Construction of a transformer:

The essential parts of a transformer are: i) core ii) primary coil and iii) secondary coil. In its general form, a core (C) consists of a soft-iron or ferrite material. This core is the form of a rectangular frame built by placing thin sheets of iron one above the other. These sheets are laminated and therefore are insulated from each other.
On one side of the rectangular core, an insulated copper wire is wound to make a coil of turns (n1). If voltage is applied across this coil, it is called a primary (P) coil. On the opposite side of the rectangular core, another copper wire is wound to make coil of turns (n2). If output voltage is delivered from this coil it is called a secondary (S) coil.
The coils P and S are wound on the same iron-core and are electrically insulated from one another.