The factors affecting this dissipation and the thermal stability of the materials are discussed together with means for achieving stable operationin particular the use of composite conductors. Some of the engineering problems associated with the further development of large superconducting magnets are set out; for example, protection against burn-out, the support of the large forces developed and the introduction of current to the magnet. The article concludes with a brief description of some currently planned superconducting magnets, their applications and their economics.