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In the usual electrical machinery the magnetic saturation characteristic of iron is a handicap and in a-c. circuits gives rise to such undesirable characteristics as wave dis-tortion and variable reactance, decreasing with increasing load. Furthermore, these characteristics are inherent in the material and can not be controlled by design to suit individual requirements or changing load conditions. Control of these characteristics can however, be accomplished the mean magnetic density in the core be controlled by means of a suitable d-c. excitation, in which event, the saturation characteristics of the iron can be put to some useful applications, (1) APPLICATION AS A FREQUENCY MULTIPLIER The second and third harmonics of magnetization have been utilized in radio to multiply the supply frequency. Using a suf-ficient number of stages, theoretically any desired radio frequency may be obtained from commercial frequencies. However, the efficiency and regulation of the scheme are very poor, and this application has therefore become almost obsolete by the advent of the vacuum tube oscillator and the high-frequency alternator. When used at all, the number of stages would be kept to a minimum. (2) APPLICATION AS A SERIES CURRENT-LIMITING REACTOR The ordinary iron-core reactor is saturated at increasing values of load, and thus having a much lower reactance at short circuit than at normal load, is undesirable as a current-limiting reactor.