Scientists found that if they wrapped two identical coils next to each other on a core of steel square plates held apart by thin sheets of insulation:
then if an alternating voltage and current is applied across coil 1 nearly 100% of the energy is transferred to coil 2. The same voltage and current results in coil 2 and at the same frequency (only if the number of turns on each coil is the same).
So energy can be transferred from one circuit to another through electromagnetic induction.
The core is not designed to have current flow through it although some is induced in the core (called eddy currents) and these are minimized with insulation layers between the plates.
The alternating current through the winding produces a continually changing and alternating magnetic field in the core that cuts through the second coil and induces an electrical voltage and current. This is why these transformers do not work for direct current.
Transformers only work using alternating current supply.