The birth of electromagnetic induction technology dates back to 1831. In November of that year, Michael Faraday wound two coils of wire onto aniron ring and noted that when an alternating current was passed through oneof the coils, a voltage was induced in the other. Recognizing the potentialapplications of transformers based on this effect, researchers working overthe next several decades concentrated on the development of equipment forgenerating high-frequency alternating current.It was not until the latter part of the 19th century that the practical appli-cation of induction to heating of electrical conductors was realized. The firstmajor application was melting of metals. Initially, this was done using metalor electrically conducting crucibles. Later, Ferranti, Colby, and Kjellin devel-oped induction melting furnaces which made use of nonconducting crucibles.In these designs, electric currents were induced directly into the charge, usuallyat simple line frequency, or 60 Hz. It should be noted that these early induc-tion melting furnaces all utilized hearths that held the melt in the form of aring. This fundamental practice had inherent difficulties brought about by themechanical forces set up in the molten charge due to the interaction betweenthe eddy currents in the charge and the currents flowing in the primary, orinduction coil. In extreme cases, a "pinch" effect caused the melt to separateand thus break the complete electrical path required for induction, and induc-tion heating, to occur. Problems of this type were most severe in melting ofnonferrous metals.Ring melting furnaces were all but superseded in the early 1900's by thework of Northrup, who designed and built equipment consisting of a cylin-drical crucible and a high-frequency spark-gap power supply. This equipmentwas first used by Baker and Company to melt platinum and by AmericanBrass Company to melt other nonferrous alloys. However, extensive appli-cation of such "coreless" induction furnaces was limited by the power attain-able from spark-gap generators. This limitation was alleviated to a certainextent in 1922 by the development of motor-generator sets which could supplypower levels of several hundred kilowatts at frequencies up to 960 Hz. It wasnot until the late 1960's that motor-generators were replaced by solid-stateconverters for frequencies now considered to be in the "medium-frequency"rather than the high-frequency range.