Tesla coils use two coils to produce current and charge capacitors, which store electric charge until the charge builds high enough to discharge as the visible blast of electrical charge that Tesla coils are known for. The primary coil is powered by a source of high voltage that a Tesla coil converts into stored charge via electromagnetic induction.
The transformer is composed of a primary and a secondary coil, each with its own capacitor. Unlike many transformers that have a magnetic core, a Tesla coil's core is composed of air separating the two coils. High voltage is applied to the primary coil, inducting a strong magnetic field in the coil and building up charge in its capacitor until the capacitor discharges as a spark across the air gap between the two coils. The sudden discharge of current causes the electromagnetic field in the primary coil to collapse, inducting a powerful electromagnetic field in the secondary coil and storing charge in its capacitor.
The secondary coil then undergoes the same process, inducting a magnetic field in the primary coil. However, the secondary coil continues to store most of the accumulated charge in its capacitor. The process repeats rapidly, increasing the charge in the secondary capacitor with each cycle until it discharges powerfully as a shower of sparks. Many Tesla coils feature a metal dome above them, which functions to direct the generated magnetic fields in an optimal manner.