A transformer works on the principle of

Explanation

Construction : A transformer consists of two coils insulated from each other. They are wound on a soft-iron core, on separate limbs of the core or either one on top of the other). They are called primary and secondary coils. In a step up transformer, the primary coil consists of few turns when compared to turns of secondary coil. It is just opposite in step down transformer.
Principle : Transformer works on "mutual induction".