English: Early resonant transformer circuit built by Prof. Henry Rowland in 1889 to generate radio frequency currents, a forerunner of the Tesla coil. The source says Rowland demonstrated it at the 1899 annual meeting of the American Institute of Electrical Engineers. It consists of a resonant transformer composed of two identical 6-turn coils of wire each attached to a glass plate capacitor (bottom) to make a tuned circuit. A high voltage from a step-up transformer was applied to one side of the circuit, creating sparks across the spark gap, visible in front of the capacitor. When the primary and secondary circuits were tuned to resonance, the secondary produced high voltage oscillations, generating sparks across the second spark gap. It functioned similarly to the Tesla coil circuit invented 2 years later by Nikola Tesla, except that it didn't generate as high voltages.

Electrical engineer Elihu Thomson, a rival of Tesla's, published this picture in an 1899 paper in the journal The Electrician to show that Tesla did not invent the Tesla coil. Thomson claimed that he, Rowland, and other scientists had been using such circuits for years before Tesla patented it in 1891. However Tesla seems to have been the first to publish a description.