A Tesla coil is a type of resonant transformer circuit invented by Nikola Tesla around 1891. It is used to produce high voltage, low current, high frequency alternating current electricity. Tesla coils produce higher current than the other source of high voltage discharges, electrostatic machines. Tesla experimented with a number of different configurations and they consist of two, or sometimes three, coupled resonant electric circuits. Tesla used these coils to conduct innovative experiments in electrical lighting, phosphorescence, x-ray generation, high frequency alternating current phenomena, electrotherapy, and the transmission of electrical energy without wires. Tesla coil circuits were used commercially in sparkgap radio transmitters for wireless telegraphy until the 1920s, and in pseudomedical equipment such as electrotherapy and violet ray devices. Today their main use is for entertainment and educational displays.
A Tesla coil transformer operates in a significantly different fashion from a conventional (i.e., iron core) transformer. In a conventional transformer, the windings are very tightly coupled and voltage gain is determined by the ratio of the numbers of turns in the windings. This works well at normal voltages but, at high voltages, the insulation between the two sets of windings is easily broken down and this prevents iron cored transformers from running at extremely high voltages without damage.
With Tesla coils, unlike a conventional transformer (which may couple 97%+ of the magnetic fields between windings) a Tesla coil's windings are "loosely" coupled, with a large air gap, and thus the primary and secondary typically share only 10–20% of their respective magnetic fields. Instead of a tight coupling, the coil transfers energy (via loose coupling) from one oscillating resonant circuit (the primary) to the other (the secondary) over a number of RF cycles.
As the primary energy transfers to the secondary, the secondary's output voltage increases until all of the available primary energy has been transferred to the secondary (less losses). Even with significant spark gap losses, a well designed Tesla coil can transfer over 85% of the energy initially stored in the primary capacitor to the secondary circuit. Thus the voltage gain of a Tesla coil can be significantly greater than a conventional transformer, since the air gap has a very high insulation.
With the loose coupling the voltage gain is instead proportional to the square root of the ratio of secondary and primary inductances.
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