A Tesla coil is a resonant transformer circuit that converts low-voltage power into very high-voltage, high-frequency alternating current. At the core are two coupled coils—the primary and secondary—tuned to the same resonant frequency. When energy oscillates between them, voltage in the secondary can rise to hundreds of thousands of volts, strong enough to ionize air and create visible plasma discharges. The effect you see—streamers and arcs—is air breaking down under an intense electric field. Electrons accelerate, collide with molecules, and create a cascade of ionization. The glow comes from excited atoms returning to lower energy states and emitting light at characteristic wavelengths. Modern desktop Tesla coils miniaturize this physics. Many use solid-state drivers instead of spark gaps, switching current rapidly through a primary coil. Some designs modulate the drive signal so the arc itself vibrates at audio frequencies—turning plasma into a speaker. Why it matters beyond th...