page_title: History of the Tesla coil
section_title: Magnifying transmitter
hierarchical_section_title: History of the Tesla coil / Magnifying transmitter
context_page_description: Nikola Tesla patented the Tesla coil circuit on April 25, 1891. and first publicly demonstrated it May 20, 1891 in his lecture "Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination" before the American Institute of Electrical Engineers at Columbia College, New York. Although Tesla patented many similar circuits during this period, this was the first that contained all the elements of the Tesla coil: high voltage primary transformer, capacitor, spark gap, and air core "oscillation transformer".
context_section_description: Tesla's wireless research required increasingly high voltages, and he had reached the limit of the voltages he could generate within the space of his New York lab. Between 1899 and 1900 he built a laboratory in Colorado Springs and performed experiments on wireless transmission there. He chose this location because the polyphase alternating current power distribution system had been introduced there and he had associates who were willing to give him all the power he needed without charging for it.  The Colorado Springs laboratory had one of the largest Tesla coils ever built, which Tesla called a "magnifying transmitter" as it was intended to transmit power to a distant receiver. With an input power of 300 kilowatts it could produce potentials of the order of 10 million volts, at frequencies of 50–150 kHz, creating huge "lightning bolts" reportedly up to 135 feet long. During experiments, it caused an overload which set fire to the alternator of the Colorado Springs power company, destroying it, and Tesla had to rebuild the alternator.
  In the magnifying transmitter, Tesla used a modified design (see circuit) which he had developed in his New York lab in the period 1895–1898, and patented in 1902, different from his previous double-tuned circuits. In addition to the primary (L1) and secondary (L2) coils, it had a third coil (L3) which he called the "extra" coil, not magnetically coupled to the others, attached to the top terminal of the secondary. When driven by the secondary it produced additional high voltage by resonance, being adjusted to resonate with its own parasitic capacitance (C2) The use of a series-fed resonator coil to generate high voltages was independently discovered by Paul Marie Oudin in 1893 and employed in his Oudin coil.
  The Colorado Springs apparatus consisted of a 51-foot-diameter (15.5 m) Tesla transformer composed of a secondary winding (L2) of 50 turns of heavy wire wound on a 6-foot-high (2 m) circular wooden "fence" around the periphery of the lab, and a single-turn primary (L1) either mounted on the fence or buried in the ground under it. The primary was connected to a bank of oil capacitors (C1) to make a tuned circuit, with a rotary spark gap (SG), powered by 20 to 40 kilovolts from a powerful utility step-up transformer (T). The top of the secondary was connected to the 100-turn 8 ft (2.4 m) diameter "extra" or "resonator" coil (L3) in the center of the room. Its high-voltage end was connected to a telescoping 143-foot (43.6 m) "antenna" rod with a 30-inch (1 m) metal ball on top which could project through the roof of the lab.  By cranking the rod up or down he could adjust the capacitance in the circuit of the extra coil, tuning it to resonance with the rest of the circuit.
caption: Circuit of magnifying transmitter at Tesla's Colorado Springs laboratory.[62][63] C2 represents the parasitic capacitance between the windings of coil L3.