Collection of Tesla coils in 1919 built by the inventor, Serbian-American engineer Nikola Tesla. They consist of an induction coil or transformer, an interrupter to repeatedly interrupt the primary current, a capacitor, and a resonant air core transformer consisting of a few turns of heavy wire primary around a secondary of many turns of fine wire, connected to the high voltage binding posts. Some contained a vibrator interrupter, some a mercury interrupter. These small compact Tesla coils were produced by Tesla for industrial purposes such as powering his gas discharge lamps and for producing ozone for water purification. A technical article describing how they work and giving circuit diagrams is "Tesla circuit controllers for obtaining oscillatory currents of high frequency" in "The Electrical Engineer", New York, Vol. 26, No. 538, August 25, 1898, p. 180-182