English: Parts of the magnifying transmitter, a huge Tesla coil high voltage generator built by inventor Nikola Tesla in his Colorado Springs laboratory 1899-1900. Picture was taken by photographer Dickinson V. Alley during a December 1899 photo shoot commissioned by Tesla to accompany his upcoming article "The Problem of Increasing Human Energy" in the June 1900 Century Magazine. The apparatus consumed 300 kilowatts and could produce potentials in the neighborhood of 12 million volts at frequencies of 50 to 150 kHz, reportedly producing 135 foot "lightning bolts". The first time Tesla turned it on, the resulting overload set fire to the generator of the Colorado Springs electric power company, destroying it. Tesla used the device in experiments on wireless power transmission.

The curving wall at left is the 53 ft diameter secondary winding of the air core resonant transformer, consisting of 50 turns of thick wire wound on a 9 ft. high circular wooden "fence" around the periphery of the lab. The primary winding is a single turn of heavy cable buried at the base of the "fence". It is connected to the bank of oil capacitors in foreground and to a rotary spark gap in background. The primary is powered by the powerful Westinghouse step-up transformer against the wall in back, which Tesla had rewound to produce from 20 to 40 kilovolts. At background right is a large adjustable coil in series with the transformer, that Tesla used to control the current to the circuit.

Caption: Photographic view of the essential parts of the electrical oscillator used in the experiments described.