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[[https://commons.wikimedia.org/wiki/File:Transformer3d_col3.svg|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.

[[File:Transformer3d_col3.svg|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 i²R.  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).

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 i²R.  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).