[[File:Transformer3d_col3Transformer-polemount-singlephase-closeup.svgjpg|thumb|right|A transformer hung on a pole to serve a few houses in a neighborhood.]]A '''transformer''' is a device that transfers [[electrical energy]] from one [[circuit]] to another, usually at different voltages.
Early electrical systems were developed using [[direct current]]. However, the heat loss limited the ability to send electric power a long distance from the generating station. This caused the industry to adopt [[alternating current]]. With alternating current, electric power is generated at a low voltage. A "step up transformer" then raises the voltage to a much higher level (for example 220,000 volts). Power can travel down a transmission line at a high voltage with less energy being converted to heat.<ref>Joule heating equal i<sup>2</sup>R. If the voltage doubles, the current is cut in half to transmit the same power. This will cut the heat loss by 1/4.</ref> When the electric power arrives at a local substation a "step down transformer" will lower the voltage. Finally, at a point near an individual customer's house, a final transformer will lower the voltage to 120 volts (in the United States).