English: An experimental Tesla coil circuit invented by Elihu Thomson and published in February 1892, The Tesla coil was a spark-excited resonant transformer circuit invented by Serbian American inventor Nikola Tesla and patented by him (US patent 454,622) April 25, 1891, which produced very high voltage high frequency current. However, many sources confirm that Tesla was not the first to invent this circuit. Although this circuit was published a year after Tesla's invention, Thomson claimed to have been experimenting with similar circuits in 1890, including one which could produce 60 inch sparks, and many other sources confirm this. This circuit, essentially identical to Tesla's circuits of the time, consists of: (A) high voltage step-up transformer producing 15-20 kV (b) primary capacitor, (J) spark gap with compressed air blowout to prevent it from arcing continuously, (C) air-core resonant transformer consisting of a 15-20 turn helix of heavy wire surrounded by a glass cylinder on which are wound 150 turns of fine wire, immersed in a tank of oil for insulation (D) output spark gap. Thomson wrote that the output voltage was 150,000 volts, but was limited only by the coil insulation.