Abstract
FOR several years past iron-cored coils for oscillatory circuits have been used by telephone engineers at audio frequencies and at the low radio frequencies which are used in carrier current telephony. In order to reduce the losses which would accompany the use of iron at frequencies of the order of 50,000 cycles per second, the magnetic material was made up in the form of iron dust or filings embedded in a wax or cement in such a way that each particle of iron was insulated from its neighbours. In this way the eddy currents were very restricted in their paths, and the resulting iron losses were reduced to a minimum, so that the presence of the iron increased the inductance of the coil to a much greater extent than its resistance. When attempts were made to use such cores for coils required for higher radio frequencies, however, they were found to be a disadvantage, and the best design of coil for use in the medium broadcasting band of frequencies has been found to be a single-layer air-core solenoid of suitable proportions of length to diameter.
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Iron-cored Coils for Radio Frequencies. Nature 130, 748 (1932). https://doi.org/10.1038/130748a0
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DOI: https://doi.org/10.1038/130748a0