Editing Tesla coil (section)

===Resonant transformer===
{{Further|Resonant inductive coupling}}
{{Further|Transformer types#Resonant transformer}}The specialized transformer used in the Tesla coil circuit ''(L1,L2)'', called a [[resonant transformer]], oscillation transformer, or radio-frequency (RF) transformer, functions differently from ordinary transformers used in AC power circuits.<ref name="Gerekos"/><ref name="Gottlieb"/><ref name="BurnettOperation"/> While an ordinary transformer is designed to ''transfer'' energy efficiently from primary to secondary winding, the resonant transformer is also designed to ''temporarily store'' electrical energy. Each winding has a [[capacitance]] across it and functions as an [[LC circuit]] (resonant circuit, [[tuned circuit]]), storing oscillating electrical energy, analogously to the way a [[tuning fork]] stores vibrational mechanical energy. The [[primary winding|primary coil]] ''(L1)'' consisting of a relatively few turns of heavy copper wire or tubing, is connected to a [[capacitor]] ''(C1)'' through the [[spark gap]] ''(SG)''.<ref name="Haddad"/><ref name="Naidu"/> The [[secondary winding|secondary coil]] ''(L2)'' consists of many turns (hundreds to thousands) of fine wire on a hollow cylindrical form inside the primary. The secondary is not connected to an actual capacitor, but it also functions as an LC circuit, the inductance of ''(L2)'' resonates with stray capacitance ''(C2)'', the sum of the stray [[parasitic capacitance]] between the windings of the coil, and the capacitance of the [[toroid]]al metal electrode attached to the high-voltage terminal. The primary and secondary circuits are tuned so that they have the same [[resonant frequency]],<ref name=":0">{{cite journal |last=Cvetić |first=Jovan M. |date=October 2016 |title=Tesla's High Voltage and High Frequency Generators with Oscillatory Circuits |journal=Serbian Journal of Electrical Engineering |volume=13 |issue=3 |pages=301–333 |doi=10.2298/SJEE1603301C |s2cid=55561957 |doi-access=free}}</ref> so they exchange energy, acting like a [[coupled oscillator]]; during each spark the stored energy rapidly oscillates back and forth between the primary and secondary.

The peculiar design of the coil is dictated by the need to achieve low resistive energy losses (high [[Q factor]]) at high frequencies,<ref name="Sprott"/> which results in the largest secondary voltages:

* Ordinary power transformers have an [[magnetic core|iron core]] to increase the magnetic coupling between the coils. However at high frequencies an iron core causes energy losses due to [[eddy current]]s and [[hysteresis]], so it is not used in the Tesla coil.<ref name="BurnettOperation"/>
* Ordinary transformers are designed to be "tightly coupled".  Both the primary and secondary are wound tightly around the iron core.  Due to the iron core and close proximity of the windings, they have a high [[mutual inductance]] ''(M)'', the [[coupling coefficient (inductors)|coupling coefficient]] is close to unity 0.95&nbsp;– 1.0, which means almost all the magnetic field of the primary winding passes through the secondary.<ref name="Gerekos"/><ref name="BurnettOperation"/> The Tesla transformer in contrast is "loosely coupled",<ref name="Haddad"/><ref name="BurnettOperation"/> the primary winding is larger in diameter and spaced apart from the secondary,<ref name="Naidu"/> so the mutual inductance is lower and the coupling coefficient is only 0.05 to 0.2.<ref name="BurnettCouplingCoefficient"/><ref name="Sprott"/> This means that only 5% to 20% of the magnetic field of the primary coil passes through the secondary when it is open circuited.<ref name="Haddad"/><ref name="Denicolai2"/> The loose coupling slows the exchange of energy between the primary and secondary coils, which allows the oscillating energy to stay in the secondary circuit longer before it returns to the primary and begins dissipating in the spark.
* Each winding is also limited to a single layer of wire, which reduces [[proximity effect (electromagnetism)|proximity effect]] losses. The primary carries very high currents. Since high-frequency current mostly flows on the surface of conductors due to [[skin effect]], it is often made of copper tubing or strip with a large surface area to reduce resistance, and its turns are spaced apart, which reduces proximity effect losses and arcing between turns.<ref name="BurnettParts"/><ref name="Gerekos1"/>

{{Multiple image
| total_width       = 400
| image1            = Homemade Tesla coil closeup.jpg
| caption1          = Unipolar coil design widely used in modern coils. The primary is the flat red spiral winding at bottom, the secondary is the vertical cylindrical coil wound with fine red wire. The high-voltage terminal is the aluminum [[torus]] at the top of the secondary coil.
| image2            = Bipolar Tesla transformer 1908.jpg
| caption2          = Bipolar coil, used in the early 20th century. There are two high-voltage output terminals, each connected to one end of the secondary, with a spark gap between them. The primary is 12 turns of heavy wire, which is located at the midpoint of the secondary to discourage arcs between the coils.
}}

The output circuit can have two forms:

* ''Unipolar'': One end of the secondary winding is connected to a single high-voltage terminal, the other end is [[ground (electricity)|grounded]]. This type is used in modern coils designed for entertainment. The primary winding is located near the bottom, low potential end of the secondary, to minimize arcs between the windings. Since the ground (Earth) serves as the return path for the high voltage, streamer arcs from the terminal tend to jump to any nearby grounded object.
* ''Bipolar'': Neither end of the secondary winding is grounded, and both are brought out to high-voltage terminals. The primary winding is located at the center of the secondary coil, equidistant between the two high potential terminals, to discourage arcing.