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Presently, insufficient material data is available to predict core losses at high flux densities with complex flux waveforms. A steel tester is described whereby this data can be obtained from electrical steel at densities in excess of 20 kG. The flux wave-form can be selected and controlled to study the influence of harmonics and to simulate actual flux conditions existing in magnetic circuits. An electromechanical waveform generator is capable of synthesizing waveforms of 30 to 90 Hz fundamental with phase and amplitude selected odd harmonics up to the eleventh. This signal is amplified to excite Epstein sample strips placed in a water-cooled test yoke up to 140 kA/m. Feedback technique is applied to keep total distortion between input signal and flux waveform below 4 percent at peak excitation. The flux density is measured with air-flux compensated search coils. A calorimetric method is used to measure core losses by recording the temperature rise of thermistors placed between sample strips. Some test results are presented to demonstrate the flexibility and usefulness of the steel tester.