Tesla coil

A Tesla coil is a type of resonant transformer circuit invented by Nikola Tesla around 1891. It is usually 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, but is difficult to define precisely, as 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.

Source: Wikipedia - Tesla coil