History
The Tesla coil transformer employed a capacitor which, upon break-down of a short spark gap, became connected to a coil of a few turns (the primary winding), forming a resonant circuit with the frequency of oscillation, usually 20–100 kHz, determined by the capacitance of the capacitor and the inductance of the coil. The capacitor was charged to the voltage necessary to rupture the air of the gap, about 10 kV by a line-powered transformer connected across the gap. The line transformer could tolerate the short circuit occurring while the gap remained ionized, or for the few millisecond until the high frequency current had died away. A more prominent secondary winding, with vastly more turns of thinner wire than the primary, was positioned to intercept some of the magnetic field of the primary. The secondary was designed to have the same frequency of resonance as the primary using only the stray capacitance of the winding itself and that of any "top hat" placed at the upper end.
The later and higher-power coil design has a single-layer primary and secondary. These Tesla coils are often used by hobbyists and at venues such as science museums to produce long sparks. The "American Electrician" gives a description of an early Tesla coil wherein a glass battery jar, 15 × 20 cm (6 × 8 in) is wound with 60 to 80 turns of AWG No. 18 B & S magnet wire (0.823 mm²). Into this is slipped a primary consisting of eight to ten turns of AWG No. 6 B & S wire (13.3 mm2) and the whole combination immersed in a vessel containing linseed or mineral oil. (Norrie, pg. 34–35)
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