There are two basic types of transformer – a step up and a step down. In a step-up transformer, the potential difference across the secondary coil is greater than the potential difference across the primary coil. Step-up transformers have more turns in the secondary coil than in the primary coil. In a step-down transformer, the potential difference across the secondary coil is less than that across the primary coil. Step-down transformers have fewer turns in the secondary coil than in the primary coil.

The induced potential difference across the secondary coil is directly proportional to the number of turns in the secondary coil. This means that if the number of turns in the secondary coil doubles, the induced potential also doubles, and if the number of turns is five times greater, then the induced potential is five times greater.

Step-up transformers are used to increase the voltage generated in power stations to 400 000 volts for transmission in the National Grid.

Step-down transformers later reduce the voltage to 230 volts for use in homes and offices and factories.