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A tesla coil is a resonant transformer invented by the great and eccentric scientist Nikola Tesla, also credited with the invention of AC power and the radio. Its primary purpose is to make large, beautiful sparks, make the hair of children stand on end, and generally demonstrate the power of electricity to humanity. The cloud of free electrons it produces can also wirelessly power a fluorescent lamp held near it, but the device is too loud, expensive, and close-range to be a practical power transmitter.
Nikola Tesla developed his coil in the early 1890s, inspired by early results of research into electricity. The tesla coil is based on a device called an induction coil, spark coil or, archaically, a Ruhmkorff coil. This consists of a two coils of insulated copper wire wrapped around a central iron core. The first coil, the primary, consists of a few dozen turns of a coarse copper wire. The secondary coil is many thousands of turns of much finer copper wire. When a current is fed through the coil, a powerful magnetic field builds in the primary. An abrupt cut-off in the current causes the field to collapse and release energy into the surrounding environment, through the secondary. Because of the secondary's many turns, the release causes a discharge of thousands of volts, producing a visible spark as it jumps across a gap to a terminal.
The main component of the tesla coil is the induction coil, but it also includes a specially designed disruptive coil, two powerful capacitors, and a discharge target, usually a metal ball connected to a wire that connects back up with the coil, creating a complete circuit. The capacitors are used to store large quantities of electrical energy for an abrupt and visually stunning discharge.
Today, building tesla coils and observing their beautiful sparks is a common activity among electricity enthusiasts and science hobbyists. The largest tesla coil in the world releases 1.5 megavolts of current and produces sparks 20 ft long.
1.5 megavolts of current? Current is measured in Amperes, not in Volts.