English: Diagram by Nikola Tesla of a Tesla coil circuit similar to his "magnifying transmitter" which he used to create very high voltages at his Colorado Springs laboratory 1900-1901.

It shows an air-core resonant transformer (spiral coils) with its primary driven by an alternating current generator. The high voltage secondary is grounded at one end, and the other end drives a third coil (long thin helix, center), not magnetically coupled to the others, which Tesla called the the "extra coil", generating high voltage. This is connected through a spark gap with an elevated metal ball capacitive electrode (top), which is the high voltage terminal. Except for the addition of the spark gap, this is identical to the circuit shown in Tesla's US patent No. 1119732A Apparatus for transmitting electrical energy, filed January 18, 1902, granted December 1, 1914, his "magnifying transmitter" patent which was the design used in his Colorado Springs transmitter. Tesla used this coil in experiments to try to achieve long distance wireless power transmission. This also may be the circuit used in his Wardenclyffe tower prototype transatlantic wireless telegraphy transmitter and wireless power station built in 1901 at Shoreham on Long Island.

The common type of Tesla coil circuit, which Tesla patented in 1891, has only two coils, making up a resonant transformer, with the high voltage output taken from the secondary. The magnifying transmitter, which Tesla invented around 1896, adds the third coil, which generates additional high voltage by resonating with its own parasitic capacitance either at the same frequency as the primary and secondary, or at the second harmonic.