File:EarlyTeslaCoil.svg

English: An experimental Tesla coil circuit to duplicate the discharges of a Holtz electrostatic machine, from Tesla's 1892 lecture before the Institute of Electrical Engineers, London, England. Following the initial research of voltage and frequency by William Crookes, Nikola Tesla developed a series of coils that produced high-voltage, high-frequency currents. In the majority of Tesla's experiments, he used machinery of his own design to produce the Tesla effect. These early coils would use the "disruptive" action of a spark gap in their operation. The setup can be duplicated by a Ruhmkorff coil, two condensers (now called capacitors), and a second, specially constructed, disruptive coil.

The Ruhmkorff coil, being fed from a main source, is wired to capacitors on both ends in series. A spark gap is placed in parallel to the Ruhmkorff coil before the capacitors. The discharge tips were usually metal balls under one inch in diameter, though Tesla used various forms of dischargers. The capacitors were of a special design, small with high insulation. These capacitors consisted of plates in oil that were movable. The smaller the plates, the more frequent the discharge of this early coil apparatus. The plates also help nullify the high self inductance of the secondary coil by adding capacity to it. Mica plates were placed in the spark gap to establish an air current jet to go up through the gap. This helped to extinguish the arc, making the discharge more abrupt. An air blast was also used for this objective.

The capacitors are connected to a double primary (each coil in series with a capacitor). These are part of the second specially constructed disruptive coil. The primaries each have twenty turns of No. 16 B & S rubber covered wire and are wound separately on rubber tubes not less than a 1/8th inch thick. The secondary has 300 turns of No. 30 B & S silk-covered magnet wire, wound on rubber tube or rod, and the ends encased in glass or rubber tubes. The primaries must be large enough to be loose when the secondary coil is place between the coils. The primaries must cover around two inches of the secondary. A hard rubber division must be placed between these primary coils. The ends of the primaries not connected with the capacitors are lead to a spark gap.

Tesla's later coils were considerably larger and operated at much higher power levels. These later systems were powered from large high voltage power transformers, used banks of glass bottle capacitors immersed in oil to reduce corona losses, and used rotating spark gaps to handle the higher power levels. Tesla also dispensed with using oil to insulate the transformer coils, relying instead on the insulating properties of air. Tesla coils achieve great gain in voltage by loosely coupling two resonant LC circuits together, using an air-core (ironless) transformer. Unlike a conventional transformer, whose gain is limited to the ratio of the numbers of turns in the windings, Tesla coils' voltage gain is proportional to the square root of the ratio of secondary and primary inductances.