A Tesla coil is a type of resonant transformer, invente by Serbian-American scientist Nikola Tesla around 1891, used to generate very high voltage, low current, high frequency alternating current electricity. A Tesla coil consists of two, or sometimes three, coupled resonant electric circuits. A Tesla coil is difficult to define, as Nikola Tesla experimented with a large variety of coils and configurations. 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.