Transformer, a device that transfers electric energy from one circuit to another, usually with 

 a change in voltage. Transformers work only with a varying electric current, such as 

 alternating current (AC). Transformers are important in the distribution of electric power. 

 They raise the voltage of the electricity generated at a power plant to the high levels 

 needed to transmit the electricity efficiently. Other transformers reduce the voltage at the 

 locations where the electricity is used. Many household devices contain transformers to raise 

 or lower house-current voltage as needed. Television sets and stereo equipment, for 

 example, require high voltages; doorbells and thermostats, low voltages.
How A Transformer Works

 A simple transformer consists essentially of two coils of insulated wire. In most transformers, the 

 wires are wound around an iron-containing structure called the core. One coil, called the 

 primary, is connected to a source of alternating current that produces a constantly varying 

 magnetic field around the coil. The varying magnetic field, in turn, produces an alternating 

 current in the other coil. This coil, called the secondary, is connected to a separate electric 

 circuit.
The ratio of the number of turns in the primary 

 coil to the number of turns in the secondary coil—the turns ratio—determines the ratio of the 

 voltages in the two coils. For example, if there is one turn in the primary and ten turns in the 

 secondary coil, the voltage in the secondary coil will be 10 times that in the primary. Such a 

 transformer is called a step-up transformer. If there are ten turns in the primary coil and one 

 turn in the secondary the voltage in the secondary will be one-tenth that in the primary. 

 This kind of transformer is called a step-down transformer. The ratio of the electric current 

 strength, or amperage, in the two coils is in inverse proportion to the ratio of the voltages; 

 thus the electrical power (voltage multiplied by amperage) is the same in both coils.

 The impedance (resistance to the flow of an alternating current) of the primary coil depends 

 on the impedance of the secondary circuit and the turns ratio. With the proper turns ratio, the \

 transformer can, in effect, match the impedances of the two circuits. Matched impedances are 

 important in stereo systems and other electronic systems because they permit the maximum 

 amount of electric power to be delivered from one component to another.

 In an autotransformer, there is only one coil and both circuits are connected to it. They are 

 connected at different points, so that one circuit contains a larger portion of the coil (that is, has 

 more turns) than the other.