Tesla coil is one of the many inventions of Nikola Tesla, made in1891. In essence, it is a high frequency air-core transformer, which generates an electric field of a 1 to 100 megavolt potential, which dissipates into the surrounding space through the arcs of electrical discharges. In some cases, the electric field is sufficient to activate fluorescent lamps from a distance of 15 meters.
Modern Tesla coils are created by a few enthusiasts, and used for educational purposes. Some of them, however, are created for the sake of artistic expression.
The largest Tesla coil in the world at the moment, called Electrum, is located on a farm near Auckland, New Zealand. It was commissioned by the famous art patron Alan Gibbs and created by the artist and sculptor Eric Orr and Greg Leyh, the electrical engineer. The Electrum tower, tall as a four-story building, produces a voltage of three million volts, and its arc discharges extend 15 meters into space. It is amazing to see Electrum in action, but the luckiest ones are those who get to sit inside the spherical Faraday cage on top of Electrum. However, as a rule, that place is already occupied by Greg Leyh, who loves to watch the arcs of the coil during the discharge shows. The cage itself is safe, but should you dare to poke your head or hand through the cage, the arcs from the coil would kill you instantly.
Being the biggest Tesla coil at present, Electrum it's not the biggest and the tallest one the history of mankind. The biggest one was the 18-floor tall transformer tower, which has being built by Tesla himself in 1903 in Wardenclyffe, Long Island. However, this tower was never completed, it was torn down before it was finished. Greg Leyh plans to restore this unfinished structure and also to create another Tesla coil tower, twice as big and more powerful.