Figure shows a soft iron core in the center. This is in the form of a rectangular ring. There are windings on its left arm, connected to a voltage source. These are labeled N subscript p turns. The current through them is i subscript p parentheses t parentheses. The voltage across two ends of the windings is v subscript p parentheses t parentheses. The windings on the right arm of the core are connected to a resistor R subscript s. The windings are labeled N subscript s turns. These are more in number than the windings on the left arm. The current in the right circuit is i subscript s parentheses t parentheses. The voltage across the windings is v subscript s parentheses t parentheses. The current in the left circuit flows into the windings from the top. The current in the right circuit flows out of the winding from the top.