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An analysis of the core losses in induction motors is the object of the research described in this paper. The authors subdivide the core losses into six elements and point out that only one of the six is ordinarily calculated by designing engineers, with resulting large discrepancies between expectation and fact. The belief is expressed that to calculate other elements approximately is entirely feasible and that such calculations would explain most of the variations in core loss now attributed to imperfections in manufacture. Rational formulas are developed for the losses in the core back of the teeth, which are found to give larger values than the more approximate formulas now in use, and the conclusions are presented in the form of charts giving the correction factors to apply to the usual formulas for all ordinary conditions. A Webb floating dynamometer, which was constructed for the measurement of the extremely small torques due to these core losses, is described: and results on the losses in a smooth-core rotor obtained with it are presented. These results and those obtained by an independent electrical method of testing are compared with the expected values based on the theoretical conclusions previously derived, reasonable agreement being found.