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This paper describes the first "polyphase" (more than one phase) system developed for the distribution of alternating current (AC) power. This two-phase system was subsequently rendered obsolete, however, by the superior three-phase system that is now universally used throughout the world. Today, the large-scale generation, transmission, and distribution of electric power is by means of the three-phase AC system; that is, three individual single-phase voltages and currents having a 120° phase relationship to each other and intermingled on three wires. The three-phase system has been adopted because it provides for a constant rather than pulsating power flow to motors, and because it is an efficient system as far as the amount of copper required per kilowatt transmitted. The theoretical complexity of the three-phase system, however, delayed its complete acceptance in the early days of electric power system development.