English: Tesla coil built by MIT student. A Tesla coil, invented by Nikola Tesla in 1891, is a spark excited resonant transformer circuit which generates high voltage radio frequency electric current al very low current levels. This example illustrates a common design used for entertainment displays. The primary of the coil is the flat spiral of thick red wire at bottom. Not visible under it are the other parts of the primary circuit; a high voltage step up transformer, spark gap and capacitor, which excite oscillations in the primary coil. The secondary winding is the narrow vertical cylinder wound with fine red wire. Connected to the top is the high voltage terminal; a torus made of spun aluminum. In operation, dramatic streamer arcs and brush discharges burst from the top terminal. The circle of copper tubing surrounding the primary is a grounded "arc rail" to prevent arcs from the high voltage terminal from hitting the primary.

Alterations to original image: cropped out portions of the bed and other peripheral parts of image; rotated CCW small amount to show upright.