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[[File:Transformer-polemount-singlephase-closeup.jpg|thumb|right|A transformer hung on a pole to serve a few houses in a neighborhood.]]
[[File:Transformer3d col3 XML stripped.jpeg|thumb|right|The ratio of the number of turns around the core determines the ratio of the voltage on the two circuits.]]

A '''transformer''' is a device that transfers [[Electricity|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) or 230 volts (in Europe).

==Operation==
A transformer works because the two circuits are wound around a common magnetized metal core. As the current alternates on the first circuit, it creates a varying [[magnetic field]].  That field, in turn, induces a current in the coils of the second circuit.  The ratio of the voltage on the two circuits is determined by the ratio of the number of times coils on the two circuits are wrapped around the metal core that serves as a magnet.

==Threats==
[[Newt Gingrich]] and other [[conservative]]s have expressed concerns about the viability of power grid transformers against [[threat]]s from modern adversaries such as [[Islam]]ic [[jihad]] [[terrorists]], [[China]], [[Russia]] and [[North Korea]]. They consider the current "hardening" of transformers to be a major issue requiring investment.<ref>[[One Second After]] written by [[William Forstchen]] with a forward by [[Newt Gingrich]] discusses this potentially major threat to modern power transformers: [https://www.amazon.com/One-Second-After-William-Forstchen Amazon.com], Tor Books, 2011. Accessed January 5, 2015.</ref>

If there is a major disaster, damaged transformers may have to be replaced, and both homeland security experts as well as the electric utilities are worried that the inventory and placement of spare transformers may not allow the electric grid to be reenergized quickly.  As a result, the industry is attempting to arrange for nationwide cooperation in the distribution of spare transformers and parts to respond to such emergencies.

==See also==
* [[Rectifier]] - Converts [[alternating current]] into [[direct current]]
* [[Inverter]] - Converts [[direct current]] into [[alternating current]]

==Bibliography==
{{cite book
 | first = James
 | last = Harlow
 | title = Electric Power Transformer Engineering
 | publisher = CRC Press
 | year = 2004
 | isbn = 0-8493-1704-5
 | url=https://books.google.com/?id=THO9U9f2m9EC}}

==External links==
* [http://www.allaboutcircuits.com/vol_2/chpt_10/6.html Three-phase transformer circuits] from All About Circuits
* [http://mysite.du.edu/~jcalvert/tech/transfor.htm  Inside Transformers, composed by J. B. Calvert, from Denver University]
* [http://www.elkor.net/pdfs/AN0305-Current_Transformers.pdf Introduction to Current Transformers]

==References==
<references />

[[Category:Electronics]]
[[Category:Energy]]
[[Category:Electricity]]
[[Category:Radio]]
[[Category:Electricity]]