English: A Tesla coil built by Serbian-American inventor Nikola Tesla at his New York laboratory around 1898. A Tesla coil is a resonant transformer circuit that produces high voltage, alternating current electricity at radio frequencies. The source says this coil is producing 2.5 million volts, resulting in a brush discharge 16 feet across. The source says, and Tesla confirmed, that this is a prototype of Tesla's "magnifier" circuit, a smaller version of huge "magnifying transmitter" which Tesla installed in his Colorado Springs laboratory a year later. This is a more elaborate Tesla coil circuit than the two-coil "resonant transformer" circuit used in most Tesla coils, in addition to the two coupled coils in most Tesla circuits, it had a third "resonator" coil which was not magnetically coupled to the other two, to produce higher voltage. In the picture, the large spiral "spiderweb" coil in the background is the secondary, and the small cylinder mounted on the vertical rod from which the spark discharge emanates is the third "resonator" coil; note that its axis is vertical, while the "spiderweb" axis is horizontal, so they are not magnetically coupled.

Caption: View of model transformer, or "oscillator" in action. Actual width of space traversed by luminous streams issuing from a circular single terminal, over 16 feet. Area covered by the streamers, approximately 200 square feet. Estimated effective electrical pressure, 2 1/2 million volts.