• Transformer : It is an electrical device (essentially AC device) which is used for changing the AC voltages. A transformer which increases the AC voltages is called a step up transformer. A transformer which decreases the AC voltages is called a step down transformer.

• Principle : It is based on the principle of mutual induction (when there is a change in magnetic flux of the primary coil, an e.m.f is induced in the secondary coil).

• Construction : A transformer consists of a rectangular iron core made up of laminated sheets, well insulated from one another. Two coils and are wound on the same core, but are well insulated from each other. Both of the coils are also insulated from the core. The source of alternating e.m.f is connected to the primary coil and a load resistance R is connected to the secondary coil through and open switch. Thus, there can be no current through the secondary coil so long as the switch is open.

• Working : The alternating e.m.f supplied by the AC source connected to primary be

= sin ω t.

In a purely inductive circuit, power factor is cos Φ = cos 90° = 0, therefore no power is dissipated.