A Tesla coil is an electrical resonant transformer circuit invented by Nikola Tesla around 1891. It is used to produce high-voltage, low-current, high frequency alternating-current electricity.
Tesla coils can produce higher voltages than other artificial sources of high-voltage discharges, electrostatic machines. Tesla experimented with a number of different configurations consisting of two, or sometimes three, coupled resonant electric circuits.
Tesla used these coils to conduct innovative experiments in electrical lighting, phosphorescence, x-ray generation, high frequency alternating current phenomena, electrotherapy, and the transmission of electrical energy without wires. Tesla coil circuits were used commercially in sparkgap radio transmitters for wireless telegraphy until the 1920s, and in pseudomedical equipment such as electrotherapy and violet ray devices. Today their main use is for entertainment and educational displays. Wikipedia, Tesla Coil
"12 Million volts." Article about Robert Golka's work in Wendover, Utah, replicating and modernizing Tesla's generator. Called "Project Tesla." Secondary coil has resonance of 50 kHz. An additional coil resonates at the second harmonic, 100 kHz. (Radio Electronics. 47, June 1976.)
"Scientific aspects of intense magnetic fields & high voltages." (Nature (London). 120(3031), Dec. 3, 1927. p. 809-811.)
Bearden on Tesla and EM Source Charge
Figure 16.04.05 and Figure 16.04.06 - Nikola Tesla and Lord Kelvin
Figure 16.06 - Russells Genero-Radiative Coils - Electricity Expands and Contracts
Figure 8.3 - Coiled Spring showing Longitudinal Wave
Tesla - Electricity from Space
14.35 - Teslas 3 6 and 9
16.04 - Nikola Tesla describing what electricity is
16.17 - Negative Electricity - Tesla