A transformer is a four-terminal device comprising of two (or more) magnetically coupled coils. It is composed of two coils:
- a primary coil of resistance R1 and self-inductance L1
- a secondary coil of resistance R2 and self-inductance L2
A transformer is said to be linear if the coils are wound on a magnetically linear material for which the magnetic permeability is a constant. Some of linear materials are air, plastic, Bakelite, wood, etc.
Circuit representation of a linear transformer is shown in figure.
Circuit Representation of Linear Transformer