A Tesla coil is a resonant electromagnetic transformer invented by Nikola Tesla.
It uses coupled coils and resonant oscillation to produce high-voltage, relatively high-frequency alternating current. The primary and secondary circuits are tuned so energy can transfer efficiently at a selected frequency. The output can create striking electrical discharges, which made the apparatus important in early demonstrations of wireless energy and radio-frequency technology.
Tesla developed and patented versions of the device in the 1890s. A Tesla coil is not a practical replacement for ordinary power transformers: its output is generally high in voltage but low in available current. Modern examples are often used for education, entertainment, and research into high-frequency electrical behavior.