Often the primary coil is the input coil
and the secondary coil is the output coil of the transformer FIGURE 7 16 Two arrangements for winding of primary and secondary coil in a transformer:
(a) two coils on top of each other, (b) two coils on separate limbs of the core When an alternating voltage is applied to the  primary, the resulting
current produces an alternating magnetic flux which links the secondary
and induces an emf in it