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Electrical steels, which are used to form the magnetic flux paths of electrical machines, are categorised according to the specific total loss of the material determined by the Epstein frame method, under defined conditions of magnetic excitation. A brief review is given of the types of steels now produced, with a description of the primary magnetic parameters; e.g. permeability, power loss and apparent power which are of importance to the constructor of electrical machines. Because specific total loss and specific apparent power are derived units of measurement, the standards system which has been developed to provide an industry-based reference for these parameters is built on the traceability of base units to national standards. The problems associated with the measurement of the magnetic parameters of electrical steels are related primarily to the magnetic nonlinearity of these materials, so that uncertainty budgets must take into full account the characteristic behaviour of the test piece as well as instrumentation contributions.