What is a transformer? The voltage of an alternating current can be changed using a device called a transformer. A transformer contains two coils that are wound around a soft iron core. iron core The alternating current in the primary (input) coil produces an alternating magnetic field. This alternating magnetic field induces an alternating current in the secondary (output) coil. primary coil secondary coil How does a transformer change voltage? The voltage induced in the secondary (output) coil depends on the number of turns on the primary and secondary coils. A step-up transformer has more turns on the secondary coil and so increases voltage. A step-down transformer has fewer turns on the secondary coil and so decreases voltage. Where are transformers used? A television needs a very high voltage to operate. It contains a step-up transformer, which increases the voltage of the electricity supplied to the television. This outdoor transformer decreases the voltage of the electricity carried by the national grid. It is an example of a step-down transformer. How are transformers used to supply electricity? How does electricity get to your home?