Editing Tesla coil (section)

===Number of coils===
Tesla circuits can also be classified by how many resonant [[electromagnetic coil|coils]] ([[inductor]]s) they contain:<ref name="Gerekos4"/><ref name="Denicolai4"/>
* '''Two coil''' or '''double-resonant''' circuits: Virtually all present Tesla coils use the two coil [[resonant transformer]], consisting of a primary winding to which current pulses are applied, and a secondary winding that produces the high voltage, invented by Tesla in 1891. The term "Tesla coil" normally refers to these circuits.
* '''Three coil''', '''triple-resonant''', or '''magnifier''' circuits: These are circuits with three coils, based on Tesla's "magnifying transmitter" circuit which he began experimenting with sometime before 1898 and installed in his Colorado Springs lab 1899–1900, and patented in 1902.<ref name="Patent1119732"/><ref name="Sarkar2"/><ref name="Reed"/> They consist of a two coil air-core step-up transformer similar to the Tesla transformer, with the secondary connected to a third coil not magnetically coupled to the others, called the "extra" or "resonator" coil, which is series-fed and resonates with its own capacitance.  The output is taken from the free end of this coil.  The presence of three energy-storing [[tank circuit]]s gives this circuit more complicated resonant behavior.<ref name="de Queiroz"/> It is the subject of research, but has been used in few practical applications.