Past exam of the physics 2 paper of the University of Oxford / 2013 resit / 9 / v

Use north-minus-south terminal voltages for both oppositely wound coils, with coil A measured at high input impedance so its AC current is zero. Then $v_B=L_B{\dot{I}}_B$ and $v_A=M{\dot{I}}_B$, with $M<0$. HenceThe magnitude is the turns ratio times the drive, and these terminal references give a phase reversal. Reversing one voltmeter lead reverses the displayed sign. A fixed DC bias in A does not change this incremental result, while short-circuiting A would require solving both coupled current equations.