Tesla coil and gas tubes. Footage of a Tesla coil producing high-voltage electrical sparks (top centre), with the purple glow caused by ionisation of air molecules to form a plasma. The strong electric fields created by the Tesla coil cause the gases in discharge tubes to glow. Several examples of this are shown here: first a neon gas tube (orange glow), then a krypton gas tube (purple glow), then a nitrogen gas tube (red glow). The discharge in the nitrogen gas tube is shown being manipulated by touching the glass and moving a finger up and down the tube.