What is Alternating current?
[[File:City lights in motion.jpg|thumb|right|250px|City lights viewed in a [[motion blur]]red exposure. The AC blinking causes the lines to be dotted rather than continuous.]] An '''alternating current''' ('''AC''') is an [[electric current]] of which [[Magnitude (mathematics)|magnitude]] and direction vary, unlike [[direct current]], whose direction remains constant. This means that the direction of current flowing in a [[Electrical circuit|circuit]] is constantly being reversed back and forth. This is done with any type of AC voltage source. The usual [[waveform]] of an AC power circuit is a [[sine wave]], because this leads to the most efficient transmission of energy. However, in certain applications different [[waveform]]<nowiki/>s are used, such as triangular or square waves. Inexpensive power [[inverter]]s produce a square wave with a pause between the change in direction. When one speaks of alternating current one mostly refers to the form in which [[electricity]] is delivered to businesses and residences. The AC comes from a [[power plant]]. The direction of the electricity switches back 60 times every second (or 50 times in some parts of the world). This happens so fast that a light bulb does not stop glowing.<ref>{{cite web|url=http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/alternatingcurrent.htm|title=Alternating current|access-date=24 May 2012|archive-date=21 April 2012|archive-url=https://web.archive.org/web/20120421171448/http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/alternatingcurrent.htm|url-status=dead}}</ref> Both [[Audio frequency|audio]] and [[Radio frequency|radio]] signals carried on electrical [[wire]] are also examples of alternating current. In these applications, an important goal is often the recovery of [[information]] encoded (or [[modulated]]) onto the AC signal. == History == [[Nikola Tesla]] experimented with electrical [[resonance]] and studied various lighting systems. He invented an induction motor, new types of generators and tran