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THE measurement of impedance of power distribution circuits is a continuing problem as long as materials, installation methods, and ultimate use of such circuits are constantly undergoing changes. In contrast to transmission lines, power distribution circuits are relatively short and operate at utilization voltages. This simplifies the impedance measuring problem because the effects of capacitance and leakage are negligible at power frequencies. Consequently, the literature shows that the analysis of the problem has been thorough and measurement techniques have been painstaking; however, the measurements have usually been made at 60 cycles and, in the case of 3-phase measurements a neutral wire or earth return has been used. The 3-phase 4-wire electric systems rapidly being developed for aircraft in this country are operated at 400 cycles, and the neutral return is made through the aircraft structure, particularly the conducting aluminum skin. For increased physical reliability, the wire used in aircraft distribution circuits has a different construction than that commonly used in other applications. These differences are sufficient to hinder the use, even by extrapolation, of the results of much research on the general problem of wire impedance measurement.