Transformer adopts iron core to improve magnetic permeability. The closed iron core is used to reduce the magnetic resistance (similar to the resistance in the circuit) and improve the electromagnetic induction efficiency. But the transformer cores are all assembled, so it is impossible to realize an ideal closed magnetic circuit.

Because the cross section and shape of the magnetic circuit of the transformer core are not the same everywhere, and there are other technological problems, there is a certain leakage flux in the transformer core (that is, some magnetic lines run outside the core), which is attributed to "leakage reactance" in electrical terminology. If the closed magnetic gap of the iron core is too large, it will force the magnetic field lines to pass through the air gap and increase the magnetic leakage component.