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In today's electrical engineering curriculum, the use of the circle diagram as a teaching and analytical tool is all but forgotten. It has been neglected by the majority of recent textbooks on electromagnetic devices and electric circuits. Yet there are electrical devices whose behavior can easily be understood and analyzed by the use of circle diagrams. In fact, in some cases the use of circle diagrams makes the solution almost a trivial exercise. The graphical and constructional procedures of circle diagrams are presented, and then the circle diagram is employed to analyze two problems: 1) the variation of the mutual and net coil fluxes of a two-winding transformer with varying secondary load impedance, and 2) the study of the variation of the secondary and load voltages of an autotransformer whose load varies from open to short circuit.