Determine the sign of the mutual voltage A current entering the dotted terminal of one coil produces an open-circuit voltage between the terminals of the second coil which is sensed in the direction indicated by a positive voltage reference at the dotted terminal of this second coil V2 O=M M 十 十 M dtDetermine the sign of the mutual voltage dt di M 1 2 = dt di M 1  2 = − dt di M 1  2 = − dt di M 1 2 =   − +  2 1 i M   − +  2 1 i M   − +  2 1 i M   − +  2 1 i M A current entering the dotted terminal of one coil produces an open-circuit voltage between the terminals of the second coil which is sensed in the direction indicated by a positive voltage reference at the dotted terminal of this second coil