Transformer is a device that increases or decreases the voltage associated with an alternating current. Alternating current is electric current that regularly reverses its direction. Transformers provide a simple, inexpensive way to change such voltage. They enable electric power companies to transmit alternating current easily and efficiently. Transformers also ensure the proper voltage for the circuits of home appliances, lights, industrial machinery, and other electric equipment.
Most transformers consist of two coils of insulated wire. One coil, known as the primary winding, is connected to the source of the voltage that is to be changed. This voltage is the input voltage of the transformer. The other coil, called the secondary winding, supplies the output voltage to the desired circuit. In most transformers, the primary and secondary windings are wound around a hollow core made of thin iron or steel sheets. Most cores have the shape of a ring or a square. The two wire coils are not connected to each other.
Transformers work by means of electromagnetic induction (see Electromagnetism). When the input alternating voltage is applied to the primary winding, it generates alternating current in the coil. As the current flows, it sets up a changing magnetic field in the core of the transformer. When this changing magnetic field passes through the secondary winding, it produces alternating voltage in the coil. If the secondary winding is connected to a circuit, the output voltage causes alternating current to flow through the circuit.
The ratio between a transformer’s maximum output and input voltages equals the ratio of the number of turns in the secondary winding to the number in the primary winding. If V represents the voltage and N stands for the number of turns, then 
The output voltage will be greater than the input voltage in a transformer whose secondary winding has more turns than the primary winding. Such a transformer is called a step-up transformer. If the secondary coil has fewer turns than the primary, the output voltage will be less than the input voltage. This type of transformer is called a step-down transformer.
In power plants, step-up transformers increase the voltage of the alternating current produced by generators. High voltages and small currents make it possible to transmit the current over long distances with only a small loss in power. When the current reaches the area where it will be used, step-down transformers lower the voltage to the level needed by local consumers (see Electric power (Transmitting and distributing electric power)).
Some transformers have special uses. For example, air-core transformers and powdered iron-core transformers are designed to handle high-frequency alternating currents. Instrument transformers are used in measuring extremely large alternating voltages and currents. Variable transformers vary the amount of output voltage delivered to a circuit.
