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Ralph Bown 《Journal of The Franklin Institute》1917,183(1):41-60
Not many years ago it was quite generally believed that iron was unable to follow rapid magnetic changes. Experiments which showed an apparent decrease in the permeability of the iron with an increase in the frequency of the magnetic cycle furnished a basis for a theory that iron was magnetically sluggish. Further and more accurate experiments proved, however, that the effects which had previously been ascribed to a peculiarity of the material were in reality caused by eddy currents in the sample. Theoretical calculations were made which demostrated that eddy currents in an iron test piece increased as the square of the frequency and that for even the lower frequencies it was necessary to use quite thin laminations in magnetic circuits in order to eliminate deleterious effects. Furthermore, it was found that due to eddy currents and the magnetic properties of iron, the magnetization in high frequency fields was confined to a thin surface layer of the piece. This “Magnetic Skin Effect” reduced the cross section of the iron which was magnetically active even though the laminations were extremely thin. Careful experimental measurements compared with theoretical calculations proved that the real permeability of iron remained unchanged at frequencies up to about 106 and that previous results had been is serious error due to neglect of the factors mentioned. This fact having been established, efforts were made to see what practical use could be made of iron in high frequency work and to that end some extensive experimental investigations of the saturation curves and core losses were made upon specimens laminated as thinly as was commercially practicable. The resulting data have furnished a basis for design.It is a demostrated fact that the permeability of all metals is unity for the magnetic cycles imposed upon them by heat and light waves. In the region between frequencies of about 106, where the true permeability of iron is practically the same as at zero frequency, and frequencies of about 1010 where the true permeability of iron approaches unity, the experimental values of μ decrease smoothly with the frequency. What happens to μ in the range of frequencies between the lingest heat waves and the shortest Hertzian waves which have yet been made is a question which has many interesting features but which has not yet yielded to the experimenter. 相似文献
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The article reports on a study investigating influences on Australian politicians’ decision making for early childhood education and care (ECEC) policy. The astronomical concept of dark matter is utilised as a metaphor for considering normalising, and therefore frequently difficult to detect and disrupt, influences implicated in politicians’ decision making for ECEC policy. The concept of dark matter connects with and extends Foucauldian conceptualisations of knowledge, power and discourse. The paper argues that maternalist discourses continue to shape the policy landscape for ECEC, though these are largely unacknowledged. 相似文献
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Lalage Bown 《Compare》2000,30(3):341-351
Taking a historical perspective and focusing on Low and Medium Human Development countries, the paper starts with a quotation from a nineteenth-century Scot proposing universal empowerment through education for all citizens and ends with the recognition that at the end of the twentieth century his social justice challenge has not yet been met. The concept of lifelong learning and associated ideas are unpacked and particular applications highlighted, including human rights, literacy and gender. New arenas for lifelong learning are noted, but the general imbalance between the High Human Development countries and the rest of the world is under scored. At the same time, lessons in learning from the majority world for the High Development countries are suggested, including development-related learning and popular participation. 相似文献
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