A Tesla coil (also teslacoil) is a type of resonant transformer, named after its inventor, Nikola Tesla. Tesla coils consist of two, or sometimes three, coupled resonant electric circuits. Tesla experimented with a large variety of coils and configurations, so it is difficult to define a "Tesla" as one specific mode of construction. Tesla used these coils to conduct innovative experiments in electrical lighting, fluorescence, x-rays, high frequency alternating current phenomena, electrotherapy, and wireless power for electric power transmission. The designs of his "early coils" and "later coils" were considerably different.
Tesla's early and later designs usually employed a high voltage power source, one or more high voltage capacitor(s), and a spark gap to excite the primary side of the Tesla Coil with periodic bursts of high frequency current. An important characteristic of his later, higher power coil designs was that the primary and secondary circuits were also tuned so that they resonated at the same (high) frequency (typically, but not always, between 25 kHz and 2 MHz). The later Tesla coil designs can also be used to create long electrical discharges. Today they are built by many high-voltage enthusiasts.