A constant current transformer developing 15,000 volts open circuit produces an electrical arc between two long electrodes. The current is about 0.03 Amperes. The separation of the electrodes increases from 7mm at the bottom to 26mm at the top. Air heated by the arc moves upward because it is less dense than the cooler air around it, and it moves at a speed for which the buoyant force and the drag force are equal. Because the current is alternating, there are really a succession of arcs one 120th of a second apart, with each electrode alternately plus and minus.
Press the black button on the lower left. If you wave your fingers while standing back, you can strobe it, like a TV screen.