English: An experiment demonstrating resonance, conducted by Nikola Tesla at his Colorado Springs laboratory in 1899-1900 using his magnifying transmitter, a huge Tesla coil. The secondary winding of the transmitter is visible at left as the 60 ft diameter circular "fence" surrounding all the other coils. A 50 kHz radio frequency current in this coil creates a magnetic field that induces a high voltage in all the other coils by electromagnetic induction The 20 ft tall output "resonance coil" of the transmitter is at right. Its parasitic capacitance causes it to act like a resonant circuit; it is tuned to the frequency of the secondary, generating millions of volts which creates a large discharge of "streamer" arcs at its top. The corona discharges visible at the top of all the other coils shows they are also generating high voltages. Much smaller than the large coil, they have lower inductance and capacitance and thus higher resonant frequencies. Tesla claims they are tuned to harmonics of the fundamental 50kHz frequency.

Caption from Century Magazine article: The picture shows a number of coils, differently attuned and responding to the vibrations transmitted to them through the earth from an electrical oscillator. The large coil on the right, discharging strongly, is tuned to the fundamental vibration, which is fifty thousand per second; the two larger vertical coils to twice that number; the smaller white wire coil to four times that number, and the remaining small coils to higher tones. The vibrations produced by the oscillator were so intense that they affected perceptibly a small coil tuned to the twenty-sixth higher tone.

Caption of Electrical Experimenter article: A problem in harmonics. Dr. Tesla placed a number of inductance (air core) coils within the large high frequency coil as shown. Each coil resonated or became attuned to a certain harmonic of the fundamental oscillation, glowing as shown.

Alterations to image: increased contrast to bring out discharges on the coils