English: Circuit diagram of a Tesla coil, a spark-excited resonant transformer circuit invented in 1891 by Nikola Tesla. It generates very high voltage, radio frequency alternating current electricity at low current levels. This is a modern unipolar version commonly used in entertainment coils, with a toroidal-shaped metal capacitive load E on the high voltage terminal. The primary circuit is shown connected to the primary winding L1 with a variable tap, so that the primary and secondary coils can be adjusted to resonance. C2 is not an actual capacitor, but represents the capacitance of the secondary winding L2 plus the capacitance to ground of the toroidal electrode E.