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## Main Question or Discussion Point

Consider a solenoid which is almost toroidal, with a few turns missing (say you have 1000 turns on 99% of the circumference of the toroid) - they will form the primary winding. Let us have 10 turns on 1% of the circumference of the toroid - they will form the secondary winding. Since practically the whole flux of the primary winding goes through the secondary and most of the flux generated by the secondary winding does not go through the primary winding, we have unequal mutual inductances, against a theorem saying that they are equal.