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Incremental Permeability of Special Steels

Nature volume 142, pages 707708 (15 October 1938) | Download Citation



THE importance now attached to the incremental permeability of silicon-steel and of nickel-iron alloys arises from the use of these materials for the cores of transformers and chokes in communication apparatus. Under these conditions, the magnetization is due to a continuous steady current upon which a variation is imposed either by modulation or by separate coils carrying alternating currents. Generally, the amplitude of the imposed wave is not great enough to reduce the steady magneto-motive force to zero, so that the value of the induction rises and falls as the resultant magneto-motive force changes ; but it does not reverse. The calculation of the induced electromagnetic force in a secondary coil in these circumstances is fraught with peculiar difficulties. Adopting the usual convention and indicating magnetic induction by B, magnetic force by H and permeability, B/H, by µ, then if the mean value of B is not high and its variation small, the mean ratio B/H over the change will approach the value of µ as measured and used for simple alternating currents. But as the mean value about which B varies gets nearer to the saturation region of the iron, and particularly when the imposed variation is large, not only does the mean value of µ fall rapidly, but also its instantaneous value varies considerably. The consequences are (1) that the prediction of a secondary induced E.M.F. is difficult ; (2) that if the imposed variation of H be sinusoidal, the wave form of B will be distorted ; (3) that if the imposed variation of H be sinusoidal, the wave form of B will be distorted ; (4) that losses associated

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