What is a transformer? Write its principle and describe its construction.

(N/A) transformer is an electrical device that increases or decreases the $AC$ voltage.
Transformers that increase the voltage are called step-up transformers,while transformers that decrease the voltage are called step-down transformers.
Principle: $A$ transformer works on the principle of mutual electromagnetic induction.
Construction: $A$ transformer consists of two sets of coils insulated from each other.
As shown in figure $(a)$ and $(b)$,the coils are wound on a soft-iron core,either one on top of the other or on separate limbs of the core. One of the coils,called the primary coil,has $N_{p}$ turns.
The other coil is called the secondary coil and has $N_{s}$ turns. The primary coil is the input coil,and the secondary coil is the output coil of the transformer.
The coils are wound in two ways:
$(1)$ Core type: Coils are wound on a soft-iron core on separate limbs of the core as shown in figure $(b)$.
$(2)$ Shell type: Coils are wound on a soft-iron core,either one on top of the other as shown in figure $(a)$.