What is the name of Tesla's high-voltage resonant transformer invention?

Tesla's high-voltage resonant transformer invention is called the Tesla coil. It is an electrical circuit that uses resonance and electromagnetic induction to produce very high voltages at high frequency.

Tesla developed the device in 1891 while investigating high-frequency alternating currents and wireless lighting. A classic Tesla coil contains a primary circuit with a capacitor and spark gap, plus a secondary coil. When the two circuits are tuned to the same resonant frequency, energy transfers between them and the voltage rises dramatically.

Tesla coils typically produce high voltage but relatively low current. They can create spectacular sparks, illuminate gas-discharge tubes, and demonstrate wireless energy transfer. Early versions also influenced radio transmitters and high-frequency medical equipment.

The name can cause confusion because a Tesla coil is not simply an ordinary induction coil or a Van de Graaff generator. Its defining feature is resonance between tuned electrical circuits. Tesla patented related oscillation-transformer designs, although similar resonant-transformer experiments had been conducted by other researchers.